University of Southern California Dissertations and Theses

Electrodeposition of platinum-iridium coatings and nanowires for neurostimulating applications: fabrication, characterization and in-vivo retinal stimulation/recording EIS studies of hexavalent a...

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Electrodeposition of platinum-iridium coatings and nanowires for neurostimulating applications: fabrication, characterization and in-vivo retinal stimulation/recording EIS studies of hexavalent a...

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ELECTRODEPOSITION OF PLATINUM-IRIDIUM COATINGS AND
NANOWIRES FOR NEUROSTIMULATING APPLICATIONS:
FABRICATION, CHARACTERIZATION AND IN-VIVO RETINAL
STIMULATION/RECORDING
EIS STUDIES OF HEXAVALENT AND TRIVALENT CHROMIUM BASED
MILITARY COATING SYSTEMS

by

Artin Petrossians

A Dissertation Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(MATERIALS SCIENCE)

May 2012

Copyright 2012 Artin Petrossians

ii

DEDICATION

To my Family

iii
ACKNOWLEDGMENTS

I would like to mention my great appreciation to my two advisors through my entire
doctoral research, Prof. Florian Mansfeld and Prof. James Weiland for their outstanding
guidance toward my PhD degree. They have not been just advisors on my research
project, but the ones who helped me to achieve my dream goals to be someone whose
achievements could someday provide help for better treatment for the patients around the
world. While working all these years with these two scientists, I always felt that I am
dealing with my second family.

I would like to greatly thank Prof. Mark Humayun who has pioneered this effort to
restore vision for the blind and his outstanding work that has provided such a great
opportunity for me and others to be part of.

I would like to thank Dr. Jack Whalen III, who has been a real friend through all these
years and patiently has answered all my sporadic questions also helped me to write my
papers like my third adviser .

I would like to thank The Mork Family Departments of Chemical Engineering and
Materials Science faculty members Prof. Edward Goo, Prof. Steven Nutt, Prof. Anupam
Madhukar and Prof. Ted Lee for their continuous guidance.

iv
I also would like to express great appreciation to my previous advisor at California State
University of Northridge (CSUN), Prof. Behzad Bavarian; who has continued his help
and advice after I had graduated from CSUN.

Ne x t I would li ke to t ha n k m y c oll e a g ue s a nd a ll m y fr i e nds i n P rof . Ma ns fe ld’s a nd P rof.
W e il a nd’s groups and the staff at the Mork Family Department of Chemical Engineering
and Materials Science at UPC and the staff of the BMES_ERC and Doheny Vision
Research Center at Health Science Campus at USC.
I also want to say thanks to my family for their support for all these years.

And last but not least I have few words for my Mom. Mom, I miss you so much. It has
been 19 years since your final journey to heaven. You have been on my shoulders as a
guardian angel and I am sure that you are watching me right now. I did it! I hope that I
ha ve made y ou v e r y prou d …

v
TABLE OF CONTENTS

DEDICATION .................................................................................................................... ii
ACKNOWLEDGMENTS ................................................................................................. iii
LIST OF TABLES ............................................................................................................. ix
LIST OF FIGURES ............................................................................................................ x
ABSTRACT ..................................................................................................................... xvi

CHAPTER I: Introduction and Background ....................................................................... 1
1.1 Introduction ................................................................................................................... 1
1.2 Project Motivation ........................................................................................................ 1
1.3 Research Approach ....................................................................................................... 4
1.3.1 Electrodeposition from Sodium Hexachloroiridate and
Sodium Hexachloroplatinate Mixed Solutions ............................................................... 5
1.3.2 Study of the Charge Injection Properties of Electrodeposited
High Surface-Area Platinum-Iridium Coatings .............................................................. 5
1.3.3 Electrodeposition of Platinum-Iridium Nanowires ................................................ 5
1.3.4 In-vivo Electrical Stimulation of Rat Retina using High
Surface-Area Platinum-Iridium Coated Single Electrodes ............................................. 6
1.4 Historical Background on Electrical Neurostimulation ................................................ 6
1.5 Visual Neuroprosthesis ................................................................................................. 8
1.6 Microelectrodes........................................................................................................... 12
1.7 Materials Selection...................................................................................................... 15
1.7.1 Technologies for Electrode Optimization ............................................................ 17
1.7.2 Electrodeposition ................................................................................................. 19
1.7.3 Electrodeposition of Alloys ................................................................................. 19
1.7.4 Effect of Ultrasound in Electrochemistry ............................................................ 20
1.8 References ................................................................................................................... 22

CHAPTER II: Electrodeposition and Characterization of
Dense Platinum-Iridium Alloy Films................................................................................ 25
2.1 Introduction ................................................................................................................. 25
2.2 Background ................................................................................................................. 25
2.3 Materials and Methods ................................................................................................ 28
2.3.1 Substrate Preparation ........................................................................................... 28
2.3.2 Electrodeposition Solution Preparation ............................................................... 28
2.3.3 Electrochemical Measurements ........................................................................... 28
2.3.4 Electrodeposition Process .................................................................................... 31
2.3.5 Weight Gain and Thickness Measurements of
Platinum-Iridium Films ................................................................................................. 31
2.3.6 Surface and Compositional Analysis ................................................................... 32
vi
2.3.7 X-Ray Diffraction ................................................................................................ 32
2.3.8 Nanoindentation Measurements ........................................................................... 32
2.3.9 Cyclic Voltammetry ............................................................................................. 33
2.3.10 Impedance Measurements .................................................................................. 33
2.3.11 Potentiodynamic Polarization Measurements .................................................... 34
2.4 Results and Discussion ............................................................................................... 34
2.4.1 Electrodeposition of Platinum-Iridium Dense Films ........................................... 34
2.4.2 Effect of Ultrasound in Electrochemistry ............................................................ 42
2.4.3 Scanning Electron Microscopy (SEM) ................................................................ 43
2.4.4 Material Properties and Characterization of Thin Films...................................... 46
2.4.5 X-Ray Diffraction ................................................................................................ 46
2.4.6 Nanoindentation ................................................................................................... 50
2.4.7 Cyclic Voltammetry for Electroplated Platinum-Iridium
Films ............................................................................................................................. 52
2.4.8 EIS Data Analysis ................................................................................................ 55
2.4.9 Potentiodynamic Polarization Measurements ...................................................... 59
2.5 Conclusions ................................................................................................................. 61
2.6 References ................................................................................................................... 63

CHAPTER III: In-vitro Characterization of Safe Charge Transfer
Characteristics of Electrodeposited Platinum-Iridium Coatings ....................................... 65
3.1 Introduction ................................................................................................................. 65
3.2 Background ................................................................................................................. 65
3.2.1 Safety Limits for Neural Electrostimulation ........................................................ 68
3.3 Electrochemical Measurements .................................................................................. 70
3.3.1 Cyclic Voltammetry ............................................................................................. 70
3.3.2 Electrochemical Impedance Spectroscopy .......................................................... 72
3.4 Materials and Methods ................................................................................................ 74
3.4.1 Microelectrode Array (MEA) Fabrication Methods ............................................ 74
3.4.2 Electrodeposition ................................................................................................. 76
3.4.3 Surface and Compositional Analysis ................................................................... 77
3.4.4 Cyclic Voltammetry ............................................................................................. 78
3.4.5 Impedance Measurements .................................................................................... 78
3.4.6 Charge Injection Measurements .......................................................................... 79
3.5 Results and Discussion ............................................................................................... 81
3.5.1 SEM and EDS of Platinum-Iridium Coatings ...................................................... 81
3.5.2 Cyclic Voltammetry for Electroplated Platinum-Iridium
Coatings ........................................................................................................................ 82
3.5.3 EIS Measurements ............................................................................................... 85
3.5.4 Voltage Response to Applied Currents Pulses..................................................... 87
3.5.5 Reliability Test of the Platinum-Iridium Electroplated
Electrode ....................................................................................................................... 94
3.6 Conclusions ................................................................................................................. 96
3.7 References ................................................................................................................... 97

vii

CHAPTER IV: Electrodeposition of Platinum-Iridium
Alloy Nanowires for Hermetic Packaging of Microelectronics ..................................... 100
4.1 Introduction ............................................................................................................... 100
4.2 Background ............................................................................................................... 100
4.3 Materials and Methods .............................................................................................. 105
4.3.1 Alumina Template Preparation .......................................................................... 105
4.3.2 Platinum-iridium Nanowire Electrodeposition .................................................. 105
4.3.3 SEM Characterization ........................................................................................ 109
4.3.4 TEM Characterization ........................................................................................ 110
4.3.5 Electrical Testing ............................................................................................... 110
4.3.6 Helium Leak Testing.......................................................................................... 111
4.5. Results and Discussion ............................................................................................ 112
4.5.1 Platinum-Iridium Nanowire Electrodeposition .................................................. 112
4.5.2 SEM Characterization ........................................................................................ 114
4.5.3 TEM Characterization ........................................................................................ 117
4.5.4 Effect of Deposition Potential on the Chemical
Composition of the Nanowires ................................................................................... 120
4.5.5 Conductivity Measurements .............................................................................. 121
4.5.6 Helium Leak Testing.......................................................................................... 123
4.6 Conclusions ............................................................................................................... 124
4.7 References ................................................................................................................. 126

CHAPTER V: Biological Experiments Using Electroplated
Platinum-Iridium Microelectrodes: ................................................................................. 128
5.1 In-vivo Electrical Stimulation of Retinal Cell Tissue Using
Platinum-Iridium Coated Microelectrodes ..................................................................... 128
5.2 In-vitro Recording Using Platinum-Iridium Coated MEAs...................................... 128
5.1.1 Introduction ............................................................................................................ 128
5.1.2 Materials and Methods ........................................................................................... 128
5.1.2.1 Animal Preparation and Surgery ..................................................................... 128
5.1.2.2 Stimulation Electrode...................................................................................... 129
5.1.3 Results and Discussion .......................................................................................... 129
5.1.4 Conclusions ............................................................................................................ 132
5. 2 In-vitro Recording Using Platinum-Iridium Coated MEAs..................................... 133
5.2.1 Introduction ............................................................................................................ 133
5.2.2 Background ............................................................................................................ 133
5.2.3 Materials and Methods ........................................................................................... 135
5.2.4 Results and Discussion .......................................................................................... 136
5.2.5 Conclusions ............................................................................................................ 143
5.3 References ................................................................................................................. 144

viii

CHAPTER VI: Research Summary and Future Work ................................................... 145
6.1 Summary ................................................................................................................... 145
6.2 Suggestions for Future Work .................................................................................... 148
6.2.1 Chronic In-vivo Stimulation .............................................................................. 148
6.2.2 In-vivo Neural Recording .................................................................................. 148
6.2.3 Other Biomedical Applications.......................................................................... 148

EIS STUDIES of Hexavalent and Trivalent Chromium Based
Coating Systems for Al 2024 .......................................................................................... 149
2.1 Introduction ............................................................................................................... 149
2.2 Experimental Methods .............................................................................................. 155
2.2.1 Sample Preparation ............................................................................................ 155
2.2.2 Electrochemical Impedance Spectroscopy (EIS) Measurements ...................... 158
2.3 Results and Discussion ............................................................................................. 158
2.3.1 Analysis of Impedance Spectra .......................................................................... 158
2.3.2 Analysis of Impedance Spectra of Scribed Samples .......................................... 174
2.3.3 Optical Evaluations of Scribed Samples ............................................................ 178
2.4 Conclusions ............................................................................................................... 181
2.5 Suggestions for Future Work .................................................................................... 182
2.6 References ................................................................................................................. 183

COMPREHENSIVE BIBLIOGRAPHY ........................................................................ 185

ix
LIST OF TABLES

Table 2.1 Average chemical compositions and standard deviation
of 80 –20% platinum-iridium foil and the platinum-iridium films
electroplated for 4, 8, 16 and 32 min for 3 sets of samples
de ter mi ne d b y W DS…… ………… ……………………………… …………..36

Table 2.2 Structural characteristics of Au, Pt, Ir and Pt-Ir samples
using XRD…………… … …..……………… …………...………...….……...48

Table 2.3 Hardness and reduced modulus values for platinum foil
and platinum-iridium electroplated film …………………..………… …..…...51

Table 2.4 Fit parameters for the impedance spectra shown in
Figure 2.13 … ………………………………………………………………….58

Table 4.1 Chemical composition of platinum-iridium nanowires in
different applied potential ranges. ……… …………...……… ……..………..121

Ta ble 2.1 D iff e re nt c oa ti ng s a ppli e d on Al 2024 sa mpl e s……………………………...156

x
LIST OF FIGURES

Figure 1.1 Schematic diagram of an epiretinal implant. Adopted from
IEEE Engineering in Medicine and Biology, 2 4, 15 ( 2005) ………… .. ………3

Figure 1.2 Demonstration of animal nerve-muscle experiments by
Galvani in the late eighteenth century. Picture taken from
Neural Prosthesis: Fundamental Studies, Agnew W.F.,
McCreery D.B., Prentice Hall, NJ (1990) ……………………………………..7

Figure 1.3 Retinal implants: a) Argus I with 16 electrodes (human subject)
[Mahadevappa et al. 2005] and b) next generation implant with 1000
electrodes (animal subject) [Weiland et al. 2009] …………………………...10

Figure 1.4 Schematic example of a microelectrode array (contact pads are
not shown). Designed and fabricated by Artin Petrossians (USC)
and Mandheerej Nandra (Ca lt e c h)…… ………… …………….…… ………..12

Figure 2.1 Electrochemical cell. Deposition chamber (A), reference electrode
chamber (B), interconnection via a capillary (C), substrate holder
(D), clamping bed (E), toothless clip (F) and substrate (G) ………………… .30

Figure 2.2 Comparison of the surfaces of platinum-iridium films prepared
from high pH (a) and low pH (b) electroplating baths ……………………….37

Figure 2.3 Cyclic voltammograms recorded during electrodeposition of
platinum-iridium films ……………………………………………………….38

Figure 2.4 Anodic (a) and cathodic (b) maximum currents as a function
of number of cycles ………………………………………… ……...………...40

Figure 2.5 Measured thicknesses of electroplated
platinum-iridium films.....................................................................................41

Figure 2.6 Plots of c oa ti n g s thi c kne ss ( □ )
and coatings ma ss (■ ) vs. de posi ti on ti me ….…… …………………………..42

Figure 2.7 Comparison of deposition cycles for sonicated
mode (a) and silent mode (b) ………….………… …… …….. ……………....43

Figure 2.8 SEM micrographs of Au substrate (a) and platinum-iridium
films (b –e)with 4, 8, 16 and 32 min. deposition time, respectively.
Micrographs (f –j) are of the same samples as (a –e), respectively,
but at higher magnification (scale bars at bottom are for all
mi c rogr a phs i n c olum n) ………… ………..……………………………….…45

xi
Figure 2.9 X-ray diffraction patterns for platinum-iridium
electroplated film …………………………………………………………….47

Figure 2.10 Comparison of XRD patterns of platinum-iridium
film, Au substra te a nd c ontrol samples………… ………… ………...……...48

Figure 2.11 Representative load-displacement curves for a platinum
foil and a platinum- iridi um ele c tropla ted f il m… ………… ………..……….51

Figure 2.12 Cyclic voltammograms in 0.05 M H
2
SO
4
of Au substrate
(a), control samples (b) –(d) and platinum-iridium
films (e) with different deposition times ………… …….…………………....54

Figure 2.13 Bode plots for Au substrate, Pt, Ir, 80 –20 Pt-Ir foils and
platinum-iridium films measured in 0.05 M H
2
SO
4
……… ………..………56

Figure 2.14 Equivalent circuit for analysis of impedance data shown in
Figure 2.16 ……… …………………………………………………………..57

Figure 2.15 Bode plots of experimental ( …) and fit data ( ―) of
platinum-iridium film electro plate d for 32 mi n………………… ....……….57

F ig u re 2.16 1/R p (□ ) a nd C (■ ) a s a func ti on of de posi ti on ti me
(da ta f rom T a ble 2.4) … …………………………………… ..……………...58

Figure 2.17 Polarization curves for the platinum foil and the
platinum-iridium fil m…… ……………………..……….………………..…60

Figure 3.1 A typical voltage response to biphasic current pulse waveform.
(Ohmic resistance (OR) and charge transferred (CT) ………… ……………..69

Figure 3.2 Cyclic voltammogram for polycrystalline platinum
electrode in acidic solution. (Conway REF …… …………………………….71

Figure 3.3 Typical Bode-plots for platinum foil in 0.05 M H
2
SO
4
………… ….………..73

Figure 3.4 Fabrication processes of parylene- ba s e d MEAs… .…………………………..76

Figure 3.5 Representative biphasic constant current pulse waveform used for
mi c roe lec trod e testing (n ot t o sc a le) …………… ……………………………80

Figure 3.6 Scanning electron micrographs of platinum thin film (a) and
platinum-iridium electrodeposited MEAs(b) …… ……………. ….…………82

xii
Figure 3.7 Cyclic voltammograms of uncoated platinum and
platinum-iridium electroplated electrodes in 0.05
M H
2
SO
4
(pH = 2.0) (a) and in PBS solution (pH = 7.4) (b) ………… ….…..84

Figure 3.8 Bode plots for uncoated platinum and an electroplated
platinum-iridium electrode measured in 0.05 M
H
2
SO
4
(pH =2.0) at the OCP ……. ……………… …………………………...86

Figure 3.9 Voltage response of a Pt microelectrode to a biphasic
s y mm e tric c urr e nt pul se with ampli tude I = 60 μ A
(frequency = 400 Hz) ……………...……………………….………………..88

Figure 3.10 Voltage responses of platinum and platinum-iridium
electroplated MEAs to applied current pulses of
amplitudes: (a) 30 μ A ( b) 60 μ A ( c ) 90 μA a nd ( d) 12 0 μ A… …..…....……89

Figure 3.11 Comparison of voltage response magnitudes (V) for
platinum and platinum-iridium microelectrodes measured
in PBS at room temperature …………………… ……………… …..……….90

Figure 3.12 Potential response of platinum-iridium electroplated
mi c roe lec trod e (Φ = 200 µ m) to 800 μ A c urr e nt p ulses
in P B S a t room e mper a t ure …… ……………… ………………...........…….92

Figure 3.13 Potential response of platinum-iridium electroplated
mi c roe lec trod e (Φ = 75 µ m) in PBS at room temperature
with the applied current amplitude to reach CIC
max
.......................................93

Figure 3.14 Impedance data obtained at frequencies of 1 kHz,
0.1 Hz and 0.01 Hz as a function of time in 0.05 M H
2
SO
4

at room temperature … ………………………………………………... …….95

Figure 4.1 The top diagram shows a interconnect substrate with a case bonded
over the top of the chip. Bumps on both the chip and the interconnect
substrate facilitate electrical connections. This interconnect substrate
could also be used with a conformal coating technology (bottom).
Dr a win g not t o sc a le ………………………………………………………..103

Figure 4.2 Electrochemical cell used for nanowire deposition in nano-channeled
aluminum oxide (Al
2
O
3
) template ………………………………………….107

Figure 4.3 Schematic of representing the fabrication processes of metallic
nanowires in AAO nanopores . Dr a win g not t o sc a le……… .………….…...108

Figure 4.4 Schematic of the isolation processes of electrodeposited
na nowir e s in AAO t e mpl a tes. D ra wing not t o sc a le…… .………………….109
xiii

Figure 4.5 (left) Schematic of electrochemical template fabrication of metallic wires.
(right) Demonstration of the reduction of metal ions on the working
electrode at the base of a single channel.
Dr a win g s not to sca le… ………………………… ………….……….……..113

Figure 4.6 SEM micrograph of the cross section of platinum-iridium
na nowir e s g ro wn in AA O por e s…… …………… ……………….. ..………114

Figure 4.7 SEM micrograph of isolated nanowires electrodeposited at
higher pH with brittle structures....................................................................115

Figure 4.8 SEM micrograph of isolated nanowires electrodeposited at
lower pH with shorter lengths.......................................................................116

Figure 4.9 SEM micrograph of isolated dense nanowires electrodeposited at
pH of 3.1 without discontinuities...................................................................116

Figure 4.10 Bright-field (a) and dark-field (b) images of platinum
na nowir e s………… ……………………………...………………...……...118

Figure 4.11 Electron beam diffraction pattern of platinum-iridium
na nowir e with f c c struc t ure …………… .……… …………………………..119

Figure 4.12 HRTEM image of the individual platinum- iridi um nanow ire … ……… .….120

Figure 4.13 SEM micrograph of the testing device used for electrical
conductivity measurement of a) single platinum nanowire
and b) single platinum- ir idi um nanow ire …………………………… ..........122

Figure 4.14 Current vs. voltage plots demonstrating the improved
conductivity of platinum-iridium nanowires … …………………………...123

Figure 5.1 Current stimulation of the retinal with platinum-iridium modified
and unmodified microelectrodes. (a) Electrodes were touching
the retina. (b) Electrodes were close to the retina..........................................131

Figure 5.2 Representative Figure demonstrating different types of neural
recording microelectrodes (Center for Microelectrode
Technology_CenMeT., University of Kentucky...........................................134

Figure 5.3 Comparison of MEAs before (a) and after (b) electrodeposition of
platinum-iridium alloy coating on the recording sites...................................136

xiv
Figure 5.4 Representative cyclic voltammograms recorded in 0.05 M H
2
SO
4
showing that the MEA recording surface with an
electrodeposited platinum-iridium coating has a larger
active area than that of the uncoated thin film platinum
MEA recording surface …………………………………………………......137

Figure 5.5 Impedance measurements for platinum and electrodeposited
platinum-iridium MEA recording surfaces in 0.05 M H
2
SO
4
……………....139

Figure 5.6 Comparison of the sensitivity of the platinum (blue bars) and
platinum-iridium (black bars) microelectrodes to peroxide
concentration (a-d). …… ……………………… ……………. ……………..142

Figure 2.1 Impedance spectra for sample # 1 at different exposure times ……………..159

Figure 2.2 Impedance spectra for sample # 4 at different exposure times ……………..160

Figure 2.3 Impedance spectra for sample # 7 at different exposure times ……………..161

Figure 2.4 Equivalent circuit for the coating model ……………………………………163

Figure 2.5 One-time-constant model (OTCM )…… ……………………………………163

Figure 2.6 Comparison of the impedance spectra of a polymer coated metal
with an intact coating (curve 1) and a deteriorated coating (curve 2)
(Mansfeld 2006 ……… ……………………………………………………..164

Figure 2.7 Comparison of the impedance spectra of samples # 1, 4 and 7 after
(a )1, ( b ) 14 a nd ( c ) 30 d a y s of e x posure … .………………………………...168

Figure 2.8 Time dependence of E
corr
of samples # 1, 4 and 7 during exposure
to 0.5N NaCl ………………………………………………………………..169

Figure 2.9 Time dependence of C
c
of samples # 1, #4 and #7 during exposure
to 0.5N NaCl ………………………………………………………………..170

Figure 2.10 Time dependence of Rpo of samples # 1, 4 and 7 during exposure
to 0.5N NaCl ………………………………………..……………………..172

Figure 2.11 Time dependence of C
dll
of samples # 1 and 4 during exposure to
0.5N NaCl …………………………………………………………….…...173

Figure 2.12 Time dependence of R
p
of samples # 1, #4 and #7 during exposure
to 0.5N NaCl ……………………………………………..………………..174

xv
Figure 2.13 Impedance spectra for scribed sample # 1 at different
exposure times ………………... …………………………………………...175

Figure 2.14 Impedance spectra for scribed sample # 4 at different
exposure times ………………………...…………………………………...176

Figure 2.15 Impedance spectra for scribed sample # 7 at different
exposure times …….…………………………… ………………………….177

Figure 2.16 Optical micrograph of scribed sample # 1 after 3 days of
exposure to 0.5N NaCl ………………………………… … ……………….179

Figure 2.17 Optical micrograph of scribed sample # 4 after 3 days of
exposure to 0.5N NaCl …………………………………… …...…………..179

Figure 2.18 Optical micrograph of scribed sample # 7 after 3 days of
exposure to 0.5N NaCl …………………………………………………….180

xvi
ABSTRACT

The studies presented in this thesis are composed of two different projects demonstrated
in two different parts. The first part of this thesis represents an electrochemical approach
to possible improvements of the interface between an implantable device and biological
tissue. The second part of this thesis represents electrochemical impedance spectroscopy
(EIS) studies on the corrosion resistance behavior of different types of polymer coated
Al2024 alloys.

In the first part of this thesis, a broad range of investigations on the development of an
efficient and reproducible electrochemical deposition method for fabrication of thin-film
platinum-iridium alloys were performed. The developed method for production of dense
films was then modified to produce very high surface area coatings with ultra-low
electrochemical impedance characteristics. The high-surface area platinum-iridium
coating was applied on microelectrode arrays for chronic in-vitro stimulation.
Using the same method of producing dense films, platinum-iridium nanowires were
fabricated using Anodized Aluminum Oxide (AAO) templates for hermetic packaging
applications to be used in implantable microelectronics. The implantable microelectronics
will be used to perform data reception and transmission management, power recovery,
digital processing and analog output of stimulus current.
Finally, in-vivo electrical stimulation tests were performed on an animal retina using high
surface-area platinum-iridium coated single microelectrodes to verify the charge transfer
characteristics of the coatings.

xvii
In the second part of this thesis, three different sets of samples with different
combinations of pretreatments, primers with the same type of topcoat were tested in 0.5
N NaCl for period of 30 days. The surface changes measured by EIS as a function of time
were then analyzed. The analysis of the fit parameters of the impedance spectra showed
that the different primers had the most effect on the corrosion protection properties of the
coatings in which the primers with hexavalent chromium ions (Cr
6+
) provided better
corrosion protection compared to primers with trivalent chromium ions (Cr
3+
).
After 30 days of the exposure of the samples in 0.5 N NaCl, one sample from each set of
samples was scribed and exposed to 0.5 N NaCl for 3 days. Analysis of the impedance
spectra revealed that the samples with chromium conversion coating pretreatment and
hexavale nt chr omi um prim e r show e d “ s e lf - h e a li ng ” c ha r a c te risti c s a nd pr o vided be tt e r
corrosion protection on the scribed areas compared to the scribed samples with trivalent
chromium pretreatment and non-hexavalent chromium primer.

1
CHAPTER I
Introduction and Background

1.1 Introduction

The objective of Part I of this thesis which is part of an ongoing project dealing with the
investigation of advanced retinal prosthesis is to develop an electrochemical metal alloy
coating deposition process to improve the properties of microelectrodes to be used in
retinal prosthesis.
This chapter includes five major sections: 1) Project Motivation, 2) Research Approach
3) Historical Background of Electrical Neurostimulation 4) Visual Neuroprosthesis 5)
Anatomy and Physiology of the Eye, 6) Microelectrodes and 7) Materials Selection.

