Ping Du's Research Highlight

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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

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Ping Du's Research Highlight

  1. 1. Research Highlight Ping Du, PhD Mechanical Engineering Boston University May 2013
  2. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 17. Conducting Polymer (1) 17/30 Actuation animation Work density Experiment setup Voltage and Current
  18. 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. 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. 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. 21. SiOC (1) 21/30 EDX spectra of SiOC films SEM Residual stress Scale bar: 100nm Thickness reduction FTIR spectra of SiOC films
  22. 22. SiOC (2) Modulus Hardness 22/30 FTIR peak shift
  23. 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. 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. 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. 26. CAD (3) Certified SolidWorks Associate (CSWA) 26/30
  27. 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. 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. 29. Questions and Comments 29/30

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