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

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Presentation on theme: "Geometrically Optimized mPAD Device for Cell Adhesion Professor Horacio Espinosa – ME 381 Final Project Richard Besen Albert Leung Feng Yu Yan Zhao Fall."— Presentation transcript:

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 Geometric and Mechanical Analysis  Bending Stress  Bending Moment H

11 Fall 2006ME 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 Optimization Criterion  Maximization of post density  Minimization of spring constant

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

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

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

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

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

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

20 Fall 2006ME 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 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 m Height H 4 m Distance between posts L 2 m Aspect ratio3.2945

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

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

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

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

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

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

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

34 Fall 2006ME Thank You!  Questions?


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