My research at Boston University (May 2013) 1. Thesis: Viscoelastic testing and modeling of PDMS micropillars for cellular force measurement 2. Side Projects 1) Conducting polymer actuators 2) PDMS and conducting polymer nanowire composites 3) Silicon oxycarbide thin films 4) Tribological study of DLC coatings

1. Research Highlight
Ping Du, PhD
Mechanical Engineering
Boston University
May 2013

2. Outline
 Thesis
– Viscoelastic testing and modeling of PDMS micropillars
 Side Projects
– Conducting polymer actuators
– PDMS and conducting polymer nanowire composites
– Silicon oxycarbide thin films
– Tribological study of DLC coatings
 CAD and FEA Experience
2/30

3. 3/30
Viscoelastic Testing and
Modeling of PDMS Micropillars
• Measure the viscoelastic properties of PDMS.
• Developed enhanced cantilever beam bending model.
• Application to PDMS micropillar transducer for cellular
force measurement
⃰ Ping Du et al., Journal of Microelectromechanical Systems, 22, 44-53, 2013.
⃰ Ping Du et al., Applied Physics Letters, 99, 083701, 2011.
⃰ Ping Du et al., Journal of Micromechanics and Microengineering, 20, 095016, 2010.

4. Background
4/30
 Cells: complex entities
1. Sense cues:
respond to stimuli: beneficial, harmful
2. Regulate cell functions
division, growth, apoptosis, migration, etc.
3. Biomechano-transductions: mechanical forces
develop micro/nano sensors, medical devices.
4. Detection of interactions
unique index to probe trivial changes in cells
5. Application fields
physiology, medicine, cell biology.
C.S. Chen, J. Cell Sci., 2008.
X. Zheng and X. Zhang, JMM, 2011.

5. Background
5/30
 Methods to measure sub-cellular forces
Wrinkles on thin film (Harris, 1980)
Embedded beads (Dembo, 1999)
Shallow markers (Balaban, 2001)
High pillars (Tan, 2003)

6. Motivation
6/30
 Polydimethylsiloxane (PDMS)
- bio-compatibility, mechanical compliance,
optically transparency.
- fabrication with ease (low cost, high fidelity)
 PDMS micropillars
- behave as simple cantilever beams, bends
upon cell contraction.
- direction and magnitude of force: deflection
on top of pillars.
Traditional
Enhanced
Geometry
Euler beam
(high aspect ratio)
r=L/d > 10
Timoshenko beam
(low aspect ratio)
Material property
Linear elastic
Linear viscoelastic

7. Research overview
7/30
Young’s relaxation
modulus
Time
domain
Viscoelastic
Timoshenko beam
Case study
Stress relaxation
nanoindentation
Micro-beam
bending test
Loading rate effect
PDMS
Cellular contraction
Freq
domain
Complex modulus
Cellular force
Fourier series
Dynamic
nanoindentation
Finite element
analysis

8. Time domain: Relaxation modulus
 Stress relaxation test
8/30
 Young’s relaxation modulus of PDMS
Hysitron TI 900 Triboindenter
N
Transducer
E (t )  E   E j e
 j t
(𝝀 𝒊 = 𝟏/𝝉 𝒊 )
j 1
Microscope
tj (sec)
Ej (kPa)
X-Y moving stage
10-1.5
201.8 58.2 53.7 31.2 25.7
134
1500
Load (nm)
Load (mN)
Test 1
Test 2
Test 3
Test 4
Holding
2000
Disp (nm)
1
10
100
136
2500
Loading
1000
Unloading
500
0
0.1
132
130
128
126
124
0
20
40
60
Time (sec)
80
100
Holding
0
20
40
60
80
Time (sec)
100
120

9. Time domain: Viscoelastic Timoshenko beam model
9/30
Combine low aspect ratio and viscoelasticity
N E
3IAv0
 t
j
P(t ) 
[ E t   (1  e j )]
L[ AL2  6aI (1  )]
j 1  j
200 mm
12
Force, ( N)
Force, P Pm(m N)
10
10
8
8
a : shear coefficient
G: shear modulus
Experiment
Elastic Euler
Elastic Timoshenko
Viscoelastic Euler
Viscoelastic Timoshenko
Reaction force predictions
from different formulas
a) At the same loading rate (250 nm/s)
6
6
4
4
Euler: overestimate
(violate the slender beam)
2
2
100 mm
0
0
0
0
500
500
10
10
1000
1500
Deflection,  (nm)
1000
1500
Deflection,  (nm)
2000
2500
2000
1000 nm/s 500 nm/s
2500
250 nm/s
Force, P (m (m N)
Force, P N)
8
b) At different loading rates
8
6
6
Viscoelastic: loading rate dependent
4
4
2
Experiment
Elastic Timoshenko
Viscoelastic Timoshenko
2
0
0
0
0
2
2
4
6
4
6
Time, t (sec)
Time, t (sec)
8
8
10
10