1.2 Project Motivation

Neurostimulating/recording implantable devices have been extensively used for partial or
complete recovery of the functionality of disabled human body organs. These devices
include retinal prosthesis for vision restoration [Weiland et al. 2005], cochlear implant for
hearing restoration [Wilson et al. 2003] as well as deep brain stimulators (DBS) used for
e pil e ps y , P a rkinson’ s di se a se , c h ronic pa in, d y sto nia and depression [Benabid et al 2003],
neuromuscular prosthesis [Navarro et al, 2005], and pacemakers for people with heart
diseases. All implantable medical devices communicate with nerve cells of the body
called device/tissue interface, where the microelectronics transfer electrical signals to the
neurons through microelectrodes. The properties of the interface material are one of the
most important parameters to be considered during the design of the device.
2
Human vision restoration by electrical stimulation of the retina is feasible based on
physiology, anatomy and functional description of the human vision system. Intraocular
prosthesis can restore sight even in one eye by using the remaining functional optic
nerves, inner retina and remaining retina-geniculo-cortical pathways. Cortical prosthesis
can be used if the visual pathway from the retina to the cortical area is damaged.

There are four main approaches to restore the human sight. Those include visual cortex
stimulation, optic never stimulation, sub-retinal stimulation and epi-retinal stimulation.
Visual cortex prostheses on human subjects were first implanted in 1995, epiretinal
prostheses in 1998 and subretinal in 2000 [Finn et al. 2003].

Implantation of these devices can partially restore the vision for the blind. For a better
understanding of the visual prosthesis concepts, the physiology of the visual system is
discussed next.

In cortical visual prosthesis, stimulating electrodes are directly inserted into the visual
cortex. The degree of success depends on the development of the visual cortex electrode
array. In optic nerve prosthesis, an electrode array penetrates into the optic nerves to a
certain depth and locations in the brain. In sub-retinal prosthesis electrode arrays are
implanted in the sub-retina space where the photoreceptors were prior to degeneration.
This position requires that connections to the electrode array need to be made across the
choroid blood vessels or across the retina, both of which pose long-term risks. Instead of
photoreceptors, electrode arrays stimulate the bipolar and intermediate cells. This device
is implanted underneath of the inner cell layer of the retina.

3
Epiretinal prosthesis is another type of device in which the electrode array is in direct
contact with the ganglion cell layer of the inner layer of the retina (Figure 1.1).
The work presented in Part I of this dissertation describes the development of an
electrochemical deposition method for surface modification of microelectrodes to be used
in the epiretinal prosthesis.

Figure 1.1 Schematic diagram of an epiretinal implant. Adopted from IEEE Engineering
in Medicine and Biology, 24, 15 (2005).

The epiretinal prosthesis is composed of two main components that contain an external
image capture and a processing system as well as an implanted stimulator. The external
component is composed of a camera mounted on eyeglasses that captures images and
wirelessly transmits the captured image information along with the power to the
implanted antenna. The internal device is composed of a microchip, an antenna and a
4
microelectrode array which is tacked to the retina. The antenna conducts the received
data to the microchip to be processed. Based on the input data, the microchip produces an
output as an electrical current that is applied to the retina through the attached
microelectrodes. The ganglion cells sense the stimulated current and transfer the signals
to the visual cortex through the optic nerves.

The resolution of the stimulated image is directly related to the number and the size of the
implanted microelectrodes. This could be compared to the resolution of the picture on a
TV in which the higher the number of the pixels, the higher the resolution of the picture
on the TV.

1.3 Research Approach

The main goal of this project was to develop a reproducible electrochemical deposition
method for production of thin film platinum-iridium alloy coatings with specific
properties such as high surface area, excellent adhesion, 60:40% ratio of platinum-
iridium with controlled deposition rate and surface microstructure.
To date, very few studies of the general methods of electrodeposition of platinum-iridium
thin film coatings have been reported. In this approach, the steps described below were
taken.

5
1.3.1 Electrodeposition from Sodium Hexachloroiridate and Sodium Hexachloroplatinate
Mixed Solutions

Extensive studies of an electrochemical deposition method of platinum-iridium films
using platinum and iridium salts such as sodium hexachloroiridate (IV) hexahydrate and
sodium hexachloroiridate (III) hydrate mixed with sodium hexachloroplatinate(IV)
hexahydrate were performed. The experimental characterizations were applied and the
results were compared to Figure out the optimum conditions to electroplate alloy thin
films. These experimental procedures include several sets of platinum and iridium salt
mixtures with different ratios and concentrations with different pH values for fabrication
of dense and defect-free thin alloy films.
1.3.2 Study of the Charge Injection Properties of Electrodeposited High Surface-Area
Platinum-Iridium Coatings

After electrodeposition of platinum-iridium thin films, slight changes on the
electroplating conditions were applied to create very high surface area platinum-iridium
coatings. Using electrochemical methods the charge transfer properties of the high
surface-area platinum-iridium coated microelectrodes were evaluated. These methods
included electrochemical impedance spectroscopy, cyclic voltammetry and galvanostatic
current pulsing.
1.3.3 Electrodeposition of Platinum-Iridium Nanowires

Dense platinum-iridium nanowires were electrodeposited using anodized aluminum oxide
templates. The electrical conductivity of single platinum-iridium nanowires was
compared to that of pure platinum nanowires.
6
1.3.4 In-vivo Electrical Stimulation of Rat Retina using High Surface-Area Platinum-
Iridium Coated Single Electrodes

In the final step, a platinum-iridium electroplated needle type electrode probe was used to
stimulate the retina of a rat in an in-vivo experiment. The goal of this step was to
demonstrate the improved charge transfer properties of modified microelectrodes and
compare them with that of non-modified electrodes.

1.4 Historical Background on Electrical Neurostimulation

Electrophosphenes was first attempted in 1755 by the French physician and chemist
Charles Leroy to cure blindness. He discharged a leyden jar which is a high voltage
capacitor from the head of a blind person [LeRoy, 1755]. In the late eighteenth century
L ui g i Ga lvani p e rf o rme d a se rie s of e x pe rime nts on the musc les of de a d fr o g s’ le g s in
which he observed the contraction of the leg muscles when they were struck by a spark.
I n 1791 h e pre s e nted the theor y o f “ a nim a l ele c tri c it y ” [ Ga lvani 1 791]. Figure 1.2 shows
a n e x a mpl e of G a lvani’ s a nim a l ex pe rime nts. Ga lvani then postul a ted the e x is tenc e of
some t y pe o f “ a nim a l e le c tric it y ” . Ne a rl y a t t he s a me tim e Ale ssandr o Vol ta pr e se nted
the voltaic battery which was made of a stack of dissimilar metals that generated an
e lec tric a l pot e nti a l. He c l a im e d that the ba tt e r y w a s the sour c e of e lec tri c it y for Ga lvani’ s
experiments. Volta then electrically stimulated his ear and produced a crackling noise by
insertion of the battery terminal into his ear channel [Volta, 1800, Ratty 1990].

There was some controversy for some time concerning the interpretation of the results of
Ga lvani’ s e x pe rime nts. Volta e ve ntuall y showe d that e lec tric it y c ould be ge ne ra t e d b y
7
placing two dissimilar metals in an electrolyte and completing the electrical circuit [Volta
1800]. These stimulation methods were labeled as Galvanic stimulation [Hambrecht et al.
1990].

Figure 1.2 Demonstration of animal nerve-muscle experiments by Galvani in the late
eighteenth century. Picture taken from Neural Prosthesis: Fundamental
Studies, Agnew W.F., McCreery D.B., Prentice Hall, NJ (1990).

From 1820 to 1821 Orsted and Michael Faraday discovered the relation between the
direct current and magnetism that contributed to the development of electrostimulation.
Magnetic induction discovery by Faraday led to the development of transformers and
indu c ti on c oil sti mul a tors . F a r a da y ’s stim ulator s pr oduc e d puls e tra ins and c a use d
contraction of the tetanic muscle [Hambrecht, 1990]. Physiology, anatomy and pathology
8
of human muscles were then studied by Guillame Benjamin using a Faradaic stimulator.
He studied the function of the muscles by pulsing current to their motor points. His work
which was published in 1867 provided the fundamentals for research on the
neuromuscular stimulations carried out today [Hambrecht 1990].

In 1874 Roberts Bartholow performed a series of experiments on the brain of an
unconscious woman by inserting electrodes in different spots of the cerebral cortex with
high amplitudes of current. He observed different involuntary motor movements on her
arms and legs. His findings demonstrated the first electrical excitation of neurons in the
cerebral cortex, the first physical demonstration of motor function due to electrical
stimulation and - most importantly - revealed that high amounts of electric current may
significantly damage the tissue.

1.5 Visual Neuroprosthesis

Retinal prosthesis is an implantable device for restoration of sight to patients suffering
from degenerative retinal diseases which has been successfully fabricated and implanted
for over 35 patients around the world as of the writing of this dissertation. The device
named Argus has two generations with two different resolutions. Argus I has 16
electrodes (Figure 1.3a), while Argus II has 60 electrodes. The patients with the
implanted retinal prosthesis have improved mobility and can distinguish between large
letters. In order to improve the prosthesis resolution to enable the blind to be able to read
at a reasonable rate and recognize faces, a large group of scientists has been trying to
improve the resolution of the device by developing the third generation of Argus with
9
nearly 1000 electrodes (Figure 1.3b). However, due to limitations of the surface area on
the retina, the total size of the electrode arrays cannot be larger than 5 mm x 5 mm
geometric area. Therefore, the number of microelectrodes needs to be increased and
consequently the geometric size of the individual microelectrodes needs to be decreased.
However, the amount of charge to be transferred to the retina has to remain constant. As a
result, due to the decreased geometric area of each electrode, the impedance on the
electrode surface will increase and will generate heat and an excessive voltage drop
across the electrode/tissue interface. The voltage drop may induce electrochemical
irreversible faradaic reactions, toxic by-products; decomposed body fluid and produce
permanently damaged surrounding cells.

10

Mahadevappa et al. 2005

Weiland et al. 2009

Figure 1.3 Retinal implants: a) Argus I with 16 electrodes (human subject)
[Mahadevappa et al. 2005] and b) next generation implant with 1000
electrodes (animal subject) [Weiland et al. 2009].

Optic
Disk
a
b
11
The size and the shape of the implantable prosthetic device for an eye are important
factors that need to be carefully evaluated. Biological cells, optical nerves and blood
vessels can undergo permanent damage due to the insertion and implantation of a device
that is too rigid and not compatible with the soft tissue inside of the eyeball.
Displacement of the eyeball could also result from insertion of a bulky device which in
turn would change the intraocular pressure (IOP) and consequently lead to nerve fiber
layer damage. As a result, miniaturization of the implantable electronic device to place it
entirely inside the eye is highly desired.
These issues have created new challenges on the design of the device and the electrode
array fabrication. In addition to the existing fabrication issues, the limitations of the
geometric area on the retina of the eye and the increased electrode/tissue interface
impedance have limited progress of the device fabrication.
In order to overcome the existing and undeniable charge transfer difficulties without
damaging the interface tissue cells, surface modifications of the electrodes has the only
solution. By surface modification of the electrodes using a proper material, the real
surface area on the electrode/tissue interface will increase with higher safe charge transfer
capabilities due to improved electrode material properties. The increased real surface of
the electrode decreases the electrode/tissue interface impedance and as a result will
decrease the created interface voltage and the heat, thus improving device safety.

12
1.6 Microelectrodes

There are two major types of stimulating/recording microelectrodes. Electrodes can be
individual single electrodes or grouped into electrode arrays. Single electrodes are
needle-like with bipolar and monopolar design in which platinum and iridium wires are
extruded through insulators such as glass or polymer with micron scale in probe size
[Guld, 1963]. Microelectrode arrays are typically fabricated in clean rooms using
microlithography and thin film physical vapor deposition (PVD) techniques under ultra-
high vacuum (UHV) conditions (Figure 1.4).

Figure 1.4 Schematic example of a microelectrode array (contact pads are not shown).
Designed and fabricated by Artin Petrossians (USC) and Mandheerej Nandra
(Caltech).

13
Stimulation microelectrodes used in implantable neuromodulating microelectronic
devices for chronic stimulations need to minimize electrochemical damage to the
surrounding nerves or tissues. The most important and common requirements for
fabricating microelectrode arrays include a large electrode density, the ability of injecting
high electrical charge, biocompatibility and corrosion resistance [Margalit et al., 2002].
A neurostimulating processes through microelectrodes need to provide low electrode
impedance, high charge storage capacity and a low voltage drop across the
electrode/electrolyte interface.

Microelectrodes are fabricated using one of the four most common methods. These
include electrodes made of conductive nanowires, electrochemical deposition of thin
films, micro-machining and sputter-deposited under vacuum conditions. The need for
high-density electrodes with advanced performance is drastically increasing in order to
fulfill the demand for more neuroprosthetic devices. In order to increase the selectivity of
the target neurons stimulation, the size of each individual electrode in the array becomes
much smaller. Therefore, the material selection and design of the electrode array are
extremely important. Prevention of electrochemical irreversible reactions, dissolution or
corrosion of the electrodes, toxic chemical reaction byproduct generation and gas
evolution are the characteristics of an implantable electrode.

Several groups have performed large number of studies in order to develop a method of
producing electrode surface conditions that provide the required electrode properties.
Noble metals such as gold or platinum are the conventional materials used in the
14
fabrication of microelectrodes. For these materials, vacuum deposition techniques for
production of thin films are the most commonly used methods for fabrication of
neurostimulating devices [Weiland, 2000, Slavecheva 2004]. Various types of conductive
materials such as Au, Pt and Pt alloys, Ti and Ti alloys and TiN have been used as an
electrode material. One of the major concerns in the materials selection is the
biocompatibility [Seo et al. 2004, Meyer 2002].

Although metal deposition under vacuum conditions is a suitable method of producing of
2D patternable electrodes, microscopic inspection reveals that the produced thin film is
composed of nanoscale particles that are piled up on top of each other. One of the major
disadvantages of these stacked-up particles is the low bonding forces between the
particles which reduces the strength of the structure and causes low structural stability of
vacuum-deposited films. These types of films also suffer from low height-to-width ratios,
high impedance and electrical resistance which decrease the charge transfer capabilities
and increase the thermal heating across the electrode/tissue interface.

Wire electrodes are used in conditions where there is structural complexity, lower space,
electrode density and position precision [Humayun et al. 1999].
Production of high-density wire electrodes is very labor intensive and difficult, and high
production volume and low cost are important considerations. As a result, high
fabrication costs and low productivity are two major deficiencies of these electrodes.

15
Fabricated electrodes including single and multi-microelectrode arrays have a smooth
surface structure that is not suitable for charge transferring purposes due to the equality of
the geometric surface area with the real surface area of the electrode. Different methods
of surface modification and alloy noble metal deposition techniques have been applied to
improve the electrode surface properties. In the present study, an efficient
electrochemical deposition method with comprehensive investigations of the
electrochemical and physical properties of the produced alloys is discussed.

1.7 Materials Selection

An ideal stimulating electrode must meet several requirements. The electrode must be
fabricated from a biocompatible material that creates no toxic byproducts. The electrode
must be mechanically stable to maintain enough strength during the implantation process
and durability for the lifetime of the patient. During electrical stimulation using
electrodes, faradaic reactions that damage the surrounding tissue must not occur. Failure
of the electrode due to irreversible reactions must be prevented. The corrosion rate must
be very low during chronic electrode stimulation for the lifetime of the implant and the
impedance of the electrode surface must be low and stable for long term stimulation
[Merrill 2005].

Historically, implantable microelectrodes have been made of noble metals [Robblee
1990, Brummer 1983]. Noble metals such as Pt, Ir, Au, Pd and Rh have been extensively
used for electrical stimulation applications due to their relatively high corrosion
resistance [Dymond et al., 1970; White and Gross, 1974, Johnson and Hench, 1977]
16
where platinum-iridium alloys and platinum are commonly used materials for electrical
stimulation purposes of excitable cells [Brummer and Turner 1975, 1977a, 1977b, 1977c]

Proper materials for microelectrodes must meet the minimum requirements for
capacitance and compatibility [Plonsey, 1988)]. These materials must transfer high
charge density to depolarize the excitable neurons without creating irreversible faradaic
redox reactions. New neuroprosthetic devices with higher data transfer resolution require
higher number and consequently much smaller electrode arrays.

Due to the softness of pure platinum it may not be acceptable for applications that require
mechanical strength. For example, implantable electrodes need to be robust for handling
during surgical implant procedures. Therefore platinum is usually alloyed with iridium to
improve its mechanical properties. Iridium has better mechanical strength and is a much
harder metal than platinum which makes it a desirable substance as an intracortical
electrode. Bare platinum and bare iridium have similar reversible charge storage
capacities. However, when an oxide film is formed on the surface of these materials, the
charge storage capacity of these metals drastically increases over platinum [Merrill
2005]. Charge injection processes of these electrodes are performed by valency changes
between two oxide states in which reduction of oxide layer is incomplete [Merrill 2005].

Platinum and its improved alloy platinum-iridium have been used as neurostimulatory
electrodes for a long time due to their biocompatibility, high charge transfer properties
and high resistance to corrosion [Margalit 2002]. For long-term implantation, platinum,
iridium and platinum-iridium alloys are being used and have shown good performance
17
characteristics in implantable microelectrode technologies [Weiland et al. 2000, Weiland
et al. 2002].

1.7.1 Technologies for Electrode Optimization

A large number of different groups over several years have performed a broad range of
investigations on the modification of implantable stimulating microelectrodes using
different metal and metal alloy coatings. In order to reduce the impedance of the
electrode, to increase the ability of the electrode for higher charge injection and to
minimize the power consumption of the implanted device, the real surface area of the
electrodes needs to be maximized. This will lower the voltage excursion during charge
injection in the electrode/electrolyte interface in the stimulation process.

Platinum is one of the most commonly used metals as a neural stimulation electrode.
However, two major weaknesses of platinum include its softness and low charge transfer
properties (100-300µC/cm
2
) [Rose et al. 1990]. Although activated iridium oxide films
(AIROF) significantly increase the charge injection limits (2-3 mC/ cm
2
) in a reversible
faradaic reaction Ir3
+
↔ I r 4
+
+ e
−
[Brummer, 1983] under high current density
stimulation the coating delaminates and particles deposit on the surrounding tissue
[Cogan et al. 2004].

Porous electrodes fabrication by sintering tantalum pentoxide is an initial approach to
increase the real surface area of the electrode [Johnson et al 1977]. Ke et al presented a
method of high-surface area electrode fabrication using vertically aligned carbon
18
nanotube (CNT) pillars [Ke et al. 2006]. In this method, phosphorus-doped polysilicon
was deposited on quartz substrates as a conductive layer using low-pressure chemical
vapor deposition (LPCVD) and patterned by plasma etching. The pre-prepared CNTs
were integrated on patterned micro-circuitry. The numerous fabrication steps and
extremely high processing costs make this method not suitable for fabrication of
microelectrodes with high charge injection properties.

Another electrode surface modification technique is coating of the electrodes using
polyethylenedioxythiophene (PEDOT). Recent studies of PEDOT coatings have revealed
the instability of PEDOT during chronic electrical stimulation [Cui et al. 2007, Jan et al.
2009]. Delamination and cracking of the PEDOT coatings under electrical stimulation
may lead to detachment of the coating and consequently to electrode failure [Xiliang et
al. 2011].

Activated Iridium oxide films (AIROFs) are formed by electrochemically activation of
iridium metal in PBS where hydrated oxide films are formed on the electrode surface by
repeating oxidation and reduction of the iridium metal. This in turn increases the charge
injection limits by reversible faradaic reactions between Ir
3+
and Ir
4+
[Mozota et al. 1983].
Sputtered iridium oxide film (SIROF) is produced by sputter deposition of iridium metal
in oxidized plasma to form oxide film on the surface to increase charge injection abilities.
Disadvantages of both of these techniques include the activation process of the deposited
iridium in hydrous solution in AIROF that is time inefficient and higher cost of sputter
deposition in oxidizing plasma in SIROF method.
19
1.7.2 Electrodeposition

In electrodeposition or electroplating processes, an applied cathodic potential or current
reduces metal ions to metal or metal alloys onto a conductive substrate. The
electroplating process is a useful technique to deposit metal films on the conductive
surface in order to improve the electrochemical properties, wear resistance, corrosion
resistance and other thermal, magnetic and optical characteristics of the surface [Zanella
2010]. The relatively low cost and the improved properties obtained using electrolytic
and electroless deposition methods have increased the interest to the application of these
techniques [Di Bari, 2002].

1.7.3 Electrodeposition of Alloys

Metal alloy electrodeposition is an old technique that has existed for as long as
electrodeposition of pure single metals. The superior properties of electrodeposited metal
alloys compared to individual electroplated metals have discussed frequently in the
literature. Metal alloy electrodeposits provide improved properties compared to those of
electrodeposited pure metals. It should be noted that certain chemical compositions
ranges of alloys have different properties. They can be stronger and rougher, possess
higher corrosion resistance, be harder and denser and provide better protection to
underlying layer, better magnetic properties and superior wear resistance applications.
The electrodeposition of alloys is considered as co-deposition of two or more metals with
their ions being present in the electrolyte. There are three main steps considered in the
cathodic alloy deposition: migration of hydrated ions in the electrolyte that is influenced
by the applied electrode potential and diffusion of the ions. Hydrated metal ions at the
20
cathode surface enter into the diffusion layer of the aligned water molecules of hydrated
ions. Moreover, the hydrated sheath is lost and consequently, ions are neutralized by
transferring electrons from the metal substrate to the ions. The adsorbed ad-atoms diffuse
to the metal surface, then cluster to form nuclei followed by the incorporation of ad-
atoms, development of morphological and crystallographic characteristics and alloy
growth [Mordechay et al. 2000].

1.7.4 Effect of Ultrasound in Electrochemistry

Many research groups have published several articles to describe the applications of
ultrasound in electrochemistry which include crystallization and degassing, dispersion of
solids, chemical processes, synthesis, and some aspects of catalysis and polymer
chemistry. The use of ultrasound in electrodeposition has shown to improve the quality of
the deposited film as well as film adhesion and morphology and decreasing the amount of
additives needed in silent mode [Walker 1993].
Ultra sound c a us e s the f or mation of “ a c oust ic str e a mi ng ” in l iqui d whic h c r e a tes
cavitation bubbles that are related to the frequency, sonication power, solution viscosity,
nucleation sites and presence of dissolved gasses [Manson et al. 1989]. Generated
cavitation bubbles cause transients of temperature and pressure and form micro-jets
impinging towards the electrode surface.

The use of ultrasound can produce a cleaning effect on the electrode surface, general
improvements on the movements and the hydrodynamics of the species [Walton 2002].
21
During electrodeposition, the following effects can be noted as [Manson et al. 1988]:
Effective and rapid surface gas bubble removal thus preventing the deposited layer from
pitting, higher rate of ions transportation, improvement on the anodic and cathodic
process efficiencies by cleaning and activation of the electrode surface and reducing the
diffusion and double layer thicknesses due to pressure waves and the agitation.

22
1.8 References

Agnew W.F., McCreery D.B., Neural Prostheses. Prentice Hall, NJ (1990).

B e na bid A. L . De e p B ra i n S ti mul a ti on fo r Par kins on’s D isea se . C ur re nt Opinion in
Neurobiology, 13, 696 (2003).

Brenstein P., Chapter 1: Macular Biology. Age Related Macular Degeneration. Eds.
Berger JW, Fine SL, Maguire MG, Mosby Inc., MO (1999).

Brummer S.B., Roblee L.S., Hambrecht F.T., Criteria for Selecting Electrodes for
Electrical Stimulation: Theoretical and Practical Considerations. Annals of
New York Academy of Sciences, 405, 159 (1983).

Brummer S.B., Turner M.J., Bioelectrochem Bioenerg, 2, 13 (1975).

Brummer S.B., Turner M.J., IEEE Trans Biomed Eng;BME, 24, 436 (1977).

Brummer S.B., Turner M.J., IEEE Trans Biomed Eng;BME, 24, 440 (1977).

Brummer S.B., Turner M.J., Cogan S. F.; Guzelian, A. A.; Agnew, W. F.; Yuen, T. G.,
McCreery, D. B. J. Neurosci. Methods, 137, 141 (2004).

Cui X.T., Zhou D.D. IEEE Trans Neural Syst Rehabil Eng., 15, 502 (2007).

Walton D.J., ARKIVOC, 198 (2002).

Di Bari G., Metal Finishing, 35 (2002).
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Finn W.E., LoPresti P.G., Introduction to Neuroprosthetics. Handbook of
Neuroprosthetic Methods. Eds. Finn WE, LoPresti PG. CRC Press, NY., 13
(2003).

Galvani L., 7, 363 (1791).

Guld C., Medical Electronics. Proc. 5th International Conf. Medical Electronics. Liege,
Belgium, 516 (1963).

Hambrecht F.T., Eds. Agnew W.F., McCreery D.B., Prentice Hall, NJ (1990).

Humayun M.S., de Juan E. Jr, Weiland J.D., Dagnelie G., Katona S., Greenberg R.,
Vision Res., 39, 2569 (1999).

23
Jan E., Hendricks J.L., Husaini V., Richardson-Burns S.M., Sereno A., Martin D.C.,
Kotov N.A., Nano Lett., 9, 4012 (2009).

Johnson P.F., Bernstein, J.J., Hunter G., Dawson W.W., Hench L.L., J. Biomed. Mat.
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Johnson P.F., Hench L.L., Brain Behav., 14, 23 (1977).

Ke W., Fishman H. A., Dai H., Harris J.S., Nano Letters, 6, 2043 (2006).

LeRoy C., Hist. Acad. Royal Sciences (Paris), Memoires math. phys. 60, 87 (1755).

Luo X.W., Cassandra L., Zhou D.D., Greenberg R., Xinyan T.C., Biomaterials, 32, 5551
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Mahadevappa, M., J. D. Weiland, et al., IEEE Trans Neural Syst Rehabil Eng 13, 201
(2005).

Margalit E., Maia M., Weiland J.D., Greenberg R.J., Fujii G.Y., Torres G., Piyathaisere
D.V., O'Hearn T.M., Liu W., Lazzi G., Dagnelie G., Scribner D.A., de Juan Jr
E., Humayun M.S., Retinal Prosthesis for the Blind, Survey of Ophthalmology
(Major Review), 47, 335 (2002).

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Meyer J.U., Sensors and Actuators A97-98, 1 (2002).

Mordechay S., Milan P., Eds. Modern Electroplating: Electrodeposition of Alloys. The
Electrochemical Society, Inc. Pennington, New Jersey (2000).

Mozota J, Conway B.E., Electrochim. Acta 28,1 (1983).

Navarro X., Krueger T.B., Lago N., Micera S., Stieglitz T., Dario P., J. Peripher Nerv.
Syst., 10, 229 (2005).

Plonsey R., Barr R.C. (Eds), Functional neuromuscular stimulation. Bioelectricity: A
quantitative approach, plenum press. New York, 271 (1988).

