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?