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Additive manufacturing of piezoelectric and composite for biomedical application

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Additive manufacturing of piezoelectric and composite for biomedical application

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Content ADDITIVE MANUFACTURING OF PIEZOELECTRIC
AND COMPOSITE FOR BIOMEDICAL APPLICATION

BY

Zeyu Chen

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

(BIOMEDICAL
ENGINEERING)

December
2017

Copyright

2017

Zeyu
Chen

i
Dedication

To
my
beloved

Parents

Zhiya
Chen
&
Lifang
Liu

II
Acknowledgements
I would like to thank the National Science Foundation (NSF) and China
Scholarship Council (CSC) for their generous funding in support of the work presented
in this dissertation. This four years journey through graduate school has brought me in
contact with many wonderful people who have contributed immensely to my personal
development as a scientist and engineer, and to the development of additive
manufacturing for biomedical applications.
First, I am sincerely grateful to my committee members. Foremost, my advisors,
Dr. Qifa Zhou, Dr. Yong Chen and Dr. K. Kirk Shung, their guidance, mentoring and
support since the day I entered into the laboratory has allowed me to excel and be
passionate about this project. They are truly great mentors who pass to me not only
their knowledge but also their life philosophy. I would also like to express my sincere
gratitude to my Dr. Wei Wu and Dr. Keyue Shen for their suggestion during my
preparation of this dissertation.
I am greatly thankful to all my colleagues and friends at the University of
Southern California, Prof. Xuan Song, Dr. Yang Yang, Dr. Mingyue Yu, Dr. Xuejun
Qian, Dr. Teng Ma, Dr. Harry Chiu, Dr. Yang Li and Ms. Mingyue Yu, Mr. Nestor
Cabrera, Mr. Runze Li, Mr. Haocheng Kang and Mr. Gengxi Lu, our lab manager Dr.
Ruiming Chen, our previous visiting scholar Dr. Chunlong Fei, Dr. Xiaoyang Chen, Dr.
Jun Zhang, Dr. Ming Qian and our budget analyst Peter Lee, Tanisha Hughes, Sydney

III
Burke. We have been working together for many years and shared a lot pains and
happiness. They supported me throughout my study and made my stay at the center an
enjoyable experience.
Last but not the least, I would like to thank my parents who raised me up and
give me unconditional love.

iv

Table of Contents
Dedication ............................................................................................................................ i
Acknowledgements ............................................................................................................. II
List of Tables ...................................................................................................................... v
List of Figures .................................................................................................................... vi
ABSTRACT ........................................................................................................................ 1
Chapter 1 Introduction ........................................................................................................ 3
1.1 Additive manufacturing and composite materials .................................................... 3
1.2 Stereolithography ...................................................................................................... 6
1.3 Piezoelectric materials and ultrasonic transducer ..................................................... 9
1.4 Optoacoustic transducer .......................................................................................... 18
1.5 Natural materials and bio-inspired composites ....................................................... 20
1.6 Outline of this work ................................................................................................ 26
Chapter 2 Additive manufacturing of piezoelectric element ............................................ 27
2.1 Mask-Image-Projection-based Stereolithography (MIP-SL) .................................. 27
2.2 Fabrication of piezoelectric composite and post processing ................................... 30
2.3 Characterization of Sintered-Parts .......................................................................... 37
2.4 Ultrasonic imaging with MIP-SL fabricated piezoelectrics .................................... 43
2.5 MIP-SL fabricated piezoelectric array .................................................................... 51
2.6 Summary ................................................................................................................. 59
Chapter 3 Additive manufacturing of Carbon Nanotube composite ................................ 61
3.1 Electrically assisted Mask-Image-Projection-based Stereolithography .................. 61
3.2 Fabrication of Menger sponge structure ................................................................. 66
3.3 Characterization of Menger model with Bouligand structure ................................. 70
3.4 Artificial meniscus with anisotropic mechanical properties ................................... 73
3.5 Additive manufacturing of optoacoustic transducer ............................................... 79
3.6 Summary ................................................................................................................. 85
Chapter 4 Summary and future work ................................................................................ 86
4.1 Summary ................................................................................................................. 86
4.2 Future work ............................................................................................................. 87
Reference .......................................................................................................................... 88

v

List of Tables
Table 1.1: Properties of important piezoelectric materials ............................................... 13
Table 2.1 Comparison our printed BaTiO
3
to reported literature values .......................... 41
Table 2.2: The measured pulse and echo characteristics for all array elements ............... 57
Table 2.3: Lateral resolutions for phantom images .......................................................... 59

vi
List of Figures
Figure 1.1: Basic principle of additive manufacturing process .......................................... 4
Figure 1.2: Different additive manufacturing processes ..................................................... 4
Figure 1.3: Different applications of additive manufacturing ............................................ 5
Figure 1.4: Schematic of modern stereolithography ........................................................... 8
Figure 1.5: Profile structure of single element transducer ................................................ 14
Figure 1.6: Different types of transducer array ................................................................. 16
Figure 1.7: Illustration of a simple time delay beamforming ........................................... 17
Figure 1.8: Illustration of photoacoustic effect ................................................................. 18
Figure 1.9: There are only four types of fibres in natural organism (Mingallon, 2012) ... 20
Figure 1.10: Bio-inspired heterogeneous structure with complex-shaped (Le, 2015) ...... 23
Figure 1.11: Freeze casting for bio-mimic nacre structure (Deville, 2006) ...................... 24
Figure 1.12: Stereolithography with nonmagnetic-reinforcing process (Martin, 2015) ... 25
Figure 2.1: Design of Mask-Image-Projection-based Stereolithography ......................... 30
Figure 2.2: Prototype system of Mask-Image-Projection-based Stereolithography ......... 31
Figure 2.3: (a) Illustration of the MIP-SL system. (b) Imaging pattern controlled by the
projection. (c) 3D geometry designed by SolidWorks. (d) Interface to control motor and
projector. (e) Optical images of Green-part fabricated by MIP-SL system. ..................... 33
Figure 2.4: Debinding process to remove the organic polymer ........................................ 35
Figure 2.5: (a) Schematics of green-part fabrication and post-processes. (b) SEM image
of sintered sample after 6h sintering process at 1330°C. (c) SEM image of debinded
sample ............................................................................................................................... 36
Figure 2.6: (a)(b)(c) Optical microscopy images of 3D-SAA with the 64 pillars annular
segment array. (d)(e)(f) Optical microscopy images of the concaved-shaped piezoelectric
element (PF-CPE) ............................................................................................................. 37
Figure 2.7: (a) Spectrum of dielectric constant and loss tangent. Impedance and phase as
the function of frequency with fundamental radial resonance (b) and the first thickness
resonance (c) ..................................................................................................................... 40
Figure 2.8: (a) Polarization–electric field hysteresis loops of the sintered ceramic. (b)
XRD patterns of BatiO
3
sintered ceramic ......................................................................... 42
Figure 2.9: (a) Profile of the transducer structure. (b) Optical image of the printed
ceramic based transducer .................................................................................................. 44
Figure 2.10: Schematic diagram of non-focused and focused transmit beam generated by
ultrasonic transducer ......................................................................................................... 45
Figure 2.11: Schematic diagram of the experimental setup for the ultrasonic sensing and
imaging ............................................................................................................................. 45
Figure 2.12: (a) Initial pulse and echo generated by the printing focused transducer. (b)
Beam profile simulation by Field II program. (c) Pulse-echo waveform (solid line) and
normalized spectrum ......................................................................................................... 47
Figure 2.13: (a) Schematic of tungsten wire phantom. (b) Phantom imaging using

vii
6.28-MHz transducer. (c) Voltage and lateral resolution as the function of depth between
the transducer and the target. ............................................................................................ 49
Figure 2.14: (a) The 6.28-MHz ultrasonic scan through porcine eyeball using the printing
focused transducer. (b) Ultrasonic imaging of porcine eyeball. ....................................... 50
Figure 2.15: Optical images of Green-part fabricated by MIP-SL system. (a) Annular
array. (b) Self-focused linear array (c) Cylinder array ..................................................... 51
Figure 2.16: Optical image of piezoelectric array after post-process. (Top) sintered
annular array. (Middle) sintered linear array (Bottom) sintered cylinder array ............... 52
Figure 2.17: Optical image of the annular array transducer ............................................. 52
Figure 2.18: Spectrum of impedance and phase ............................................................... 53
Figure 2.19: Four elements with different numbers .......................................................... 54
Figure 2.20: Pulse-echo waveform (solid line) and normalized spectrum o element 1 (a)
element 2 (b) element 3 (c) and element 4(d) ................................................................... 56
Figure 2.21: Phantom images of (a) element1 and (b) annular array after beam forming 58
Figure 3.1: Bouligand naturally evolved composite structure .......................................... 62
Figure 3.2: (a) Schematic of electric electrically assisted MIP-SL system. (b) Design of
electric electrically assisted MIP-SL method. (c) The electric field distribution controlled
by electrode. (d) The rotation of MWCNT-S under electric field ................................. 65
Figure 3.3：Electric field controlled by different types of electrodes ............................. 66
Figure 3.4: Schematic diagram of the AM process of electrically assisted MIP-SL. (a)
The Menger sponge model designed by solid works. (b) Interface to control motor and
projector. (c) and (f) Sliced pattern to define the projected images. (d) Diagram of the
electrically assisted printer. (e) Optical image of the fabricated Menger model. (g) Optical
images of more details in (e) ............................................................................................. 68
Figure 3.5: Impact resistance test for Menger models with different rotation angles. (a)
Diagram of the rotation in different layers. (b) Rotation period along z-axis with N=4. (c)
Schematic of Bouligand structure with different rotation angles. (d) Microscopic images
of the fraction under compression load. (e) The results of fracture load test for pure resin,
MWCNT-S and aligned MWCNT-S with different N values. (f) Schematic showing the
direction of crack propagation (red lines) and crack arrest (yellow lines). (g) Stress
distribution with different N values simulated by Comsol Multiphasics ......................... 69
Figure 3.6: SEM images of fracture surface of Pure polymer and different layers of
printed Menger model with N=90 .................................................................................... 70
Figure 3.7: Compression test for the Menger model with Bouligand structure when (a)
N=1, (b) N=4, (c) N=90 .................................................................................................... 71
Figure 3.8: Compressive strength of pure polymer, MWCNT-S and aligned MWCNT-S
with different N of (a) Menger model and (b) cylinder model. SEM image of fractured
Menger model for N=1(c) and Menger model for N=90(d) ............................................. 73
Figure 3.9: Schematic of artificial meniscus fabricated by electrically assisted MIP-SL. (a)
Model designed by Solidworks. (b) Model was sliced by interface to generate different
types of patterns. (c) Fabrication process. (d) Sample fabricated by the system. (e)

viii
Microscopy images show the radial and circumferential alignment MWCNT-S. (f)
Human meniscus’s structure and the dog-bone bars for tensile test. (g) Comparison of
tensile modulus in different parts. Simulation results of the vertical tear (h) and radial tear
(j) of the human meniscus. The relative strain of the artificial meniscus with the
reinforcement of aligned MWCNT-S in (i) and (k) ........................................................ 75
Figure 3.10: (a) Schematic of the tensile test sample from the artificial meniscus. (b) The
stress-strain curves of the sample. (c) Compression tests of different parts in the meniscus
and (d) compressive modulus ........................................................................................... 77
Figure 3.11: Schematic of test samples for fracture energy. (a) (b) (c) (d) Samples without
pre-cut crack. (e) (f) (g) (h) Samples with pre-cut crack. (i) Stress-strain curves of these
samples. (j) The results of fracture energy ........................................................................ 78
Figure 3.12: (a) Optoacoustic transducer model designed by Solidworks. (b) Parameters
of the optacoustic transducer. (c) Optical image of the printed transducer ...................... 81
Figure 3.13: Optoacoustic transducer and measurement setup ......................................... 83
Figure 3.14: Spectrum of (a) MWCNT-S/rubber transducer and (b) MWCNT-S/hydrogel
transducer .......................................................................................................................... 84

1
ABSTRACT
Additive manufacturing processes have obvious advantages on saving time and cost,
and enabling the complex geometry. Stereolithography is one of the most popular AM
technologies for its merits on fabricating speed, surface finish, manufacturing cost and
applicable scale.
Composite materials and structure are very critical for architectural, biomedical
application and artist design. Although composites like polymer/ceramcis and
polymer/dielectrics were successively fabricated by stereolithography, the solid loading
and properties restrict the application.