Walker R., Advances in Sonochemistry Manson Edt.,JAI Press, 3, 125 (1993).

Ratty F., Electrical Nerve Stimulation: Theory, Experiments and Applications., Springer-
Verlag/Wein NY (1990).

24
Robblee L.S., Rose T.L., Electrochemical guidelines for selection of protocols and
electrode materials for neural stimulation in Neural Prostheses: Fundamental
Studies, Eds. Agnew W.F., McCreery D.B., Prentice Hall, Englewood Cliffs,
NJ, 25 (1990).

Rose T. L., Robblee, L.S., IEEE Trans. Biomed. Eng., 37, 1118 (1990).

Seo J.M. Kim S.J., Chung H., Kim E.T., Yu H.G., Yu Y.S., Materials Sci. Eng. C24. 185
(2004).

Slavecheva E. Vitushinsky R., Mokwa W., Schnakenberg U., J. Electrochem Soc.,151,
226 (2004).

Brummer S.B., Turner M.J., IEEE Trans Biomed Eng., 24,59, (1977).

T.J. Manson, J.P.Lorimer, Sonochemistry, Theory, Applications and Uses of Ultmsound
in Chemistry,,Sonochemistry, Ellis Horwood, Chichester, (1989).

T.J.Manson, J.P.Lorimer, Theory, Application and. Uses of Ultrasound in Chemistry,
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Weiland J.D., Anderson D.J., IEEE Trans. Biomed. Eng. 47, 911 (2000).

Weiland J.D., Anderson D.J., Humayun M.S. IEEE Trans. Biomed. Eng. 49, 1574
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Weiland J.D., Humayun M.S., Eckhardt H., Ufer S., Laude L., Basinger B., Tai Y.C.,
IEEE EMBS Minneapolis, Minnesota, USA (2009).

White R.L., Gross T.J., IEEE Trans Biomed Eng., BME, 21, 487 (1974).

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25
CHAPTER II
Electrodeposition and Characterization of Dense Platinum-Iridium
Alloy Films

2.1 Introduction

Electordeposition of thin alloy films is one of the most valuable techniques that have
distinct advantages over other thin film deposition methods applied in clean rooms. Such
advantages include low cost, fabrication at room temperature and atmospheric pressure
and well-controlled structure and chemical composition of the electrodeposited films.
The following chapter discusses a detailed investigation of electrodeposition of thin-film
platinum-iridium alloys with the desired microelectrode material properties for use in
biological interfaces.

2.2 Background

Neural stimulating electrodes serve an important role as interfaces between neural tissues
and implanted neural stimulators and have the potential to be used in a variety of
biomedical devices. Low impedance and long term electrical and mechanical stability of
the interface are desirable characteristics for microelectrodes. Electrode stability is
required since implanted stimulators should last the lifetime of the patient and significant
drift in electrode properties will degrade implant performance. Low impedance electrodes
are more efficient, since less energy is required for current to be passed to tissue. Smaller
electrodes improve the selectivity of neural stimulation and recording which helps to
stimulate the target cells and to avoid activation of adjacent cells, which in turn increases
the resolution of the implantable system. However, by decreasing the geometric area of
26
the electrodes, the impedance of the electrodes increases which in turn reduces implant
efficiency [Negi et al. 2010]. Instead of increasing the geometric area, the real surface
area of the electrodes needs to be increased by selection of a proper material and
deposition process.
Electrode surface properties such as electrical and mechanical robustness depend on the
deposition conditions of the desired material. Different deposition methods such as dc
reactive magnetron sputtering [Slavcheva et al. 2004] and dc reactive sputtering [Cho et
al. 1997, Cogan et al. 2004] have been applied to modify microelectrode surfaces. Such
methods are neither time nor cost effective due to the need for high vacuum conditions,
large target size and low material deposition efficiency. In contrast, electrochemical
deposition can be performed using simple electrochemical systems, at room temperature
and under atmospheric pressure which significantly reduces the cost of such processes.
Platinum is a material that is widely used for neural stimulation. It is resistant to
corrosion and can transfer charge using reversible reactions that do not harm tissue
[Merrill 2005, Beebe et al. 1988]. High surface area platinum can be formed by
electrodeposition. Platinum black has a very high surface area, but is mechanically weak
and uses lead as a plating bath component, both of which preclude its use in implantable
devices [Schuettler et al. 2005]. Recent work has developed lead-free plating solutions
[Whalen et al. 2006] and other methods have been used to increase the active platinum
surface area [Zhou et al. 2007]. However, high surface area platinum electrodes in which
structures with high aspect ratios are employed may be subject to mechanical damage due
to the softness of platinum.
27
Platinum-iridium alloy electrodes have been widely used in biomedical and industrial
applications. Iridium adds mechanical stiffness to platinum. Only a limited number of
methods are available for producing platinum-iridium films, including methods such as
Chemical Vapor Deposition (CVD) and plasma sputtering, which are inefficient, as well
as electrodeposition methods, which have not been studied as comprehensively.

Electrodeposition of platinum-iridium films is a valuable alternative to more conventional
techniques for thin film deposition and metal plating. Apart from the study by Tyrell on
electrodeposition of platinum-iridium alloy films [Tyrell 1967] a lack of literature exists
on the electroplating of this alloy. The recent development of more elaborate complexing
agents has made it possible to keep platinum and iridium salts stable for longer periods of
time under different electrolyte conditions.

A reproducible method is described for deposition of platinum-iridium coatings with a
composition of approximately 60% platinum: 40% iridium on gold or platinum base
electrodes over a wide range of deposition times. The process developed can be easily
applied to a wide variety of biomedical sensors as well as neural recording and
stimulation devices.

28
2.3 Materials and Methods
2.3.1 Substrate Preparation

Thin films were electrodeposited on 25x75x1 mm glass slide substrates coated with a
1000 Å gold layer (EMF Corp., Ithaca, NY, USA). The substrates were chemically
cleaned using trichloroethylene, followed by acetone, then methanol and finally rinsed
with DI water to remove organic impurities. Control samples of Pt (99.9%), Ir (99.8%)
and 80:20% platinum-iridium (99.9%) 25x25 mm foils (AlfaAesar, Ward Hill, MA,
USA) were mechanically polished and electrochemically cleaned by applying potential
steps at E = +1.0 V and E=-1.0 V vs. Ag/AgCl for 30 s at each potential which were
repeated for 5 cycles.

2.3.2 Electrodeposition Solution Preparation

The platinum-iridium film electroplating solution contained 0.2 g/L of sodium
hexachloroiridate (III) hydrate, (Na
3
IrCl
6
_H
2
O) and 0.186 g/L sodium
hexachloroplatinate (IV) hexahydrate (Na
2
PtCl
6
_ 6H
2
O) in 0.1 M nitric acid (HNO
3
). The
plating solution was heated until it boiled and the color changed to reddish. The solution
was left to cool down to room temperature.

2.3.3 Electrochemical Measurements

All electrochemical experiments were performed using a three-electrode setup with a
working electrode geometric area A=0.7 cm
2
. A Gamry potentiostat (FAS1, Gamry
Instruments, Warminster, PA, USA) was used to control the electrochemical tests. Figure
2.1 shows the custom made Teflon electrochemical cell. The cell consisted of two
29
chambers with a larger diameter (a) and smaller diameter (b) designed for the electrolyte
and the reference electrode, respectively. The two chambers were connected by a small
capillary (c). The Teflon cell was placed on top of the substrate (d) and the larger
chamber was fixed over an o-ring using a steel spring-clamp (e). The column was filled
with the electroplating solution or 0.05 M H
2
SO
4
during electrodeposition and
electrochemical measurements, respectively. Substrates were connected to the working
electrode lead (WE) using a toothless copper alligator clip (f). A spiral platinum wire (D
= 1.0 mm) was suspended as the counter electrode and an Ag/AgCl reference electrode
was placed in the smaller chamber of the cell. The small capillary between the two
chambers allowed the potential measurement between reference electrode and the
working electrode through the electrolyte.

30

Figure 2.1 Electrochemical cell. Deposition chamber (A), reference electrode chamber
(B), interconnection via a capillary (C), substrate holder (D), clamping bed
(E), toothless clip (F) and substrate (G).

31
2.3.4 Electrodeposition Process

Prior to electrodeposition, the platinum-iridium solution was preheated to 56ºC. The
solution was agitated using an ultrasonic homogenizer (Misonix, Inc. Newtown, CT,
USA) at a frequency of 20 kHz with a power of 5 W in order to maintain constant mass
transport during electrodeposition and to keep the temperature constant. The potential
range for electrodeposition was first identified by cycling the applied potential over
various 200 mV ranges from E = -0.6 V up to E = +0.4 V vs. Ag/AgCl until thin films
were produced. The range E = +0.1 to -0.1 V vs. Ag/AgCl was subsequently selected for
all deposition experiments since it included potentials at which platinum-iridium alloy
was deposited. Electroplating was performed at a scan rate of 500 mV/s for 300, 600,
1200 or 2400 cycles (equivalent to 4, 8, 16 and 32 min of deposition, respectively).

2.3.5 Weight Gain and Thickness Measurements of Platinum-Iridium Films

Au substrates were weighed before and after each deposition process to calculate the
deposited film mass (MT XS105DU, Mettler-Toledo Inc., Columbus, OH). Film
thicknesses were measured on two different portions of each film using a profilometer
(Ambios XP-2 Stylus, KT,UK) and averaged to determine the average film thickness.
The total length scanned was 11 mm for each test.

32
2.3.6 Surface and Compositional Analysis

Control samples and electroplated platinum-iridium alloy films were imaged using a field
emission scanning electron microscope (SEM) (ZEISS 1550VP) with an accelerating
voltage of 4 kV at magnifications of 70 x and 1,00,000 x . The chemical composition of
the control foils and electrodeposited platinum-iridium films was characterized using an
Electron Probe Micro-analyzer (JEOL JXA-8200). All samples were analyzed at three
separate locations on each film.

2.3.7 X-Ray Diffraction

The platinum-iridium thin film microstructure and crystallographic orientation were
characterized by XRD analysis (Bruker D8 Advance X-ray diffractometer, Texas, USA)
using Cu Ka-radiation. Control samples and platinum-iridium electrodeposited films were
mounted on a sample holder. Scans were taken on the surface of all samples over the
a ng l e ra n ge 2ϴ ( 20º -100º) using a 0.02
o
step size. X-ray beam energy was set to 40kV
and 45A.

2.3.8 Nanoindentation Measurements

A semi-quantitative analysis of hardness and stiffness was performed on the platinum-
iridium thin films and the commercially available Pt foil for relative comparative
purposes only. Hardness and stiffness were measured using an automated
nanomechanical test system (Hysitron TriboIndentor, Minneapolis, MN, USA). Sixteen
nano-indentations were run on each sample. Forces with amplitude of 1000 µN at a
33
loading rate of 500 µN/s with a 5s hold segment were applied. A three-sided pyramidal
Berkovich nanoindentation probe (tip radius = 150 nm) was utilized as this is the most
commonly accepted and widely used standard for mechanical properties characterization
[Lund et al. 2004].
2.3.9 Cyclic Voltammetry

The electrodeposited films were chemically cleaned using the steps described previously
and then electrochemically cleaned in 0.05 M H
2
SO
4
by cycling the working electrode
potential over the range E = -0.3 to +1.2 V vs. Ag/AgCl at a scan rate of 250 mV/s until
the cyclic voltammograms reached a steady-state condition. The properties of the
electrodeposited platinum-iridium thin films, the control foils and the Au substrate were
characterized using cyclic voltammetry (CV) in 0.05 M H
2
SO
4
at a scan rate of 50 mV/s
in the potential range of E = -0.3 to +1.2 V vs. Ag/AgCl. Prior to characterization, control
samples were mechanically polished using 4000-grit sandpaper and rinsed with ethanol
and DI water followed by electrochemical cleaning as described previously.

2.3.10 Impedance Measurements

Electrochemical impedance spectroscopy (EIS) was performed in 0.05 M H
2
SO
4
at the
open-circuit potential (OCP) with a ±10 mV amplitude ac signal over a frequency range
100 kHz –10 mHz for control samples, and from 100 kHz to 5 mHz for electrodeposited
thin films, using a Gamry FAS1 potentiostat. Experimental data were fitted to a one-time
constant equivalent circuit (EC) and the values of the solution resistance (R
S
),
polarization resistance (R
P
) and capacitance (C) were determined using the ANALEIS
software [Marcus et al. 2006, Mansfeld et al. 1992, Mansfeld et al. 1993].
34

2.3.11 Potentiodynamic Polarization Measurements

Anodic and cathodic potentiodynamic polarization measurements were performed for the
Pt foil and the platinum-iridium alloy film that was electroplated for 32 minutes. The
polarization curves were recorded in 0.05 M H
2
SO
4
at a scan rate of 0.2 mv/s. Anodic
polarization curves were measured in potential range of E = OCP and E = +1 V vs.
Ag/AgCl, while the cathodic polarization measurements were recorded in potential range
of E = OCP and E = 0V vs. Ag/AgCl, where OCP is the open-circuit potential. Before
the electrochemical test for the platinum foil the sample was polished with sand paper
(4000) and rinsed with methanol and DI water followed by electrochemical cleaning
using potential cycling between E = -0.25 V to E = + 1.25 V vs. Ag/AgCl at scan rate of
100 mv/s. For the electroplated platinum-iridium thin film only electrochemical cleaning
was applied. In order to reach a stable OCP for each measurement both samples were
exposed to .05 M H
2
SO
4
for about 45 minutes prior to the start of the potentiodynamic
measurements.

2.4 Results and Discussion
2.4.1 Electrodeposition of Platinum-Iridium Dense Films

A number of deposition parameters can be varied to control the properties of the
electrodeposited thin films. Deposition bath temperature, deposition potential range,
agitation of the solution, solute concentration and pH of the plating solution are all
examples of process variables that can be adjusted to affect thin film composition,
morphology and structure [Tyrell 1967, Sheela et al. 2005].
35
In this work the objective was to identify deposition conditions that would create thin
platinum-iridium films with desirable characteristics for neurostimulation applications
such as sufficient Ir content, good films substrate adhesion and low electrochemical
impedance with minimal surface defects. Thin films were deposited using a potential
cycling technique over a 200mV potential range. The cycle was shifted to different
200mV intervals from E
min
= -0.5 V to 0.5 V vs. Ag/AgCl and the resulting films were
characterized to identify the optimal potential range. Previous work in other binary
plating systems has shown that multi-layered metallic deposits of alternating metal A and
metal B composition can be synthesized by stepping the potential between two potentials
below the equilibrium potentials of the two plating species [Chen et al. 2006]. For these
reasons a cycling potential regimen was used and the limits were varied over ranges
below the equilibrium potentials of both platinum and iridium.
Preliminary experiments revealed that under ambient conditions (room temperature and
without sonication), the rate of deposition was only a few mA/m
2
. When sonication was
introduced (which also elevates the solution temperature), the deposition current density
increased the rate to the order of a few mA/cm
2
, which is consistent with publications on
platinum-group metals deposition currents [Levason et al. 1998, Gregory et al. 1993,
Meyer et al. 2001, Wu et al. 2004].
Based on these observations, all further deposition experiments were conducted under
sonication. The effect of solution temperature on the deposition rate was also studied.
Table 2.1 shows that a desirable iridium content of about 42% was obtained at 56
o
C.
Other studies characterizing the electroactive surface of platinum-iridium alloy electrodes
of different composition have shown that platinum-iridium alloys with a composition of
36
60–40% have the largest electroactive surface and lowest charge transfer resistance
[Holt-Hindle et al. 2008].

Table 2.1 Average chemical compositions and standard deviation of 80 –20% platinum
iridium foil and the platinum-iridium films electroplated for 4, 8, 16 and 32
min for 3 sets of samples determined by WDS.

Sample Pt (a%) Ir (a%) STDV (%)
Pt-Ir foil (80-20) 80.3 19.7 0.32
Pt-Ir film (4 min) 59.1 40.9 3.01
Pt-Ir film (8 min) 56.9 43.1 1.6
Pt-Ir film (16 min) 57.4 42.6 2.2
Pt-Ir film (32 min) 58.6 41.4 1.4

Studies of the effects of electroplating bath pH were also performed. Baths prepared with
more neutral pH by adding NaOH with Na
2
HPO
4
buffer resulted in stressed films with
significant cracking (Figure 2.2a). In comparison, thin films deposited from acidic
solutions (pH=1.5 –2.0) showed more mechanical integrity and no cracking as shown by
SEM analysis (Figure 2.2b). Plating solution temperature and sonication had little effect
on thin film mechanical stability.
However, both elevated temperature and sonication of the bath during deposition
increased the Ir concentration. Once the optimum deposition parameters were defined
deposition rates were characterized independently by profilometry and by measuring
deposited film mass as a function of deposition time.

37

Figure 2.2 Comparison of the surfaces of platinum-iridium films prepared from high pH
(a) and low pH (b) electroplating baths.

38
Figure 2.3 shows a typical series of current-voltage curves recorded during
electrodeposition of a platinum-iridium thin film for 2400 cycles, which equals 32 min of
deposition (every 100th cycle is shown, scan rate=500 mV/s). In the negative potential
sweep direction, the cathodic current increased steadily over the 32 min time interval,
suggesting that the film increased in active surface area with each cycle, despite having a
fixed geometric area.
-0.1 -0.06 -0.02 0.02 0.06 0.1
Potential (V)
-6
-4
-2
0
2
I (mA)

Figure 2.3 Cyclic voltammograms recorded during electrodeposition of platinum-iridium
films.

Time
39
Figure 2.4a shows the maximum anodic currents and Figure 2.4b shows the maximum
cathodic currents recorded during deposition as a function of number of cycles. The
currents observed in the anodic and cathodic directions showed a steady increase with
increasing cycle number (i.e., deposition time). This result is consistent with previously
reported data on electrodeposition of other iridium-containing thin films [Kasem et al.
2004]. The platinum-iridium film thickness was measured using profilometry at two
separate spots on each film. Figure 2.5 shows representative segments of the profilometry
scans used to calculate average thickness for each film. The average film thickness
plotted as a function of deposition time in Figure 2.6 suggests that platinum-iridium films
were deposited at a relatively constant rate. The deposition rate was also calculated by
measuring the film mass as a function of deposition time.

40

300 600 900 1200 1500 1800 2100 2400
Number of cycles
-0.5
0
0.5
1
1.5
2
Imax (mA)
a

300 600 900 1200 1500 1800 2100 2400
Number of cycles
-6
-5
-4
-3
-2
-1
Imax (mA)
b

Figure 2.4 Anodic (a) and cathodic (b) maximum currents as a function of number of
cycles.

41

6000 6200 6400 6600 6800 7000
Distance across the samples (µm)
0
2000
4000
6000
1000
3000
5000
Thickness (Å)

32 min
16 min
8 min
4 min

Figure 2.5 Measured thicknesses of electroplated platinum-iridium films.

Thin film mass and thickness vs. deposition time plots support the observation that the
thin film deposition rate was constant over the measured time interval at 14.8 µg/min and
16.5 nm/min. These data are useful for fabrication of electrodeposited neurostimulating
electrodes as they suggest that platinum-iridium films of known thickness can be
predictably deposited using the plating solution and plating method reported here.

42

5 10 15 20 25 30 35
Deposition time (min)
100
200
300
400
500
Thickness (nm)
100
200
300
400
500
Mass (µg)
16.5
14.8

Figure 2.6 Plots of coatings thickness ( □) and coatings mass ( ■) vs. deposition time.

2.4.2 Effect of Ultrasound in Electrochemistry

Figure 2.7 shows the effect of electroplating bath sonication during the thin films
deposition. The magnitude of the peak current in the sonicated mode higher than that of
the maximum current measured under silent condition which could be explained by the
increased number of transported ions due to sonication which causes higher mass
transport. The comparison of the Y axes shows over 30 time higher current amplitudes in
the sonicated mode. All other electroplating parameters such as solution concentration
and the electrodeposition bath temperature were in the same condition.

43

-0.1 -0.05 0 0.05 0.1
Potential (V)
-4.2
-3.6
-3
-2.4
-1.8
-1.2
-0.6
0
0.6
I (mA)
-0.14
-0.12
-0.1
-0.08
-0.06
-0.04
-0.02
0
0.02
I (mA)

Figure 2.7 Comparison of deposition cycles for sonicated mode and silent mode.

2.4.3 Scanning Electron Microscopy (SEM)

SEM images were used to investigate the surface morphology of the Au substrate and the
electrodeposited platinum-iridium films as a function of deposition time. Figure 2.8
shows micrographs for the Au substrate and the platinum-iridium films grown by
potential cycling at different electrodeposition times with two different magnifications
(70X for Figures. 2.8a –2.8e and 100,000X for Figs. 2.8f –2.8j). Figure 2.8a shows the
surface of the Au substrate for comparison. Figures 2.8b–2.8e correspond to deposition of
platinum-iridium films formed on the gold substrate for 4, 8, 16 and 32 min. With
increasing deposition time (Figures 2.8c –2.8e) the platinum-iridium films showed a slight
increase in the irregularity on the surface of the films, but in general the appearance at
Sonicated
Silent
44
low magnification is little changed. The higher magnification SEM micrographs exhibit
the structure of the texture of the films. Comparison of the Au substrate surface
morphology with that of the 4 min platinum-iridium film (Figures 2.8f and 2.8) confirms
that the electrodeposited film on the Au substrate is dense and uniform and possesses a
nanostructure coating with an average grain size smaller than 10 nm. With increasing
deposition time (Figures 2.8h–2.8j) the surface of the platinum-iridium films shows a
rougher appearance.

45

Figure 2.8 SEM micrographs of Au substrate (a) and platinum-iridium films (b –e) with 4,
8, 16 and 32 min. deposition time, respectively. Micrographs (f –j) are of the
same samples as (a –e), respectively, but at higher magnification (scale bars at
bottom are for all micrographs in column).
46
2.4.4 Material Properties and Characterization of Thin Films

Quantitative compositional analysis was performed on the electrodeposited thin films
using an electron microprobe equipped with a wavelength dispersive spectroscopy
(WDS) system. Table 2.1 summarizes the average chemical composition of the different
electrodeposited thin films and compares them with the 80:20% platinum-iridium control
sample and the electroplated platinum-iridium films. The data show an average chemical
composition of about 58:42% platinum-iridium for all electroplated samples suggesting
that the relative amounts of platinum and iridium in the films remain almost constant over
the deposition time interval studied. Historically, platinum-iridium pacemaker electrode
compositions have ranged from 5% iridium to 40% [Piersma et al. 1987]. These
electrodes are typically fabricated through high temperature casting, extrusion or particle
sintering to create high surface areas and low impedance. In the present study a novel
electrochemical method for fabricating platinum-iridium thin films with near 40% Ir
composition has been developed. This process can be used for low-cost coating of
biomedical electrodes, but also for other applications such as fuel cell electrodes.

2.4.5 X-Ray Diffraction

The crystal structure of the electroplated platinum-iridium alloy thin films and control
samples was examined by XRD (Figure 2.9), and compared side-by-side in Figure 2.10.
The re sult s for the pu re P t foil’s sca n show the (11 1), ( 200), (220 ), a nd (2 22 ) pe a ks a t
their respective locations with preferential (220) orientation over the other planes.
Similarly, the Ir foil scan exhibited (111), (200), (220), and (222) peaks at their expected
positions for polycrystalline iridium,
47

40 50 60 70 80 90
Two-Theta (deg.)
Intensity
(111) Au
(111) Pt-Ir
(200) Au
(200) Pt-Ir
(220) Au
(220) Pt-Ir
(311) Au
(222) Au
(311) Pt-Ir

Figure 2.9 X-ray diffraction patterns for platinum-iridium electroplated film.

48

40 50 60 70 80 90
Two theta (deg.)
Intensity (a.u.)
Ir
Au
Pt-Ir (foil)
Pt
Pt-Ir (film)
(111)
(200)
(220)

Figure 2.10 Comparison of XRD patterns of platinum-iridium film, Au substrate and
control samples.

Table 2.2 Structural characteristics of Au, Pt, Ir and Pt-Ir samples using XRD.

Sample D (Å) FWHM Average particle size (nm) STDV
Au 1.7 0.57 14.25 2.3
Pt 1.66 0.52 17.6 2.3
Ir 1.68 0.54 15.25 2.06
Pt-Ir foil 1.68 0.54 16.5 2.6
Pt-Ir film 1.84 1.08 2.66 0.5

with preferential orientation in the (200) and (222) directions. The mixed composition
platinum-iridium foil interestingly showed (200), (220) and (222) with almost no (111)
49
pre fe rr e d o rie ntation. Thi s sample’ s c ha ra c te risti c pe a ks we re posi ti one d dire c tl y be t ween
the corresponding platinum and iridium peaks, confirming its mixed platinum-iridium
composition.
These results were then compared with the XRD scans of the electrodeposited films to
further assess their composition. The peaks for the platinum-iridium thin films in the X-
ray spectra were also located at angles between their corresponding peaks in the pure
platinum and pure iridium scans. They aligned well with the diffraction peaks of the
mixed composition foil, further supporting the hypothesis that the thin films were mixed
alloys in nature. These results were similar to XRD results reported on galvanostatically
deposited platinum-iridium thin films [Wu et al. 2004].
The electrodeposited films showed preferred (111) orientation and the peaks were broad
in profile as compared to the control foils which is attributable to both the mixed nature
of the alloy and also to the nanocrystalline microstructure commonly seen in
electrodeposited thin films [Erb 1995]. To further characterize the microstructure, the
average grain size for the electrodeposited thin films and control samples was estimated
using the Debye-Scherer equation:
) 1 (
cos  


K


where K is constant, k is the wavelength, b is the full width at half maximum (fwhm) and
h is the incident angle of the x-ray beam. Calculations from the X-ray diffraction data
obtained for the Pt-I thin films estimated the average grain sizes in the range of 1 –3 nm.
Table 2.2 summarizes the average grain sizes of all samples for comparison. It is likely
that these numbers are skewed to lower values than the actual grain sizes since platinum-
50
iridium alloying will also contribute to peak broadening [Langford et al. 1978]. TEM
studies are needed to more accurately assess the grain size of these films.
2.4.6 Nanoindentation

The relative hardness of the electrodeposited thin films was assessed with respect to the
commercial Pt foil using nanoindentation measurements. Figure 2.11 shows
representative load versus displacement curves for the platinum-iridium film and the Pt
foil. Hardness and reduced modulus values are listed in Table 2.3. These results
demonstrate that alloying platinum and iridium significantly hardens the material as
compared to the pure Pt substrate, while the modulus is unaffected. This finding is
consistent with standard hardness values for Pt and Ir. However, it is important to note
that a thorough examination of mechanical properties would entail multiple indentation
profiles across many samples. A definitive characterization of the mechanical properties
of platinum-iridium films was beyond the scope of this thesis.

