A novel MEMS platform for a cell adhesion tester

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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

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Overview

• Basic design
• Advantages of biaxial testing
• Stretcher coatings
• Manufacturing process
• Force calculation
• Air and water operation
• Summary

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Structure

• X structure applies biaxial force
• 3 lower sections move (top stationary)
• Driven by single comb drive actuator

Return
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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)

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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

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Advantage of biaxial testing

• Possible method of displacement affecting cell
  stiffness
• Buckling of inner cytoskeleton causes a more
  linear response

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Advantage of Biaxial Testing

• Linear response seen in graph A
• Biaxial stretching can stop this behavior by
  eliminating lateral strain, seen in graph B

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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)

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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)

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Doping and Insulation

• n-type Si wafer is doped further to prevent
  charge feedthrough
• An insulating layer of Si3N4 is deposited using
  LPCVD

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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

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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

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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

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PSG 2

• Second sacrificial layer applied and patterned to
  surround platform features
• Will provide support for final layer of PolyS

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PolyS 2

• This layer of PolyS creates the linkage arms for
  the device
• Four separate arms are used to connect the
  platform quadrants

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Final Outcome

• Side and Top Views
• Linkage to comb drive can be observed

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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

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PolyS 0

• PolyS 0 layer creates the stator bases and the
  posts for the folded springs

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PSG 1

• PSG is used to provide support for main comb
  drive structure

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PolyS 1

• PolyS 1 layer creates both the rotor and stator
  heads and combs
• Folded springs also come from PolyS 1 layer

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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

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Adhesion force F Fcomb - kx
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How to calculate x
If small displacements are assumed, it can be
inferred that ? Bx ?Cy / 2 ? By ?Cy / 2
? x ? x0 2 ? Bx ? x0 ? Cy ? y ? y0 2
? By ? y0 ? Cy ? x0 , ? y0 tip distances in
the undeformed configuration.
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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
• K6 KbKx

Structure
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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 NetV2/g
• N the number of comb electrodes,
• e the permittivity constant
• t the comb electrode thickness
• V the driving voltage
• g the comb electrode gap.

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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.

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Underwater operation

• Underwater challenges
• Electrolysis
• water is broken down into hydrogen and oxygen
  at the anode and cathode, 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 silicon-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.

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Underwater solutions ?

• 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.

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Underwater solutions ?

• 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.
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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.

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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

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Questions?