This research employed a novel Mask-Image-Projection-based Stereolithography
(MIP-SL) technology to fabricate the piezoelectric ceramics. After post-prosessing, the
density of 5.64g/cm
3
was obtained, which corresponds to 93.7% of the density of bulk
BaTiO3 (6.02 g/cm
3
). The printed ceramic exhibits a piezoelectric constant and relative
permittivity of 160 pCN
-1
and 1350 respectively. Different types of transducer with
printed piezoelectric ceramics were fabricated. The printed single element transducer
realized the energy focusing and ultrasonic sensing. The annular array transducer with 4
elements improved the lateral resolution and signal-to-noise ratio.
An electrically assisted MIP-SL system is reported to fabricate the reinforcement
architectures with anisotropic properties. Multi-walled Carbon Nanotubes (MWCNT)
was combined with photocurable resin to serve as printable slurry. A bio-mimic

2
Bouligand structure and an artificial meniscus with improved mechanical properties were
fabricated using this AM process and the printable slurry.
Besides, an optoacoustic transducer can be printed by the system using MWCNT
and photocurable resin. This transducer can converse laser energy to ultrasonic wave. The
maximum acoustic pressure measured was 0.37MPa.

3
Chapter 1 Introduction
1.1 Additive manufacturing and composite materials
In 1980’s, rapid prototyping, the first form of fabricating layer by layer a three
dimensional object using computer-aided design was developed to create models and
prototype parts. It is considered as the earlier additive manufacturing (AM) process.
Because the AM processes can save time and cost, and provide the possibility to create
the parts with complex geometry, it is widely used among researchers, artists and
designers (Noorani, 2006; Flowers 2002; Chua,1998 ) .
Nowadays, these technologies have other names like 3D printing, but the basic
principles are similar to rapid prototyping (Kochan, 1997): A three-dimensional model and
the corresponding parameter are designed；the computer aided design (CAD) model is
sliced into several two-dimensional figures；A 3D model can then be obtained by
overlaying the 2D figures one by one (shown in Figure 1.1).
The popular additive manufacturing processes include but are not limit to
Stereolithography (Halloran, 2011), 3DP (Cooper, 2001), Fused Deposition Modeling
(Noorani, 2006), Prometal (Kruth, 1991), Selective Laser Sintering (Tang, 2011), Electron
Beam Melting (Murr, 2012), Laser Engineered Net Shaping (Xiong, 2009), Polyjet (Singh,
2011) and Laminated Object Manufacturing (Liao, 2006). Figure 1.2 shows the overview
of the different additive manufacturing processes that are promising for the future of the
industry. The criterion is to classify these processes into liquid base, solid based, and

4
powder based (Wong, 2012).

Figure 1.1: Basic principle of additive manufacturing process
(From http://www.iti-global.com/)

Figure 1.2: Different additive manufacturing processes

AM
processes
Liquid
based
Melting

FDM
Polymerization
SL
Polyjet
Solid
based
LOM
Powder
based
Binding
3DP
Prometal
Melting
SLS
EBM
LENS

5
With additive manufacturing technologies, it is possible to fabricate lightweight parts
with complex cross section like honeycomb cell (Bletzinger, 2001), or the other porous
structure, which reduce the weight-strength relation. The AM technologies can create
special architectural (Williams, 2002) model for the architects to study the functionality
and convince the customers to make their design a reality. In the biomedical area
(Fielding, 2012), researchers making high quality bone transplants, artificial tissue and
biosensors with AM processes. Furthermore, the low-cost 3D printers are very powerful
tool to artist and nonindustrial users in do-it-yourself project (Wallich, 2010). Figure 1.3
shows the applications of additive manufacturing.

Figure 1.3: Different applications of additive manufacturing
(From Google images (free to share or use))

6
However, existing AM technologies are focusing on polymer and metal. Compared
to the polymer and metal, current AM processes for composites, which have diverse
physical or chemical properties, are still limited. The goal of this research is to fabricate
the piezoelectric ceramic and anisotropic reinforcement architectures through the AM
technology of composites.

1.2 Stereolithography
Stereolithography is one of the most popular AM technologies for its advantages on
fabricating speed, surface finish, manufacturing cost and applicable scale. When the
photosensitive polymer is exposure to UV light, visible light or laser, the light can offer
energy, which lead to a chemical reaction, bonding the small molecules and forming a
highly cross-linked structure. Then the curing or solidification of the liquid polymer will
happen across its surface (Figure 1.4).
Lithography comprising different techniques, such as photographic reproduction,
photosculpture, xerography and microlithography is the method of reproduction of
graphic objects. Modern stereolithography AM technology combined the
computer-designed graphics with photosensitive materials to produce three- dimensional
product. The early significant work associated with modern photolithographic AM
systems reported in 1970s. In 1977, Swainson (Swainson, 1977) presented photochemical
machining, a patent, which produce a phase change in the photosensitive polymer to

7
build 3D structure by two intersecting beams of radiation. Although this process can
cross-link and degrade the polymer, the photonic absorption occurs along the paths of
each laser, which leading the difference between the built part and designed one. Kodama
(Kodama, 1981) reported an automatic technology for fabricating 3D structures with
photocurable resin by layered stepped stages. In this method, the designed shape of a
layer was projected on the surface of polymer by a mask or optical fiber, which provides
a more precise shape control. Herbert also presented two sets of apparatus with
layer-by-layer strategies and photocurable resin. The apparatus rotating the polymer
surface and focusing the light spot one the layer. The second one has the ability to build
part with any designed cross-section. Hull (Hull, 1986) came up with the idea of the first
modern stereolithography in 1986. It builds the part based on photopolymerization of
liquid resin. When exposed to UV light, the energy with specific wavelength is absorbed
by the photoinitiator, which results in the free radicals in the polymer. The highly reactive
free radicals with unpaired electron can attract one electron from the carbon-carbon
double bond in the monomer (Wang, 2005) and leave the other electron in the carbon
radical. This reaction will be repeated to take the electron from monomers and use up all
the monomers. After the photopolymerization process of this free radical, polymer chains
can be formed consequently.

8

Figure 1.4: Schematic of modern stereolithography
The entire process of fabricating a 3D part using modern stereolithography
comprises the following steps:
1 Surface or solid design by a CAD system
2 Export the data of CAD model
3 Add supporting structures
4 Define the specific variables and parameters for slicing
5 Slice the CAD model into 2D patterns and generate the information that
controls the SL apparatus
6 Build the part using the 2D patterns
7 Clean the part

9
8 Post process
Recently, composites like polymer/ceramics and polymer/dielectrics
(Hinczewski,1998) were successively fabricated by stereolithography. The process requires
the formulation of a photosensitive medium comprising a photocurable resin and fillers
prior to light exposure. The photopolymer forms a matrix around the fillers after the
polymerization.

1.3 Piezoelectric materials and ultrasonic transducer
The applications of piezoelectric ceramics are far reaching, ranging from acoustic
imaging to energy harvesting. (Ma, 2015; Ma, 2014; Yang, 2012; Chen, 2014; Hwang,
2014)

The most popular piezoelectric materials include Pb-(Mg1/3Nb2/3)O3-PbTiO3
single crystal, Lead zirconate titanate (PZT), (Levassort, 2007; Fu, 2000; Jaffe, 2012)
polyvinylidene fluoride (PVDF) (Liu, 2011) and lithium niobate (LiNbO
3
) (Guo, 2004),
due to their high piezoelectric constant, processability, and affordability. Among the
properties of piezoelectric materials, the most critical properties for ultrasonic
applications are electromechanical coupling coefficient (k
t
), acoustic impedance ( ),
piezoelectric constants (d), and clamped dielectric permittivity ( ).
The thickness mode electromechanical coupling factor k
t
is an indicator of the
effectiveness of a piezoelectric material converting electrical energy into mechanical
energy and vice versa. Therefore, in transducer designs, a high k
t
value is desirable for
a
Z
33 0
/
s
εε

10
higher energy conversion efficiency. The value of can be calculated by equation 1-1
(Shung, 2015):

1-1
where is the series resonant frequency at which the conductance reaches the
minimum and is the anti resonant frequency at which the resistance reaches the
maximum.
The piezoelectric constants d, whose unit is coulomb per newton or meter per volt,
relating to the mechanical strain are the strain constant. It can be expressed as the
polarization generated per unit when mechanical stress applying to the piezoelectric, or
the mechanical strain caused by an external electric field. The former phenomenon is the
piezoelectric direct effect, while the second one is the piezoelectric converse effect. A
high sensitivity piezoelectric material should have high .

1-2

represents that the direction of excitation (force or electric signal) and
corresponding response (charge or displacement) are along the same direction, which are
along the Z axis (polarization axis).
t
k
tan( )
22
ar r
t
aa
ff f
k
ff
ππ −
=
r
f
a
f
33
d
33
d

11
The piezoelectric parameter relating to the electric field produced by a mechanical
stress is termed as the voltage constant or “g" coefficients. The unit can be expressed as
volt-meter per newton.

1-2
indicates that the direction of electric field and the mechanical stress are both
along the polarization axis. A piezoelectric with high energy transform efficiency should
have high . is represented as:
1-3
where is the dielectric constant measured at 1 kHz.
The acoustic impedance ( ) of a material is defined as the product of its density
( ) and longitudinal acoustic velocity ( ):
1-4
is determined by the elastic constant ( ):
1-5
is related to the density ( ), the thickness of the piezoelectric element ( ), and
the anti-resonance frequency ( ):

1-6

open circuit electric field
applied mechanical stress
g =
33
g
33
g
33
g
33
33
0 33
T
d
g
εε
=
033
T
εε
a
Z
ρ
t
v
at
Zv ρ = ⋅
t
v
33
c
33
t
c
v
ρ
=
33
c ρ
t
a
f
2
33
(2 )
a
c tf ρ =

12
Meanwhile, the piezoelectric material itself can be considered as a capacitor. The
clamped capacitance and electrical impedance are related to the clamped dielectric
permittivity and the area ( ) and thickness of the piezoelectric materials (t).

1-7
For a miniaturized transducer, materials with high dielectric permittivity are more
desirable. To maximize the power transmission, the input electrical impedance of the
ultrasonic transducer is designed to match the electrical impedance of the source (50 Ω in
termination). Piezoelectric with a low dielectric permittivity is more desirable for the
design of large element area ultrasonic transducers. The properties of some widely used
piezoelectric materials are shown in Table 1-1 (Zipparo, 1997; Zhang, 2001; Chen, 2010; Zhou,
2014).
Ultrasound refers to the sound wave that has a frequency higher that the human
hearing range (20~20,000 Hz). Ultrasonic transducer, converting electrical signal to
mechanical vibration (acoustic wave) and vice versa via the piezoelectric direct effect and
piezoelectric converse effect, is a very important device for clinical diagnose and
nondestructive inspection. When an electrical signal is applied on the transducer, the
dipoles in it were realigned, which produces the vibration of the piezoelectric layer.
Likewise, when a mechanical force such as acoustic wave is applied on the transducer,
there is an electrical potential generated (Shung, 1996).