51

20 40 60 80 100 120 140
Displacement (nm)
100
200
300
400
500
600
700
800
900
1000
Load (µN)

Figure 2.11 Representative load-displacement curves for a platinum foil and a platinum-
iridium electroplated film.

Table 2.3 Hardness and reduced modulus values for platinum foil and platinum-iridium
electroplated film.

Material Modulus (GPa) Hardness (GPa)
Pt-Ir foil 105.75 ± 6.8 2.39 ± 0.45
Pt-Ir film 103.92 ± 5.38 4.7 ± 0.43
Pt-Ir Pt
52
2.4.7 Cyclic Voltammetry for Electroplated Platinum-Iridium Films

Cyclic voltammograms were recorded for the electrodeposited thin films to assess their
electrochemical properties and real surface area. Control electrodes of Au, Pt, Ir and 80 –
20 Pt-Ir were first scanned and the representative cycles are shown in Figures 2.12a –
2.12d. All four control electrode materials exhibited CV profiles characteristic of their
composition in a sulfuric acid solution validating the test system [Pell et al. 2002,
Farebrother et al. 1991, Glarum et al. 1980, Hefny et al. 1996, Ureta-Zanartu et al.
2001]. These results were used as references for the voltammetric behavior of the
electrodeposited films. Figure 2.12e shows representative CV curves for each of the four
electrodeposited platinum-iridium films. The profiles of all four CVs are most similar to
the 80 –20 Pt-Ir CV profile (Figure 2.12c), exhibiting some characteristics of a platinum
electrode in sulfuric acid solution, but lacking distinct hydrogen peak pairs in the anodic
and cathodic sweeps. The CVs also show a broad platinum oxidation shoulder in the
anodic sweep without distinct peaks associated with specific oxide transition states [Pell
et al. 2002, Farebrother et al. 1991]. The measured currents were also significantly larger
over the entire potential range as compared to the control electrodes of equal geometric
area. The blurring of peaks coupled with the significantly larger currents suggests that the
surface area of the samples increased with increasing deposition time [Elliott et al. 1999,
Whalen et al. 2005].
Additionally, the mixed platinum-iridium composition of the thin films eliminates the
presence of two discrete hydrogen adsorption peaks, as is typically seen for pure platinum
electrodes.
53
For neurostimulation applications, the area enclosed by the CV curve corresponds to the
charge storage capacity of the platinum-iridium films, which is one indicator of the
performance of a neurostimulating electrode material [Robblee et al. 1990]. While the
electrode size here is much larger and the current densities much lower, CV testing has
been used with many electrodes to assess their performance relative to other well-studied
electrode materials [Robblee et al. 1990]. The CV plots of the platinum-iridium thin films
have a significantly larger active area than that of the control samples, indicating higher
charge storage capacity of electroplated platinum-iridium films.

54
-0.2 0 0.2 0.4 0.6 0.8 1 1.2
Potential (V)
-0.4
-0.2
0
0.2
0.4
0.6
0.8
I (mA)
Au
-0.2 0 0.2 0.4 0.6 0.8 1 1.2
Potential (V)
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
I (mA)
Ir
b

-0.2 0 0.2 0.4 0.6 0.8 1 1.2
Potential (V)
-0.8
-0.6
-0.4
-0.2
0
0.2
I (mA)
Pt-Ir (80-20)
-0.2 0 0.2 0.4 0.6 0.8 1 1.2
Potential (V)
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
I (mA)
Pt
d

-0.2 0 0.2 0.4 0.6 0.8 1 1.2
Potential (V)
-6
-4
-2
0
2
4
6
I (mA)

4 min
8 min
16 min
32 min

Figure 2.12 Cyclic voltammograms in 0.05 M H
2
SO
4
of Au substrate (a) control samples
(b) –(d) and platinum-iridium films (e) with different deposition times.

c
a
e
55
2.4.8 EIS Data Analysis

EIS data for the electrodeposited films in sulfuric acid solution are shown in the Bode
plot format in Figure 2.13. The impedance was found to be greatly reduced for the
elctrodeposited platinum-iridium films as compared to the uncoated Au substrate and the
control samples. The impedance of the electroplated platinum-iridium films decreased
significantly with increasing deposition time for all samples. Compared to the smooth Au
substrate, the platinum-iridium film deposited for 32 min showed a more than two orders
of magnitude decrease in the impedance modulus ( Z ) in the capacitive region. Similar
impedance results have been reported for electroplating of platinum black [Franks et al.
2005] and poly (3,4-ethylenedioxythiophene) (PEDOT) [Richardson-Burns et al. 2007]
on platinum electrode surfaces. Since in the capacitive region Z is inversely proportional
to the electrode capacitance C ( ) 2 /( 1 C f Z   ), the observed decrease of Z is due to
an increase of C as a result of increased real surface area. Electroplated platinum-iridium
films initially form a thin, dense layer, but with increasing deposition time a nanorough,
nodular morphology is formed as shown in Figure 2.8.

The impedance spectra for all electroplated platinum-iridium films were fit to a one-time-
constant model (OTCM) according to the equivalent circuit (EC) shown in Figures 2.14.
Figure 2.15 shows excellent agreement between the Bode plots of the experimental
results and the Bode plots of the fit data for the platinum-iridium film electroplated for 32
min. The fit parameters C, R
P
and R
S
are given in Table 1.4. The time constant (s) values
shown in Table 1.4 can be calculated as τ = C* R
P
. The unit of C is equal to s/ (ohm.cm
2
)
and the unit of R
P
is ohm.cm
2
. The time constant τ therefore has the unit of time (s). The s
56
values had very similar values for all electroplated platinum-iridium films which proves
that the increase of the real surface area is the only effective factor for the observed
changes of the C and R
P
values in Table 2.4.

-1 0 1 2 3 4
log f (Hz)
2
4
6
log lZl (ohm)

Au
Pt
Pt-Ir 80-20
Ir
4 min
8 min
16 min
32 min

-1 0 1 2 3 4
log f (Hz)
-10
-20
-30
-40
-50
-60
-70
-80
-90
Phase angle (degree)

Figure 2.13 Bode plots for Au substrate, Pt, Ir, 80–20 Pt-Ir foils and platinum-iridium
films measured in 0.05 M H
2
SO
4
.

57

Figure 2.14 Equivalent circuit for analysis of impedance data shown in Figure 2.16.

-1 0 1 2 3 4
log f (Hz)
0
1
2
3
4
log lZl (ohm)
-10
-20
-30
-40
-50
-60
-70
-80
-90
Phase angle (deg.)

Exp.
Fit

Figure 2.15 Bode plots of experimental ( …) and fit data ( ―) of platinum-iridium film
electroplated for 32 min.

58
Table 2.4 Fit parameters for the impedance spectra shown in Figure 2.13.

Sample C (F) R
P
(kΩ) R
S
(Ω) Τ ( s)
Au 4.57 × 10
-5
650 21.7 29.7
Pt foil 9.55 × 10
-5
815 22.1 77.8
Pt-Ir foil (80-20) 6.3 × 10
-5
820 22.5 51.7
Ir foil 3.57 × 10
-4
241 23.1 86.3
Pt-Ir film (4min) 1.55 × 10
-3
78 21.0 120.9
Pt-Ir film (8min) 5.21 × 10
-3
17 21.1 88.4
Pt-Ir film (16min) 1.11 × 10
-2
7.8 21.3 85.8
Pt-Ir film (32min) 2.01 × 10
-2
4.3 21.1 86.4

5 10 15 20 25 30 35
Deposition time (min)
5E-005
0.0001
0.00015
0.0002
1/Rp (ohm-1)
5
10
15
20
C (mF)

Figure 2.16 1/R p (□ ) a nd C (■ ) a s a func ti on of de posi ti on ti me ( da ta f rom Ta ble 2.4).

Figure 2.16 demonstrates linear relationships between C, 1/R
P
and deposition time
indicating that the increase in platinum-iridium film roughness occurred at a constant
rate.
59
2.4.9 Potentiodynamic Polarization Measurements

Figures 2.17 shows the anodic and cathodic polarization curves recorded in 0.05 M
H
2
SO
4.
The comparison of the anodic polarization measurements for the Pt foil and the
platinum-iridium film shows more than a 1.15 order of magnitude higher oxidation
current density for the platinum-iridium alloy. The intersection of anodic polarization
curve with cathodic polarization curve for the platinum-iridium electroplated film shows
nearly a 1.6 order of magnitude higher exchange current density compared to that of the
Pt foil. For both the anodic and cathodic potentiodynamic curves higher current densities
were recorded in the entire potential range for the platinum-iridium alloy film.

60
-8 -7.5 -7 -6.5 -6 -5.5 -5 -4.5 -4 -3.5
Log i (A/cm
2
)
0.2
0.4
0.6
0.8
1
1.2
Potential (V)
Pt
Pt-Ir

Figure 2.17 Polarization curves for the platinum foil and the platinum-iridium film.

61
2.5 Conclusions

An efficient method for electrodeposition of binary platinum-iridium alloys has been
developed. Deposition process variables have been thoroughly investigated.
Electroplating conditions such as potential limits, bath temperature and the ultrasonic
homogenizer amplitude play an important role in determining the properties and the
bstructure of the electroplated platinum-iridium films. Deposition rates using this method
appear to be constant through the time-period used for electroplating, suggesting that
platinum-iridium thin films of predictable thickness can be fabricated with this approach.
Compositional analysis of films deposited for different times suggests that they have the
same composition at approximately 60:40% platinum-iridium. Additionally, the fact that
no heavy metal compounds are used in the electroplating bath supports the potential use
of these films in biomedical applications.
Nanoindentation measurements on the platinum-iridium films demonstrated improved
hardness in comparison to the Pt foil suggesting a robust biological interface.
Electrochemical characterizations of these films suggested that they are proper for
biomedical sensing/stimulating applications. The films exhibit significantly reduced
electrochemical impedance as evidenced by both the voltammetric data and the
impedance spectroscopy data. Furthermore, the increase of the capacitance of the
electroplated platinum-iridium films compared to control samples indicates that the real
surface area of the films was significantly increased despite the electrode maintaining its
geometric area. These results may provide additional motivation to use these films in
neurostimulating applications where arrays of microelectrodes must be arranged within a
small geometric area to target specific cells and/or tissues. The electrodeposited
62
platinum-iridium binary alloy thin films reported here show great promise as an electrode
coating for neural stimulating electrodes.

63
2.6 References

Beebe X., Rose T. L., IEEE Trans. Biomed. Eng., 35, 494 (1988).

Chen M.,Chien C. L, Searson P., Chem. Mater., 18, 1595 (2006).

Cho H. J.,Horii H., Hwang C. S., Kim J. W., Kang C. S., Lee B. T., Lee S. I., Koh Y. B.,
Lee M. Y., J. Appl. Phys., 36, 1722 (1997).

Cogan S. F., Ehrlich J., Plante T. D., Smirnov A., Shire D. B., Gingerich M., Rizzo J. F.,
Conf. Proc. IEEE Eng. Med. Biol. Soc., 6, 4153 (2004).

Elliott J. M., Birkin P. R., Bartlett P. N., Attard G. S., Langmuir, 15, 7411 (1999).

Erb U., Nanostruct. Mater., 6, 533 (1995).

Farebrother M., Goledzinowski M., Thomas G., Birss V.I., J. Electroanal. Chem., 297,
469 (1991).

Franks W., Schenker I., Schmutz P., Hierlemann A., IEEE Trans. Biomed. Eng., 52, 1295
(2005).

Glarum S. H., Marshall J. H., J. Electrochem. Soc., 127, 1467 (1980).

Gregory A. J., Levason W., Pletcher D., J. Electroanal. Chem., 348, 211 (1993).

Hefny M. M., Abdel-Wanees S., Electrochim. Acta, 41, 1419 (1996).

Holt-Hindle P., Yi Q., Wu G., Koczkur K., Chen A., J. Electrochem. Soc., 155, K5
(2008).

Kasem K. K. Huddleston L., Platinum Met. Rev., 48, 159 (2004).

Langford J. Wilson A. J. C., J. Appl. Crystallogr., 11, 102 (1978).

Levason W., Pletcher D., Smith A. M., J. Appl. Electrochem., 28, 18 (1998).

Lund A. C., Hodge A. M., Schuh C. A., Appl. Phys. Lett., 85, 8 (2004).

Mansfeld F., H. Shih, Greene H., Tsai C. H., ASTM STP, 37, 1188 (1993).

Mansfeld F., Tsai C. H., Shih H., ASTM STP, 186, 1154 (1992).

Mansfeld F., Electrochemical Impedance Spectroscopy. Analytical Methods in Corrosion
Science and Engineering. Marcus P. and Mansfeld F., Eds. CRC Press, 463
(2006).
64

Merrill D. R., Bikson M., Jefferys J. G. R., J. Neurosci. Methods, 141, 171 (2005).

Meyer R. D., Cogan S. F., Nguyen T. H., Rauh R. D., IEEE Trans. Neural Syst. Rehabil.
Eng., 9, 2 (2001).

Negi S., Bhandari R., Rieth L., Solzbacher F., Biomed. Mater., 5, 015007 (2010).

Pell W. G., Zolfaghari A., Conway B., J. Electroanal. Chem., 532, 13 (2002).

Piersma B. J. Greatbatch W., J. Electrochem. Soc., 134, 2458 (1987).

Richardson-Burns S. M., Hendricks J. L., Foster B., Povlich L. K., Kim D. H., Martin D.
C., Biomaterials, 28, 1539 (2007).

Robblee L.S., Rose T.L., Electrochemical guidelines for selection of protocols and
electrode materials for neural stimulation in Neural Prostheses: Fundamental
Studies, Eds. Agnew W.F., McCreery D.B., Prentice Hall, Englewood Cliffs,
NJ, 25 (1990).

Schuettler M., Doerge T., Wien S. L., Becker S., Staiger A., Hanauer M., Kammer S.,
Stieglitz T., 10th Annual Conference of the International FES Society (2005).

Sheela G., Pushpavanam M., Pushpavanam S., Trans. Inst. Met. Finish., 83, 77 (2005).

Slavcheva E., Vitushinsky R., Mokwa W., Schnakenberg U., J. Electrochem. Soc., 151,
226 (2004).

Tyrell C. J., Trans. Inst. Met. Finish., 45, 53 (1967).

Ureta-Zanartu M. S., Bustos P., Diez M. C., Mora M. L., Gutierrez C., Electrochim. Acta,
46, 2545 (2001).

Whalen J. J., Weiland J. D., Searson P. C., J. Electrochem. Soc., 152, 738 (2005).

Whalen J. J., Young J., Weiland J. D., and P. C. Searson, J. Electrochem. Soc., 153, 834
(2006).

Wu F., Yamamoto Y., Yamabe-Mitarai Y., Murakami H., Hirosaki N., Harada H.,
Katayamac H., Yamamotoa Y., Surf. Coat. Technol., 184, 24 (2004).

Zhou D. M., U. S. Pat. WO 2007/050212 (2007).

65
CHAPTER III
In-vitro Characterization of Safe Charge Transfer Characteristics of
Electrodeposited Platinum-Iridium Coatings

3.1 Introduction

It was shown in the previous chapter that platinum-iridium alloy thin films can be
obtained by electroplating with a chemical composition ratio of approximately 60:40%
(Pt:Ir) using a cyclic voltammetry technique. One of the most important applications of
the platinum-iridium thin films is modification of neurostimulating microelectrodes.
Transferred charge across the electrode/tissue interface is usually high enough to evoke
neural responses from target cells without causing damaging the tissue by irreversible
faradaic electrochemical processes.

In this chapter, high-surface area platinum-iridium coatings have been electrodeposited
on platinum microelectrodes to increase the charge capacity of the electrode.
Electrochemical measurements were carried out using electrochemical impedance
spectroscopy (EIS), cyclic voltammetry (CV) along with galvanostatic current pulse
measurements. Experimental results obtained for platinum thin films and platinum-
iridium coating were compared.

3.2 Background

Implantable microelectronic devices that deliver electrical current to targeted tissue are
being developed for the treatment of a variety of diseases. Medical devices that stimulate
tissue include cardiac pacemakers for cardiovascular conditions, cochlear implants for
66
deafness, spinal cord neuromodulators to regulate chronic pain, and deep brain
sti mul a tors to t re a t P a rkinson’ s a nd e pil e psy [Heiduschka et al. 1998, Horch et al. 2004].
Retinal prostheses and brain-machine interfaces that are now under development will
require sophisticated circuitry and a large number of electrode contacts with tissue
[Zrenner et al. 1999, Yagi et al. 1999].

Stimulating microelectrodes in implantable devices are used to transfer electrical charge
and thereby elicit a response from neurons. The charge transfer process needs to be
conducted through an electrochemically stable electrode material in which some
properties such as low impedance, high charge injection capability, excellent corrosion
resistance and biocompatibility are critical [Margalit et al. 2002]. The ability of the
electrode to efficiently transfer charge can also decrease the power consumption of the
implantable devices, extending battery life and reduce component size. For these
reasons, materials selection in determining the proper electrode material becomes a key
factor impacting device design.

Different electrode materials have been used for neural stimulation. These include
titanium nitride, platinum, iridium, platinum-iridium alloys, iridium oxide, poly
(ethylenedioxythiophene) (PEDOT) and carbon nanotubes [Cogan 2008]. Platinum is the
most common material used in devices approved for human use. However, platinum
electrodes may be subject to mechanical deformation due to the softness of pure
platinum.
67
Platinum can be strengthened by alloying with iridium [Petrossians et al. 2011]. In an
application like neurostimulation, mechanical strength protects against damage during
insertion. A damaged electrode will have altered properties.
A mechanically stable connection between the electrode surface and the neurons can
prevent the formation of a nonconductive organic coating at the electrode/tissue interface
whic h in t urn w il l pre ve n t di mi nishi ng the e lec tro de ’s e f fic ienc y a t eliciti ng a c e ll ular response [Turner et al. 1999]. There is significant motivation to understand the
properties of platinum-iridium alloy microelectrodes in detail.

Methods of producing platinum-iridium thin films including chemical vapor deposition
(CVD), hydrothermal methods and plasma sputtering are both material- and time-
inefficient. Electrodeposition approaches have not been studied comprehensively.
Advantages of using electrodeposition methods include low cost and operating conditions
at near room temperature. These reasons provided a strong motivation for the work
reported here.

In this investigation, in vitro experiments were performed to evaluate the electrochemical
properties of microelectrode arrays treated with an electrodeposited high surface area
platinum-iridium coating. 58:42% platinum-iridium coatings were produced using
similar electrodeposition method discussed in an earlier comprehensive study
[Petrossians et al. 2011]. The platinum-iridium coating was deposited on polymer
substrate microelectrodes and subsequently characterized and tested with electrochemical
impedance spectroscopy, cyclic voltammetry and long-term pulsing. The electrodeposited
68
high-surface area platinum-iridium alloy coatings showed low impedance and improved
mechanical properties capable of transferring large amounts of electrical charge within a
voltage range that avoids potentially harmful hydrolysis of body fluid.
In many reversible reactions used for neural stimulation at least one product of the
reaction remains absorbed on the electrode surface. During irreversible faradaic reactions
the electrolyte undergoes permanent changes such as water decomposition which is
highly non-desired reaction. Irreversible reactions in electrical stimulation of the neural
cells must be prevented since the reaction byproducts might be toxic and harmful to the
surrounding tissue. The potential of the electrode surface controls the rate and type of the
electrochemical reactions of the electrode/electrolyte interface. Such electrochemical
reactions can take place in phosphate buffered saline solution within safe potential limits
in which no irreversible reactions take place. These potential limits are approximately E =
-0.6 V to E = +0.8 V vs. Ag/AgCl [Cogan 2004]. Hydrogen and oxygen gas evolutions
start if the applied potential on the surface of the electrode is below -0.6 V or above +0.8
V, respectively. Within this potential range, double layer charging, hydrogen
absorption/desorption and reversible oxidation and reduction of the platinum electrode
can occur.

3.2.1 Safety Limits for Neural Electrostimulation

Neural electrostimulation is often performed using controlled current instead of
controlled potential [Agnew, 1990, Brummer, 1983, Brummer, 1975]. Current is applied
to the target cells using short duration pulses (typically less than 2 ms). In this process,
the current can only be applied briefly since a constant current flow will increase the
69
voltage across the electrode/tissue interface to the extent of which irreversible faradaic
reactions may occur.

The amount of injected charge density can be calculated by multiplying the applied
current with amount of time divided by the geometric area of the electrode surface.
Figure 3.1 shows a biphasic symmetric stimulation waveform [Petrossians et al. 2011].
Cathodic (negative) current is applied until the measured voltage reaches the maximum
safety limit. The polarity of the applied current is then reversed to anodic (positive) until
the safety limit of opposite measured potential is measured. This process is repeated at a
certain frequency.

0.5 1 1.5 2 2.5
Time (ms)
-0.8
-0.4
0
0.4
Voltage (V)
OR
CT
OR

Figure 3.1 A typical voltage response to biphasic current pulse waveform.
(Ohmic resistance (OR) and charge transfer (CT).

70
3.3 Electrochemical Measurements
3.3.1 Cyclic Voltammetry

In cyclic voltammetry the current is measured by cycling the applied potential within two
potential limits at a constant scan rate. This technique is used to determine the safe
neurostimulatory charge transfer limits and to calculate the charge storage capacity
(CSC) of the electrode. Cathodic charge storage capacity (CSC
C
) is determined by
integrating the cathodic area under the curve and anodic charge storage capacity (CSC
a
)
is determined by integrating the anodic area under the curve of the cyclic voltammogram.
Scanning the potential in one direction identifies the safe potential limits in which the
measured current increases drastically [Weiland, 2002, Robblee, 1990].

Figure 3.2 shows a typical cyclic voltammogram for a polycrystalline Pt electrode
measured in acidic solution H
2
SO
4
. In the anodic sweep the two H
A1
and H
A2
peaks are
due to hydrogen desorption on the platinum electrode. The low current in the following
potential region in the anodic direction of the sweep represents the double layer charging
region. Increasing the potential from 0.8 V to 1.25 V hydroxyl groups start to absorb on
the Pt surface and the measured current increases due to oxide formation. By increasing
the potential beyond 1.25 V (not shown here) the current will increase due to the
formation of oxygen gas.

71

Figure 3.2 Cyclic voltammogram for polycrystalline platinum electrode in H
2
SO
4
.
[Conway].

In the cathodic sweep, platinum oxide is reduced to platinum represented by the reduction
peak in the O
C
region close to 0.8 V. At more negative potentials than about -0.3 V and
after the small region of double layer charging, the two distinct current peaks (H
C2
and
H
C1
) are associated with the adsorption of hydrogen on the platinum electrode surface
[Whalen 2005].
In order to better understand the surface reactions of the electrode in biological
environments, cyclic voltammograms are obtained in a PBS solution. The different pH of
the solution and the differences in the ionic composition of the solution compared to that
of acidic media results in changes of the required potential range and the observed
adsorption/desorption peaks.

72
The charge storage capacity (CSC) is the total amount of electrical charge in coulombs on
the electrode surface that can be transferred to the electrolyte via a faradaic process
[Weiland, 2002]. CSC can be calculated by integration of the area under the current-
voltage curve in both the anodic (+) and cathodic ( ) direction.

3.3.2 Electrochemical Impedance Spectroscopy

Electrochemical impedance spectroscopy (EIS) is a powerful technique that to evaluate
of an electrode/electrolyte properties as a function of a small AC signal frequency that is
applied at a fixed working point [Mansfeld 2006].

Impedance measurements can be performed using either at a constant applied potential or
current. For neurostimulation electrodes impedance measurements were performed at
OCP with a +/- 10 mV amplitude ac signal of over a wide range of frequencies.
Impedance spectra can be shown as Bode-plots in which the logarithm of the impedance
|Z| and the phase angle are plotted vs. the logarithm of the frequency of the applied ac
signal.

The Bode plot of a platinum electrode obtained in 0.05 M H
2
SO
4
is shown in Figure 3.3.
The impedance magnitude is low and independent of frequency at high frequency ranges
above 10 kHz. The phase angle near zero degree represents the solution resistance R
s

[Geddes, 1997, Pell 2002, Marcus and Mansfeld 2006].

The impedance modulus begins to increase at mid-range to lower frequencies due to
capacitive charging of the electrode surface and the impedance magnitude becomes
73
frequency dependent with a slope close to -1. In this region, the phase angle was found to
be close to -75
o
. For an ideal capacitance the phase angle is -90
o
.

-1 0 1 2 3 4 5
Log f (Hz)
1
2
3
4
5
Log lZl (ohm)

-1 0 1 2 3 4 5
Log f (Hz)
-10
-20
-30
-40
-50
-60
-70
-80
Phase angle (deg.)

Figure 3.3 Typical Bode-plots for platinum foil in 0.05 M H
2
SO
4.

74
3.4 Materials and Methods
3.4.1 Microelectrode Array (MEA) Fabrication Methods

Two sets of microelectrode arrays with different microelectrode size and different
substrates were used in this study. The first was a parylene flex microelectrode array with
200µm diameter microelectrodes (Caltech Micromachining Laboratory, Pasadena, CA).
The 200µm microelectrode array fabrication is based on a process described previously
[Rodger et al. 2006], with a sandwich structure of parylene-platinum-parylene. Parylene-
C is a USP class VI biocompatible material and its mechanical properties provide the
necessary flexibility to make good contact with non-planar sensory tissue such as the
retina.

Figure 3.4 illustrates the microfabrication process, which begins with a layer of sacrificial
photoresist being spun onto a wafer followed by deposition of 5 µm thick parylene-C.
The metal layer was patterned using a liftoff process with the photoresist being composed
of a layer of LOR3B (Microchem Corporation, Newton, MA, USA) and an AZ1518 layer
(AZ Electronic Materials, Branchburg, NJ, USA). The surface of the parylene was
roughened with oxygen plasma (200mTorr at 50W for 2 minutes) prior to metallization.
A platinum layer (d = 2000Å) was directly deposited onto it using e-beam evaporation
without an adhesion layer. The second layer of parylene-C was deposited to 20µm
thickness and about 30µm thick AZ9260 photoresist was spun on and patterned as an
etch mask. Reactive ion etching with oxygen plasma at 200mTorr and 400W was
performed for about 60 minutes to create the openings in the parylene for the electrodes
75
and contact pads. The completed devices were released from the wafer using acetone and
annealed in a vacuum oven at 200
o
C for 48 hours.
The second array was a polyimide flex microelectrode array with 75µm diameter
microelectrodes (Premitec Inc. Raleigh, NC), fabricated under clean room conditions.
The microelectrode arrays consisted of gold traces connecting electrode sites with
respective bondpads on opposing ends sandwiched between two patterned layers of
polyimide films (each about 10mm thick). Standard microfabrication techniques were
utilized to deposit and pattern the various layers involved, i.e. base polyimide film, gold
traces and top polyimide film. A photo-definable polyimide material was spin-coated
onto a sacrificial substrate material, e.g. glass plate or silicon wafer, in order to maintain
enough mechanical stability during subsequent processing steps. A thin film of chromium
served as an adhesion promoting film between the polyimide surface and gold layer with
thicknesses of 10nm and 200nm, respectively. The second polyimide film contained
openings to define the size of individual electrode sites as well as openings for
corresponding bond pads. As a final processing step, each device was removed from the
sacrificial substrate by immersion into a metal etch bath, which removed the metal
release film with which the sacrificial substrate was coated. The parylene C MEAs were
fabricated at C a lt e c h ’s MEMS Lab under supervision of Prof. Y.C. Tai.
76

Rodger D. C. et. al., Sensors and Actuators B 132 (2008) 449 –460

Figure 3.4 Fabrication processes of parylene-based MEAs.