10. Time domain: Application to cardiac myocytes
10/30
In-situ force probing system for living cells
Feedback controller
Liquid pump
Heating
rod
Inlet
Thermometer Vacuum
pump
Outlet
Perfusion chamber
PDMS
chip
Waste solution
Buffer
solution
Computer system
For imaging analysis
Micropillar displacements
Displacement (m m)
0.1
0
10
-0.1
9
Stress (KPa)
EE
VT
3
32.2
Aspect ratio, r=L/d
1
-3
10
32.6
Time (sec)
33
33.4
-2 0
1
4
-2
10
-1
-5
-3.5
31.8
-3 0
2
-3
1.5
5
5
0 15
2
Zone 1
-10
-8
Relaxation
-10
3
6
-5
3.5
7
-8
4
-20
8
Contraction
-3 0
-0.2
4.5
2.5
-2.5
(c)
PEE  PVT
 100%
PVT
Parametric study Diff 
-5
(b)
0.2
Stress (KPa)
(a)
CCD
camera Inverted
microscope
0
10
10
Loading rate, v0 (m m/s)
-3 0
Zone 2 -30
-20 -2020
-10 -1010
-8 -8-8
-5 0- 5
0 -5 0
5 5
1
2
10
10

11. Freq. domain: Complex modulus (1)
 Dynamic nanoindentation
11/30
 Instrument dynamics
Agilent G200 Nanoindenter
Herbert, J. Phys. D, 2008
Coil/magnet
assembly
K
Leaf spring
𝑃0 𝑒 𝑖𝜔𝑡 = 𝑚ℎ + 𝐷ℎ + 𝐾ℎ
D
ℎ 𝑡 = ℎ0 𝑒 𝑖(𝜔𝑡−𝜙)
Capacitance
gauge
 Material model for dynamic indentation
- Black box: no constitutive law involved
- General formula: applicable to all linear
viscoelastic solids.
3.35
3.34
3.33
3.31
12.18
12.16
12.14
12.12
12.1
12.08
12.06
3.3
3.29
240
242
244
246
Time (s)
248
250
Disp (m m)
Disp (mm)
Force (mN)
3.32
𝐸
Test
sample
′
1 − 𝜈 2 ∆𝑃0
𝜔 =
cos 𝜙
2𝑅 ∆ℎ0
1 − 𝜈 2 ∆𝑃0
𝐸" 𝜔 =
sin 𝜙
2𝑅 ∆ℎ0

12. Freq. domain: Complex modulus (2)
1. Compare to previous results
12/30
2. Mathematical expression
Generalized Maxwell model
𝑁
𝐸 𝜔 =
𝐸∞ +
𝑗=1
Conte (flat)
𝐸 𝑗 𝜔2
+𝑖
𝜆2 + 𝜔 2
𝑗
𝑁
𝑗=1
𝐸𝑗 𝜆 𝑗 𝜔
𝜆2 + 𝜔 2
𝑗
Conte (berk)
1
2
3
4
5
i (1/sec)
0.1
1
10
100
1000
Ei (kPa)
Du (flat, time) Du (flat, freq)
i
2.2×10-11
18.4
94.1
119.1
742.3
1.1
E'-Du (flat,time)
E''-Du (flat,time)
E'-Du (flat,freq)
E''-Du (flat,freq)
E'-Conte (berk)
E''-Conte (berk)
E'-Conte (flat)
E''-Conte (flat)
1. C.C. White et al., J. Poly. Sci. B, 2005
2. N. Conte, V. Jardret, MRS Proc. 2001.
E' (MPa)
0.9
0.3
0.8
0.25
0.2
0.7
0.15
0.1
0.05
1
10
2
10
Angular freq (rad/s)
Angular freq. (rad/s)
0
Loss factor
Loss tangent
1

13. Freq. domain: Application to cellular force
 Cellular contraction data
 Contraction force
from FEA simulation
- Decompose to Fourier series: sum of
trigonometric functions with different
amplitudes and frequencies.
c e
k 0
i
2nk
N
k
N 1
ck  FFT[ f n ]   f n e
i
2jk
N
𝑁−1
𝑓𝑘 𝑒
𝑖
2𝜋𝑘𝑡
𝑇
𝑘=0
j 0
 Two representative states
(a)
3 min: stimulated state, much regulated contraction.
7 min: desensitized state.
Power spectra of FFT coefficients ck
20
3 min
15
10
0.3
5
0.2
0
0.1
Force (nN)
Force (nN)
N 1
1
𝐹 𝑡 =
𝑁
Disp (m m)
1
yn 
N
13/30
0.4
0.3
0
3 min
7 min
3.62 Hz
0.25
Disp (m m)
Power
(b)
0.2
0.15
0.1
0.05
0
0
1.84 Hz
Nyquist freq
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
Time (sec)
7 min
15
10
0.3
5
0.2
0
0.1
0
10
20
30
40
Freq. (Hz)
50
60
0
0.2 0.4 0.6 0.8 1
Time (sec)
Time (sec)
1.2 1.4 1.6
Force (nN)
Force (nN)
0.35

14. Conclusion
14/30
 A comprehensive characterization was conducted on the viscoelastic properties
of PDMS in both time domain (relaxation modulus) and frequency domain
(complex modulus) using advanced nanoindentation techniques.
 Developed an enhanced viscoelastic Timoshenko beam formula to investigate
the effects of loading rate and pillar aspect ratio on the cellular contraction force
calculation.
 Converted the cyclic cardiac myocytes contraction into Fourier series, and
simulated the contraction force in the frequency domain by finite element
analysis.
 Publications during PhD study.
 6 peer-reviewed journal papers published, 1 paper under review.
 24 conference proceeding papers and posters
(Transducers, MicroTAS, MRS, etc.).