33 0
/
s
εε A
33 0 33
1
e TT
t
Z
CA εε
∝ =

13
Table 1.1: Properties of important piezoelectric materials
PMN-PT PIN-PMN-PT LN NBT-BT PZT-5H
ρ (kg/m
3
)
d
33
(pC/N)
k
t
ε
s

c
p
(m/s)
Z
a
(MRayl)
T
c
(°C)
8060
2820
0.58
680-800
4610
37.1
130
8198
2742
0.59
659
4571
37.5
200
4700
49
0.39
39
7340
34.5
1140
5400
210
0.52
80
5550
30
150-180
7500
593
0.51
1470
4580
36.9
180

The criteria of a high quality ultrasonic transducer for imaging applications involve
wide frequency response or broad bandwidth, good impedance matching to biological
tissues, and high efficiency as a transmitter and high sensitivity as a receiver. The
aperture size of the transducer is also very critical. Figure 1.5 shows the profile structure
of the most elementary single-element transducer, which consists of the piezo-element,
electrodes on both surfaces and backing materials. Sometime, the lens used for focus the
ultrasonic beam to a desired distance and a matching layer to compensate for their
acoustic impedance mismatch will be added on the transducer. For a piston transducer
with radius of a and wavelength of λ in the loading medium, there is a distance which is
the last maximum of the axial pressure. The distance was found to be:

14

1-9
The region between the transducer to is called near-field zone or Fresnel zone,
in which the axial pressure oscillates. In the far-field region, the region beyond the

distance, the axial pressure decreases gradually. Which suggests that the target of interest
should be located beyond .

Figure 1.5: Profile structure of single element transducer
Image resolution is important in ultrasonic clinical diagnose. Axial resolution and
Lateral resolution refers to the minimum reflector spacing along the axis of a beam and
the ability to distinguish two closely spaced reflectors perpendicular to the beam,
respectively (Cobbold, 2006).
1- 10

1-11
2
0
a
Z
λ
=
0
Z
0
Z
0
Z
#
0
lateral
c
Rf
f
= ⋅
6
2
axial
dB
c
R
BW
−
=
⋅

15
where is defined as the ratio of focal depth to aperture size.

is the speed of
sound, f
!
is the center frequency of transducer, and 𝐵𝑊
!!!"
is the -6dB bandwidth of
transducer. Equation 1-10 is valid for unfocused transducer and focused transducer, while
the Equation 1-11 is only valid in the focal zone. The Equations shows that high center
frequency transducer can improve the axial and lateral resolutions.
The ultrasound beam originates from mechanical oscillations of the piezoelectric
element in the transducer. Echoes are produced due to the interaction of ultrasound with a
target, and some of these echoes travel back to the transducer. The returned echoes are
converted back into electrical impulses by the transducer and are further processed in
order to form the ultrasound image. The position of each target’s interface can be timely
resolved according to the time delay of echo signal:

1-12
where 𝑑 is the distance between transducer and the target, 𝑐 is sound speed in the
front medium, and 𝑡 is the time delay of echo signal. The echo signals received along
one propagation and receiving route is one A-line. After linearly or rotationally scan of
the transducer and incorporating multiple A-lines, brightness mode (B-mode) can be
formed, which shows the signal amplitude as the brightness of a point and the real-time,
2-D image representing cross-sectional slice.
Ultrasonic array transducer has the same working principles compared with
single-element transducer. As the names imply, the difference between single-element
#
f c
2
tc
d =

16
transducer and transducer array is that transducer array has more than one element.
Based on the geometry, it can be divided into linear array (one-dimensional array),
two-dimensional (2D) array and annular array (Chiu, 2016). Based on the transmission
mode, it can be classified into linear array and phased array.

Figure 1.6: Different types of transducer array
Although there are many different types of transducer array (shown in Figure 1.6),
the operations have same basic concepts, which involve aperture control and
beamforming. Because transducer array has separated elements connected to electrode
for independent transmission and reception, the transducer aperture size is highly flexible.
Time delay is a common method for beamforming. By controlling the transmission
timing for each element, the excitation timing at specific location can be controlled
(shown in Figure 1.7).

17
Although ultrasonic transducer arrays not only increase frame rate but also improve
image quality, it also has challenges encountered in the fabrication process. For example,
in order to decrease the grating lobes, the separation size between each element should
less than λ/2, where λ is the wavelength of ultrasonic wave generated by the elements.
Therefore, new manufacturing method is expected for transducer array’s fabrication,
especially for high frequency (>20MHz) ultrasonic device.

Figure 1.7: Illustration of a simple time delay beamforming

18
1.4 Optoacoustic transducer
The traditional material to generate ultrasound is the piezoelectric. Besides,
researchers are also interested in generating ultrasound by other materials except the
piezoelectric. There has been considerable interest in the production of ultrasound by
irradiation of a solid with a laser pulse. This optoacoustic transducer converts absorbed
laser or modulated light into heat energy and temporally changes the temperatures at the
loci where radiation is absorbed. This process results in an expansion and contraction of
surrounding materials following the temperature changes, which are translated to acoustic
waves (shown in Figure 1.8).
Figure 1.8: Illustration of photoacoustic effect
(From http://www.bioopticsworld.com/)

19

The laser ultrasound transduction structure usually consists of a laser (light)
absorption layer and a thermal expansion layer. High acoustic pressure is expected in
medical application such as drug delivery (Ferrara, 2007) and High-Intensity Focused
Ultrasound (HIFU) (Kennedy, 2005). In order to obtain high performance and
high-pressure acoustic, many efforts have been made on the absorbing materials with a
high light absorption ratio and expanding layer or matrix with a high thermal expansion
coefficient. Polydimethylsiloxane (PDMS), with a thermal coefficient of volume
expansion (α=0.92×10-3K-1) higher than metal, water and other polymers, is the most
popular thermal expanding layer. Some main absorbing materials involve carbons In
2014, Colchester et al. achieved 4.5 MPa acoustic pressures using Carbon
Nanotube-PDMS composite (Colchester, 2014). In 2015, Jiang group successfully
generated ultrasound with 12.15 MPa pressure using the carbon nanofibers/PDMS
composite (Hsieh, 2015). Baac et al. fabricated gold-coated CNT-PDMS into concave
shape (focused shape) and generated 50 MPa in peak positive (Baac, 2012). Carbons,
especially CNT and its Assemblies, have been shown to possess the highest thermal
conductivities of any known material, which is consider as the best absorption materials
for optoacoustic transducer.

20
1.5 Natural materials and bio-inspired composites
Self-organization is a process through which the internal organization of the system
adapts to the environment to promote a specific function without being controlled from
outside. Biological systems have adapted and evolved over several billion years into
efficient configurations, which are symbiotic with the environment (Mann, 2001).
Although living tissues have the capability to adapt to complexly changing environmental
conditions, cellulose, collagen, chitin and silks are the only four types of tissues found in
most natural constructions (Mingallon, 2012) (Figure 1.9).
Figure 1.9: There are only four types of fibres in natural organism (Mingallon, 2012)

21
The natural materials such as wood, shells and bone supported the early stage of
technological development of humanity. Although as history advanced, the natural
materials were replaced by synthetic compounds that can provide better mechanism
properties. Scientists and researchers still continue to be attracted by the unique
properties of the elegant and complex biological architectures. At present, people need
new structural and functional materials that can support more advanced technologies to
serve a variety of strategic fields, such as architecture, transportation, military equipment
and energy transform and storage. However, in order to adapt the requirement of new
technology, yet-to-be-developed materials such as steel and alloy face many challenges
like unprecedented combinations of strength, stiffness, and toughness at a low density. It
is an open question how do we achieve the goal. Although remarkable materials and
architectures have been came up with from the laboratory, the practical applications
remain uncertain without a long time testing (Vincent, 2008). As the development of
contemporary characterization and simulation technology, researchers now can study the
intricate interplay of structure and properties acting at different scales, from nano-scale to
the macroscopic. Which encourage us to find the answer in natural materials and
structures that have great number diversity of solutions, perfected over millions of years
of evolution.
The classic materials-design dilemma of artificial materials is that strength and
toughness (Ritchie, 2011), two key mechanical properties that describe their ability to
withstand applied loads and displacements, are mutually exclusive. The fundamental

22
relationship of these properties is the constitutive law. The normalized relative
displacement (strain) is related to the load normalized by area (stress). It is usually
measured by a uniaxial tensile test to characterize the properties such as stiffness,
toughness and strength. Strength and toughness can be defined as the yield stress at the
maximum load before fracture and the area under the load-displacement curve,
respectively. Strong materials are inevitably brittle, while the materials with high
toughness are usually weak. But for highly mineralized, mostly ceramic, natural
structures, such as tooth, nacre, can take advantages of different component or structural
orientations, to resist wear and adapt the increased deformation at the same time (Wegst,
2004; Aizenberg, 2009; Dunlop, 2010; Li, 2014). The other type of natural composite such as
fish scales, lobster cuticle and bamboo can incorporate nanofibres and gradient structure
(Weaver, 2012; Wegst, 2008; Wegst, 2011; Wegst, 2007). Such natural materials combine the
desirable properties of their components often with properties that far exceed their
material constitute.
Although biological materials and architecture are multifunctional, mimicking the
features of a natural material is a difficult work. In order to fulfill functional demands,
researchers developed many methods to fabricate the bio-inspired materials (Gehrke, 2005;
Oaki, 2005; Tseng, 2009; Aizenberg, 2004 Aizenberg 2003; Ethirajan, 2008; Perkin, 2005).
Inspired by the natural tooth, Hortense Le (Le, 2015) developed the magnetically
assisted slip casting to fabricate bio-inspired heterogeneous structure with
complex-shaped that involves the deposition of a fluid suspension of particles into a

23
porous mould of pre-defined geometry and pore dimensions typically smaller than the
particle size (Figure 1.10). Capillary force of the pores in the mould can remove the
liquid phase from the suspension to build a layer of jammed particles nest to the mould
wall, as know as the cake layer. Besides the capillary forces, vacuum can also be used to
extract the liquid phase through the walls of porous mould. The anisotropic particles are
coated with superparamagnetic iron oxide nanoparticles to be magnetically responsive.
An external magnetic field was used to control the orientation of the anisotropic particles.

Figure 1.10: Bio-inspired heterogeneous structure with complex-shaped (Le, 2015)

24
A recent attempt at mimicking nacre in a bulk material is the ice templating (also know as
freeze casting) (Deville, 2006). Which involves the freeze of ceramic suspensions to form
lamellar ice crstals that results in a homogeneous ceramic scaffold. The second soft phase
was filled between the scaffolds to from the hard-soft layered composite. The nacre-like
PMMA/alumina composite fabricated by the freeze casting shows yield strengths of 200
MPa. Which is 300 times higher than those of either the component of ceramic or
polymer (Figure 1.11).

Figure 1.11: Freeze casting for bio-mimic nacre structure (Deville, 2006)
Joshua J. presents an additive manufacturing approach (Martin, 2015) that combines
real-time colloidal assembly with stereolithography to fabricate fibre composites. Iron
oxide nanoparticles were coated on the traditionally nonmagnetic-reinforcing materials
such as alumina, silica and calcium phosphate. The orientation of the ceramic-reinforcing
particles was controlled by a magnetic-labeling technique. With this technique,

25
researchers created layered structures of nacre, cholesteric structure of shrimp and
concentric structures of the osteom (Figure 1.12).
Among these methods, additive manufacturing may provide a path towards
fabricating the bio-inspired structural composites with high surface quality and complex
microstructure in future.
Figure 1.12: Stereolithography with nonmagnetic-reinforcing process (Martin, 2015)

26
1.6 Outline of this work
The aim of this research is to study the fabrication methods of composites using
slurry based SLA process and the application of the corresponding composite. More
specifically, the major goals of this research include:
(1) Develop a projection-based SLA process for ceramic/polymer composites
with high solid loading of ceramic.
(2) Study the post process for ceramic fabrication and test the piezoelectric
properties.
(3) Study the potential application of the printed piezoelectric ceramic in
ultrasonic imaging and sensing.
(4) Develop an electrically assisted additive manufacturing approach for carbon
nanotube alignment in composite.
(5) Fabrication of anisotropic reinforcement architectures and test the
corresponding mechanical properties.
(6) Fabrication of optoacoustic transducer and test the acoustic pressure.