3.4.2 Electrodeposition

Platinum-iridium coatings were electrodeposited from a solution containing 0.2 g/L
sodium hexachloroiridate (III) hydrate (Na
3
IrCl
6
•xH
2
O), 0.186 g/L sodium
hexachloroplatinate (IV) hexahydrate (Na
2
PtCl
6
•6 H
2
O) and 40 g/L potassium sulfate
(K
2
SO
4
) in 0.1 M nitric acid (aq) (HNO
3
) (AlfaAesar, Ward Hill, MA, USA) at pH 2.0.
The solution preparation details have been reported elsewhere [Petrossians et al. 2011].

Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS)
experiments were performed using a three-electrode system inside a Teflon cell
controlled by a Gamry potentiostat (FAS1, Gamry Instruments, Warminster, PA, USA).
Platinum disc microelectrodes on a parylene C substrate (section 2.1) with a geometric
area of 3.14 *10
-4
cm
2
(Φ = 200µ m) se rve d a s the wor king e lec trod e s. A s pira l pl a ti nu m
77
wire counter electrode and an Ag/AgCl reference electrode were used to complete the
cell. Prior to electrodeposition of the platinum-iridium coatings, the platinum
microelectrodes were cleaned to remove impurities using trichloroethylene followed by
rinsing with acetone, methanol and DI water.

A potential sweep technique in the potential range of E = +0.2V to -0.2V vs. Ag/AgCl at
used for electrodeposition produced 60:40% platinum-iridium coatings as reported
previously [Petrossians et al. 2011, Holt-Hindle et al. 2008]. The solution was agitated
using an ultrasonic homogenizer (Misonix, Inc. Newtown, CT, USA) at a frequency of 20
kHz with a power of 8 W to maintain constant mass transfer during electrodeposition and
to maintain the temperature of the plating solution between 56ºC to 65ºC.
3.4.3 Surface and Compositional Analysis

Structural and morphological characterization of the platinum-iridium coated electrode
surfaces were performed using a field emission scanning electron microscope (ZEISS
1550VP). The chemical composition of the electrodeposited platinum-iridium coatings
was characterized using energy dispersive spectroscopy (Oxford EDS - HKL EBSD).
EDS analysis of the MEAs was performed at three separate locations for each electrode.

78
3.4.4 Cyclic Voltammetry

The electrochemical properties of the uncoated Pt control electrodes and the electroplated
platinum-iridium electrodes were characterized using CV in N
2
-purged 0.05M H
2
SO
4

(pH = 1.5) and in phosphate buffered saline (PBS) solution at room temperature. The
PBS solution was prepared by dissolving PBS powder (Sigma-Aldrich Corp., St. Louis,
MO, USA) with a composition of 0.138M NaCl and 0.0027M KCl in one liter of de-
ionized water producing a PBS solution with a concentration of 0.01M and a pH of 7.4 at
room temperature.

The potential was swept at a scan rate of 50mV/s

in the potential ranges of -0.2V to 1.2V
and -0.6V to 0.8V vs. Ag/AgCl for the two different solutions, respectively. The
cathodic charge storage capacity (CSC
C
) in PBS was measured by integrating the
measured cathodic current between the potential limits of -0.6V to 0.8V vs. Ag/AgCl
[Cogan et al. 2004]. For neurostimulation applications, the area enclosed by the cathodic
portion of the CV curve has been defined as CSC
C
of the electrode [Robblee et al. 1990]
(in this case the platinum-iridium coating).

3.4.5 Impedance Measurements

EIS measurements were performed in 0.05M H
2
SO
4
at room temperature at the open-
circuit potential (OCP) with a +/-10 mV amplitude ac signal in a frequency range of 100
KHz to 10 mHz. Uncoated Pt and platinum-iridium coated electrodes with a geometric
surface area of 3.14*10
-4
cm
2
(D = 200 µm) were used for these measurements. The
experimental data were fitted to a one-time constant equivalent circuit (EC) and the
79
values of the solution resistance (R
s
), polarization resistance (R
p
) and capacitance (C)
were determined using the ANALEIS software [Mansfeld et al. 2006, Mansfeld et al.
1992, Mansfeld et al. 2005].

3.4.6 Charge Injection Measurements

Microelectrode voltage responses to biphasic current pulses were determined for the
uncoated and platinum-iridium coated microelectrodes in-vitro in PBS solution to
simulate conditions similar to nervous systems electrical charge stimulation. Charge
injection experiments were performed on the platinum and platinum-iridium disc
microelectrodes in a two-electrode (working electrode – counter electrode) configuration.
Spiral Pt wire electrodes were used as counter electrodes in these measurements.

An 8-channel voltage/current stimulus generator (Multi-Channel Systems, STG 2008,
MCS GmbH, Germany) was used to drive biphasic current pulses equal to those used for
neurostimulation applications [Mahadevappa M. 2005]. Cathodic-first, symmetric,
charge-balanced, biphasic pulse profiles were employed. Each cathodic and anodic half-
pulse was 1 ms in duration with a 100 µs delay between each phase and a 400 µs latent
period following each anodic phase for a 2.5 ms total cycle duration (Figure 3.5). The
pulse waveform was delivered at 400 Hz. Potential responses to the applied current
pulses were recorded with an oscilloscope (Tektronix, TDS 5034B, Beaverton, OR,
USA).
80

Figure 3.5 Representative biphasic constant current pulse waveform used for
microelectrode testing (not to scale).

The current amplitude was varied to measure, characterize and compare the voltage
responses of the two microelectrode types as a function of current amplitude. Current
amplitude pulses of 30µA, 60µA, 90µA and 120µA were applied to 200µm diameter
uncoated Pt and platinum-iridium microelectrodes to compare voltage responses. Higher
current amplitude pulses of 800µA were applied only on 200µm platinum-iridium coated
microelectrodes to assess its maximum charge injection capacitance (CIC
max
) which is the
maximum charge that can be transferred at the electrode-electrolyte interface before the
surface potential rises enough to initiate water hydrolysis at the interface [Cogan et al.
2004]. CIC was calculated based on Eq.1:
) 1 (
m ax
m ax
A
t I
CIC
pulse



1ms
1ms
100 µs
- I
+ I
400 µs
81
where CIC
max
is the maximum transferred charge without creating hydrolysis, I
max
is the
maximum applied current amplitude, t
pulse
is the pulse duration and A is the geometric
surface area (cm
2
).

3.5 Results and Discussion
3.5.1 SEM and EDS of Platinum-Iridium Coatings

SEM images were used to evaluate the surface morphology of the modified
microelectrodes. Figure 3.6 shows the micrographs of uncoated Pt and electrodeposited
high surface area platinum-iridium coatings. It is evident from the micrographs that the
surface of the platinum microelectrode is completely covered by the platinum-iridium
coating. Comparison of the uncoated Pt microelectrode surface morphology (Figure
3.6a) with that of platinum-iridium coated microelectrode (Figure 3.6b) suggests that the
electrodeposited coatings are rough with a nanostructure coating indicating that the real
surface area of the microelectrodes was increased by electrodeposition. The
electrodeposited coatings are granular consisting of numerous nodules with sizes ranging
from 50 to 500nm resulting in a high surface area platinum-iridium alloy coating. SEM
micrographs of the surface of high surface area platinum-iridium coatings also showed a
uniformly coated platinum-iridium structure without evidence of delamination.

82

Figure 3.6 Scanning electron micrographs of Pt thin film (a) and platinum-iridium
electrodeposited MEAs (b).

Quantitative compositional analysis by EDS performed on the electrodeposited coatings
showed an average chemical composition of 57.9%Pt and 42.1%Ir with a standard
deviation of approximately +/- 1.2 % for both elements. Alloys of this composition have
exhibited very high electroactive surface area [Holt-Hindle et al 2008].

3.5.2 Cyclic Voltammetry for Electroplated Platinum-Iridium Coatings

Cyclic voltammograms were collected for the electrodeposited platinum-iridium coatings
to evaluate their electrochemical properties and to determine their charge storage
capacitance in both sulfuric acid and PBS solutions (Figure 3.7). Figure 3.7a compares
representative CVs for uncoated platinum and electroplated platinum-iridium electrodes
in 0.05M H
2
SO
4
. There was almost a two orders of magnitude difference in current
150 nm
150 nm
a
b
83
amplitude between the platinum and the platinum-iridium microelectrodes. The CV of the
platinum-iridium coating exhibits some of the characteristics of a platinum electrode in
sulfuric acid solution, but lacks the typical platinum oxide formation peaks and distinct
hydrogen adsorption and desorption peaks [Pell et al. 2002]. The platinum-iridium CV
shows a wide platinum oxidation peak in the anodic direction [Pell et al. 2002,
Farebrother et al. 1991].

The CV of the platinum-iridium coating showed a larger active area as evidenced by its
significantly larger current density amplitudes across the entire potential range scanned
compared to that of the uncoated platinum electrode which suggests significantly higher
CSC
c
of the electroplated platinum-iridium coatings compared to the platinum thin film
electrodes.
Figure 3.7b compares the CV profiles for platinum and an electroplated platinum-iridium
microelectrode in PBS (pH 7.4). The CSC
C
of the platinum-iridium coated
microelectrode was calculated from the time-integral of the cathodic current:
) 2 ( ) / (
1
2
cm C IdE
VA
CSC
Ef
Ei
c


where E is the electrode potential (V vs. Ag/AgCl),V is the scan rate (mV/s), A is the
surface area of the electrode (cm
2
), I is the measured current (A) and E
i
and E
f
are the
initial and final potential limits (V), respectively. The average values of the CSC
C
of the
uncoated platinum and platinum-iridium coated microelectrodes were found to be 6.28
mC/cm
2
and 243 mC/cm
2
, respectively. Other studies have demonstrated that sputtered
iridium oxide coatings had CSC
C
values of nearly 194 +/- 2 mC/cm
2
[Cogan 2009]. The
result suggests that the platinum-iridium coatings that were electrodeposited at room
84
temperature and ambient pressure can provide a 25% larger CSC
C
than sputtered iridium
oxide microelectrodes.
-0.2 0 0.2 0.4 0.6 0.8 1 1.2 1.4
Potential (V)
-20
-10
0
10
20
I (µA)
-0.4
-0.2
0
0.2
0.4
I (µA)

Pt-Ir
Pt
a

-0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1
Potential (V)
-10
-5
0
5
10
I (
µA)
-0.2
0
0.2
I (
µA)
Pt-Ir
Pt
b

Figure 3.7 Cyclic voltammograms of uncoated platinum and platinum-iridium
electroplated electrodes in 0.05 M H
2
SO
4
(pH = 2.0) (a) and in PBS solution
(pH = 7.4) (b).
85
3.5.3 EIS Measurements

Figure 3.8 shows impedance spectra for the two microelectrodes as Bode-plots with the
log of the impedance modulus (|Z|) and the phase angle ( ) plotted versus the log of the
frequency (f). The impedance magnitudes for both microelectrode types were
independent of frequency above 10 kHz indicating that the measured impedance
represents the solution resistance R
S
. Below 10 kHz the impedance magnitude becomes
frequency dependent for the platinum microelectrodes and below about 10Hz for the
platinum-iridium microelectrode with a slope close to n = -1 indicating capacitive
behavior.
As shown in Figure 3.8, platinum-iridium coated microelectrodes have much lower
impedance compared to the uncoated platinum microelectrodes in the intermediate
frequency range. The impedance spectra for the platinum-iridium coated microelectrode
were fit to a one-time-constant model (OTCM) in which R
S
is in series with a parallel
combination of the electrode capacitance C and the polarization resistance R
P
. The
observation of only one time constant in the impedance measurements indicates that the
surface of the microelectrode was completely covered with the non-porous platinum-
iridium coating. The values of C for uncoated and platinum-iridium electroplated
microelectrodes were determined to be 7.67*10
-7
F and
8.82 *10
-5
F, respectively. The observed low interface impedances can be explained by
considering the relation between the capacitance C and the impedance |Z| of the electrode
( ) 2 /( 1 C f Z   ). The observed decrease of |Z| is due to an increase of C as a result of
the increased real surface area, since in the capacitive region |Z| is inversely proportional
to the electrode capacitance. C depends on the surface area A as shown in Eq. 3:
86
) 3 (
0
d
A
C
 


whe re ε is the die l e c tric c onst a nt of the c oa ti n g , ε
0
is the permittivity of free space and d
is the coating thickness.
-2 0 2 4
-1 1 3 5
Log f (Hz)
3
4
5
6
7
8
9
Log IZI (ohm)

Pt
Pt-Ir

-2 0 2 4
-1 1 3 5
Log f (Hz)
-20
-40
-60
-80
-10
-30
-50
-70
-90
Phase angle (deg.)

Pt
Pt-Ir

Figure 3.8 Bode plots for uncoated platinum and an electroplated platinum-iridium
electrode measured in 0.05 M H
2
SO
4
(pH =2.0) at the OCP.
a
b
87
3.5.4 Voltage Response to Applied Currents Pulses

Figure 3.9 shows a representative potential response recorded for one of the platinum
mi c roe lec trod e s. The po rtions of the volt a ge re sp onse s labe led “ OR” (oh mi c re sis tanc e )
correspond to the potential drop associated with the ohmic resistance of the electrolyte
between the two electrodes. OR can therefore be subtracted from the voltage transients to
calculate the potential associated with polarization of the microelectrode surface, i.e. the
charge transfer (CT) region of the plot in Figure 3.9 [Robblee L. S. et al. 1990].
Safe electrical stimulation is achieved when the potential magnitude of the CT region is
less than the potential diff e re n c e b e twe e n th e e lec t rode ’s ope n -circuit potential and the
potential for water h y dro l y sis (∆ E
Safe
< E
Hydrolysis
-E
OCP
), i.e . ∆E
Safe
= 700mV as discussed
in other studies where no hydrogen gas evolution was observed [Cogan et al. 2009].
Therefore for any given current pulse profile the output voltage profile is analyzed to
assess whether the CT voltage difference is within these safety limits.

88
0.5 1 1.5 2 2.5
Time (ms)
-0.8
-0.4
0
0.4
Voltage (V)
OR
CT
OR

Figure 3.9 Voltage response of a platinum microelectrode to a biphasic symmetric current
pulse w it h a mpl it ude I = 60 μ A ( fr e qu e nc y = 400 Hz ).

Figure 3.10 compares the potential response of platinum and platinum-iridium
microelectrode as a function of current pulse amplitude. A significant difference was
observed between the two microelectrodes at all four currents tested. At the lowest
current pulse amplitude tested (I = 30 µA), the platinum-iridium microelectrode exhibited
a potential response less than 10% in magnitude than the platinum microelectrode pulsed
at the same current amplitude. This difference increased with increasing current
amplitude, with the platinum microelectrode showing significant current dependence and
the platinum-iridium microelectrode showing virtually no current dependence in the
amplitude range tested.

89
0.5 1 1.5 2 2.5
Time (ms)
-0.4
-0.2
0
0.2
Voltage (V)
a
Pt-Ir
Pt

0.5 1 1.5 2 2.5
Time (ms)
-0.8
-0.4
0
0.4
Voltage (V)
b
Pt-Ir
Pt

0.5 1 1.5 2 2.5
Time (ms)
-0.8
-0.4
0
0.4
Voltage (V)
c
Pt-Ir
Pt

0.5 1 1.5 2 2.5
Time (ms)
-1.2
-0.8
-0.4
0
0.4
Voltage (V)
d
Pt-Ir
Pt

Figure 3.10 Voltage responses of platinum and platinum-iridium electroplated MEAs to
a ppli e d c urr e nt pulses of a mpl it ude s: (a ) 30 μ A (b ) 60 μ A (c ) 90 μ A a nd (d )
120 μ A.

As shown in Figure 3.10, deposition of the platinum-iridium alloy coating greatly
reduced the voltage responses of the microelectrode to current pulses which imply that
the safety margin was increased for microelectrodes treated with this coating since even
when significant current was applied the voltage was well within the safe limits. The
platinum-iridium microelectrode surface potential never increased more than 50mV in
response to the applied current even with a 120µA current pulse. A 120µA and 1ms
current pulse on the 200µm platinum-iridium microelectrode generated a charge density
of 380µC/cm
2
which is less than the safe charge density of 400-500µC/cm
2
that has been
90
reported by Brummer and Turner for real surface area platinum disk electrodes [Brummer
et al. 1975] and 50-150µC/cm
2
for polished platinum disk microelectrodes [McCreery
D.B. et al. 2004]. The platinum-iridium microelectrode only generated a 50mV
polarization in response to this current pulse. Figure 3.11 shows a comparison of voltage
response magnitudes (V) for platinum and platinum-iridium microelectrodes pulsed at
four different current amplitudes in PBS at room temperature. This result suggests that it
is possible to reach significantly higher charge densities with this microelectrode material
with minimal risk of irreversible reactions of the microelectrode surface.

Figure 3.11 Comparison of voltage response magnitudes (V) for platinum and platinum-
iridium microelectrodes measured in PBS at room temperature.

1.0
0.8
0.6
0.4
0.2
0.0
Potential Response (V)
5.6mV
10mV
16mV
24mV
392mV
800mV
864mV
920mV
                    
Pt-Ir
Pt
91
In order to confirm this observation, the experiments were repeated on the same
platinum-iridium microelectrode using 800 µA current pulse amplitudes. Figure 3.12
shows the potential transients recorded for a platinum-iridium electroplated
microelectrode. The measured voltage drop was approximately 0.2V, which is well
below the safe potential limits of -0.6V and 0.8V vs. Ag/AgCl [Cogan S.F. et al. 2004].
This resulted in a charge density of 2.54 mC/cm
2,
which is five times greater than the
values reported for rough platinum disk electrodes [McCreery D.B. et al. 2004].

The transferred charge in the pulse (Q = 800nC) was well above the typical threshold for
retinal stimulation [De Balthasar et al. 2008]. These results suggest that current pulse
amplitudes and total charge injected using these microelectrodes can be driven to values
well above those typically accepted as safe with minimal potential response of the
electrode and almost no risk of inducing hydrolytic reactions on the microelectrode
surfaces.

92
0.5 1 1.5 2 2.5
Time (ms)
-2
0
2
Voltage (V)

Figure 3.12 Potential response of platinum-iridium electroplated mi c ro e le c trode s (Φ = 200 µ m) to 800 μ A c urr e nt pul se s in P B S a t room t e mper a tur e .

The potential limits of the platinum-iridium microelectrodes were determined by
repeating the same experiment with a smaller 75µm microelectrode. Figure 3.13 shows
the voltage response at which the water window limit was reached. This corresponded to
a 280µA current pulse amplitude and a CIC
max
of the platinum-iridium coating of
6.11mC/cm
2
which is more than ten times larger than the higher estimates of charge
density limits [Brummer et al. 1975] for rough platinum electrodes.

Irreversible reactions generated on a microelectrode surface is a major concern in the
development of implantable microstimulators for sustained use as it can lead to corrosion
and failure of the microelectrodes and the device. The findings presented here suggest
that sustained pulsing with these platinum-iridium microelectrodes would have a low
93
probability of causing accumulated polarization over time thereby improving the
performance characteristics significantly.

0.5 1 1.5 2 2.5
Time (ms)
-2
0
2
Voltage (V)

Figure 3.13 Potential response of platinum-iridium electroplated microelectrode ( Φ= 75
µm) in PBS at room temperature with the applied current amplitude to reach
CIC
max
.

In addition to the safety advantages gained there are also potential energy savings gained.
The electrochemical impedance data and current pulse data show that substantially lower
electrochemical impedance is encountered during charge transfer at the platinum-iridium
microelectrode-electrolyte interface as compared to a platinum microelectrode of similar
geometric area. This result suggests that the efficiency of an implanted device equipped
with this type of microelectrode would be higher, thus minimizing power requirements
for current stimulation. As a result the battery lifetime would be extended for an
implanted device with this technology and hence would potentially delay subsequent
surgeries for battery replacements. Another observation related to energy efficiency is
94
that microelectronic component sizes scale with power. Therefore devices equipped with
platinum-iridium microelectrodes of this type would require lower power components,
e.g. diodes, to drive them. Lower power components would thus make it possible to
decrease the total implant size.

3.5.5 Reliability Test of the Platinum-Iridium Electroplated Electrode

Since delamination caused by mechanical instability and/or poor film adhesion to the
underlying substrate may be exacerbated by sustained current pulsing, it is important to
evaluate microelectrodes for this type of failure. To evaluate the durability of the
electroplated platinum-iridium coatings over the time course of sustained current pulse
testing, EIS measurements were performed at intermittent time points in 0.05 M H
2
SO
4

at room temperature. The impedance magnitude was recorded at three different
frequencies to monitor any changes in electrochemical impedance that may result from
delamination or other changes in the platinum-iridium coating properties.

Figure 3.14 shows the impedance magnitude values measured as a function of time at f =
1 kHz, 0.1 Hz and 0.01 Hz over 329 days. Average |Z| values were calculated to be 1.20
± 0.043 k Ω, 44.13 ± 2.91 k Ω and 477.84 ± 32.96 k Ω at the three frequencies,
respectively. These data correlate to 3.6%, 6.6% and 7.3% standard deviations,
respectively, over the test period suggesting relatively stable impedance values at low,
mid, and high frequencies. These results confirm that the platinum-iridium coatings did
not fail due to delamination and that the electrochemical properties of the coatings
showed little change over 11.23 billion pulses.
95

100 200 300 400
Time (day)
3
4
5
6
log lZl (ohm)
Graph 1
0.01 Hz
0.1 Hz
1 kHz

Figure 3.14 Impedance data obtained at frequencies of 1 kHz, 0.1 Hz and 0.01 Hz as a
function of time in 0.05 M H
2
SO
4
at room temperature.

96
3.6 Conclusions

In this study in-vitro experiments were performed to evaluate the properties of
microelectrodes with high-surface area platinum-iridium coatings in comparison with
conventional platinum thin film microelectrodes for sustained neural stimulation
applications. Using a slightly modified electroplating bath composition as used in
previous work for developing thin dense platinum-iridium films [Petrossians A. et al.
2011] an efficient and reproducible method for electrodeposition of rough platinum-
iridium coatings was developed. CV and EIS measurements of these platinum-iridium
coatings showed a 40-fold increase in CSC and a reduced interface impedance of almost
two orders of magnitude. These improvements can be attributed to the increased real
surface area of the platinum-iridium microelectrodes as compared to pure platinum. The
electrode alloy materials exhibited consistent, reversible charge transfer under square
wave biphasic current pulses for more than 329 days of sustained pulsing. The platinum-
iridium coatings also exhibited mechanical stability as confirmed by periodic EIS
measurements over the same 329 days of current pulsing. Future biological and
cytotoxicity testing of this material will be required in order to adequately validate its use
for sustained, implantable neurostimulation applications.

97
3.7 References

Agnew W.F., McCreery D.B., Neural Prostheses. Prentice Hall, NJ (1990).
Brummer S.B., Roblee L.S., Hambrecht F.T., Criteria for Selecting Electrodes for
Electrical Stimulation: Theoretical and Practical Considerations. Annals of
New York Academy of Sciences, 405, 159 (1983).

Brummer S.B., Turner M.J., Electrical Stimulation of the Nervous System: The Principle
of Safe Charge Injection with Noble Metal Electrodes. Bioelectrochemistry
and Bioenergetics. 2, 13 (1975).

Brummer S.B., Turner MJ. Electrical Stimulation of the Nervous System: The Principle
of Safe Charge Injection with Noble Metal Electrodes. Bioelectrochemistry
and Bioenergetics. 2, 13 (1975).

Cogan S.F., Ehrlich J, Plante T.D., Smirnov A, Shire DB, Gingerich M, Rizzo JF., J
Biomed Mater Res Part B: Appl Biomater; 89B, 353 (2009).

Cogan S.F., Neural stimulation and recording electrodes. Annu Rev Biomed Eng., 10,
275 (2008).

Cogan S.F., Plante T.D., Ehrlich J., Conf Proc IEEE Eng Med Biol Soc., 6, 4153 (2004).

Cogan S.F., Plante T.D., Ehrlich J., Sputtered iridium oxide films (SIROFs) for low-
impedance neural stimulation and recording electrodes. Conf Proc IEEE Eng
Med Biol Soc, 6, 4153 (2004).

de Balthasar C., Patel S., Roy A., Freda R., Greenwald S., Horsager A., Mahadevappa
M., Yanai D., McMahon M.J., Humayun M.S., Greenberg R.J., Weiland J.D.,
Fine I., IOVS, 49, 2303 (2008).

Farebrother M., Goledzinowski M., Thomas G., Birss V.I., J. Electroanal. Chem., 297,
469 (1991).

Geddes L.A., Historical Evolution of Circuit Models for the Electrode-Electrolyte
Interface. Annals Biomed.Eng.25, 1 (1997).

Heiduschka P., Thanos S., Prog. Neurobiol., 55, 433 (1998).

Holt-Hindle P., Yi Q., Wu G., Koczkur K., Chen A., J. Electrochem. Soc.; 155, K5,
(2008).

Horch, K.W., Dhillon G.S. Eds., Neuroprosthetics: Theory and Practice; River Edge, NJ,
World Scientific (2004).

98
Mahadevappa M., Weiland J.D., Yanai D., Fine I., Greenberg R.J., Humayun M.S.,
IEEE Transactions on Neural Systems & Rehabilitation Engineering; 13, 201
(2005).

Mansfeld F., Shih H., Greene H., Tsai C.H., Analysis of EIS data for common corrosion
processes, Electrochemical impedance: Analysis and interpretation ASTM
STP 1188, 37 (1993).

Mansfeld F., Tsai C.H., Shih H., Software for simulation and analysis of electrochemical
impedance spectroscopy (EIS) data, Computer modeling in corrosion ASTM
STP 1154, 186 (1992).

Mansfeld F., Electrochemical Impedance Spectroscopy. Analytical Methods in Corrosion
Science and Engineering. Marcus P. and Mansfeld F., Eds. CRC Press, 463
(2006).

