Geometrically Optimized mPAD Device for Cell Adhesion

1. Geometrically Optimized mPAD Device for Cell Adhesion Professor Horacio Espinosa – ME 381 Final Project Richard Besen Albert Leung Feng Yu Yan Zhao Fall 2006

2. Introduction Cellular Adhesion Force • For a cell to move, it must adhere to a substrate and exert traction • Traction forces are concentrated at focal points between the cell and substrate Cellular Functions Biological Mechanism ME 381

3. Cellular Adhesion Video ME 381

4. Literature Review Adhesion Force Measurement Continuous Substrate Method Wrinkle Method • Sensitive to nano-Newton forces • Force calculations difficult because of complexity of wrinkle pattern • Model does not show adhesion force focal points ME 381

5. Literature Review Adhesion Force Measurement Continuous Substrate Method Gel imbedded with fluorescent markers • Highly sensitive to adhesion forces • Markers aid in optical detection of surface deformation • Difficult to manufacture uniform fluorescent marker pattern ME 381

6. Proposed Design Adhesion Force Measurement mPADs (micro Pillar Array Detectors) • Discrete individual force sensors • Direct calculations from cantilever deflection theory • Highly detailed force vector field • Precise and simple manufacturing ME 381

7. Proposed Design Adhesion Force Measurement Customization mPAD design depends on the type of cell being used Variable Parameters: • Material Selection • Aspect ratio • Pillar density • Cell to pillar contact area ME 381

8. Proposed Design Adhesion Force Measurement mPAD Sensing • Pillar is modeled as a cantilever beam with uniform diameter • Pillar geometry, quantity of pillars per area, material choice can be modified to match known ranges of a cell’s adhesion force • Force vector field shows magnitude and direction of discrete forces exerted by the cell on the array ME 381

9. Geometric and Mechanical Analysis • Force and Displacement • Area Percentage ME 381

10. Geometric and Mechanical Analysis • Bending Stress • Bending Moment H ME 381

11. Optimization • Material: • Flexible to cell adhesion forces • Optically measurable displacements • Geometry and Spatial Arrangement: • Minimize cell flow down sides of posts • Detailed vector field representation • Manufacturable ME 381

12. Optimization Criterion • Maximization of post density • Minimization of spring constant   ME 381

13. Optimization Theory • Cost function: • Optimization Problem: • Lagrangean: C1, C2- Weighting Coefficients subject to ME 381

14. Constraints • System Dynamics: • Material: • Properties: • Yield Stress: ME 381

15. Height (H) 4 μm -150 μm Diameter (D) 100 nm – 5 μm Distance between posts (L) >2Δmax Constraints continued • Spatial & Geometric Parameters: • Optical Resolution: R=50nm ME 381

16. Optimization trends Density as a function of diameter holding height constant at 4m ME 381

17. Optimization trends continued Density as a function of the distance between adjacent posts holding diameter constant at 1.2141 m ME 381

18. Optimization trends continued Spring constant as a function of diameter holding height constant at 4m ME 381

19. Optimization trends continued Spring constant as a function of post height holding diameter constant at 1.2141m ME 381

20. Optimization trends continued Spring constant as a function of distance between adjacent posts where K=2Fmax/L and Fmax=10nN ME 381

21. Results ME 381

22. Materials • PDMS - polydimethylsiloxane • Desirable chemical, physical, and economic properties ME 381

23. Chemical Properties • Cell friendly • Chemically inert • Thermally stable • Non-toxic • Can be made hydrophilic for adhesion purposes ME 381

24. Physical Properties • Extremely flexible • (.87MPa < E < 3.6MPa) • Scalability • Conforms to nano-scale structures • Necessary for micro-molding • Transparent within visible spectrum • Cheap! Around $50 per pound to process • Adjustable stiffness and aspect ratio based on mixing ratio and curing time ME 381

25. Mask Oxide Si substrate UV light Mask and pattern 1 μm photoresist using UV lithography Mask 1 – quartz plate with 800Å chromium layer Photoresist Transfer pattern to mask oxide with HF isotropic etching Microfabrication Deposit mask oxide with LPCVD (SiO2) ME 381

26. Passivation oxide Deposit .3 μm passivation oxide with PECVD After vertical oxide etch, deep Si etch alternating with passivation Microfabrication (cont’d) • Bosch Process First deep anisotropic silicon etch (DRIE) with Cl2/BCl3 ME 381

27. mono-Si base substrate Cured PDMS structure soft bonded to mono-silicon substrate (E ~ 100 GPa), removed from mold Microfabrication (cont’d) • Micromolding Liquid PDMS prepolymer Liquid PDMS poured into silanized micromold ME 381

28. Defects • Scalloping from imperfect etch selectivity in DRIE (~100 nm) • Variable diameter (conic shape) ME 381

29. Preparation and Fluorescent Labeling • Oxidize structure in air-plasma to make hydrophilic • Create flat PDMS stamps for top of each pillar • Microcontact print fluorescent label • Coat pillars and stamps in adhesive ME 381

30. mPAD Calibration • Spring Constant (K) • AFM Curves • Young’s Modulus (E) • Compression • Height/Diameter • SEM analysis ME 381

31. Optical Sensing • Phase-Contrast Microscopy • Epifluroescence Microscopy ME 381

32. Optical Sensing (cont’d) • Pillar Deflection Detection • Force Analysis Package ME 381

33. Future Studies • 3D Analysis – Software improvements ME 381

34. Thank You! • Questions? ME 381