27
Chapter 2 Additive manufacturing of piezoelectric element
2.1 Mask-Image-Projection-based Stereolithography (MIP-SL)
Piezoelectric ceramics are widely used in acoustic imaging, sensing and energy
harvesting. However, conventional manufacturing processes such as etching and dicing
for piezoelectric ceramics fabrication have limited capability of achieving complex
geometry and high resolution (X-Y resolution<25µm). Moreover, mechanical stress
caused by the traditional machining processes can result in grain pullout, strength
degradation and depolarization of the near surface region, which will led to significant
degradation in piezoelectric device performance (Goat, 1999).

In decades, researchers
developed tape casting, soft molding, transfer/pad printing, direct inkjet based printing,
extrusion-based direct write technique and light based stereolithography apparatus to
enable the manufacturing of multiple functional ceramics (Chartier, 1997; Wu, 2015; Lewis,
2006; Smay, 2004; Lee, 2004; Noguera, 2005; Sun, 2010). Generally, the AM process of
ceramic fabricating involves two steps: (1) ceramic particle/polymer composite
fabrication to define the geometry of green part; and (2) post process involves debinding
and sintering of composite to remove the polymer and achieve high density components.
However, the molding techniques and pad printing are indirect processed with many
steps that require significant effort. The inkjet printing and extrusion-based direct write
techniques have the limitation on geometry and resolution. Because of the complex
geometry and high resolution (X-Y resolution<25µm) of SLA, many efforts have been

28
made on using it in ceramic composite fabrication. But the solid loading using SLA
process limits by the viscosity and photosensitivity of liquid resin, which results in a low
density of the sintered part and degrades the piezoelectric properties.
For ceramic sintering, the decrease in free energy caused by reduction in surface
area is the most important driving force. Therefore, nano-scale ceramic particle with high
free energy has been an active topic to increase the density of sintered ceramic.
For SLA, the challenge of using slurry with a high solid loading nanoparticles comes
from the reduction of cure depth. According to Jacob's equation (Jacobs, 1992):

2-1

Where C
d
is the cure depth, E is the energy density of incident light, E
c
is the critical
energy density, and D
p
is the resin sensitivity defined as:
𝐷
!
=
!!
!"
!!
!
!
!
∆!
!
2-2

Where d
50
is the average particle size; Δn is the refractive index difference between
the ceramic particle (n
p
) and the liquid resin (n
0
, i.e.Δn
2
= (n
p
–n
0
)
2
). When the average
particle size d
50
decreases from micro-scale to nanoscale, the resin sensitivity D
p
and cure
depth C
d
decrease dramatically. For SLA, each layer needs to be sufficiently over-cured
to firmly attach to the previously solidified layers, thus the decrease of cure depth brings
significant difficulty.
𝐶
!
=𝐷
!
𝑙𝑛(
𝐸
𝐸
!
)

29
In order to overcome the problems and fabricate the high solid loading
ceramic/polymer composite with nanoparticle, this work present a bottom-up
Mask-Image-Projection-based Stereolithography (MIP-SL) method. Different with the
system shown in figure 1.4, the mask images are projected through a transparent resin
tank coated with transparent materials such as PDMS or Teflon, to cure the layers that are
hung upside-down on the platform. Bottom-up projection based SLA process has two
benefits:
(1) Because this method does not required a tank fulfilled with slurry during the
fabrication process, less building material is needed compared with traditional SLA.
(2) Every new layer is sandwiched between previously solidified layer and the
bottom of the tank. Hence the thickness is accurately controlled by the height of
building platform. Which can results in an ultrathin layer.
Figure 2.1 shows the design of MIP-SL. The film collector moving along X-axis has
the PDMS coating. The imaging patterns controlled by a DMD are only projected onto
the film collector coating with the PDMS. After a layer is cured, the Z stage moving
along the Z-axis moves up for a small distance to separate the cured layer from PDMS.
Right after the lift-up, the film collector slides from the left to right. Then the dispenser
will add the new slurry on the PDMS. The building platform move up for a distance
(0.5~1mm) to allow the slurry spread by blade to be set underneath the previous cured
layer. The next layer is fabricated in the same manner. Figure 2.2 shows the prototype
system of MIP-SL.

30

Figure 2.1: Design of Mask-Image-Projection-based Stereolithography
2.2 Fabrication of piezoelectric composite and post processing
The most popular piezoelectric ceramic such as Pb-(Mg1/3Nb2/3)O3-PbTiO3 single
crystal, Lead zirconate titanate (PZT),

and lithium niobate (LiNbO
3
) have been expelled
from many applications especially in clinic diagnoses, due to concerns regarding its
toxicity. Compared with Lead-based piezoelectric, lead-free piezoelectric materials such
as Barium Titanate (BaTiO
3
) and Sodium potassium niobate (KNN) are competitive for
the same types of applications (Lau, 2011; Cheung, 1999).

31

Figure 2.2: Prototype system of Mask-Image-Projection-based Stereolithography

In this study, we report that BaTiO
3
, a widely used lead-free ceramic material, can
be three-dimensional (3D) printed into complex 3D geometry with sufficient ferroelectric
and piezoelectric properties that are suitable for the application of piezoelectric device
such as ultrasonic transducer.
An azeotropic mixture was prepared by combining ethanol (34v/v%, 99.5%,
Sigma-Aldrich, Saint Louis, MO) with methylethylketone (66 v/v%, 99%, MEK,
Sigma-Aldrich, Saint Louis, MO). In order to get the dry BTO powders with dispersant
absorbed uniformly on the surface, the BaTiO
3
powders (25v/v% solid loading, 100nm,
Sigma-Aldrich St. Louis, MO) and azeotropic mixture with dispersant (Triton x-100,

32
0.5-0.8wt% on a dry weight basis of ceramic powders) were planetary milled
(pulverisette 5, FRITSCH Idar-Oberstein, Germany) with 200 rpm rotation speed. After
that, the mixture is dried at 50°C for 12 h.
The deagglomerated BaTiO
3
powder (100nm) and photocurable resin SI500,
(EnvisionTec Inc., Ferndale, MI) were mixed with a mass ratio 7/3 by ball milling for 1h.
Figure. 2.3a shows the illustration of the MIP-SL system. The slurry (0.045ml) is
dispensed onto the film collector and spread out to a thin layer (50µm) by a doctor blade
when the film collector slides towards the left. After that, a Computer-Aided Design
(CAD) model (Fig. 2.3c) is sliced into two-dimensional (2D) images, each of which is
then projected onto the bottom of the film collector through a Digital Micromirror Device
(DMD) (Fig. 2.3a and b). Figure 2.3d shows the interface to control the motor and
projector. Under the projected visible light, the cross-linked matrix and strong bond
between BTO particles and polymer network are formed, which results in the
photopolymerization of the SI500. Figure. 2.3e shows the piezoelectric-composite
fabricated by the MIP-SL process.

33

Figure 2.3: (a) Illustration of the MIP-SL system. (b) Imaging pattern controlled by the
projection. (c) 3D geometry designed by SolidWorks. (d) Interface to control motor and
projector. (e) Optical images of Green-part fabricated by MIP-SL system

In order to achieve a high-density piezoelectric ceramics, the post-processing steps
involves debinding to remove the organic polymer from the composite and high temperature
sintering to increase the density. In the debinding process, the piezoelectric/polymer

34
fabricated by MIP-SL method were set in a muffle furnace with Argon under a temperature of
600°C for 3 hours (Fig. 2.4). In the pyrolysis process without oxygen, polymer degrades into
a range of decomposition products including char, combustible liquids and gases. After the
furnace cooling, the combustible gas and liquid were moved out without ignition. The
sintering of the debinded sample was conducted in a regular muffle furnace at 1330°C for 4 to
6 hours. When the temperature increases again in the sintering process, the char reacted with
oxygen and escaped as a form of carbon dioxide, which would not damage the geometry of
the sample. In the sintering process, the BTO particles undergo initial stage sintering, which
leads to neck formation at particle-particle connection (Fig. 2.5a). As the temperature
increases and atoms diffuse, more BTO particles connect with each other, which results in
grain size as well. The sintering process leads to the consolidation of powdery structure, the
considerable shrinkage and the significant reduction of porosity. Fig. 2.5c and b show the
Scanning Electron Microscope (SEM) images of the debinded and sintered sample
respectively. As the figure shows, the particle size in debinded sample is in nanoscale and the
pores interconnected at various dimensional planes are irregular. The sintered samples show a
larger particle size and the less porosity, compared to the debinded sample. The increasing
density of sintered samples is caused by the higher free energy and large driving force of
nanoparticles.
The 3D printed segment annular array (3D-SAA) and the printing focused
concaved-shape piezoelectric element (PF-CPE) are fabricated by the MIP-SL method and

35
the post-processing. The 3D-SAA consists of 64 fan shape pillars (1 mm height) and a 1mm
basement (shown it Fig 2.6a, b, c). For the traditional matching process like dicing and
etching, the shaping of fan pillar is very difficult. So the pillars are usually designed in
rectangular or square shape. Fig 2.6d, e, f show the detailed of the PF-CPE surface under
microscope. The aperture (concaved area), thickness and arc length are 5mm, 390µm and
5.2mm, respectively. The concaved shape of PF-CPE can be used to focus the ultrasonic
wave for the application of ultrasonic imaging and therapy.

Figure 2.4: Debinding process to remove the organic polymer

36

Figure 2.5: (a) Schematics of green-part fabrication and post-processes. (b) SEM image
of sintered sample after 6h sintering process at 1330°C. (c) SEM image of debinded
sample

37

Figure 2.6: (a)(b)(c) Optical microscopy images of 3D-SAA with the 64 pillars annular
segment array. (d)(e)(f) Optical microscopy images of the concaved-shaped piezoelectric
element (PF-CPE)

2.3 Characterization of Sintered-Parts
Several cylindrical BTO samples with 10mm diameter and 390µm thickness
fabricated by MIP-SL method and post-process were used to measure the properties.
Gold is sputtered at two surfaces of BTO sample to serve as the electrode. An electric
field with 30kV/cm voltage is added on the electrode at 100°C for 30 min. The
impedance and phase of the sample will change with the frequency when the alternating
current is applied on the piezoelectric. Figure 2.7a depicts the spectrum of dielectric

38
constant and loss tangent. The values of dielectric constant and loss tangent are about
1300 and 0.01 respectively when the frequency ranging from 40kHz to 1MHz. The peak
values are caused by the fundamental radial resonance (300kHz~500kHz), which
corresponds to the motion in the largest dimension of the piezoelectric ceramic. The
fundamental radial resonance consists of a resonant and an anti-resonant frequency
(shown in Figure 2.7b). The impedance drops to a minimum at the resonant frequency
(fr=419kHz) and rises to a maximum at the anti-resonant frequency (fa=429kHz). The
phase angle rises to a peak value roughly at the center of the resonance. The first
thickness resonance appears quite different from the radial harmonics (Figure 2.7c). The
impedance drops to a much lower value and the span in frequency is far wider. The
resonant frequency (𝑓
!
) and anti-resonant frequency (𝑓
!
) are 6.00MHz and 6.67MHz
respectively. The thickness mode frequency often appears much rougher than the radial
mode. It is because a strong thickness mode vibration excites that harmonics. The
electromechanical coupling factor K
t
can be calculated as (Kim, 2015):
𝐾
!
=
!!
!
!!
!
∗ cot
!!
!
!!
!
=47% 2-3

39

40

Figure 2.7: (a) Spectrum of dielectric constant and loss tangent. Impedance and phase as
the function of frequency with fundamental radial resonance (b) and the first thickness
resonance (c)

The dielectric constant and clamped dielectric constant at 1KHz measured by an
impedance analyzer (Agilent 4294A) are 1350 and 1030. The Density measured by
ASTM B962-14 standard of 3D-printed BaTiO
3
is 5.64 g/cm
3
, which corresponds to 93.7%
of the density of bulk BaTiO
3
(6.02 g/cm
3
). The loss tangent of the printed ceramics is
0.012. Table 2.1 compares some BTO properties of our MIP-SL method with typical
BTO, and laser sintering, bind jetting and injection moulding. The results indicate that the
MIP-SL method and post processing fabricated sample shows a higher density and lower
dielectric loss compared with other AM method (Vijatović, 2008; Kasai, 1989; Gaytan, 2015).