Margalit E., Maia M., Weiland J.D., Greenberg R.J., Fujii G.Y., Torres G, Piyathaisere
D.V., O'Hearn T.M., Liu W., Lazzi G., Dagnelie G., Scribner D.A., de Juan E.
Jr, Humayun M.S., Survey of Ophghalmology, 47, 335 (2002).

McCreery D.B., Tissue reaction to electrodes: The problem of safe and effective
stimulation of neural tissue in Neuroprosthetics Theory and Practice, Horch
KW, Dhillon GS, Eds. Singapore: World Scientific (2004).

Pell W.G., Zolfaghari A., Conway B.E., J. Electroanal Chem., 532, 13 (2002).

Petrossians A., Whalen III J.J., Weiland J.D., Mansfeld F., J. Electrochem. Soc.; 158, 269
(2011).

Robblee L.S., Rose T.L., Electrochemical guidelines for selection of protocols and
electrode materials for neural stimulation in Neural Prostheses: Fundamental
Studies, Eds. Agnew W.F., McCreery D.B., Prentice Hall, Englewood Cliffs,
NJ, 25 (1990).

Rodger D.C., Li W., Ameri H., Ray A., Weiland J.D., Humayun M.S., Tai Y.C., Proc.
IEEE-NEMS, 743 (2006).

Turner J.N., Shain W., Szarowski D.H., Andersen M., Martins S., Isaacson M., Craighead
H.G., Exp Neurol, 156, 33 (1999).

Weiland J.D., Anderson D.J., Humayun M.S., IEEE Trans.Biomed.Eng., 49, 1574
(2002).

99
Yagi T. Hayashida Y., Implantation of the artificial retina, Nippon Rinsho, 57,1208
(1999).

Zrenner E., Stett A., Weiss S., Aramant R.B., Guenther E., Kohler K., Miliczek K.D.,
Seiler M.J., Hammerle H., Vision Res., 39, 2555 (1999).

100
CHAPTER IV
Electrodeposition of Platinum-Iridium Alloy Nanowires for Hermetic Packaging of
Microelectronics

4.1 Introduction

The electrochemical deposition method of platinum-iridium thin films were described in
chapter 2 in which platinum-iridium alloy thin films can be electroplated from 1:1 ratio of
mixed solution of hexachloroiridate (III) hydrate and sodium hexachloroplatinate (IV)
hexahydrate in 0.1 M nitric acid. In chapter 2, the electrochemical, mechanical, surface
structure and the effect of the pH and the electroplating potential range on the chemical
composition of the alloy were comprehensively investigated. The goal of this chapter is
to fabricate platinum-iridium alloy dense nanowires using similar conditions developed
for electrodeposition of platinum-iridium thin films.

4.2 Background

The use of implantable devices for treatment of medical disorders such as movement
disorders, deafness and urinary incontinence has drastically been increased [Horch et al.
2004]. Such devices include microelectronics embedded in protective biocompatible
cases. Microelectronic components such as integrated circuits in implantable devices
need to be protected inside a hermetic packaging in order to prevent the penetration of
mobile ions such as K
+
, Na
+
and Cl
-
from the body fluid. However, in order to stimulate
tissue the hermetic package must have a means to conduct stimulation current from the
electronics inside the package to the tissue outside the package. These conducting paths
communicating the encapsulated microchip to the outside of the hermetic case are called
101
feed-throughs or interconnect. Humidity has been one of the major concerns causing
failure to the microelectronics [Thomas 1976, Osenbach1993].

Earlier developed implantable electronic devices such as cardiac pacemakers have few
relatively large electrical interconnects, while newer devices such as cochlear implants
contain nearly 20 interconnects. Brain machine interfaces and visual prostheses will
require hundreds of interconnects to make a parallel connection with the brain. Advanced
interconnect technology for implantable microelectronics can be achieved by applying
proper fabrication processes.

Several research groups are trying to overcome the existing issues on the fabrication of
hermetic packages for implantable microelectronics [Ramesham et al. 2000, Roy et al.
2001, Roy et al. 2003, Ferrara 2003]. Since these implanted devices are to remain all
sealed for the lifetime of the patients, many important factors need to be considered in
hermeticity of the fabricated packages. These include the use of an advanced and proper
interconnect fabrication technology. Also selection of biocompatible materials for both
the hermetic case and the metallic interconnects in which the electrical, mechanical and
corrosion resistance a property of these interconnects have critical roles on the
functionality of the implantable microelectronics. However, a limited number of proper
methods for the fabrication of high density interconnects are available.

Figure 4.1 shows a biocompatible and hermetic microelectronics packaging with
electrodeposited platinum-iridium nanowire interconnects embedded in an anodized
102
aluminum oxide wafer. Data are transferred from the encapsulated silicon microchip
through the interconnect platform to the outside leads.

A novel fabrication process using an electrochemical deposition method has been applied
for the fabrication of platinum-iridium feed-through which is an alternative to existing
conventional processing methods with many advantages over complex and costly
nanowire fabrication routes. Advantages include the cost effectiveness and simplicity of
the process which makes it a more suitable method for industrial applications. In
comparison with most of the existing nanofabrication techniques at high temperatures,
the electrodeposition baths are operated at low-temperatures, usually below 80
o
C. Also
non-equilibrium phases can be produced by electrodeposition which cannot be achieved
by thermal processing techniques [Yu-Zhang et al. 2006].

103

Figure 4.1The top diagram shows a interconnect substrate with a case bonded over the
top of the chip. Bumps on both the chip and the interconnect substrate
facilitate electrical connections. This interconnect substrate could also be used
with conformal coating technology (bottom). Drawings not to scale.

Using the electrodeposition method, higher-aspect ratio (400:1) nanowires are achievable
in comparison with thin film sputtering and other microfabrication processes. Since sub-
micron deposits are desired, dense and continuous interconnects can be electrodeposited.
Thicker interconnect platforms can be fabricated by deposition of longer interconnects.
This in turn will further increase the package hermeticity by increasing the length of the
pathway that water and ions must penetrate to enter the inside of the package. Using this
104
technology, ultra-high density interconnect arrays can be fabricated. Thus, nanowire
based microelectronics packaging offers a novel approach for the fabrication of
tissue/device interface technology such as neural stimulating/recording electrodes and
biological sensors, while the requirements such as biocompatibility and hermeticity of the
implantable medical devices are considered.

To date, no study has been reported on the fabrication of ultra-high-density platinum-
iridium nanowires using electrodeposition methods. Few research studies have been
accomplished in the fabrication of platinum nanowires. Different techniques used toward
the electrodeposition of platinum nanowires include focused ion beam [Rothkina et
al.2003], photoreduction in mesoporous silicides, [Sasaki et al. 1999, Sasaki et al. 1998,
Fukuoka 2001, Husain et al. 2003, Yang et al. 2002], colloidal synthesis [Hippe et al.
1999, Fu 2003], self-assembly [Kimizuka 2000, Gurlu et al. 2003], and nanopore filling
by electrochemistry [Luo et al. 2003, Llopis et al. 1976]. The effect of high or low pH of
the electroplating solution needs to be considered since it may dissolve the alumina
template during electrodeposition.

This study focuses on the fabrication and evaluation of platinum-iridium dense nanowires
with improved electrical and mechanical properties to be used as feed-through
technology in hermetically packaged implantable microelectronics. In this study, an
electrochemical deposition method for the fabrication of platinum-iridium thin films was
used [Petrossians et al. 2011] and the effects of the pH and the electroplating potential
105
range on the chemical composition and the surface structure of the alloy as well as the
hermeticity of the fabricated nanowires were comprehensively investigated.

4.3 Materials and Methods
4.3.1 Alumina Template Preparation

“ Anodisc ” N a nopor ous a nodiz e d a lum inum filt ra ti on membr a ne s (W ha tm a n I n c .UK )
were used as templates for the electrodeposition of the nanowires. The templates have an
a pprox im a te thickne ss of 60 μ m and 47 mm i n dia me ter. For ease of membrane
transportation all templates were fitted with a plastic annular ring, attached to one side of
each membrane. SEM inspections on both sides of the membranes revealed that the pore
sizes were different on both side of the membranes. The pore sizes on the side with the
plastic ring attached were found to be 20 nm in diameter while the pore sizes of the other
side were approximately 200 nm in diameter, randomly distributed with larger spaces
between the pores. On the side with 20 nm end, large pore sizes were branched to smaller
finger like channels with an approximate length of 50 nm.

4.3.2 Platinum-iridium Nanowire Electrodeposition

Nanowires were prepared using electrochemical deposition into nanoporous anodized
aluminum oxide (AAO) membranes with a pore size of 200 nm in diameter. Ti thin films
as adhesion layers with a thickness of 40nm followed by Au thin films with a thickness of
80 nm were sputter-deposited on one the side of the AAO membranes with the smaller
pore size to make it the cathode.
106
Platinum-iridium nanowires were electrodeposited from an electroplating bath
(explained in chapter 2) using a three electrode electrochemical cell which contained two
vertical chambers with different size and a horizontal small via that connected the two
chambers in the bottom (Figure 4.2). The larger chamber contained the electrodeposition
solution and the smaller chamber held the reference electrode. The alumina templates
were placed on a copper plate that was gold coated. The electrochemical cell was held on
top of the template. In order to seal the cell, a polymer O-ring was placed between the
cell and the substrate using a steel spring-clamp.

The Ti/Au coated alumina template was placed on the copper plate on its coated side and
the working electrode (WE) lead was connected to the copper plate. An Ag/AgCl
electrode was used as the reference electrode (RE) and a spiral platinum wire with 1 mm
diameter was placed in the larger chamber as the counter electrode (CE). The small via
between two chambers connected the two chambers through the solution. The copper
plate was polished with fine sand paper to remove existing oxide layers followed by
rinsing with DI water.

107

Figure 4.2 Electrochemical cell used for nanowire deposition in nano-channeled
aluminum oxide (Al
2
O
3
) template.

108
The electrodeposition potential range was selected based on the results from
electrodeposition of platinum-iridium alloy thin films (chapter 2) except that the
deposition was performed on titanium thin films instead of gold substrates). Platinum-
iridium nanowires were electrodeposited within different potential ranges in which
different atomic percentage of iridium was measured.
Parallel platinum-iridium nanowires were electrodeposited using a potential cycling
technique within the potential range of 0.1 V to -0.15 V vs. Ag/AgCl using AAO
templates (Figure 4.3). The solution preparation details have been explained in chapter 2.

Figure 4.3 Schematic of the fabrication processes of metallic nanowires in AAO
nanopores. Drawing not to scale.

Nanowires were isolated for further analysis. The electrodeposited AAO template was
immersed in an aqueous solution of NaOH in a vial. In this process, AAO was totally
dissolved and individual nanowires were floating in the solution (Figure 4.4). A certain
amount of time was allowed for free nanowires to sink. Then the solution was pipetted
off and DI water was added to the vial to rinse the nanowires. This process was repeated
three times until a neutral solution was achieved.

109

Figure 4.4 Schematic of the isolation processes of electrodeposited nanowires in AAO
templates. Drawing not to scale.

4.3.3 SEM Characterization

SEM images were used to characterize the surface morphology, structure and size of the
electrodeposited platinum and platinum-iridium nanowires using a field emission
scanning electron microscope (ZEISS 1550VP) with an accelerating voltage of 4 kV.
The electrodeposited nanowires were isolated from the alumina templates using several
dissolving processes. The sputter deposited gold thin films were etched using gold
etchant (AlfaAesar). The alumina templates were broken to smaller pieces and were fit in
10 ml vials filled with 8 ml of 1.0 M NaOH. After completion of the dissolution process
of the alumina wafers in NaOH, vials were left untouched for one hour until all wires
sank to the bottom of the vial. Then the etchant (NaOH) was pipetted off and had filled
with DI water. This process was repeated three times to neutralize the solution containing
nanowires. Isolated platinum-iridium nanowires were distributed on the surface of SEM
aluminum flat stubs and gently dried with nitrogen gas.

110
4.3.4 TEM Characterization

Isolated platinum-iridium nanowires (preparation method explained in the previous
section) were distributed on carbon coated copper grids with 300 mesh size (Ted Pella
Inc.). Copper grids with platinum and platinum-iridium nanowires were separately loaded
in TEM (JEOL 2100, Japan) to acquire images and diffraction patterns. Images were
taken on the thinner edges of the nanowires with smaller branches of 20 nm for
transmission of the electron beam through the samples in dark and bright field modes.
Diffraction patterns were taken for the isolated platinum and platinum-iridium nanowires
using beam widths smaller than the width of the nanowires.

4.3.5 Electrical Testing

Electrical testing was performed on single platinum and single platinum-iridium
nanowires. In this process, randomly oriented single nanowires between two Ti/Au
contact pads were trapped in order to measure the conductivity through the length of the
single nanowires. Platinum and platinum-iridium nanowires were soaked into methanol
in separate vials. Diluted amounts of each set of nanowires were dispersed on the Si/SiO
2

substrates. After the desired nanowire density was achieved, photolithography was
applied to pattern the surface of the SiO
2
substrates. Using e-beam evaporation, a thin
layer of Ti/Au with a thickness of about 5/50nm was deposited on top of the substrates.
Metal lift-off technique was applied to remove the excess of deposited metal from the
surface. After device fabrication, a voltage bias was applied cross the nanowires and the
current was measured.
111
Electrical conductivity of the nanowires was measured across the contact pads on the
fabricated device. The electron transport measurements of the single nanowires were
performed using an Agilent 4156B semiconductor parameter analyzer.

4.3.6 Helium Leak Testing

Helium leak tests were performed using an ASM 182-TD (Alcatel, Inc.), helium leak
detector with capability of detecting helium leak rates down to 5 x 10
-12
mbar L s
-1
. A
custom sample mounting fixture was designed for NIA sample testing. The fixture with a
flat and ultra-fine surface was mounted to the inlet aperture of the leak detector using a
standard vacuum seal and clamp. A small circular inlet was placed in the center of this
fixture in order to serve as a conduit for helium gas that leaks through the test sample.
In He leak testing, a small amount of vacuum oil was applied to a polymer O-ring which
was placed between the surface of the fixture and the sample, over the aperture of the
fixture. The applied vacuum by the leak detector at the inlet created a tight seal between
the fixture and the sample. Measurements were taken after the flow was below 1 x 10
-11

mbar L s
-1
and a stable flow was read. A continuous small dose of He was sprayed on the
top side of the sample exposed to ambient air. He gas was sprayed from nearly 10 cm
from the sample surface with a pressure of 20 lbs for 10 seconds.
After helium exposure, the highest observed leak rates were measured and recorded. Leak
rate measurements were repeated three times for each sample. After each measurement,
time delay was given in order to return the leak rate to below 1 x 10
-11
mbar L s
-1
.
112
4.5. Results and Discussion
4.5.1 Platinum-Iridium Nanowire Electrodeposition

Metallic nanowires can be fabricated by electrochemical template synthesis [Martin 1994,
Xia et al. 2003]. In this method, platinum and iridium ions dissolved in the electroplating
solution are reduced to platinum and iridium inside the porous channels of the AAO
substrate (Figure 4.5). Platinum-iridium nanowires with approximately 60:40% in
chemical composition were electrodeposited in a potential range from 0 V to -0.15 V vs.
Ag/AgCl at a scan rate of 250 mv/s.

The optimum potential range for electrodeposition of the nanowires was first identified
by cycling the applied potential over various 150 mV ranges from E = -0.2 V to E = 0.2V
vs. Ag/AgCl until visibly robust platinum-iridium nanowires were achieved. The
potential range of E=0 to -0.15 V vs. Ag/AgCl was subsequently selected for the
deposition of 60:40% platinum-iridium nanowires. Another factor effecting the
electrodeposition of the nanowires was found to be the pH of the electroplating bath.
Alumina templates showed corrosion and surface wear as well as destruction of the pores
due to low pH of the electroplating bath. On the other hand, pH level close to neutrality at
temperatures close to 60
o
C caused the formation of clusters larger than the membranes
pores size in the electrodeposition solution which in turn stopped platinum-iridium
na nowir e s g ro wth a fte r 2 μ m i n leng th b y blockin g the por e s.

113

Figure 4.5 (left) Schematic of electrochemical template fabrication of metallic wires.
(right) Demonstration of the reduction of metal ions on the working electrode
at the base of a single channel. Drawings not to scale.

It was previously shown in Chapter 2 that the platinum-iridium alloy thin films can be
electroplated without any surface defects and delamination. It was also shown that the
chemical composition of the electroplated platinum-iridium films can be controlled by
changing the electroplating potential range. Further in chapter 3, the study on the high
surface area platinum-iridium coatings showed improved charge transfer properties in
114
comparison with pure platinum thin films. In this chapter, platinum-iridium nanowries
were electrodeposited using similar deposition conditions to verify the similarity of the
nanowires properties compared to that of platinum-iridium thin films.

4.5.2 SEM Characterization

The size and surface structure of platinum-iridium nanowires were characterized using
scanning electron microscopy (SEM). Figure 4.6 shows a SEM micrograph of platinum-
iridium nanowires inside of AAO template. The cross sectional view represents the
profile of the nanowires formed in the AAO nano-pores indicating the parallel formation
and growth of the nanowires.

Figure 4.6 SEM micrograph of the cross section of platinum-iridium nanowires grown in
AAO pores.
5 µm
115
SEM micrographs showed that the pH of the electrodeposition bath directly affected the
morphology of the nanowires. Several sets of nanowire samples were prepared using
different electrodeposition baths with different pH levels until platinum-iridium
nanowires with the desired surface structures were achieved.
Nanowires deposited in electroplating baths with pH levels of 5 and higher appeared to
be brittle with discontinuities (Figure 4.7), while nanowires deposited from the pH baths
between 1.5 to 2.5 appeared to be short in length because of suppression of the deposition
process due to wear on the surface of the templates during stirring of the
electrodeposition solution which caused the pores to fill up with AAO particles (Figure
4.8).

Figure 4.7 SEM micrograph of isolated nanowires with brittle structures electrodeposited
at higher pH.

1 µm
116

Figure 4.8 SEM micrograph of isolated nanowires with shorter lengths electrodeposited
at lower pH.

Successful results were obtained using electroplating baths with pH of 3.1. Samples
prepared using this electroplating bath (pH 3.1) showed dense and long nanowires
without discontinuities (Figure 4.9).

Figure 4.9 SEM micrograph of isolated dense nanowires with dense structures
electrodeposited at pH = 3.1.
1 µm
1 µm
117
4.5.3 TEM Characterization

Figures 4.10a and b are bright-field and dark-field TEM images of platinum nanowires,
respectively. The contrast of the images demonstrates the effect of the AAO on the
surface morphology of the nanowires. Diffraction patterns of the deposited nanowires
(Figure 4.11) corresponded to a polycrystalline structure without preferred orientation.
The uniform and diffuse ring diffraction patterns indicate the small grain size and
untextured polycrystalline morphology of the nanowires.

The radius(r) of the diffraction rings varies with h, k, l as shown in equation (1):

2 2 2
l k h r    (1)
where h, k and l are the Miller indices that represent the crystallographic plane. The
results of the calculations using equation (1) showed that the rings correspond to the
planes (111), (200), (220) and (311) which represent a typical face-centered cubic (FCC)
structure (Figure 4.11).

118

Figure 4.10 Bright-field (a) and dark-field (b) images of platinum nanowires.

a
b
119

Figure 4.11 Electron beam diffraction pattern of platinum-iridium nanowire with fcc
structure.

A HRTEM image of an individual platinum-iridium nanowire is shown in Figure 4.12.
Since the values of the lattice parameters of platinum and iridium are too close to each
other, the ( −1 −1 1), (1 1 1) and (0 0 2) planes labeled in Figure 4.12 represent the fcc
crystal structure that may belong to either platinum or iridium (JCPDS 04-0802) or an
alloy. Consequently, the twin at the grain boundary might be due to the effect of platinum
or iridium alloying element, indicating a bimetallic particle [Nitani 2005].

120

Figure 4.12 HRTEM image of an individual platinum-iridium nanowire

4.5.4 Effect of Deposition Potential on the Chemical Composition of the Nanowires

The EDS analysis of the electrodeposited platinum-iridium nanowires revealed that the
chemical composition of the nanowires was highly dependent on the deposition potential.
Table 4.1 shows the effect of different potential ranges on the atomic ratio of platinum-
iridium of the electrodeposited platinum-iridium nanowires. The results of Table 4.1
suggest that small changes in the applied potential ranges in both the positive and
negative direction of the potential sweeps caused relatively large changes in the amount
121
of iridium content of the nanowires. For example a change of the positive limit from 0 to
-0.05 V changed the Pt:Ir ratio from 62:38% to 85:15% (Table 4.1)

Table 4.1 Chemical composition of platinum-iridium nanowires in different applied
potential ranges.

Potential range (V vs. Ag/AgCl) Average Pt:Ir ratio (%)
0.2 to -0.15 68:32 %
0.02 to -0.15 67:33 %
0 to -0.15 62:38 %
0 to -0.2 68:32 %
-0.05 to -0.15 85:15 %

4.5.5 Conductivity Measurements

The conductivity measurements of the platinum and platinum-iridium nanowires were
carried out by applying an electrical potential across single nanowires. Figure 4.13 a and
b show the scanning electron micrographs of the fabricated devices for platinum and
platinum-iridium nanowires, respectively, where two thin-film Ti/Au stripes were
sputter-deposited on top of dispersed nanowires as contact pads. The distance of the gap
between the two parallel source and drain electrodes was 2 μ m.
122

Figure 4.13 SEM micrograph of the testing device used for electrical conductivity
measurement of a) single platinum nanowire and b) single platinum-iridium
nanowire.

a
123
The representative current-voltage plots in Figure 4.14 show the current vs. voltage
measured across the length of the platinum and platinum-iridium nanowires. The ratio of
the average currents measured for platinum-iridium nanowires to that of platinum
revealed approximately 2 times higher conductivity for the platinum-iridium nanowires.

Figure 4.14 Current vs. voltage plots demonstrating the improved conductivity of
platinum-iridium nanowires.

4.5.6 Helium Leak Testing

The helium leak testing method described in the previous section (4.3.6) is performed
using a military test protocol (MIL-STD-883F) that is commonly applied in the
microelectronics packaging industry. In this method, the flow rate of helium through a
package material or microelectronic package is measured using a helium leak detector.
Small amount of helium gas is used to determine the existing paths in the package
through which fluids or moisture may penetrate

-8.E-03
-6.E-03
-4.E-03
-2.E-03
0.E+00
2.E-03
4.E-03
6.E-03
8.E-03
-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5
Potential (V)
I (A)
Pt-Ir
Pt
124
Helium leak testing performed on the electrodeposited platinum-iridium nanowires using
the template fabrication technique showed hermetic characteristics. The results suggest
that fabrication of hermetic interconnect structures with desirable platinum-iridium
composition is possible. Helium test measurements using helium leak detector showed
leak rates as low as 1 x 10
-11
mbar L s
-1
indicating that dense nanowires were synthesized
inside of the nanoporous membranes. Measured helium leak rates for platinum-iridium
nanowires showed that hermeticity was achieved by lowering the electrodeposition bath
pH from neutral to nearly 3. Although further investigation is required, the results
demonstrated that hermetic platinum-iridium alloy interconnect arrays can be fabricated
using this nanowire interconnect construct.

4.6 Conclusions

Platinum-iridium alloy nanowires were fabricated in nanoporous aluminum oxide
templates using an electrodeposition method with a solution containing platinum and
iridium salts [Petrossians et al. 2011]. Hermeticity test results indicated that the fabricated
platinum-iridium nanowires were continuous with the required density necessary to
provide hermetic interconnects. This study also demonstrated some advantages of
platinum-iridium nanowires over conventional platinum nanowires. In a previous study,
mechanical characterizations using nanoindentation tests on platinum-iridium alloys thin
films revealed that the measured hardness was increased nearly100% compared to pure
platinum control sample [Petrossians et al. 2011]. Electrical measurements on single
platinum and platinum-iridium nanowires revealed increased conductivity of platinum-
125
iridium nanowires. These results will have great impact on the efficiency of implantable
microelectronics by reducing the device power consumption, while the resolution of the
data transferred between the device and the biological cells will be improved.

126
4.7 References

Ferrara L.A., Fleischman A.J., Togawa D. Bauer TW, Benzel EC, Roy S:
An in vivo biocompatibility assessment of MEMS materials for spinal fusion
monitoring. Biomed Microdev 5, 297 (2003).

Fu X., Wang Y., Wu N., Gui L., Tang Y., J. Matter. Chem. 13, 1192 (2003).

Fukuoka A., Higashimoto N., Sakamoto Y., Inagaki S., Fukushima Y. Ichikawa M.,
Micropor. Mesopor. Mater. 48, 171 (2001).

Gurlu O., Adam, O.A.O., Zandvliet H.J.W., Poelsema B., Applied Physics Letters., 83,
4610 (2003).

Hippe C. Wark M. Lork E., Schulz-Ekolf G., Platinum-Filled Oxide Nanotubes.
Microporous and Mesoporous Materials. 31, 235 (1999).

Horch K., Dhillon G.S. Eds, Neuroprosthetics: Theory and Practice. Series on
Bioengineering & Biomedical Engineering, World Scientific Publishing.
River Edge, NJ, (2004).

Husain A.,Hone J., Henk W., Postma Ch., Huang X. M. H., Drake T., Barbic M., Scherer
A., Roukes M. L., Appl. Phys. Lett. 83, 1240 (2003).

Kimizuka N., Advanced Materials, 12, 1461 (2000).

Llopis J.F., Colom F., Chapter 4: Platinum, Encyclopedia of Electrochemistry of the
Elements. Ed. Bard A.J., VI, 169 (1976).

Luo J., Zhang L, Zhu J., Advanced Materials, 15, 579 (2003).

Martin C.R., Science. 266, 1961 (1994).

Nitani H., Yuya M., Ono T., Nakagawa T., Seino S., Okitsu K., Mizukoshi Y.,
Emura S., Yamamoto T.A., J. Nanoparticle Res. 8, 951 (2005).

Osenbach J.W., J. Electrochem. Soc., 140, 12 (1993).

Ramesham R., Ghaffarian R., Conference Proceedings. 50 , 666 (2000).

Rothkina L., Lin J.F., Bird J.P., Applied Physics Letters, 83, 4426 (2003).

Roy S., Ferrara L.A., Fleischman A.J., Benzel E.C., Neurosurgery 49, 779 (2001).

Roy S., Fleischman A.J. Sens and Mat., 15, 335 (2003).

127
Petrossians A., Whalen III J.J., Weiland J.D., Mansfeld F., J. Electrochem. Soc.; 158, 269
(2011).

Sasaki M., Osada M., Higashimoto N., Yamamoto T., Fukuoka A., Ichikawa M., J. Mol.
Catal. A 141, 223 (1999).

Sasaki M., Osada M., Sugimoto N., Inagaki S., Fukushima Y., Fukuoka A., Ichikawa M.,
Microporous Mesoporous Mater. 21, 597 (1998).

