Transcript A novel MEMS platform for the biaxial stimulation of cells

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
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Basic design
Advantages of biaxial testing
Stretcher coatings
Manufacturing process
Force calculation
Air and water operation
Summary
Structure
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X structure applies
biaxial force
3 lower sections
move (top
stationary)
Driven by single
comb drive
actuator
Return
Operation
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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
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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
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Possible method
of displacement
affecting cell
stiffness
Buckling of inner
cytoskeleton
causes a more
linear response
Advantage of Biaxial Testing
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Linear response seen in graph A
Biaxial stretching can stop this behavior by
eliminating lateral strain, seen in graph B
Stretcher Coating
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Required force for cell
detachment can be
reduced
Coating with 1dodecanethiol (DDT), 1hexadecanethiol (HDT)
and 1-octadecanethiol
(ODT) on Au substrate
can decrease
detachment force (curve
peaks)
Microfabrication
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Based off of the PolyMUMPS fabrication process.
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PolyMUMPS – Multi-User MEMS Processes
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Provides fabrication of cost-effective, proof-of-concept MEMS
devices
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Multi-step process utilizing interchanging layers of polycrystalline
silicon and a sacrificial layer (in this case Phosphosilicate Glass)
Doping and Insulation
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n-type Si wafer is
doped further to
prevent charge
feedthrough
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An insulating layer of
Si3N4 is deposited
using LPCVD
PolyS 0
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Initial layer of Polycrystalline
Silicon (PolyS 0) deposited
with LPCVD
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Photolithography to create
support posts for device
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Positive mask along with
RIE to make pattern
PSG 1
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Sacrificial layers of
PhosphoSilicate Glass
(PSG) are used to provide
intermediate layers
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Can be patterned to
surround the PolyS 0
features
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Eventually will be removed
to release structure
PolyS 1
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New layer of PolyS
added in order to build
the suspended cell
stretching platform
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Transverse bar seen at
bottom of mask is
actually connected to
comb drive actuator
PSG 2
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Second sacrificial layer
applied and patterned
to surround platform
features
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Will provide support for
final layer of PolyS
PolyS 2
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This layer of PolyS
creates the linkage
arms for the device
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Four separate arms are
used to connect the
platform quadrants
Final Outcome
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Side and Top Views
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Linkage to comb drive
can be observed
Comb Drive Actuator
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Connects to the transverse
bar of the test device
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PolyS and PSG labels are
the same as for test device
fabrication
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Analogous process to Cell
Stretcher
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Begins on same doped and
insulated wafer
PolyS 0
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PolyS 0 layer creates
the stator bases and
the posts for the folded
springs
PSG 1
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PSG is used to provide
support for main comb
drive structure
PolyS 1
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PolyS 1 layer creates
both the rotor and
stator heads and
combs
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Folded springs also
come from PolyS 1
layer
Final Release of Device
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The device is ready for release after PolyS 2
layer is applied
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A ~49% HCl mixture in water is most effective
etch to remove the PSG layers
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Once PSG removed, the moving pieces of
the device are freed
Adhesion force: F = Fcomb - kx
How to calculate x
If small displacements are
assumed,
it can be inferred that
≈ ΔCy / 2
Δ By = ΔCy / 2
Δ Bx
= 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.
Δx
How to calculate k
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Six folded springs are connected to the central bar
of the vertical moving structure of the device to
provide restoring force
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The spring stiffness
Kb = 24EI / (l13 + l23)
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The stiffness k of the “X” structure
Kx ≈ 8 times the one of each single folded
spring
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K=6 Kb+Kx
Structure
Comb drive is used to operate the cell
stretcher
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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
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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
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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.
Underwater solutions Ⅰ
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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
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Surface tension
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.
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Electrical conductivity
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
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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
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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?