41
Table 2.1 Comparison our printed BaTiO
3
to reported literature values
Fabrication method Density (g/cm3) ε(1kHZ) d
33
(PC N-1) tanδ(1kHz)
Typical BaTiO3 6.02 1700 190 <0.1
MIP-SL (This study) 5.64 1350 160 0.012
Laser-sintering 5.59 −−− −−− −−−
Binder Jetting 3.93 640 74.1 0.03
Injection Moulding 5.86 −−− −−− −−−

The ferroelectric hysteresis loop of the sintered sample is shown in Figure 2.8a.
When the electric fields are 10kV/cm, 20kV/cm and 30kV/cm, the remnant polarizations
Pr of the samples are 2.2µC/cm
2
, 2.4µC/cm
2
and 7.0µC/cm
2
,

respectively. Figure 2.8b
shows X-ray powder diffraction (XRD) measurement of sintered sample. The results
show that the BaTiO
3
sintered samples have a similar pattern to the BaTiO
3
powder and
keep a perovskite structure after a high temperature sintering process. Therefore, they are
suitable for piezoelectric applications. This performance of MIP-SL method fabricated
piezoelectric components suggests that it have great potential to be used in piezoelectric
devices especially for medical ultrasound.

42

Figure 2.8: (a) Polarization–electric field hysteresis loops of the sintered ceramic. (b)
XRD patterns of BatiO
3
sintered ceramic

(a)

(b)

43
2.4 Ultrasonic imaging with MIP-SL fabricated piezoelectrics
The focused piezoelectric elements can offer not only high intensity for high
intensity focused ultrasound (HIFU), but also the high sensitivity and smaller lateral
resolution for ultrasonic imaging and nondestructive testing. The problem is that,
traditional machining process for focusing features inevitably leads to cracking and
ferroelectric domain reorientation, which results in a high loss tangent and decreased
working life of piezoelectric layer. To realize the energy conversion under the focusing
effect without the damage caused by the machining process, PF-CPE (shown in Fig. 2.6
d，e and f) was fabricated by the MIP-SL method and post-process. To demonstrate its
focusing ability and the potential applications in medical ultrasound, the PF-CPE was
assembled in an ultrasonic transducer, whose profile is shown it Figure 2.9. The Cr/Au
layer was sputtered on the two faces of PF-CPE to serve as the electrodes. Cr layer
between the piezoelectric layer and Au layer can bind the two layers more tightly. A
conductive epoxy with an acoustic impedance of ~6 MRayl (E-Solder 3022, Von Roll
Isola Inc., New Haven, CT) was attached to one of the Au surface as the backing layer. A
10µm-thick parylene was vapor-deposited by Specialty Coating System (SCS, Indiana,
USA) on the other Au surface to protect the transducer.
As mentioned in introduction, the mechanical oscillations of the piezoelectric layer
in the ultrasonic transducer can generate an ultrasound wave. A mechanical wave was
rebound after the interaction between the ultrasound wave and the target. The rebounded
echoes can generate oscillation of piezoelectric layer, which converts the oscillation in to

44
electrical impulses. The electrical signals are further processed to form the ultrasonic
imaging. Figure 2.10 reveals the sketch of a non-focused and focused transmit beam.
Because the ultrasonic wave are focused in a small region, the transmit bean has the
highest intensity and the minimum lateral resolution in the focus zone.

Figure 2.9: (a) Profile of the transducer structure. (b) Optical image of the printed
ceramic based transducer

The actual performance of the ultrasonic transducer with MIP-SL fabricated
piezoelectric element was test by the ultrasonic system consisted of GUI system, gage
card, panametrics-ndt and motor (shown in Fig. 2.11). The quartz target is 5mm away
from the transducer. Figure. 2.12a shows the initial pulse generated by the piezoelectric

45
element and the echo rebounded by the quartz target. The pulse and echo indicates the
MIP-SL fabricated ceramics have the direct and converse piezoelectric effects to convert
compressive/tensile stresses to an electric charge, and vice versa.
Figure 2.10: Schematic diagram of non-focused and focused transmit beam generated by
ultrasonic transducer

Figure 2.11: Schematic diagram of the experimental setup for the ultrasonic sensing and
imaging

46

(b)

47

Figure 2.12: (a) Initial pulse and echo generated by the printing focused transducer. (b)
Beam profile simulation by Field II program. (c) Pulse-echo waveform (solid line) and
normalized spectrum

Field II program

(Jensen, 1992)

was used to simulate the azimuthal beam profile
generated by the transducer focused by the MIP-SL fabricated ceramic. Figure 2.12b
shows normalized pressure (color bar) and the distribution of the pressure. Which
indicates that, in depth direction, the focus point with the maximum intensity is about
14~18mm away from the transducer.
The impulse–response when the quartz target is set at different location were tested
by the ultrasonic system. The received echo has the highest intensity (peak to peak output
voltage is 0.301V) when the distance between transducer and quartz is 15.5mm. Which is
corresponding to the results predicted by Field II program. Figure 2.12c shows the
(c)

48
receive-echo response in time domain (solid line). A Fast Fourier Transform applied on
response in the time domain results in the frequency spectrum (dash line). The results
show that at focused point, center frequency and bandwidth (magnitude >-6dB) are
6.28MHz and 41.28%, respectively.
In order to test the lateral and axial resolution of the transducer with the MIP-SL
fabricated piezoelectrics, a phantom with tungsten wire (50µμm diameter) replaces the
quartz target in the ultrasonic system with a 20dB amplifier portion and -30dB dynamic
range. (Fig. 2.13a and b) Figure. 2.13c shows the relationship between lateral resolution,
output voltage and depth of the target.
Figure. 2.14 shows the ultrasonic imaging of porcine eyeball using the 6.28-MHz
transducer. With the 770µμm lateral resolution and 240µμm axial resolution, the imaging
demonstrates the crucial biometric information regarding eyeball structures with the
cornea, its constituent layers, and the anterior and posterior chambers. The test proves
that the MIP-SL focused transducers have great potential in medical ultrasound
application.

49

Figure 2.13: (a) Schematic of tungsten wire phantom. (b) Phantom imaging using
6.28-MHz transducer. (c) Voltage and lateral resolution as the function of depth between
the transducer and the target

50

Figure 2.14: (a) The 6.28-MHz ultrasonic scan through porcine eyeball using the printing
focused transducer. (b) Ultrasonic imaging of porcine eyeball

(a)

51
2.5 MIP-SL fabricated piezoelectric array
Piezoelectric arrays were fabricated using the same printing and sintering method
(shown in Figure 2.15) to show the potential of printed piezoelectric in biomedical
engineering and ultrasonic imaging. In this part, green parts of annular array, self-focused
linear array and cylinder array were fabricated by this method. Figure 2.16 shows the
optical images of sintered annular array (top), linear array (middle) and cylinder array
(bottom). Annular array has five elements, which were designed with a same area
(13.5mm
2
). Linear array with concave shape can focus ultrasound without extra lens. And
we can use the cylinder array to create 1-3 composite with epoxy filled in the space
between each pillars.

Figure 2.15: Optical images of Green-part fabricated by MIP-SL system. (a) Annular
array. (b) Self-focused linear array (c) Cylinder array

52

Figure 2.16: Optical image of piezoelectric array after post-process. (Top) sintered
annular array. (Middle) sintered linear array (Bottom) sintered cylinder array

Figure 2.17: Optical image of the annular array transducer

53

Figure 2.18: Spectrum of impedance and phase

The annular array was used to characterize the properties of the printed piezoelectric
array. The thickness is 400µμm, while the areas for elements 1,2,3,4 are 13.5 mm
2
, 13.6
mm
2
, 13.7 mm
2
and 13.2mm
2
. Epo-Tek 301 was the epoxy used for all bonding steps.
Epoxy was filled into the space between each element. The 150 nm Cr/Au layer was
sputtered on the one face of annular array to serve as the electrode. Four electric cables
are connected on the elements with conductive epoxy (E-Solder 3022, Von Roll Isola Inc.,
New Haven, CT). The housing was Aluminum Silicate. Epoxy with an acoustic
impedance of ~6 MRayl was filled into the housing to serve as the backing layer. The
other Cr/Au layer was sputtered on the other side of the annular array with a cable
attached on the layer. A 10µm-thick parylene was vapor-deposited by Specialty Coating

54
System (SCS, Indiana, USA) on the other Au surface to protect the transducer array.
Figure 2.17 shows the optical image of the annular array transducer. The poling field is
30kV/cm at 100°C for 30 min.
Impedance
analyzer
(Agilent
4294A)
was
used
to
measure
the
spectrum
of

impedance
and
phase.
The
four
elements
have
similar
spectrums
shown
in
Figure

2.18.
These
elements
were
numbered
as
Figure
2.19.
The
impulse-‐response
of
each

element
was
measured
by
the
ultrasonic
system
mentioned
before.
Figure
2.20

a,b,c,d
represent
the
Pulse-‐echo
waveform
(solid
line)
and
normalized
spectrum
of

element
1,2,3,4,
respectively.
Table
2.2
shows
the
measured
pulse
and
echo

characteristics
for
all
annular
array
elements.