Thomas R.W., IEEE Transactions on Parts, Hybrids, and Packaging, 12, 167 (1976).

Xia Y., Yang P., Sun Y., Wu Y., Mayers B., Gates B. Advanced Materials. 5, 353,
(2003).

Yang C.M., Sheu H.-S, Chao K.J., Advanced Functional Materials, 12,143 (2002).

Yu-Zhang K., Guo D. Z., Mallet J., Molinari M., Loualiche A., Troyon M., Proc. of SPIE
6393, 63930B (2006).

128
CHAPTER V
Biological Experiments Using Electroplated Platinum-Iridium Microelectrodes:
5.1 In-vivo Electrical Stimulation of Retinal Cell Tissue Using Platinum-Iridium
Coated Microelectrodes
5.2 In-vitro Recording Using Platinum-Iridium Coated MEAs

5.1.1 Introduction

The first part of this chapter demonstrates in-vivo stimulation and the second presents in-
vitro recording applications of the high-surface area platinum-iridium coated
microelectrodes which include retinal stimulation using a single metal wire
microelectrode in rat retina (in-vivo) and recording measurements using ceramic-based
MEAs (in-vitro). In both cases results for surface modified electrodes were compared to
those for electrodes that were not modified.

5.1.2 Materials and Methods
5.1.2.1 Animal Preparation and Surgery

All in-vivo studies were performed in Long-Evans rats that were 3-4 months old. The
animals were initially anesthetized with an intramuscular injection of ketamine/xylazine
mixture (80mg/kg of ketamine and 10mg/kg of xylazine: 4 parts of ketamine per 1 part of
xylazine by volume) and then with sevoflurane inhalation through a nose cone. The left
eye was dilated with a few drops each of 1% tropicamide and 2.5% phenylephrine. A
0.5mm scleral incision was made using a 25-guage needle near the limbus. The electrode
was then inserted into the vitreous cavity and positioned epiretinally with the help of a
129
surgical microscope. After positioning the electrode in the vitreous cavity, it was
advanced closer to the retina [Ray et al. 2011]. The electrode was held in position
throughout the experiment with the use of the articulating arm of a micromanipulator.
Electrical stimulations were performed using a Q-Stim pulse generator (Virginia
Technologies, Inc. Charlottesville, VA, USA). All surgical and experimental procedures
were in accordance with the guidelines of the Institutional Animal Care and Use
Committee (IACUC) at the University of Southern California.

5.1.2.2 Stimulation Electrode

The electrode used for epiretinal stimulation was a bipolar electrode with platinum-
iridium being the inner pole and stainless steel being the outer pole. The inner pole with a
diame ter of 75 μ m wa s us e d a s the sti mul a ti on e lec trode . I n ord e r to inc re a s e s the
capacitance of the stimulating electrode and thus injecting more charge to the tissue, the
surface of the inner electrode was modified by electroplating with a high-surface area
platinum-iridium coating (see details in Chapter 3).

5.1.3 Results and Discussion

The current flow through the electrode-retina interface and the voltage drop across the
interface were recorded for each stimulation mode. In each stimulation mode, 3 stimulus
pulses of varying amplitude were delivered. Results from one stimulus pulse per
stimulation mode are shown below. Figure 5.1 a shows the comparison of the stimulus
c urr e nt ( I = 40 μ A a nd a pha se dura ti on of 0.5 ms ) a nd the r e sult a nt vol ta g e drop s a c ross
the platinum-iridium modified and un-modified electrode tissue interfaces in I-Stim mode
130
in closer distance to the retina. Figure 5.1 b shows the voltage responses applied across
the electrode-retina interface using the same current amplitude and the same electrode in
I-Stim mode near the retina interface.

Figures 5.1 (a,b) shows that the retinal stimulation by using platinum-iridium coated
microelectrodes lowered the electrode/tissue impedance compared to the uncoated
microelectrode. In implantable electrical stimulation devices it is important to safely
stimulate the tissue. This is possible by lowering the impedance of the electrode/tissue
interface. The reduced voltage across the electrode/tissue interface due to the lowered
electrode impedance allows an energy efficient design that can potentially reduce the size
of components associated with powering the implant such as inductive coils in an
inductive power supply. In battery powered implants, energy efficient stimulation also
reduces the size of the battery, extends the battery and reduces the number of times the
battery has to be recharged as well as the number of times for battery replacements.

131
0.5 1 1.5 2
Time (ms)
-1
0
1
2
Voltage (V)

0.5 1 1.5 2
Time (ms)
-1
0
1
2
Voltage (V)

Figure 5.1 Current stimulation of the retinal with platinum-iridium modified and
unmodified microelectrodes. (a) Electrodes were touching the retina. (b)
Electrodes were close to the retina.

Pt-Ir
Pt
Pt-Ir
Pt
b
a
132

The main purpose of these experiments was to prove the advantage of rough platinum-
iridium coating vs. smooth platinum-iridium electrodes in biomedical applications.
Preliminary investigations showed improvements on the in-vivo retinal electrical
stimulation by decreasing the impedance of the electrode/tissue interface using rough
platinum-iridium coating.

5.1.4 Conclusions

Electrical stimulation in the rat retina was successfully performed using platinum-iridium
modi fie d si ng le me tal wi re e le c trode s (Φ = 75µ m) with di ff e re nt cu rr e nt a mpl it ude s of
30µA, 40 µA and 50 µA. The decreased voltage excursions across the surface modified
electrode/tissue interface are indicative that transfer of high electrical charge densities to
biological tissues is now possible within the safe charge transfer limits. As a result, either
the amount of the applied current amplitudes during stimulation can be increased or the
size of the stimulating microelectrodes can be reduced to even further smaller size while
the amount of injected charge remains unchanged.

The in-vivo experiments results suggest that safe electrical charge properties along with
stimulation efficiency are achievable by using platinum-iridium surface modified
microelectrodes.

133
5. 2 In-vitro Recording Using Platinum-Iridium Coated MEAs
5.2.1 Introduction

In the following section, another application of platinum-iridium coated microelectrodes
used for recording purposes is discussed. The goal of this section is to demonstrate the
application of electroplated platinum-iridium electrodes in neural recording applications.

5.2.2 Background

Interneural communication of the Central Nervous System (CNS) is enabled by
molecular intermediaries of synaptic transmission or neurotransmitters. Neurotransmitters
are used for diagnosis and treatment of diseases such as epilepsies, metabolic
disturbances, neurodegenerative diseases and addictive disorders [Siegel 2006]. High
selectivity and sensitivity in detection of the target analyte in different concentrations of
the chemicals in the CNS is critical.

Neural recording microelectrodes with platinum connecting leads and contact pads were
fabricated on ceramic alumina (Al
2
O
3
) substrates using a photolithography technique
[Burmeister et al. 2000, Burmeister and Gerhardt 2001, Burmeister and Gerhardt 2006,
Hascup et al. 2007, Stephens et al. 2008].
Figure 5.2 shows different designs of neural recording MEAs with different geometric
surface areas (e.g. 50 x 50 m (2500 m
2
); 333 x 15 m (4995 m
2
)) in which the
spatially defined configurations can be arranged. The neural recording MEAs are capable
of monitoring the chemical and electrophysiological activity on the recording site,
134
performing multiple neuromolecule measurements, recording from multiple regions of
the brain or layered structures and quantifying the resting level of neurotransmitters.

Figure 5.2 Representative Figure demonstrating different types of neural recording
microelectrodes (Center for Microelectrode Technology _ CenMeT.,
University of Kentucky.

Improved performance of the MEAs is crucial to enhance the device capabilities,
recording lifetime of the MEAs for both chronic recording and reusability and to achieve
acute recording. Thus smaller electrodes are highly desirable for in-vivo recordings.
In order to enhance the recording sensitivity on the electrode/tissue interface and
consequently the performance of the MEAs, the surfaces of the recording electrodes were
modified with electrodepositing of high surface area platinum-iridium alloy.

135
5.2.3 Materials and Methods

Platinum-iridium (60:40%) coatings were applied on the surface of platinum base
electrodes (see details in Chapter III). Calibrations on the uncoated and platinum-iridium
coated MEAs were performed to equate a change in current from the oxidation of
peroxide to a proportional change of its concentration. A known concentration of
peroxide was added to 40 mL 0.05 M PBS solution to generate a current that was
measured using the FAST16mkII system (Pronexus Analytical Inc. Stockholm, Sweden).
The FAST16mkII software was used to record the current for each addition of analyte
which creates a calibration curve for each electrode recording site and stores the slope of
this calibration.

All calibrations were performed in-vitro using constant potential amperometry (+0.7 V
vs. an Ag/AgCl). The reference electrode and the MEAs were placed in a beaker
containing 40 ml PBS and connected to the FAST system amplifier (2nA/V). After a
stabilizing for a period of 15-20 minutes known concentrations of peroxide were added
and the changes of the current were recorded. Usually a three-point calibration was
performed. Once the calibration was ended, the slope of the increase of the current was
calculated. These slopes represent the sensitivity of the surface of the electrode to
peroxide.
All measurements were performed at the Morr is K . Uda ll P a rkinson’ s Disea se C e nter of
Excellence at the University of Kentucky under supervision of Prof. Greg Gerhardt.

136
5.2.4 Results and Discussion

Modified microelectrodes with very low impedance enhanced electrical charge transfer
and improved the mechanical properties of the recording sites. [Petrossians et al. 2011].
Figures 5.3 a,b demonstrate the MEAs before, and after surface modification with high
surface area platinum-iridium coating, respectively.

Figure 5.3 Comparison of MEAs before (a) and after (b) electrodeposition of platinum-
iridium alloy coating on the recording sites.

Figure 5.4 compares the cyclic voltammograms of platinum and platinum-iridium coated
MEAs in the potential range of -0.3 V to 1.2 V vs. Ag/AgCl obtained at a scan rate of 50
137
mv/s in 0.05 M H
2
SO
4
. The current in the cyclic voltammograms for the platinum-
iridium coated MEA was significantly larger than that for the uncoated platinum MEA
(Figure 5.3), which indicates a higher charge storage capacity [Robblee and Rose 1990]
of the platinum-iridium coated MEA.
The area under the curve of the measured cyclic voltammogram corresponds to the
charge storage capacity of the electrode surface which is one measure of the performance of
the neurostimulating electrode [Robblee and Rose 1990].

-0.2 0 0.2 0.4 0.6 0.8 1 1.2
Potential (V)
-12
-8
-4
0
4
8
I (µA)
-0.6
-0.4
-0.2
0
0.2
0.4
I (µA)
Pt-Ir
Pt

Figure 5.4 Representative cyclic voltammograms recorded in 0.05 M H
2
SO
4
showing that
the MEA recording surface with an electrodeposited platinum-iridium coating
has a larger active area than that of the uncoated thin film platinum MEA
recording surface.

138
EIS measurementsfor the uncoated and platinum-iridium coated MEAs were performed
(Figure 5.5). These spectra demonstrate that the impedance of the modified MEA was
significantly decreased compared to that of uncoated MEA indicating that the real surface
area of the modified MEA was greatly increased.
Electrodeposition of a platinum-iridium coating on the MEAs increased the electroactive
surface area that enhanced the nanofeatures of the platinum recording site which has
great impact on the performance of the modified MEAs and is an important consideration
for further development of miniaturized MEAs.

139
-2 -1 0 1 2 3 4 5
Log f (Hz)
3
4
5
6
7
8
9
10
11
Log IZI (ohm)
Pt-Ir
Pt

-2 -1 0 1 2 3 4 5
Log f (Hz)
-10
-20
-30
-40
-50
-60
-70
-80
-90
Log IZI (ohm)

Figure 5.5 Impedance measurements for platinum and electrodeposited platinum-iridium
MEA recording surfaces in 0.05 M H
2
SO
4
.

140
Figures 5.6(a-d) demonstrate the increased sensitivity of the recording MEAs after
surface modification with the platinum-iridium alloy coating. The measured currents are
in nano amps indicating the importance of the electrodes sensitivity to the measured
signal intensity.
All channels of the MEAs modified with platinum-iridium showed increased sensitivity
except in Figure 5.6 a (channel 2) and 5.6 b (channel 4) where the measured current
showed no changes or lower sensitivity. The platinum-iridium modified electrodes were
not returned for inspection after the measurements; however these results could be
explained due to possible failure or disconnection of the two electrodes.

141

0
0.05
0.1
0.15
0.2
0.25
Channel 1 Channel 2 Channel 3 Channel 4
Slope nA\µM
Pt
Pt-Ir
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
Channel 1 Channel 2 Channel 3 Channel 4
Slope (nA\µM)
Pt
Pt-Ir
b
a
142

Figure 5.6 Comparison of the sensitivity of the platinum (blue bars) and platinum-iridium
(black bars) microelectrodes to peroxide concentration (a-d).

0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
Channel 1 Channel 2 Channel 3 Channel 4
Slope (na\µM)
Pt
Pt-Ir
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
Channel 1 Channel 2 Channel 3 Channel 4
Slope (nA/µM)
Pt
Pt-Ir
d
c
143
5.2.5 Conclusions

Interneuronal communication in the Central Nerveus System of the human body is
represented by synaptic neurotransmission.
MEA microfabrication technology aims to simultaneously record and measure several
analyte s a nd qua nti f y th e ne urotr a nsmi tt e rs’ r e sti ng lev e ls.
Ultimately, an increased electroactive surface area is an important consideration for even
further miniaturization and decrease in the microelectrode size. This aspect was
addressed by surface modification of the MEAs with high surface-area platinum-iridium
coatings using the electrodeposition fabrication method.
Enhanced nano-features obtained by electroplating of rough platinum-iridium alloy on
the surface of platinum MEAs have greatly improved their performance on the recording
sites.

144
5.3 References

Burmeister, J. J. Gerhardt, G. A., Analytical Chemistry 73, 1037, 2001.

Burmeister, J. J. Gerhardt, G. A., Neurochemical arrays. In, Book, Grimes, C., Dickey, E.
& Pishko, M. V. Eds., Stevenson Ranch: American Scientific Publishers. 525
(2006).

Burmeister J. J., Moxon, K. Gerhardt, G. A., Analytical Chemistry 72, 187 (2000).

Hascup, E. R., Af Bjerkén, S., Hascup, K. N., Pomerleau F., Huettl P., Strömberg I.,
Gerhardt G.A., Brain Research 1291, 12 (2009).

Petrossians A., Whalen III J.J., Weiland J.D., Mansfeld F., J. Electrochem. Soc.,158, 269,
(2011).

Ray A., Chan L.L., Gonzalez A., Humayun M.S., Member, IEEE, James. D. Weiland,
Senior Member, IEEE, Transactions on Neural Systems and Rehabilitation
Engineering, 19, 696 (2011).

Robblee L.S., Rose T.L., Electrochemical guidelines for selection of protocols and
electrode materials for neural stimulation in Neural Prostheses: Fundamental
Studies, Eds. Agnew W.F., McCreery D.B., Prentice Hall, Englewood Cliffs,
NJ, 25 (1990).

Siegel G. J., Basic Neurochemistry: Molecular, Cellular, and Medical Aspects,
Burlington, MA, Elsevier Academic Press., 2006.

Stephens M. L., Pomerleau, F., Huettl, P., Gerhardt, G. A. & Zhang, Z., J. of
Neuroscience Methods 185, 264 (2010).

145
CHAPTER VI
Research Summary and Future Work

6.1 Summary

The major goal of this project was to evaluate a novel electrochemical deposition method
of thin film platinum-iridium alloy coatings with predetermined chemical composition
and improved charge transfer and mechanical properties for surface modification of
microelectrodes. Using the same method of electrodeposition, fabrication of platinum-
iridi um nanow ire s a s fe e dthroug hs used in h e rme t ic pa c ka g in g of th e re ti na l i mpl a nt’s
microchip was performed.

Comprehensive studies were performed of the different parameters affecting the
electrodeposition conditions of the platinum-iridium films. These included the selections
of the chemicals, their ratios and their concentrations, electroplating bath temperature,
agitation of the electroplating solution and the proper applied potential ranges for
electrodeposition. Studies of the chemical composition, the adhesion of the films and the
surface microstructure revealed that the pH and the temperature of the electroplating bath
as well as the applied potential range had the most effect on the alloy film properties.

For comparison of the electroplated platinum-iridium films with pure platinum a broad
range of investigations was performed to evaluate their electrochemical and mechanical
properties. The electrochemical impedance measurements showed that the electrode
impedance decreased with increasing deposition time as the real surface area of the films
146
increased. The nanoindentation measurements revealed that the mechanical properties of
the electroplated films were nearly 100% improved.

Further studies of the electrodeposition of platinum-iridium films were continued by
adding different salts and changing the deposition rate and the coating thickness to
increase the roughness of the electroplated coatings. EIS and cyclic voltammetry
measurements were performed on the rough platinum-iridium coatings. The charge
transfer properties and the durability of the electroplated microelectrodes with higher
surface area coatings were then measured in PBS under chronic pulse testing for over a
year.

In order to evaluate the improved charge transfer properties of the high surface-area
platinum-iridium coatings different current amplitudes were applied on coated and bare
platinum microelectrodes.

Measurements of the charge injection properties of the high-surface area platinum-
iridium coatings in PBS showed a significant decrease of the coating impedance. The
charge injection capacity (CIC) of the platinum-iridium coating was ten times higher than
that of high surface area platinum.

Using similar electrodeposition conditions as those developed for platinum-iridium film
deposition dense platinum-iridium nanowires to be used in hermetic packaging of the
re ti na l prosthesis’ mi c roc hip we re f a bric a ted. A f e w slig ht cha n ge s we re m a de in t he
147
deposition conditions due to the use of a different substrate and anodized aluminum oxide
(AAO) wafers as templates. Electrochemical growth of the nanowires on Ti substrates
required different applied potential ranges and the pH of the electrodeposition bath
needed to be adjusted in order to prevent the dissolution of the AAO wafers in acidic or
basic solutions.
The fabricated platinum-iridium nanowires showed dense and continuous structures with
a chemical composition of 60:40 %(Pt-Ir). The electrical properties of single individual
platinum-iridium nanowires were tested and compared to those of platinum nanowires.
The comparison of the conductivity measurements revealed that the platinum-iridium
nanowires have higher electrical conductivity.

A novel and efficient electrochemical deposition method for fabrication of platinum-
iridium thin films/coatings and nanowires has been developed. The results from long
term tests of the microelectrodes modified with ultralow impedance platinum-iridium
coatings suggest that they are excellent candidates for neural stimulating/recording
microelectrodes in biomedical applications.
The higher conductivity of platinum-iridium nanowires introduces a new generation of
nano-scale materials to be used in implantable microelectronics with the aim of reduced
power consumption and improved mechanical properties.

148
6.2 Suggestions for Future Work

The experimental results in this study have demonstrated the repeatability of the
electroplating method and the durability of the high surface-area platinum-iridium
coatings during long-term characterization. However, further investigations of the
properties of this alloy should include:

6.2.1 Chronic In-vivo Stimulation

Although long term in-vitro tests were performed of the platinum-iridium coated
microelectrodes, chronic in-vivo stimulations need to be performed using platinum-
iridium surface modified implanted microelectrodes. This in turn would allow evaluation
of the coatings characteristics in a real condition.

6.2.2 In-vivo Neural Recording

In-vitro recording measurements using platinum-iridium coated microelectrodes showed
outstanding improvements. In order to verify the properties of rough platinum-iridium
modified electrodes in neural recording applications in-vivo brain activities need to be
recorded.
6.2.3 Other Biomedical Applications

As a biocompatible alloy with excellent electrical charge injection capabilities the high-
surface area platinum-iridium coatings should be electroplated and tested on different
types of microelectrodes used in other medical devices such as deep brain stimulators
(DPS), cochlear implants, chronic pain modulators, cardiac pace makers, prosthetic limbs
and several others.
149
EIS STUDIES of Hexavalent and Trivalent Chromium Based Coating Systems for
Al 2024

2.1 Introduction

Corrosion causes significant losses of valuable products, affects operational safety,
product reliability and may cause plant shutdown. Aluminum alloys are used extensively
in many applications such as in cars, engineering structures and the aerospace industry
due to their high strength-to-weight ratios. The Al 2XXX series is widely used in
structural applications in which the Al 2024 series is one of the most important alloys
because it contains Cu, Mg, Mn and other elements [Pérez-Bustamante et al. 2011].
While the presence of alloying elements such as Cu increases the strength of Al 2024,
intermetallic compounds, that can be anodic or cathodic to the base metal, create local
galvanic cells which significantly decreases the corrosion resistance of Al 2024 compared
to that of pure aluminum [Buchheit et al. 1997]. Different protection techniques have
been applied to prevent localized corrosion of aluminum alloys. These methods include
cladding processes, anodizing processes and the application of polymer and chromate
conversion coatings.
Chromate conversion coatings (CCC) are very effective in providing excellent corrosion
protection for aluminum alloys. When Cr
6+
ions come in contact with an aluminum
substrate, a conversion process occurs involving the reduction of Cr
6+
and the formation
of a complex chromium oxide film that greatly increases the corrosion resistance of Al
alloys [Kendig et al. 1993]. Because of the low cost and ease of application, CCCs have
been used extensively for corrosion protection of aluminum and its alloys [Wernick et al.
1986]. In this protection process, the presence of Cr
6+
ions in the CCC causes a dynamic
150
repair of the defective regions in the coating which is considered a self-healing process.
This reaction is due to migration of Cr
6+
towards weaker spots such as coating defects
where they are reduced to Cr
3+
[Xia et al. 2000, Kiyama et al. 1999, Lytle et al. 1995,
Kendig et al. 1993].
Chromate conversion coatings are formed on aluminum and its alloys by the following
oxidation-reduction reactions [Kendig et al. 2001]:

Cr
2
O
 2
7
+ 8H
+
+6e
-
 2Cr (OH)
3
+ H
2
O (1)
Al  Al
3
+
+3e
-
(2)

Because of the presence of Cr
6+
and other chemicals such as fluorides and cyanides,
CCCs are toxic and carcinogenic agents. [Buchheit et al. 1994]. As a result, the need for
the substitution of hexavalent chromium conversion coatings by environmentally friendly
treatments has been greatly increased to the degree of importance that hexavalent
chromium has been ranked as one of the seventeen most hazardous toxic substances by
the U.S. Environmental Protection Agency (EPA) [Zeng et al. 2006].

Many alternatives to replace CCCs for protecting aluminum alloys have been proposed
[Rangel and Travassos 1992, Buchheit et al. 1994; Wang 1994]. One of the many
candidates to replace hexavelent chromium is trivalent chromium (Cr
3+
) which has been
widely studied [Song and Chin 2002; Yu et al. 2008b]. The trivalent chromium
pretreatment (TCP) has been considered as the most common environmental friendly
alternative for hexavalent chromium based CCCs. Cr
3+
ions in trivalent chromium baths
are non-toxic [Song and Chin 2002] compared to Cr
6+
ions in hexavalent chromium
baths. Trivalent chromium baths contain chromium chloride, conducting salts such as
potassium chloride, ammonium chloride and potassium sulfate, complexing agents such
151
as formate, acetate, hypophosphite, urea, brighteners, wetting agents and pH buffer such
as boric acid [Song and Chin 2000].

Trivalent chromium baths are less harmful in comparison with hexavalent chromium
baths containing Cr
6+
ions, although Cr
3+
could be oxidized to Cr
6+
at higher temperature
in presence of oxygen [Apte et al. 2006]. Comparison of trivalent chromium treated and
untreated Al 6063 showed increased corrosion resistance of the treated sample in
exposure to 3.5 wt % NaCl [Yu et al. 2008].

The investigation to be discussed in the following is a continuation of a larger project in
which the corrosion protection provided for Al 2024 samples by different hexavalent
and/or trivalent chromium containing polymer coatings was evaluated. These samples
were provided by NAVAIR, China Lake, CA [Zarras , 2010]. In part I of this study
[Manohar 2010], EIS was used to study the corrosion behavior as a function of exposure
time of Al 2024 samples that were coated with different combinations of pretreatments
and primers. Two groups of samples were evaluated. The first group contained a total of
9 samples with 3 different sets of 3 samples. These samples were coated with 3 different
combinations of Cr
6+
or Cr
3+
pretreatments and primers and a polyurethane topcoat. The
second group contained a total of 6 samples with 2 sets of 3 samples. These samples were
coated with 2 different combinations of Cr
6+
and Cr
3+
pretreatments and primers without
a topcoat. All samples were exposed to 0.5N NaCl for a period of 31 days and EIS data
were obtained periodically. One sample of each set was then scribed and exposed to 0.5N
NaCl for 3 days. The corrosion behavior of these samples was studied using EIS and
visual inspection of the scribed area after exposure using an optical microscope.
152

The first set of samples of group one had a chromated (Cr
6+
) pretreatment and a
chromated epoxy primer. The second set of samples in this group had a chromated
pretreatment and a non-chromated epoxy primer and the third set of samples of this group
had a trivalent chromium pretreatment (TCP) and a non-chromated epoxy primer. All
samples of this group had a polyurethane epoxy topcoat. In the second group, the first set
of the samples had a chromated pretreatment and a chromated epoxy primer and the
second set of the samples of this group had TCP and a non-chromated epoxy primer. This
group of samples had no topcoat. The thickness of the pretreatment layer for all samples
was about 0.1 µm, the thickness of the primer was about 23 µm and the thickness of the
topcoat about 38 to 50 µm [Zarras 2010].

The coated samples were exposed to 0.5 N NaCl and impedance measurements were
periodically performed for 31 days. The impedance spectra for samples with chromated
pretreatment and chromated epoxy primer did not change significantly with time during
31 days of exposure indicating that the coating provided very stable corrosion protection.
EIS data of the second set of samples of this group which had a chromated pretreatment,
a non-chromated epoxy primer and a polyurethane based topcoat and the third set with
TCP and the same primer and topcoat showed significant differences of the impedance
spectra between these two sets of samples and the first set of samples of group one. For
these two sets of samples two time constants in the impedance spectra were observed due
the increasing porosity of the coatings that exposed the base metal and caused localized
corrosion and delamination of the polymer coating. All impedance spectra were analyzed
153
using the COATFIT module of the ANALEIS software developed by Shih and Mansfeld
[Mansfeld et al. 1992, Mansfeld et al. 1993, Mansfeld 2006].

After 31 days of exposure a sample from each set was scribed and exposed to 0.5N NaCl.
Daily impedance measurements were performed on the samples for a period of 3 days.
The EIS spectra of the scribed samples revealed that the samples from the first and
second set did not undergo rapid corrosion, while pitting corrosion was observed in the
scribed area on the sample from the third set. This result could be explained as a result of
the use of the chromated pretreatment or primers for the first two sets of samples in
which migration of Cr
6+
i ons t owa rds the sc rib e d a re a s produ c e d a “ se lf - he a li ng ” pro c e ss
and protected the exposed metal surface from corrosion attack in the scribed regions.