15. Acknowledgments
 Advisor: Dr. Xin Zhang.
 Committee members.
 NSF grants CMMI-0826191, CMMI-0700688
 Photonics Center at Boston University (BU)
 Dr. Catherine Klapperich from BU
 Dr. Zhiyong Gu from University of Massachusetts at Lowell
 All previous and current LMST members
 Mr. Chen Cheng (UT Dallas), Mr. Ronnie Cooper (Hysitron), Mr. Jim Mason
(Solartron Analytical)
 All other people who have kindly helped me over the years
 All my families
15/40

16. 16/30
Conducting Polymer
Actuators
• Conducting polymer is a novel actuator material: low activation
voltage, large strain, operating in liquid, bio-compatibility.
• Developed a multilayer model for the trilayer bending actuator.
• Studied the effect of modulus and thickness of each layer.
* Ping Du et al., Sensors and Actuators A: Physical, 163, 240-246, 2010.

17. Conducting Polymer (1)
17/30
Actuation animation
Work density
Experiment setup
Voltage and Current

18. 18/30
PDMS and Conducting
Polymer Nanowire Composites
• Enhance the electrical responses of PDMS through
incorporation of conducting polymer nanowires, while
maintaining the desirable mechanical flexibility.
• Studied the effect of nanowire concentration on the
dielectric constant and elastic modulus of composites.
* Ping Du et al., Journal of Physics D: Applied Physics, 46, 195303, 2013.

19. Conducting Polymer (2)
Nanowire synthesis
19/30
Dielectric constant
of composites
Relaxation modulus
of composites
Percolation model
Mixture model
SEM of nanowires

20. 20/30
Silicon Oxycarbide Films
• Add silicon carbide into silicon oxides to improve the
mechanical properties, thermal stability, and chemical
resistance.
• Study the effect of carbon content and post-thermal
annealing temperature on the residual stress, modulus
and hardness of SiOC films.
* Ping Du et al., Sensors and Actuators A: Physical, 176, 90-98, 2012.

21. SiOC (1)
21/30
EDX spectra of SiOC films
SEM
Residual stress
Scale bar: 100nm
Thickness reduction
FTIR spectra of SiOC films

22. SiOC (2)
Modulus
Hardness
22/30
FTIR peak shift

23. Tribological study on DLC coatings (Entegris)
 Scratch test (linear mode)
– Critical load: normal load at which a particular failure mode between the
coating and substrate initiates.
– Evaluation methods: microscope, friction force, acoustic emission.
 Wear test (rotary mode)
– Coefficient of friction: ratio of friction force to normal force
– Wear rate: the ratio of the volume of removed debris to the work done
by friction force.
23/30

24. CAD (1)
Design Project: Automated Loading Machine for Microtiter Plates
Precision Machine Design and Instrumentation (MN560)
Chien-Hsin Chen (chchen@bu.edu)
Ping Du (pdu@bu.edu)
Nan Shao (nshao@bu.edu)
24/30

25. CAD (2)
25/30
Custom-made test fixture in accordance to the ASTM Standard D150
BNC Connector
Teflon
(insulator)
Base
Top electrode
(micrometer)
Bottom
electrode
Guard
ring

26. CAD (3)
Certified SolidWorks Associate (CSWA)
26/30

27. FEA (1)
27/30
Cross section distortion in
circular beam
Penetration effect of wedge indenter
R=1.57 mm
=2.5 mm
Original position
Indenter
Deformed position
Dynamic micropillar bending
PDMS
1) Element: C3D10 (10-node quadratic tetrahedron)
2) Boundary condition: cellular contraction data
3) PDMS modulus: complex modulus E(w)
4) Direct-solution steady-state dynamic analysis

28. FEA (2)
 Projects at Medtronic
Numerical modeling support (ABAQUS, ANSYS) for various devices and
manufacturing process development.
• Characterize the elastic/hyperelastic and viscoelastic properties of
common rubbers/plastics used in medical devices; evaluated their effects
on the critical component performance during the device life time.
- Impact of plastic housing complex modulus in the fatigue life of
feed-through wires under cyclic loadings.
- Relaxation of seal contact pressure and creep in surrounding plastic
components during 10 years.
- Weld strength of coils and failure prediction of lead/catheter during
aggressive tensile and bending tests.
• Superelastic behavior of shape memory alloy (Nitinol) components.
• Progressive sheet metal forming process under large plastic deformation.
• Molten solder flow and heat transfer (ANSYS CFX) for laser soldering of
circuit board.
28/30

29. Questions and Comments
29/30