Figure 2.19: Four elements with different numbers

55

4" 4.5" 5" 5.5" 6" 6.5" 7" 7.5"
'20"
'18"
'16"
'14"
'12"
'10"
'8"
'6"
'4"
'2"
0"
'0.25"
'0.2"
'0.15"
'0.1"
'0.05"
0"
0.05"
0.1"
0.15"
0.2"
0.25"
5" 6" 7" 8" 9" 10" 11" 12"
Frequency)(MHz) Magnitude)(dB) Voltage)(V) Time)(μs)
a)

4.5$ 5$ 5.5$ 6$ 6.5$ 7$ 7.5$
'20$
'18$
'16$
'14$
'12$
'10$
'8$
'6$
'4$
'2$
0$
'0.5$
'0.4$
'0.3$
'0.2$
'0.1$
0$
0.1$
0.2$
0.3$
0.4$
0.5$
5$ 6$ 7$ 8$ 9$ 10$
Frequency)(MHz))
Magnitude)(dB))
Voltage)(V))
Time)(μs))
b)

56

Figure 2.20: Pulse-echo waveform (solid line) and normalized spectrum o element 1 (a)
element 2 (b) element 3 (c) and element 4(d)

5" 5.5" 6" 6.5" 7" 7.5" 8" 8.5"
'20"
'18"
'16"
'14"
'12"
'10"
'8"
'6"
'4"
'2"
0"
'0.4"
'0.3"
'0.2"
'0.1"
0"
0.1"
0.2"
0.3"
0.4"
4.5" 5" 5.5" 6" 6.5" 7"
Frequency)(MHz))
Magnitude)(dB))
Voltage)(V))
Time)(μs))
c)

4.5$ 5.5$ 6.5$ 7.5$ 8.5$
(20$
(18$
(16$
(14$
(12$
(10$
(8$
(6$
(4$
(2$
0$
(0.8$
(0.6$
(0.4$
(0.2$
0$
0.2$
0.4$
0.6$
0.8$
4$ 5$ 6$ 7$ 8$
Frequency)(MHz))
Magnitude)(dB))
Voltage)(V))
Time)(μs))
d)

57

Table 2.2: The measured pulse and echo characteristics for all array elements
Element 1 Element 2 Element 3 Element 4
Center frequency (MHz) 5.72 5.86 6.39 6.12
-6dB bandwidth (%) 19.6 12.9 19.8 23.6
V
pp
(mV) 402 793 626 1039
-20dB pulse length (ns)
Area (mm
2
)
1940
13.7
1621
13.2
989
13.6
2949
13.5

The performance of the annular array transducer was tested by imaging a wire
phantom, which is similar to the phantom mentioned in Figure 2.13. It was imaged to
assess array lateral resolution. The array transducer was driven by JSR500 (Ultrasonics,
NY, USA) and triggered by the function generator with a pulse repetition frequency (PRF)
of 1kHz. The ultrasonic signals were filtered by an analog band-pass filter. A 12-bit
digitizer card (ATS9360, Alazartech, Montreal, QC, Canada) with a 1.8 GHz sampling
rate was used to record the signals. To obtain a 2D image, the annular transducer was
mounted on a 3-D stepper motor (SGSP33-200, OptoSigma Corporation, Santa Ana, CA,
USA) for mechanical scanning with 36µμm increment. In each case, signals were
acquired for all transmit-to-receive combinations (16 in total) and processed with
advanced beam forming technology (Ketterling, 2012) using all 25 transmit-receive
pairs. Figure 2.21a shows the phantom image of a single element transducer, which has a

58
similar diameter to element 1. Figure 2.21b shows the phantom image after beam forming
using all 25 transmit-receive pairs. Lateral resolutions for different depth of these images
are showed in Table 2.3. These results show improved lateral resolution and signal to
noise ratio, which proves that MIP-SL method can enable more applications in
biomedical industry.

Figure 2.21: Phantom images of (a) element1 and (b) annular array after beam forming

59
Table 2.3: Lateral resolutions for phantom images
Depth (mm) Resolution of element 1 (mm) Resolution after beam forming (mm)
5.6 1.4 1
6.8 1.5 1.05
8.0 1.1 1.1

2.6 Summary
A novel additive manufacturing process named Mask-Image-Projection-based
Stereolithography (MIP-SL) was investigated for the fabrication of high solid loading
slurry with nanoparticle. A specifically designed heat treatment involving debinding and
sintering process was applied on the green part. The sintered element showed
piezoelectric properties and can be used in biomedical imaging and other applications.
The single element transducer with the MIP-SL fabricated element results in a focused
beam profile. Examples images from Ex vivo porcine eyeball structure and a tungsten
wire phantom demonstrate the imaging capability of the transducer. The annular array
transducer printed by MIP-SL method improved the lateral resolution and signal-to-noise
ratio of wire phantom images. The results of this study indicate that the MIP-SL
technology for piezoelectric composite with nanoparticles opens an effective way toward
the realization of functional materials with three-dimensional complex structure. Enabled
by this method, more piezoelectric structure with complex geometry can be used to

60
converts electrical energy into mechanical energy and vice versa that bring us more
possibility in energy harvesting and medical ultrasound applications.

61
Chapter 3 Additive manufacturing of Carbon Nanotube
composite
3.1 Electrically assisted Mask-Image-Projection-based Stereolithography
As mentioned in chapter 1, biological architectures with low density, high strength
and toughness inspire researchers to design the next-generation structural materials.
Inspired by the dactyl clubs of peacock mantis shrimp, gigas fish scales, claws of crab
and lobster and Beetle wings, researchers found a naturally evolved composite structure
(Bouligand or twisted plywood structure) with different orientations of reinforcing fibers
and particles, which has superior mechanical properties (Figure 3.1) (Grunenfelder, 2014;
Finnemore, 2012; Espinosa, 2011; Kokkinis, 2015; Patek, 2004). This structure with ordered
collagen or chitin fibers in one layer, but heterogeneous between different layers can
offer a great contribution in the reinforcement of stopping crack. The crack does not
extend straightly and thereby increasing the energy dissipation and impact resistance.
Therefore, this bio-inspired structure shows great advantages through a long-term
evolution.
Although reinforced architectures with fibers or platelets were fabricated by
conventional machining process and additive manufacturing (Holmes, 2014; Dimas, 2013;
Compton, 2014; Erb, 2012), the natural materials with complicated structure is still difficult
to researchers, and the reinforcement mechanism of the rotating angle between aligning
direction in Bouligand structure is not found.

62
Carbon nanotubes is widely used to enhance the mechanical properties in the
composite and medical device due to its high modulus, unique structure and
biocompatibility (Schadler, 1998; Gonçalves, 2015). The nanocomposite with controlled
alignment of CNTs in polymer matrix can further improving the multifunction by the
anisotropic properties.

Figure 3.1: Bouligand naturally evolved composite structure
For the obvious advantages of dimensional accuracy, surface finish, high fabrication
speed, low machine cost and low materials cost, mask-image-projection-based
stereolithography, as mentioned in chapter 2, has a great potential in anisotropic
reinforcement architecture fabrication.

63
This study presents an electrically assisted MIP-SL AM/three-dimensional (3D)
printing technology for the fabrication of anisotropic reinforcement architecture with
surface modified Multi-walled Carbon Nanotubes (MWCNT-S) aligned along certain
orientation. The aligning orientation is precisely controlled by a rotation stage. The
mechanism of alignment structure for mechanical properties improvement is studied.
This reinforcement architecture provides the anisotropic mechanical properties and offers
more possibilities for the design and fabrication of bio-inspired materials.
The slurry containing MWCNT-S is prepared by following steps. First, 0.5 gram of
MWCNT-OH (Bucky USA. Inc) was first chemically modified by combining the
MWCNT-OH with 30ml 10N sulfuric acid and 1g potassium dichromate for 1h at 80
0
C.
The mixture was filtered and washed with hot and cold water multiple times to remove
the chromic acid and dried at 90
o
C. Then acetone with dissolved 0.5ml
3-aminopropyltriethoxysilane was added in the CNT dispersed in water. After 1h stirring
at 80
o
C, the mixture was filtered and washed by the acetone.
Photocurable resin from Makerjuice (MakerJuice Labs, Kansas, US) with high
photo-sensitivity and mechanical property is chosen as the matrix. MWCNT-S is
combined with the resin by a 2 hours magnetic stirring and then ultrasonic bath for 30
min. The mixture is degassed in vacuum for 30 min.
Figure 3.2a shows the set up process of electrically assisted 3D printing platform,
the transparent glass tank is fulfilled with nanocomposites. The imaging pattern is
controlled by the Digital Micro-mirror Device (DMD) based projection system. The

64
photo curable resin is solidified after the patterns are projected on the bottom of the tank.
The resolution of the DMD chip (Texas Instrument, Dallas, TX) is 1024×768, the output
light intensity of the projector is 3.16 mW/cm
2
.
Direct current voltage, compared with other methods like mechanical forces, shear
flows and magnetic field, is preferred for its easy processibility and high efficiency in the
alignment of CNT

(Steinert, 2009). Two parallel copper electrodes were used with DC
voltages to make sure that the alignment direction is perpendicular to the electrodes
(Figure 3.2c, gaps 3cm, 900V). The relaxation time can be calculated by the equation:
, 3-1
where and are vacuum dielectric constant and anisotropic dielectric constant.
is the matrix viscosity, G is the rotational torque. is the shape factor affected by
aspect ratio D. A photo curable resin with low viscosity (90cp, 20
0
C) serves as the
polymer resin in this study. Because according to the equation 3-1, the relaxation time is
determined by the matrix viscosity and a quick alignment time is expected.
The alignment time of MWCNT-S composite with 1.5wt% filler loading is 60s. The
torque, Coulombic and electrophoresis forces are the three key mechanisms that dominate
the rotation of CNT (Figure 3.2d). The red arrow shows the torque generated by the
polarization of CNT caused by the electric field. As the distribution of charge changes,
the opposite ends of the CNT are charged, which results in Coulomic attraction (blue
arrow). While the surface charge lead to the electrophoresis force (green arrow)
(Takahashi, 2006).
1
(F(D)/3 )G τη
−
=
2
0
/2 GE εε =
0
ε ε
η
F(D)

65

Figure 3.2: (a) Schematic of electric electrically assisted MIP-SL system. (b) Design of
electric electrically assisted MIP-SL method. (c) The electric field distribution controlled
by electrode. (d) The rotation of MWCNT-S under electric field

Comsol Multiphysics is used to demonstrate the electric field distribution that
controls the movement of MWCNT-S (Figure 3.2c). After one layer is fabricated, the
base moves up to separate the cured layer from the tank (Figure 3.2b), the container with
MWCNT-S and parallel electrodes rotated by a stepper motor. During this process,
MWCNT-S has been aligned along a new orientation that is different with the first layer.
Then the base moves down and repeats the pervious process. An accuracy of 0.5 degree
angle is set as the motor’s rotation step, to study its effects on the part. Besides, different
types of electrode such as two needle electrodes and needle-arc electrodes (300V/cm) are

66
used to control the different alignment of carbon nanotubes (Figure 3.3). This special
electrode’s set up lead to circumferential and radial alignments of carbon nanotubes
(Figure 3.9e).

Figure 3.3：Electric field controlled by different types of electrodes

3.2 Fabrication of Menger sponge structure
Menger sponge is a very popular model in mathematics and architecture. The model
with Bouligand structure is designed by Solid Works and the sliced by the ‘DMD-based
SL’ software to get different two-dimensional images. The images controlled by he DMD

67
are projected to the bottom of the tank. After one layer is solidified on the base, the base
move up, and let the following layers to be cured. At the same time, the electric field is
applied in the tank to control the orientation of MWCNT-S nanocomposite and the tank is
rotated by the motor to make sure that the alignment orientation is different with the
former layer. In this case, the movement on Z-axis is from 25 to 100µm and the curing
depth is 180µm with 1s light exposure. The fabrication process for 100 layers part takes
about 25 min. Fig.3.4g shows the microscopy images of the model (5mmx5mm) with
25µm thickness. The lengths of the different squares of the model are 250µm, 750µm and
2mm, respectively. The triangles with different colors show the different positions of the
Menger sponge. The SEM images indicate that there is no damage between adjacent
layers. The cross section is caused by the layer by layer fabrication process, and the layer
structure is uniform under the control of the electrically assisted MIP-SL method. The
results suggest that the interlayer bonding is strong and the method is stable and
repeatable for reinforced architecture ‘s fabrication.

68

Figure 3.4: Schematic diagram of the AM process of electrically assisted MIP-SL. (a)
The Menger sponge model designed by solid works. (b) Interface to control motor and
projector. (c) and (f) Sliced pattern to define the projected images. (d) Diagram of the
electrically assisted printer. (e) Optical image of the fabricated Menger model. (g) Optical
images of more details in (e)

A smaller Menger model (2.4mmx2.4mm) with the 50µm layer thickness is
fabricated to show the increased impact resistance coming from the Bouligand structure.
Schematic (Fig.3.5a) shows the Bouligand structure’s layers form n=1 to n=N+1. The
angle of adjacent CNT’S alignment orientation between n
th
and n+1
th
is α
n
, compared to
the global x-axis. Where n is the layer number. N layer complete 180˚ rotation (αN=180
0
)
through a pitch distance D=Nd. Where d is the layer thickness. Figure 3.5b shows period
of unit cell repetition along the z-axis for N=4. The MWCNT-S’s alignment orientation
rotates along the x-axis and N+1
th
layer has the same alignment orientation with the first
layer. The schematic (Figure 3.5c) shows the Bouligand structure with different N. For
N=1, α=180˚, the alignment orientations of different layers are parallel. For N=4, α=45˚,

69
the aligned CNT completes 180˚rotates when 5
th
layer is fabricated. When rotation angle
α=12˚, the N is equal to 15. When α=6˚, N=30, when α=2˚, N=90. The SEM images of
alignment MWCNT-S bundles are shown in Figure 3.6.

