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. ME 3812 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

3. Fall 2006ME 3813 Cellular Adhesion Video

4. Fall 2006ME 3814 Literature Review 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 Adhesion Force Measurement

5. Fall 2006ME 3815 Literature Review 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 Adhesion Force Measurement

6. Fall 2006ME 3816 Proposed Design mPADs (micro Pillar Array Detectors)  Discrete individual force sensors  Direct calculations from cantilever deflection theory  Highly detailed force vector field  Precise and simple manufacturing Adhesion Force Measurement

7. Fall 2006ME 3817 Proposed Design Customization mPAD design depends on the type of cell being used Variable Parameters:  Material Selection  Aspect ratio  Pillar density  Cell to pillar contact area Adhesion Force Measurement

8. Fall 2006ME 3818 Proposed Design 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 Adhesion Force Measurement

9. Fall 2006ME 3819 Geometric and Mechanical Analysis  Force and Displacement  Area Percentage

10. Fall 2006ME 38110 Geometric and Mechanical Analysis  Bending Stress  Bending Moment H

11. Fall 2006ME 38111 Optimization  Material: 1. Flexible to cell adhesion forces 2. Optically measurable displacements  Geometry and Spatial Arrangement: 1. Minimize cell flow down sides of posts 2. Detailed vector field representation 3. Manufacturable

12. Fall 2006ME 38112 Optimization Criterion  Maximization of post density  Minimization of spring constant

13. Fall 2006ME 38113 Optimization Theory  Cost function:  Optimization Problem:  Lagrangean: subject to C 1, C 2 - Weighting Coefficients

14. Fall 2006ME 38114 Constraints  System Dynamics:  Material: 1. Properties: 2. Yield Stress:

15. Fall 2006ME 38115 Constraints continued  Spatial & Geometric Parameters:  Optical Resolution: R=50nm Height (H)4 μm -150 μm Diameter (D)100 nm – 5 μm Distance between posts (L) >2Δ max

16. Fall 2006ME 38116 Optimization trends Density as a function of diameter holding height constant at 4m

17. Fall 2006ME 38117 Optimization trends continued Density as a function of the distance between adjacent posts holding diameter constant at 1.2141 m

18. Fall 2006ME 38118 Optimization trends continued Spring constant as a function of diameter holding height constant at 4m

19. Fall 2006ME 38119 Optimization trends continued Spring constant as a function of post height holding diameter constant at 1.2141m

20. Fall 2006ME 38120 Optimization trends continued Spring constant as a function of distance between adjacent posts where K=2F max /L and F max =10nN

21. Fall 2006ME 38121 Results Canine Kidney Cell Forces F1-10nN Young’s Modulus E PDMS 2MPa Spring constant K.0100 N/m Minimum deflection Δ min.1 m Maximum deflection Δ max 1 m Diameter D 1.2141 m Height H 4 m Distance between posts L 2 m Aspect ratio3.2945

22. Fall 2006ME 38122 Materials  PDMS - polydimethylsiloxane Desirable chemical, physical, and economic properties

23. Fall 2006ME 38123 Chemical Properties  Cell friendly Chemically inert Thermally stable Non-toxic Can be made hydrophilic for adhesion purposes

24. Fall 2006ME 38124 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

25. Fall 2006ME 38125 Mask and pattern 1 μm photoresist using UV lithography UV light Photoresist Microfabrication Deposit mask oxide with LPCVD (SiO 2 ) Mask Oxide Si substrate Transfer pattern to mask oxide with HF isotropic etching Mask 1 – quartz plate with 800Å chromium layer

26. Fall 2006ME 38126 Microfabrication (cont’d) First deep anisotropic silicon etch (DRIE) with Cl 2 /BCl 3  Bosch Process Passivation oxide Deposit.3 μm passivation oxide with PECVD After vertical oxide etch, deep Si etch alternating with passivation

27. Fall 2006ME 38127 Microfabrication (cont’d)  Micromolding Liquid PDMS poured into silanized micromold Liquid PDMS prepolymer Cured PDMS structure soft bonded to mono-silicon substrate (E ~ 100 GPa), removed from mold mono-Si base substrate

28. Fall 2006ME 38128 Defects  Scalloping from imperfect etch selectivity in DRIE (~100 nm)  Variable diameter (conic shape)

29. Fall 2006ME 38129 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

30. Fall 2006ME 38130  Spring Constant (K) AFM Curves  Young’s Modulus (E) Compression  Height/Diameter SEM analysis mPAD Calibration

31. Fall 2006ME 38131 Optical Sensing  Phase-Contrast Microscopy  Epifluroescence Microscopy

32. Fall 2006ME 38132  Pillar Deflection Detection  Force Analysis Package Optical Sensing (cont’d)

33. Fall 2006ME 38133 Future Studies  3D Analysis – Software improvements

34. Fall 2006ME 38134 Thank You!  Questions?