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A novel MEMS platform for a cell adhesion tester. Ethan Abernathey Jeff Bütz Ningli Yang Instructor: Professor Horacio D. Espinosa ME-381 Final Project, Dec 1, 2006. Overview. Basic design Advantages of biaxial testing Stretcher coatings Manufacturing process Force calculation - PowerPoint PPT Presentation
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A novel MEMS platform for a cell adhesion tester
Ethan Abernathey Jeff Bütz
Ningli YangInstructor: Professor Horacio D. Espinosa
ME-381 Final Project, Dec 1, 2006
Overview
Basic design Advantages of biaxial testing Stretcher coatings Manufacturing process Force calculation Air and water operation Summary
Structure
Return
X structure applies biaxial force
3 lower sections move (top stationary)
Driven by single comb drive actuator
Operation
Biaxial displacement within 5% Displacement measured with optical microscope 60 μN at driving voltage of 100 V 3.4 μm displacement at 100 V (shown below)
Advantage of Biaxial Testing
Uniaxial testing causes large elongation
Stiffness may decrease with elongation
Biaxial testing allows for much smaller displacement and avoids decreasing stiffness
Advantage of biaxial testing
Possible method of displacement affecting cell stiffness
Buckling of inner cytoskeleton causes a more linear response
Advantage of Biaxial Testing
Linear response seen in graph A Biaxial stretching can stop this behavior by
eliminating lateral strain, seen in graph B
Stretcher Coating
Required force for cell detachment can be reduced
Coating with 1-dodecanethiol (DDT), 1-hexadecanethiol (HDT) and 1-octadecanethiol (ODT) on Au substrate can decrease detachment force (curve peaks)
Microfabrication
Based off of the PolyMUMPS fabrication process.
PolyMUMPS – Multi-User MEMS Processes
Provides fabrication of cost-effective, proof-of-concept MEMS devices
Multi-step process utilizing interchanging layers of polycrystalline silicon and a sacrificial layer (in this case Phosphosilicate Glass)
Doping and Insulation
n-type Si wafer is doped further to prevent charge feedthrough
An insulating layer of Si3N4 is deposited using LPCVD
PolyS 0
Initial layer of Polycrystalline Silicon (PolyS 0) deposited with LPCVD
Photolithography to create support posts for device
Positive mask along with RIE to make pattern
PSG 1 Sacrificial layers of
PhosphoSilicate Glass (PSG) are used to provide intermediate layers
Can be patterned to surround the PolyS 0 features
Eventually will be removed to release structure
PolyS 1
New layer of PolyS added in order to build the suspended cell stretching platform
Transverse bar seen at bottom of mask is actually connected to comb drive actuator
PSG 2
Second sacrificial layer applied and patterned to surround platform features
Will provide support for final layer of PolyS
PolyS 2
This layer of PolyS creates the linkage arms for the device
Four separate arms are used to connect the platform quadrants
Final Outcome
Side and Top Views
Linkage to comb drive can be observed
Comb Drive Actuator Connects to the transverse
bar of the test device
PolyS and PSG labels are the same as for test device fabrication
Analogous process to Cell Stretcher
Begins on same doped and insulated wafer
PolyS 0
PolyS 0 layer creates the stator bases and the posts for the folded springs
PSG 1
PSG is used to provide support for main comb drive structure
PolyS 1
PolyS 1 layer creates both the rotor and stator heads and combs
Folded springs also come from PolyS 1 layer
Final Release of Device
The device is ready for release after PolyS 2 layer is applied
A ~49% HCl mixture in water is most effective etch to remove the PSG layers
Once PSG removed, the moving pieces of the device are freed
Adhesion force: F = Fcomb - kx
How to calculate xIf small displacements are assumed,it can be inferred that
Δ Bx ≈ ΔCy / 2Δ By = ΔCy / 2Δ x = Δ x0 + 2 Δ Bx ≈ Δ x0 + Δ Cy
Δ y = Δ y0 + 2 Δ By = Δ y0 + Δ Cy
Δ x0 , Δ y0 : tip distances in the undeformed configuration.
How to calculate k Six folded springs are connected to the central bar of the
vertical moving structure of the device to provide restoring force
The spring stiffness Kb = 24EI / (l13 + l23)
The stiffness k of the “X” structure Kx ≈ 8 times the one of each single folded spring
K=6 Kb+Kx
Structure
Comb drive is used to operate the cell stretcher It has 12 sets of comb, each with 42
electrodes The actuation force of a comb drive actuator F = NεtV2/g N : the number of comb electrodes, ε : the permittivity constant t : the comb electrode thickness V : the driving voltage g : the comb electrode gap.
In air operation
A DC power supply is wired to the support plate connectors.
Use a low-power, high output impedance power supply.
A high voltage generator to collect displacement information, while reading the actual voltage by means of a high input impedance multimeter.
To observe and record its behavior, the MEMS device is placed on the stage of an optical microscope equipped with a digital camera.
Underwater operation Underwater challenges Electrolysis water is broken down into hydrogen and oxygen at the anode and catho
de, respectively, can produce large amounts of gas underwater, which will lead to device failure due to bubbling
Surface tension Water is prevented from flowing under the PolyS 1 layer, since the silico
n-water interface tension is high, which in turn causes the silicon surface to behave hydrophobically
Electrical conductivity If the medium is electrically conductive, current can bypass the actuators
and the power available to the actuators is reduced, negatively affecting actuator efficiency.
Underwater solutions Ⅰ Electrolysis
Use AC driving system
Consists of a signal generator a high-frequency ac square wave that was set to drive the comb with a 1 MHz square wave signal with an average voltage of 0 V.
Underwater solutions Ⅱ Surface tension
Electrical conductivity
Consider that surfactants can reduce the surface tension of water by adsorbing at the liquid-gas interface, we can add a surfactant (sodium laureth sulphate) to reduce the silicon-water interface tension till the silicon surface became hydrophilic.
Using deionized water allowed the comparison of water properties such as thermal conductivity and dielectric constant without unusually large current bypassing the actuators.
Underwater operation
Performed applying a small drop of deionized water over the entire surface of the chip
Cover it with a microscope slide glass window.
The displacements are measured using the same optical equipment as in the air
An oscilloscope was used for the acquisition of the effective amplitude signal.
Summary
Biaxial cell stretcher design chosen for advantages of biaxial stress
Coatings chosen for ensured cell release Manufactured using reliable polyMUMPS
process Able to operate in air and in water
Questions?