Figure 3.5: Impact resistance test for Menger models with different rotation angles. (a)
Diagram of the rotation in different layers. (b) Rotation period along z-axis with N=4. (c)
Schematic of Bouligand structure with different rotation angles. (d) Microscopic images
of the fraction under compression load. (e) The results of fracture load test for pure resin,
MWCNT-S and aligned MWCNT-S with different N values. (f) Schematic showing the
direction of crack propagation (red lines) and crack arrest (yellow lines). (g) Stress
distribution with different N values simulated by Comsol Multiphasics

70

Figure 3.6: SEM images of fracture surface of Pure polymer and different layers of
printed Menger model with N=90

3.3 Characterization of Menger model with Bouligand structure
The impact resistance of Menger model with Bouligand structure is tested by
Instron-5942 (Dual Column Testing Systems, Instron, Massachusetts, USA). Figure 3.7
shows the set up of the compression tests for the Menger model with different values of
N. The sample was set at the middle of the plate with the compression force
perpendicular to the alignment orientation. The static compression was added with a
velocity of 1mm/min, and a maximum compression of 2mm. The sample deform in the
X-Y plane under the compression along the Z-axis.

71

Figure 3.7: Compression test for the Menger model with Bouligand structure when (a)
N=1, (b) N=4, (c) N=90

The schematic (Figure 3.5f) shows the crack lines in each layer under the
compression test. Usually, the cracks propagate along the path with least resistance (red
lines), and then extend between the aligned CNT. Because of the CNT bundle restricts
the extension of crack. For example, when N=1, the simulation results show that if the
crack in composite happen along the 0˚ direction at the bottom layer, the damage zone in
the subsequent layers will be affected by the stress concentration with the maximum
stress higher than 400kPa (Figure 3.5g). The crack prefers extension along the red lines,
which results in an obvious failure along the compression direction (Figure 3.5d).

72
Therefore, the alignment MWCNT-S results in a decrease of maximum load even
compared with the random distributed CNT nanocomposites. In aerospace industry
standard design and robust baseline architecture (Grunenfelder, 2014), the quasi-isotropic
layup structures (N=4, α=45˚) are widely used. Figure 3.5f show the different crack
extension direction under the compression force, and the yellow line perpendicular to the
alignment CNT is the direction of crack arrest. With the increasing of N, internal stress is
dispersed in a larger zone and the energy is dissipated, and the crack extension path is
redirected and twisted through the thickness of the composite, which results in a tortuous
crack extension path. From the Z direction view (Figure 3.5f), the crack is restricted
throughout the X-Y plane when N=90, while the crack extension lines are along one
direction when N=1. By in-plane spreading the internal stress crack extension, Bouligand
structure successfully prevent the catastrophic propagation of damage through the model.
More details of the compressive strength increasing with the increasing N are shown in
Figure 3.8. The optical images are corresponding with the previous discussion that a wide
in-plane spread of damage and the increased stiffness caused by a smaller fiber rotation
angle. Besides, no delamination between interlayers shown in the figure, which proves
that the interface bonding is strong enough for the compression process. Under a 200kPa
compression, the maximum stress is 400kPa when N=1, while for N=90, the pressure is
300kPa. In addition, because the elastic properties are a function of fiber orientation, the
large value of N results in a smooth change in the in-plane stiffness and can reduce the
shear stress that can cause delamination. Therefore, a model with smaller rotation angle

73
can improve the ability to withstand deflections. The strain also increased as the N
increase.

Figure 3.8: Compressive strength of pure polymer, MWCNT-S and aligned MWCNT-S
with different N of (a) Menger model and (b) cylinder model. SEM image of fractured
Menger model for N=1(c) and Menger model for N=90(d)

3.4 Artificial meniscus with anisotropic mechanical properties
Meniscus has multifunction such as shock absorption, load bearing, lubrication and
nutrition of articular cartilage. Which leave the meniscus susceptible to permanent post
traumatic and degenerative lesions (Tissakht, 1996). The mechanical properties of
meniscus are dependent on its special collagen with fiber-aligned structure along both
circumferential and radial directions (Figure 3.9f). The compression causes the

74
circumferential tensile, which may leads to vertical tear of meniscus. While the radial
deformation causes radial shear forces, which may leads to radial tear (Fox, 2012; Sweigart,
2001) (Fig. 3.9h, Fig. 3.9j) After the vertical or radial tear appears, the performance of
meniscus would degrade. Therefore, both the circumferential and radial modulus should
be enhanced for artificial meniscus in order to prevent the failure. Although meniscus tear
is a very common disease that affects more than 1.5million people through U.S.A and
Europe (Hutchinson, 2014), there is no efficiency method to solve the problem. The risk
of osteoarthritis restricted the meniscectomy surgery while the transplantation is
restricted by the lack of shortages of donors and tissue mismatch. Therefore, more and
more researchers focus on the substitute of the natural menisci and many types of
artificial meniscus have been made. Which is ranging from polymers to silks and aligned
scaffolds. However, an optimized design should take wedge-shaped, circumferential and
radial structure into account, while the biomechanical properties of these artificial
scaffolds is unsatisfactory and the designs only partially consider the circumferential
structure (Moutos, 2007). Because of the high modulus and bio-compatibility of carbon
nanotubes, it is widely used in medical devices to improve the mechanical properties
(Gonçalves, 2015). In this study, MWCNT-S bundle is used to mimic the aligned fiber in
natural meniscus and the bundle is aligned along the circumferential and radial direction
by the electrically assisted MIP-SL method. The artificial meniscus shows the anisotropic
mechanical properties, which are similar to the natural meniscus and offers researcher a
promising replica for the tissue engineering to solve the problem of meniscus tear.

75
Figure 3.9: Schematic of artificial meniscus fabricated by electrically assisted MIP-SL. (a)
Model designed by Solidworks. (b) Model was sliced by interface to generate different
types of patterns. (c) Fabrication process. (d) Sample fabricated by the system. (e)
Microscopy images show the radial and circumferential alignment MWCNT-S. (f)
Human meniscus’s structure and the dog-bone bars for tensile test. (g) Comparison of
tensile modulus in different parts. Simulation results of the vertical tear (h) and radial tear
(j) of the human meniscus. The relative strain of the artificial meniscus with the
reinforcement of aligned MWCNT-S in (i) and (k)

Polymer resin B (3D systems. Inc.) is combined with 1.5 wt% loading of
MWCNT-S to serve as the slurry of artificial meniscus. Compared with the collagen fiber,
CNT shows the biomechanical properties for its excellent mechanical property. The Solid
Works designed human meniscus with wedge shape and semi lunar-shape is firstly sliced
by the DMD-based SL software (Fig.3.9b) to form different patterns. The circumferential

76
alignment is controlled by the needle-arc electrodes to fabricate the first layer. Then the
radial alignment is achieved by two needle electrodes (Figure 3.3). Figure 3.9d shows the
optical images of artificial menisci with two types of aligned MWCNT-S successfully
fabricated by the electrically assisted MIP-SL process. Figure 3.9f shows the oriented
collagen fibers in natural meniscus, which inspires us to fabricate an alternative with
enhanced anisotropic mechanical property. The different colors of dog-bone samples
represent the position on the meniscus for the tensile test. Slices of the artificial meniscus
were cut into tensile samples using a 3D printed dumb bell shaped punch (central width
2mm and central length 10mm.) The velocity of extension and the maximum extension of
the tensile test are 2mm/min and 3mm, respectively. Figure 3.9g shows the increased
circumferential and radial tensile moduli. Then tensile modulus of pure resin B and the
polymer/CNT composite with random distribution (1.5 wt%) are 8.7MPa and 43MPa.
The enhanced circumferential moduli (○
2
,○
3
,○
5
) is of 176MPa, while the circumferential
moduli of human meniscus is 120MPa. The enhanced radial moduli (○
1
,○
4
,○
6
) is of
143MPa, while the radial moduli of human meniscus is 48MPa. As mentioned before, by
preventing the meniscus from crack extension, the aligned carbon nanotubes enhance the
tensile modulus of artificial meniscus. Simulation results show the circumferential
aligned MWCNT-S will prevent the vertical tear (Figure 3.9h and i) and the radial
alignment will prevent the radial and horizontal tears (Figure 3.9j and k). Which indicates
that the artificial meniscus can withstand multiple mechanical stresses and provide
enough mechanical support. The compressive properties and stress-strain curves of the

77
artificial meniscus were studied (Fig. 3.10). Three groups of samples were prepared with
pure polymer B, polymer B/random MWCNT-S and polymer B/aligned MWCNT-S
(circumferential and radial) to test the fracture energy. (Figure 3.11). The results indicate
that the artificial meniscus (0.79MPa) and human’s meniscus (0.69MPa) have similar
modulus. While the tensile strength, fracture energy and strain under rupture of artificial
meniscus are higher than human meniscus. The similar modulus demonstrated that the
artificial meniscus can act as a shock absorber. The higher tensile strength, fracture
energy and strain can increase the tear resistance that prolongs the lifetime of the
meniscus.

Figure 3.10: (a) Schematic of the tensile test sample from the artificial meniscus. (b) The
stress-strain curves of the sample. (c) Compression tests of different parts in the meniscus
and (d) compressive modulus
Besides, the additive manufacturing method offers a novel method to make

78
customized replacement with computer-aided design (CAD) models that can meet the
specific requirements from patient.

Figure 3.11: Schematic of test samples for fracture energy. (a) (b) (c) (d) Samples without
pre-cut crack. (e) (f) (g) (h) Samples with pre-cut crack. (i) Stress-strain curves of these
samples. (j) The results of fracture energy

Here we propose a way to build different bio-inspired structure by controlling the
alignment of MWCNT-S using electric field during 3D printing process. The new
printing process leads us an effective approach to the generation of reinforced
architectures. Bouligand MWCNT-S reinforced composites provide insight into
toughening mechanism and reveal guidelines for the design of new impact resistant
materials. With the precision orientation control of MWCNT-S over the reinforcing

79
architecture, more complex and functional materials created via additive manufacturing
will find application in a wide range of engineering disciplines. The potential usage of
electrically assisted 3D printed meniscus lies in the combination with Computed
Tomography (CT) and Magnetic Resonance Imaging (MRI) scans to print custom-made
individual implants (Kang, 2016). A photocurable bio-ink containing cells might also be
processed with the proposed approach. The ultimate goal is to mimic the native structure
and mechanical properties of the target tissue to develop the printed structures into a
functional tissue.

3.5 Additive manufacturing of optoacoustic transducer
Although many efforts have been made optoacoustic transducer, most of them
focus on the materials and corresponding intensity, bandwidth, and conversion efficiency.
The geometry still limits the application of these transducers. Since additive
manufacturing methods have a significant advantage on fabricating complex shape, the
3D printed optoacoustic transducer can enable more novel applications.
We can estimate a fraction of the thermal energy η within the absorbers after the
laser. A low η value means high conversion efficiency (Baac, 2012).
𝜂=
!
!"
!
!
×[1−𝑒𝑥𝑝 −
!
!
!
!"
] 3.2

where
𝜏
!"

and
𝜏
!

are
the
heat
diffusion
time
and
the
laser
pulse
duration.