The second group of samples without a topcoat was exposed to 0.5 N NaCl and EIS
measurements were performed periodically for 31 days. The impedance spectra for the
samples from the first set in this group with chromated pretreatment and chromated
epoxy primer followed a one-time-constant model (OTCM) for the first 14 days of
measurements, but for longer periods of time a second time constant was observed in the
low-frequency region. This indicated that the protective properties of the coating
decreased for longer exposure times due to increasing porosity of the coating.

Impedance spectra for the samples from the second set with TCP and non-chromated
epoxy primer showed significant changes during the entire exposure time. The EIS
measurements for 31 days for these two sets of samples showed that the impedance of the
154
second sample with TCP and non-chromated epoxy primer was always lower than that
for the first sample with chromated pretreatment and chromated epoxy primer indicating
lower corrosion resistance of the second set of sample compared to the first set of
samples.

The analysis of the EIS data for the first group of samples revealed that the Al 2024
samples with the hexavalent chromium pretreatment and hexavalent primer exhibited
higher corrosion resistance than the samples with the trivalent chromium pretreatment
and the non-chromated primer. The impedance spectra for the samples with hexavalent
chromium primer and pretreatment showed very little changes during the exposure period
indicating the stability of the coating which protected the Al 2024 samples surfaces from
corrosion.
E I S me a sure ments of th e sc ribe d sa mpl e r e ve a led the “ se lf - h e a li n g ” pro c e s s of the scribed areas on samples with hexavalent chromium based primer in which mobile Cr
6+

ions moved to the scribed regions and protected the damaged areas from corrosion.
S c ribe d sa mpl e s with t he triva lent c hromium pre tr e a tm e nt di d not s how the “ se lf -
he a li ng” c h a ra c ter ist ics a nd the sc ribe d r e g ions we re a tt a c k e d b y pit ti ng c o r rosion.

EIS analysis of the second group of samples without a topcoat showed that the impedance
spectra of these samples changed with time and showed higher rates of corrosion attack
on the metal surface compared to those with a topcoat. The Al 2024 sample with the
chromate based primer and CCC pretreatment exhibited better corrosion resistance.

155
Scribed samples with hexavalent chromium pretreatment and chromated primer showed
the “ se lf - h e a li n g ” c h a ra c t e risti c s, whil e for sa mpl e s without he x a va lent c hromium pretreatment and chromated primer coating significant corrosion attack of the scribed
regions was observed.

In the present investigation that is part II of the NAVAIR coating systems investigation, a
study of the corrosion resistance of a group of samples with a total of 9 samples was
completed. This group had 3 sets of 3 samples that were coated with 3 different
combinations of pretreatment and primer layers, but had the same topcoat. EIS
measurements were performed during exposure to 0.5 N NaCl for 30 days. One sample of
each set was then scribed and EIS data were obtained during exposure to 0.5 N NaCl for
3 days. After exposure the scribed area was observed under an optical microscope. The
results of these investigations will be reported in the following discussion.

2.2 Experimental Methods
2.2.1 Sample Preparation

All samples evaluated in this study were provided by NAVAIR, China Lake, CA [Zarras
2010]. Three sets of samples with different pretreatment and primer layers, but with same
topcoat were investigated in the first part of this study. The pretreatment layer was
applied by spray coating and the primer and topcoat were applied by HVLP (high
volume, low pressure) spray equipment [Zarras 2010]. The combinations of the applied
coatings for the three different set of samples tested in this study are shown in Table 2.1.

156

Table 2.1 Different coatings applied on Al 2024 samples.

Sample # Pretreatment Primer Topcoat
1-3 Trivalent chromium
pretreatment
(TCP)
Without hexavalent
chromium
MIL-PRF-23377N
Polyurethane
topcoat
(nonchromate)
MIL-DTL-64159
CARC Coating
4-6 Trivalent chromium
pretreatment
(TCP)
Without hexavalent
chromium
MIL-PRF-53022
Polyurethane
topcoat
(nonchromate)
MIL-DTL-64159
CARC Coating
7-9 Chromate
conversion coating
(CCC)
Hexavalent
chromium primer
MIL-PRF-23377C2
Polyurethane
topcoat
(nonchromate)
MIL-DTL-64159
CARC Coating

TCP is an alternative to the hexavalent based chromate conversion coating (CCC). MIL-
PRF-23377N and MIL-PRF-53022 are both epoxy based non-hexavalent chromium
primers used by the Department of Defense (DoD). MIL-PRF-23377C2 is a hexavalent
chromium based epoxy primer. MIL-PRF-23377N is a non-hexavalent chromium based
primer similar in formulation to MIL-PRF-23377C. MIL-PRF-53022 is another epoxy
based primer system that does not contain hexavalent chromium. They are both different
non-hexavalent formulations that the DoD uses.

Samples #1-3 had a trivalent chromium pretreatment (TCP) and an epoxy based primer
without hexavalent chromium. Samples #4-6 had the same pretreatment as samples #1-3,
but a different primer, while samples #7-9 had a chromate conversion coating
157
pretreatment and a hexavalent chromium based epoxy primer. All samples had a non-
chromate based polyurethane water dispersible aliphatic topcoat.
The thickness of the pretreatment layer for all samples was about 0.1µm, the thickness of
the primer was about 23 µm and the topcoat layer was about 38µm to 51µm thick.
The total thickness of the coating layers on these samples was about 64 µm [Zarras
2010].
The chromate conversion coating (CCC) used on samples #7-9 was prepared from
commercially available Alodine® 1200S. The CCC coating is applied on aluminum
alloys to increase their resistance to corrosion and to improve the adhesion between the
topcoat and the base metal. The epoxy based hexavalent chromium primer is highly
resistant to chemicals and corrosion due the presence of Cr
6+
. It also provides good
adhesion to the substrate and maintains a suitable layer for applying the topcoat (Deft
Inc).

The trivalent chromium treatment (TCP) used on samples #1-3 and #4-6 was prepared
from the SurTec® 650 chromitAl® TCP system (SurTec International). The TCP
pretreatment is free of environmentally toxic and carcinogenic agents such as hexavelent
chromium. All pretreatments, primers and topcoats used on the surface of the samples
were prepared using materials available from Deft Inc. [Zarras 2010].

158
2.2.2 Electrochemical Impedance Spectroscopy (EIS) Measurements

All electrochemical measurements performed in this study were conducted using a
three-electrode electrochemical cell which was specially designed for testing of coatings
at CEEL, USC. The electrochemical cell provided a fixed geometric area (A=3.97cm
2
) to
expose the samples to the corrosive medium. The exposed area of the samples was
considered as the working electrode. A saturated calomel electrode (SCE) was used as a
reference electrode and a stainless steel 316 sheet was used as the counter electrode.
Electrochemical Impedance Spectroscopy (EIS) measurements on all samples were
performed in 0.5N NaCl at the open-circuit potential (Ecorr) of the samples at room
temperature and open to air. The impedance spectra were measured in a frequency range
of 100 kHz to 1 mHz with an ac signal amplitude of +/- 10mV. EIS measurements were
performed periodically for 30 days with Gamry PCI4 or Gamry Reference 600
potentiostats using the EIS 300 software. The measured impedance spectra were fit to a
one-time or two-time-constant model and were analyzed using the COATFIT module of
the ANALEIS software [Mansfeld et al. 1992, Mansfeld et al. 1993, Mansfeld 2006].

2.3 Results and Discussion
2.3.1 Analysis of Impedance Spectra

Three samples from each different coating system were tested for a period of 30 days.
Figure 2.1 shows the measured impedance spectra for sample #1 during the entire
exposure time as representative of the set #1-3 (Table 2.1). The impedance spectra are
shown as Bode-plots, in which the logarithm of the impedance modulus |Z| and the phase
angle are plotted vs. the logarithm of the applied frequency f.
159

-2 -1 0 1 2 3 4 5
Log f (Hz)
4
5
6
7
8
9
Log lZl (ohm)
# 1
D 1. Ecorr = -0.469 V
D 7. Ecorr = -0.368 V
D 14. Ecorr = -0.349 V
D 21. Ecorr = -0.337 V
D 30. Ecorr = -0.345 V

-2 -1 0 1 2 3 4 5
Log f (Hz)
-10
-20
-30
-40
-50
-60
-70
-80
-90
Phase angle (deg.)

Figure 2.1 Impedance spectra for sample # 1 at different exposure times.

160
-2 -1 0 1 2 3 4 5
Log f (Hz)
3
4
5
6
7
8
Log lZl (ohm)
# 4
D 1 . E co rr = -0 .9 0 0 V
D 7 . E co rr = -0 .3 6 8 V
D 1 4 . E co rr = -0 .3 8 1 V
D 2 1 E co rr = -0 .4 0 0 V
D 3 0 . E co rr = -0 .4 1 1 V

-2 -1 0 1 2 3 4 5
Log f (Hz)
-10
-20
-30
-40
-50
-60
-70
-80
Phase angle (deg.)

Figure 2.2 Impedance spectra for sample # 4 at different exposure times.

161
-2 -1 0 1 2 3 4 5
log f (Hz)
4
5
6
7
8
9
10
log lZl (ohm)
# 7

D 1. Ecorr = -0.815 V
D 7. Ecorr = -0.463 V
D 14. Ecorr = -0.411 V
D 21. Ecorr = -0.398 V
D 30. Ecorr = -0.398 V

-2 -1 0 1 2 3 4 5
Log f (Hz)
-20
-30
-40
-50
-60
-70
-80
-90
Phase angle (deg.)

Figure 2.3 Impedance spectra for sample # 7 at different exposure times.

162
Samples #1-3 (Figure 2.1) have a trivalent chromium pretreatment (TCP), an epoxy based
primer coating without hexavalent chromium and a nonchromate polyurethane based
topcoat (Table 1). Sample #4-6 (Figure 2.2) have the same trivalent chromium
pretreatment (TCP) and the same topcoat, but a different primer (Table 2.1). Samples #7-
9 have a chromate conversion coating pretreatment with an epoxy based hexavalent
chromium primer (Table 2.1). This coating system provides very stable corrosion
protection and the impedance spectra of sample #7 did not show significant changes
during the 30 day exposure period (Figure 2.3)

Comparison of the impedance spectra of samples #1 and #4 to those of sample #7 shows
a significant difference in the electrochemical behavior and the stability of the coatings
(Figure 2.1-2.3). A porous structure of the coating caused a second time constant to
appear in the low-frequency region of the impedance spectra for samples #1-3 and #4-6.
The second time constant is considered due to the onset of corrosion of the underlying
aluminum surface as a result of the formation of conducting paths through pores in the
coating.

The impedance spectra of samples #1-3 and #4-6 can be fitted to the coating model
shown in Figure 2.4 [Mansfeld 1995]. This model is commonly applied as the equivalent
circuit to describe the properties of a polymer coating on a metal surface and corrosion of
the metal surface in pores of the coating. The coating model consists of the solution
resistance R
s
, the coating capacitance C
c
, the pore resistance R
po
, the polarization
resistance R
P
and the capacitance C
dl
of the exposed metal surface. Corrosion takes place
163
on the underlying metal surface due to pores in the coating layer which are filled with
corrosive electrolyte. This corrosion process is described by the polarization resistance R
p

and the surface capacitance C
dl
.

Figure 2.4 Equivalent circuit for the coating model

The impedance spectra of sample #7 (Figure 2.3) agree with the one-time-constant model
(OTCM) shown in Figure 2.5, where R
S
is the solution resistance, R
po
represents the
resistance of the electrolyte present in conductive paths in the coating that reach the metal
surface and C
c
is the coating capacitance.

Figure 2.5 One-time-constant model (OTCM).

164
Theoretical Bode plots for the coating model in Figure 2.4 and the OTCM in Figure 2.5
are shown in Figure 2.6 which corresponds to a pore-free (curve 1) and a deteriorated
coating (curve 2) system [Mansfeld 2006].

Figure 2.6 Comparison of the impedance spectra of a polymer coated metal with an intact
coating (curve 1) and a deteriorated coating (curve 2) [Mansfeld 2006].

165

Figure 2.7 a-c shows the impedance spectra for one sample of the three different coating
systems for 1, 14 and 31 days of exposure, respectively. The impedance spectra of
samples #1 and #4 showed two time constants and were much lower than those of
samples #7 during the entire 30 day test period. The impedance spectra for samples #1
and #4 which had the same pretreatment, but different primers were quite similar, while
the impedance spectra of sample #7 with chromate pretreatment and primer showed
different behavior. These results indicate that the protective properties of the coating
system are strongly dependent on the type of pretreatment and primer used in the coating
system.

166

-2 -1 0 1 2 3 4 5
Log f (Hz)
3
4
5
6
7
8
9
10
Log lZl (ohm)
Day 1
# 7. Ecorr = -0.815 V
# 1. Ecorr = -0.469 V
# 4. Ecorr = -0.469 V
a

-2 -1 0 1 2 3 4 5
Log f (Hz)
-10
-20
-30
-40
-50
-60
-70
-80
-90
-100
Phase angle (deg.)

167
-2 -1 0 1 2 3 4 5
Log f (Hz)
3
4
5
6
7
8
9
10
Log lZl (ohm)
Day 14
# 7. Ecorr = -0.411 V
# 1. Ecorr = -0.349 V
# 4. Ecorr = -0.381 V
b

-2 -1 0 1 2 3 4 5
Log f (Hz)
-10
-20
-30
-40
-50
-60
-70
-80
-90
-100
Phase angle (deg.)

168

-2 -1 0 1 2 3 4 5
Log f (Hz)
3
4
5
6
7
8
9
10
Log lZl (ohm)
Day 30
# 7. Ecorr = -0.378 V
# 1 Ecorr=-3.45E-01
# 4 Ecorr=-4.11E-01
c

-2 -1 0 1 2 3 4 5
Log f (Hz)
-10
-20
-30
-40
-50
-60
-70
-80
-90
-100
Phase Angle (deg.)

Figure 2.7 Comparison of the impedance spectra of samples # 1, 4 and 7 after (a) 1, (b)
14 and (c) 30 days of exposure.

169
Figure 2.8 shows the time dependence of the corrosion potential E
corr
of samples #1, #4
and #7 during exposure to 0.5 N NaCl. E
corr
of sample #1 and #4 seems to be stable after
7 days of exposure period, while E
corr
of sample #7 increased slowly from -0.9 V to about
-0.4 V after 15 days of exposure. At the end of the exposure, E
corr
had similar values for
all three types of coatings.

5 10 15 20 25 30
Time (day)
-0.8
-0.6
-0.4
-0.2
Ecorr (V)
# 7
# 1
# 4

Figure 2.8 Time dependence of E
corr
of samples # 1, 4 and 7 during exposure to 0.5N
NaCl.

Figure 2.9 shows the time dependence of C
c
of samples #1, #4 and #7 during the 30 day
exposure period. The coating capacitance C
c
is given by:

170
) 3 ( ,
0
d
A
C
C



whe re ε is the die l e c tric c onst a nt of the c oa ti n g a n d ε
0
is the permittivity of free space
(8.854 X 10
-12
F/cm), A is the total exposed surface area and d is the coating thickness.

5 10 15 20 25 30
Time (day)
-8.5
-8
-7.5
-7
Log Cc (F)
# 7
# 1
# 4

Figure 2.9 Time dependence of C
c
of samples # 1, #4 and #7 during exposure to 0.5N
NaCl.

Sample #7 showed much lower C
c
value than samples #1 and #4. According to Eq.3 C
c
is
a function of the dielectric constant of the coating, the thickness of the coating and the
exposed surface area. Since the surface area and the coating thickness were about the
171
same for all the three types of coating samples, the increased value of C
c
of samples #1
and #4 could be attributed to the penetration of water through pores in these coatings
which resulted in a higher dielectric constant of the coatings.

The time dependence of R
po
of the three coating systems is shown in Figure 2.10. Sample
#7 showed much higher and stable higher R
po
values compared to samples #1 and #4
which had similar R
po
values that decreased with time. The lower values of R
po
shown in
Figure 2.10 for samples # 1 and #4 indicate penetration of the electrolyte through coating
pores and formation of conductive paths that reach the underlying metal surface and
cause corrosion attack. The decrease of R
po
with exposure time for samples #1 and #4
indicate that the porosity of the coating increased with time. On the other hand, the high
and constant R
po
values for sample #7 are indicative of a very protective coating.

172
5 10 15 20 25 30
Time (day)
5
6
7
8
9
10
Log Rpo (ohm)
# 7
# 1
# 4

Figure 2.10 Time dependence of Rpo of samples # 1, 4 and 7 during exposure to 0.5N
NaCl.

Figures 2.11 and 2.12 show the time dependence of the fit parameters for samples #4 and
#7 of C
dl
and R
P
, respectively (Figure 2.4). The values of C
dl
increased with time, while
the values of R
p
decreased. These changes are due to the increase of the size of the pores
resulting from to the increasing deterioration of the coatings which leads to (localized)
corrosion of the exposed bare metal. As shown in Figures 2.11 and 2.12, samples #1 and
#4 had similar changes of C
dl
and R
p
with exposure time. The slightly higher values of R
p

and lower values of C
dl
for sample #1 indicate better protective properties of the coating
for samples #1-3 which were prepared with the primer MIL-PRF-23377N, while samples
#4-7 were prepared with the primer MIL-PRF-53022 (Table 1).
173

5 10 15 20 25 30
Time (day)
0
1E-005
2E-005
3E-005
Cdl (F)
# 1
# 4

Figure 2.11 Time dependence of C
dl
of samples # 1 and 4 during exposure to 0.5N NaCl.

174

5 10 15 20 25 30
Time (day)
1E+007
2E+007
3E+007
4E+007
5E+007
6E+007
Rp (ohm)
# 1
# 4

Figure 2.12 Time dependence of R
p
of samples # 1, #4 and #7 during exposure to 0.5N
NaCl.

2.3.2 Analysis of Impedance Spectra of Scribed Samples

After 30 days of exposure to 0.5 N NaCl, one sample from each set of the three coating
systems was scribed. The scribed samples were exposed to 0.5N NaCl for a period of 3
days and daily impedance measurements were performed on each sample during this
period. The impedance spectra of the scribed samples #1, #4 and #7 are shown in Figures
2.13, 2.14 and 2.15, respectively.

175
-2 -1 0 1 2 3 4 5
Log f (Hz)
3
4
5
6
7
8
Log lZl (ohm)
# 1
D1. Ecorr =
D2. Ecorr = -0.653 V

-2 -1 0 1 2 3 4 5
Log f (Hz)
-10
-20
-30
-40
-50
-60
-70
-80
Phase angle (deg.)

Figure 2.13 Impedance spectra for scribed sample # 1 at different exposure times.

176

-2 -1 0 1 2 3 4 5
Log f (Hz)
2
3
4
5
6
7
8
Log lZl (ohm)
# 4
D1. Ecorr = -0.620 V
D2. Ecorr = -0.733 V

-2 -1 0 1 2 3 4 5
Log f (Hz)
-10
-20
-30
-40
-50
-60
-70
-80
Phase angle (deg.)

Figure 2.14 Impedance spectra for scribed sample # 4 at different exposure times.

177

-2 -1 0 1 2 3 4 5
Log f (Hz)
4
5
6
7
8
Log lZl (ohm)
# 7
D1. Ecorr = -0.692 V
D 2. Ecorr = -0.685 V

-2 0 2 4
Log f (Hz)
0
-10
-20
-30
-40
-50
-60
-70
-80
-90
Log lZl (ohm)

Figure 2.15 Impedance spectra for scribed sample # 7 at different exposure times.

178

The E
corr
values for all three types of samples were close to those observed for bare Al
2024. The impedance spectra of the three scribed samples (Figure 2.13-2.15) suggest that
sample #7 did not undergo rapid corrosion of the scribed area. The impedance for the
scribed sample #1 was quite high and did not change much with time, while the spectra
for sample #4 were much lower and decreased with time. These results suggest that the
primer used for preparing the coating system on samples #1-3 provides better corrosion
protection than the primer used for samples # 4-6 in agreement with the results obtained
for the unscribed samples. Sample #1 and #4 exhibited signs of different degrees of
pitting corrosion on the underlying aluminum surface as observed by optical evaluation
(see below). Sample #7 has a chromate pretreatment which improved the corrosion
resistance in the scribed condition. One of the properties of the chromate conversion
c oa ti ng s is their “ se lf - h e a li ng ” c h a ra c ter ist ics. I n t his proc e ss, chr om a te ions i n the
surrounding coating start to migrate towards the scribed region on the metal surface and
act as inhibitors.

2.3.3 Optical Evaluations of Scribed Samples

Figure 2.16 shows the optical image for scribed sample #1 after 3 days of exposure to 0.5
N NaCl which indicates that corrosion of the substrate had occurred in the scribe lines.
Small amounts of corrosion products can be observed at higher magnification inside and
near the area where the two lines of the scribe meet.

179

Figure 2.16 Optical micrograph of scribed sample # 1 after 3 days of exposure to 0.5N
NaCl.

Figure 2.17 Optical micrograph of scribed sample # 4 after 3 days of exposure to 0.5N
NaCl.

100 µm
100 µm
180

An optical micrograph of scribed sample #4 after 3 days of exposure to 0.5 N NaCl is
shown in Figure 2.17. Closer examination of the micrograph reveals some corrosion
products in the area surrounding the scribe. As a result, lifting off and undercutting of the
coating occurred that explains the low impedance values observed for this sample (Figure
2.15)

Figure 2.18 Optical micrograph of scribed sample # 7 after 3 days of exposure to 0.5N
NaCl.

Figure 2.18 shows an optical image of the scribed sample #7 after 3 days of exposure to
0.5N NaCl. No corrosion products or delamination of the coating were observed in the
area where the two scribes intersect.

100 µm
181
2.4 Conclusions

EIS has been used to investigate the corrosion behavior of three sets of samples with
different pretreatments and primer layers, but with the same topcoat. All samples were
exposed to 0.5N NaCl for a period of 30 days. Analysis of the impedance spectra
revealed that the Al 2024 sample with the hexavalent pretreatment and a hexavalent
chromium based primer showed much better corrosion resistance than the samples with
the trivalent chromium pretreatment and two different non-chromate primers. The
impedance of the samples with the chromate pretreatment and the hexavalent chromium-
based primer did not change significantly with time indicating the stability of the coating
which provided excellent corrosion protection of the underlying metal surface.

The “ se lf - he a li n g ” c ha r a c ter ist ics of the he x a va le nt chr omate prime r we r e re ve a l e d b y analysis of the impedance spectra of the scribed samples. After scribing the sample, the
chromate ions from the surrounding areas of the scribe migrated towards the scribed
region and protected the bare aluminum surface. This behavior was not observed for the
other samples. On scribed samples with the trivalent chromium pretreatment pitting of
the scribed area occurred as a result of the electrolyte attack on the bare aluminum
substrate. The extent of pitting was less for samples #1-3 than for samples #4-7 which
have different primers (Table 2.1).
The analysis of the results obtained in part II of this project lead to conclusions that are
similar to those for the results in part I [Manohar 2010] demonstrating that the coatings
that contain Cr
6+
provide much better corrosion protection.

182
2.5 Suggestions for Future Work

The investigations performed in this study on the possible replacement of hexavalent
chromium with trivalent chromium coating systems showed that the hexavalent
chromium coating system provides much better corrosion protection for Al2024. These
results indicate that the existing candidates to replace Cr
6+
such as coatings with Cr
3+

pretreatments and non-chromate primers do not protect Al2024 from corrosion to the
same degree as coatings containing hexavalent chromium. Therefore, further
investigations on the development of coating systems for replacement of Cr
6+
with
different varieties of pretreatments, primers and topcoats are needed. Emphasis has to be
placed on simplifying the different steps in the preparation of a coating system and
reducing the time required to apply a coating system in order to make any candidate
system for replacement of Cr
6+
attractive for industrial applications.

183
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Abstract (if available)

Abstract The studies presented in this thesis are composed of two different projects demonstrated in two different parts. The first part of this thesis represents an electrochemical approach to possible improvements of the interface between an implantable device and biological tissue. The second part of this thesis represents electrochemical impedance spectroscopy (EIS) studies on the corrosion resistance behavior of different types of polymer coated Al2024 alloys. ❧ In the first part of this thesis, a broad range of investigations on the development of an efficient and reproducible electrochemical deposition method for fabrication of thin-film platinum-iridium alloys were performed. The developed method for production of dense films was then modified to produce very high surface area coatings with ultra-low electrochemical impedance characteristics. The high-surface area platinum-iridium coating was applied on microelectrode arrays for chronic in-vitro stimulation. ❧ Using the same method of producing dense films, platinum-iridium nanowires were fabricated using Anodized Aluminum Oxide (AAO) templates for hermetic packaging applications to be used in implantable microelectronics. The implantable microelectronics will be used to perform data reception and transmission management, power recovery, digital processing and analog output of stimulus current. ❧ Finally, in-vivo electrical stimulation tests were performed on an animal retina using high surface-area platinum-iridium coated single microelectrodes to verify the charge transfer characteristics of the coatings. ❧ In the second part of this thesis, three different sets of samples with different combinations of pretreatments, primers with the same type of topcoat were tested in 0.5 N NaCl for period of 30 days. The surface changes measured by EIS as a function of time were then analyzed. The analysis of the fit parameters of the impedance spectra showed that the different primers had the most effect on the corrosion protection properties of the coatings in which the primers with hexavalent chromium ions (Cr⁶⁺) provided better corrosion protection compared to primers with trivalent chromium ions (Cr³⁺). ❧ After 30 days of the exposure of the samples in 0.5 N NaCl, one sample from each set of samples was scribed and exposed to 0.5 N NaCl for 3 days. Analysis of the impedance spectra revealed that the samples with chromium conversion coating pretreatment and hexavalent chromium primer showed “self-healing” characteristics and provided better corrosion protection on the scribed areas compared to the scribed samples with trivalent chromium pretreatment and non-hexavalent chromium primer.

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Creator Petrossians, Artin (author)

Core Title Electrodeposition of platinum-iridium coatings and nanowires for neurostimulating applications: fabrication, characterization and in-vivo retinal stimulation/recording EIS studies of hexavalent a...

School Viterbi School of Engineering

Degree Doctor of Philosophy

Degree Program Materials Science

Publication Date 05/09/2012

Defense Date 01/12/2012

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag electrodeposition,implantable,microelectrodes,OAI-PMH Harvest,platinum-iridium,thin film

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Mansfeld, Florian (committee chair), Goo, Edward K. (committee member), Humayun, Mark S. (committee member), Nutt, Steven R. (committee member), Weiland, James D. (committee member)

Creator Email artinpetros@yahoo.com

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c3-39687

Unique identifier UC11289902

Identifier usctheses-c3-39687 (legacy record id)

Legacy Identifier etd-Petrossian-837.pdf

Dmrecord 39687

Document Type Dissertation

Rights Petrossians, Artin

Type texts

Source University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

Access Conditions The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the a...

Repository Name University of Southern California Digital Library

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