Where
𝜏
!"
=𝑑
!
/16𝒳
for
a
cylindrical
structure
with
the
diameter
𝑑
and
the

80
thermal
diffusivity
of
the
surrounding
medium
𝒳.
In
the
regime
of
𝜏
!"
<<𝜏
!
,

absorbers
transfer
most
of
the
thermal
energy
to
the
surrounding
medium.
While 𝜂

reaches
a
high
value
(<100%)
and
the
absorbers
confines
the
most
heat
when

𝜏
!"
>>𝜏
!
.

This
equation
explains
that
why
carbon
nanotube
is
one
of
the
most
popular

absorber
and
why
Polydimethylsiloxane (PDMS)
is
the
widely
used
expanding
layer,

for
optoacoustic
transducer.
Due
to
the
nanoscale
diameter,
heat
transition
from

CNTS
is
rapid
and
efficient,
the
generation
of
thermoacoustic
pressure
is
dominated

by
the
surrounding
PDMS.
The
PDMS
with
a
high
thermal
coefficient
of
volume

expansion
(0.92×10
-‐3
K
-‐1
)
can
generated
high
intensity
ultrasound.

As
mentioned
before,
CNT/polymer
composite
can
be
3D
printed
by
our

stereolithography
method.
However,
the
PDMS
is
still
difficult
to
be
printed
by
this

method.
Two
alternative
elastomers
were
used
to
print
the
optoacoustic
transducer.

The
first
one
is
Molecule
Ra
Rubber
(mUVe
3D
LLC).
The
other
one
is
photocurable

hydrogel.
The
hydrogel
solution
made
by
60
wt%
poly(ethylene
diacrylate)
(PEGDA,

Mw
700,
Sigma-‐Aldirich).
The
1wt%
visible
light
photoinitiator
(Irgacure
819,
BASF)

was
used
to
induce
polymerization.
The
thermal
coefficient
of
volume
expansion
are

about 5
x
10
-‐4
/K
and
2x10
-‐4
/K,
respectively.

The slurry containing MWCNT-S is prepared by the same method mentioned in 3.1
without the assisted electrical field. Two different MWCNT-S/polymer slurrys were
prepared by combined the MWCNT-S with Ra Rubber and hydrogel, respectively.

81
MWCNT-S is combined with the resin by a 2 hours magnetic stirring and then ultrasonic
bath for 30 min. The mixture is degassed in vacuum for 30 min.
Solidworks
was
used
to
design
the
printing-‐focused
optoacoustic
transducer.

The
focus
length
was
3mm
and
the
aperture
was
5mm.
The
thickness
designed
was

of
0.5mm.
The
printed
optoacoustic
transducers
with
MWCNT-‐S/rubber
and

MWCNT-‐S/hydrogel
have
a
similar
shape
shown
in
Figure
3.12.

Figure 3.12: (a) Optoacoustic transducer model designed by Solidworks. (b) Parameters
of the optacoustic transducer. (c) Optical image of the printed transducer

A
6ns
pulsed
laser
beam
with
532nm
wavelength
(Surelite
I-‐20,
Continuum,

Santa
Clara,
CA)
was
used
for
ultrasound
generation.
The
beam
diameter
was
5mm

(shown
in
figure
3.12).
The
laser
beam
passed
through
a
transparent
wall
of
the

82
water
tank
and
shined
on
the
printed
transducer.
The
MWCNT-‐S
absorbed
the
laser

energy
and
conversed
the
energy
to
heat
that
was
then
transferred
to
the

surrounding
rubber
or
hydrogel.
Which
resulted
in
ultrasound
generation.
A

hydrophone
(up
to
30MHz)
was
utilized
to
receive
the
ultrasonic
signals.
The
signals

were
recorded
by
an
oscilloscope
(LC534,
LeCroy
Corp.,
Chestnut
Ridge,
NY).
During

the
acoustic
characterization,
the
hydrophone
was
moved
along
the
propagation

direction
of
ultrasonic
wave
to
measure
the
focus
point
and
the
maximum
acoustic

pressure.
Figure
3.13
shows
the
measurement
setup
of
laser-‐generated
ultrasound.

When
the
hydrophone
was
at
a
position
of
3mm
away
from
the
printed
focus

optoacoustic
transducers,
two
types
of
transducer
had
the
maximum
acoustic

pressure.
The
peak
pressure
generated
from
the
MWCNT-‐S/rubber
transducer
is

0.37MPa,
while
the
peak
pressure
of
MWCNT-‐S/hydrogel
transducer
measured
is
of

0.23MPa
(shown
in
Figure
3.14).
The
dash
lines
showed
the
spectrum
of
the

optoacoustic
transducers.
The
difference
between
two
transducers
may
be
caused

by
different
thermal
coefficient
and
Young’s
Modulus
of
these
materials.
Which
will

be
studied
in
future.

83

Figure 3.13: Optoacoustic transducer and measurement setup

!15$
!13$
!11$
!9$
!7$
!5$
!3$
!1$

!0.4$
!0.3$
!0.2$
!0.1$
0$
0.1$
0.2$
0.3$
Magnitude*(dB)*
Frequency*(MHz)*
Pressure*(MPa)*
Time*(μs)*
a)

84

Figure 3.14: Spectrum of (a) MWCNT-S/rubber transducer and (b) MWCNT-S/hydrogel
transducer

!25$
!20$
!15$
!10$
!5$
0$

!0.26$
!0.16$
!0.06$
0.04$
0.14$
0.24$
Magnitude*(dB)*
Frequency*(MHz)*
Pressure*(MPa)*
Time*(μs)*
b)

85
3.6 Summary
An electrically assisted MIP-SL method was developed to fabricate the reinforced
architecture with anisotropic mechanical properties. The Menger sponge model with
bio-inspired Boulgand structure is successfully fabricated by this method. Aligned
MWCNT-S in Bouligand structure mimics the collagen or chitin fibers. The maximum
load is obviously increased due to the fracture resistance of the Bouligand pattern. The
smaller rotation angle of Bouligand structure results in a greater energy dissipation and a
higher impact resistance. What is more, the artificial meniscus with improved mechanical
properties is fabricated using the electrically assisted AM process. Therefore, this study
enables the design and fabrication of reinforced architectures with arbitrary 3D
geometries and anisotropic properties, which provides tremendous possibilities for
bio-inspired structure, aerospace engineering and tissue engineering.
Optoacoustic transducers were fabricated by the same method without assisted
electrical field. Two different photocurable resins enabled the 3D printed transducer and
the corresponding laser-generated ultrasound. These transducers have great potential in
cell manipulation, drug delivery and high intensity focused ultrasound.

86
Chapter 4 Summary and future work
4.1 Summary
In this research, a novel additive manufacturing process named
Mask-Image-Projection-based Stereolithography (MIP-SL) has been employed and
further developed for composites fabrication. Three types of composites were designed
and fabricated for piezoelectric and reinforced architecture. The results demonstrated the
possibilities for biomedical applications.
For piezoelectric application, piezoelectric composite with photocurable resin and
nanoparticle was fabricated into ultrasonic transducer by the MIP-SL process. A specific
post-processing involving debinding and sintering process was employed. The sample
after sintering shows piezoelectric effect. The Ex vivo porcine eyeball image and the
tungsten wire phantom image demonstrate the MIP-SL fabricated single element and
array transducer have great potential in biomedical imaging and energy transform.
For reinforced architecture, an electrically MIP-SL system has been employed to
fabricated MWCNT-S composite. Controlled by the electric field, the MWCNT-S can be
aligned along different orientations. A Menger sponge model with Bouligand structure
was fabricated by the method. The maximum load is obviously increased for the fracture
resistance of Bouligand pattern. What is more, the artificial meniscus with improved
mechanical properties is fabricated using the same AM process. Therefore, this study
enables the design and fabrication of reinforced architectures with arbitrary 3D

87
geometries and anisotropic properties.
For optoacoustic transducer, the MIP-SL system without electrical field has been
used for MWCNT-S composite. These printing focused transducer successfully
conversed laser energy into acoustic wave. The maximum acoustic pressure can be 0.37
MPa. This study shows that additive manufacturing of optoacoustic transducer can bring
more possibilities to ultrasonic applications

4.2 Future work
Novel piezoelectric array, composite and metamaterials, in which the piezoelectric
response of the materials originates from their special geometries or structures, will be
proposed in future. The MIP-SL method will be conducted to design and fabricated the
piezoelectric/polymer composite. The piezoelectric materials with complex geometry can
be used to realize ultrasonic imaging application and enhance the piezoelectric properties.
Carbon-nanotube (CNT)-polymer composites can enable laser-generated ultrasound.
The high-amplitude ultrasound, going into a therapeutic regime, is obtained due to an
efficient energy conversion process by the CNT-composites and a high focal gain in the
optoacoustic lens platform. However, limited by the thermal expansion coefficient of
existing photocurable materials, the acoustic pressure is low. Hence, a MIP-SL method
for materials with high thermal expansion coefficient such as PDMS will be study for 3D
printed optoacoustic transducer with high intensity laser-generated ultrasound.

88
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Abstract (if available)

Abstract Additive manufacturing processes have obvious advantages on saving time and cost, and enabling the complex geometry. Stereolithography is one of the most popular AM technologies for its merits on fabricating speed, surface finish, manufacturing cost and applicable scale. ❧ Composite materials and structure are very critical for architectural, biomedical application and artist design. Although composites like polymer/ceramcis and polymer/dielectrics were successively fabricated by stereolithography, the solid loading and properties restrict the application. ❧ This research employed a novel Mask-Image-Projection-based Stereolithography (MIP-SL) technology to fabricate the piezoelectric ceramics. After post-prosessing, the density of 5.64g/cm³ was obtained, which corresponds to 93.7% of the density of bulk BaTiO3 (6.02 g/cm³). The printed ceramic exhibits a piezoelectric constant and relative permittivity of 160 pCN⁻¹ and 1350 respectively. Different types of transducer with printed piezoelectric ceramics were fabricated. The printed single element transducer realized the energy focusing and ultrasonic sensing. The annular array transducer with 4 elements improved the lateral resolution and signal-to-noise ratio. ❧ An electrically assisted MIP-SL system is reported to fabricate the reinforcement architectures with anisotropic properties. Multi-walled Carbon Nanotubes (MWCNT) was combined with photocurable resin to serve as printable slurry. A bio-mimic Bouligand structure and an artificial meniscus with improved mechanical properties were fabricated using this AM process and the printable slurry. ❧ Besides, an optoacoustic transducer can be printed by the system using MWCNT and photocurable resin. This transducer can converse laser energy to ultrasonic wave. The maximum acoustic pressure measured was 0.37MPa.

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Asset Metadata

Creator Chen, Zeyu (author)

Core Title Additive manufacturing of piezoelectric and composite for biomedical application

School Viterbi School of Engineering

Degree Doctor of Philosophy

Degree Program Biomedical Engineering

Publication Date 11/16/2017

Defense Date 11/16/2017

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

Tag additive manufacturing,carbon nanotube,OAI-PMH Harvest,photoacoustic transducer,piezoelectric,ultrasonic transducer

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Zhou, Qifa (committee chair), Shung, Kirk (committee member), Wu, Wei (committee member)

Creator Email zeyuchen@usc.edu

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c40-456235

Unique identifier UC11266426

Identifier etd-ChenZeyu-5925.pdf (filename),usctheses-c40-456235 (legacy record id)

Legacy Identifier etd-ChenZeyu-5925.pdf

Dmrecord 456235

Document Type Dissertation

Rights Chen, Zeyu

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

Repository Location USC Digital Library, University of Southern California, University Park Campus MC 2810, 3434 South Grand Avenue, 2nd Floor, Los Angeles, California 90089-2810, USA