Transcript Latching SMA Microactuator

Multilayer Microfluidics
ENMA490
Fall 2003
Brought to you by:
S. Beatty, C. Brooks, S. Dean, M. Hanna, D. Janiak, C. Kung,
J. Ni, B. Sadowski, A. Samuel, K. Thaker
Problem Definition
Motivation
– BioMEMS research is
growing rapidly, but
restricted to single layer
microfluidics
– Development of a multilayer
microfluidic design would
increase flexibility
Goal
– Design, construct, and test
a controllable microfluidic
device with at least two fluid
levels
– Identify appropriate
materials, processes, and
device geometries
Problem Scope
Requirements
– To make a two-level microfluidic device
– To incorporate active control elements
Constraints
– Assume external fluid control
– Neglect biochemical reactions in channels
– Keep design feasible for manufacturing
Initial Material Choices
Desired Characteristics
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Ease of patterning and use in
microfabrication
Chemically inert
Low Cost / Obtainable
Optically transparent
Specific Elastic modulus (flexible, rigid)
Initial Material Choices
Substrate Material
• Silicon
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Relatively inexpensive
Commonly used in microelectronics
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Well known properties and processing
techniques
Pyrex
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Transparent to visible light
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Allows visual monitoring of micro channels
More expensive than silicon
Initial Material Choices
Microchannel Material
• Poly(dimethylsiloxane) or PDMS
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Inexpensive
Poor surface adhesion – releasable from mold
Highly flexible
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modulus of 2.5 MPa
SU-8
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Is a photoresist
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High aspect ratios obtainable
Good surface adhesion to silicon and pyrex
Very rigid – complementary to PDMS
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modulus of 4000 MPa
Defined
Problem
Project Development
Divided into research groups
(BioMEMS, Materials, Devices, and Circuits)
Developed Stage 1
(Initial Microchannel Design Concept)
Developed fluid control device to
manipulate fluid flow
Developed and tested Stage 3
(Final Design: Pressure Actuated Valve Design)
Summarized
manufacturing and
experimental results
of final design
Modified design to
integrate vertical vias
for multilevel fluid flow
Developed and tested Stage 2
(Modified Microchannel Design)
Device Design: Stage 1
(Initial Microchannel Design Concept)
• Objective
– To create an initial design
for a multilayer micro
fluidic device
• Initial design elements
– 90o orientation of fluid
layers
– Vertical interconnects at
channel intersections
– Each layer has same
design- reduces number
of molds
– Versatility of fluid paths
I/O
Bottom layer
I/O
Middle layer
Top layer
Device Design: Stage 1
(Initial Microchannel Design Concept)
• Materials
– Stackable PDMS layers
– Silicon substrate
– SU-8 molds
• Processes
– Create a channel mold and an interconnect mold
using SU-8
– Create PDMS layers from SU-8 mold: two layers
from channel mold, one interconnect layer
– Stack layers on substrate starting with a channel
layer, interconnect layer and second channel layer
at 90o orientation
Device Design: Stage 2
(Modified Microchannel Design)
Objective
– To test the viability of a two-level passive micro-fluidic device
Modifications from Stage 1
– Moved reservoir positions to fit existing packaging
– Created discrete flow paths to test flow on individual layers
and between layers
– Increased all dimensions to facilitate fabrication and testing
Device Logic
– Five distinct fluid paths
– 11 I/O
– Two distinct channel
levels
– One interconnect level
– One Top Cover level
Device Design: Stage 2
(Modified Microchannel Design)
Device Geometry
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–
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Critical Dimension
PDMS Layer Height
Chosen for process
Micro-channel
compatibility
Width
Rectangular micro-channelsInterconnect Width
Square Interconnects
Interconnect Depth
Circular reservoirs
Reservoir Diameter
Value
100mm
500mm
1000mm
1000mm
0.4 cm
Materials
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SU-8 used as a mold for the PDMS layers
All PDMS layers stacked on a Silicon substrate
Device Design: Stage 2
(Modified Microchannel Design)
Process Sequence and Mask Design
1.
2.
3.
Begin with four polished Si wafers
Spin SU-8 (negative photoresist) on the Si wafers and prebake at 95°C
Align each of the four wafers with one of the four masks
shown below and expose the SU-8 to ultraviolet light, then
post-bake at 95°C
Micro-Channel Layer 1
4.
Interconnect Layer
Micro-Channel Layer 2
Top Cover Layer
Develop the SU8 so that the unexposed areas are
removed
–
Results in four distinct SU8 molds
Device Design: Stage 2
(Modified Microchannel Design)
5.
Spin PDMS on the SU8 molds less than the vertical
dimension of the SU-8 protrusions
–
–
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6.
Mix PDMS (Sylgard 184, Dow-Corning) 10:1 with curing agent
Spin on PDMS
Dip the Si wafer in a sodium dodecyl sulfate(SDS) adhesion barrier
and allow it to dry naturally
Bake in box furnace for 2 hours at 70°C
Delaminate and stack all four PDMS layers in the
following order: Micro-channel Layer 1, Interconnect
Layer, Micro-channel layer 2, Top Cover Layer
Micro-Channel Layer 1
Interconnect Layer
Micro-Channel Layer 2
Top Cover Layer
Device Design: Stage 2
(Modified Microchannel Design)
Final Expected Result:
Stage 2: Experimental Trial 1
• Top layer interconnects did not go through, so
we had to poke holes with the needle
– Caused problems with sealing around the needle
and getting pressure to move liquid through the
channels and not out at the opening
• Interconnects did not connect, so liquid would
not flow at all in any of those channels
• Many layers were delaminated or had giant
air bubbles, which liquid would spread out
into
Stage 2 Experimental Trial
• Fabrication
– Successfully made and aligned four layers
– Layers had very few defects
– All interconnects joined two different layers
– Entire wafer looked very good- no rough
edges, no air bubbles between layers, no
craters
Stage 2 Experimental
Successes & Problems
• Successfully got liquid
to flow in all channels
using pressure from a
syringe
• Were able to push liquid
all the way through 2
out of five channels
• Used three different
colors of food coloring,
so we can see the
areas where the fluid
reached
• Needle opening was too
large, and angle of
needle opening made
use difficult
• Had to force the needle
into the channels to
obtain any pressure,
and this sometimes
delaminated the layers
around the channel
– Solved this by inserting
the needle opening into
the channel and sealing
the opening with a finger
on top of the needle
2nd Experimental Problems
Continued
• No capillary action effects were observed (all
liquid movement was obtained with pressure)
• In many cases, the pressure required to push
the liquid through the channels exceeded the
adhesion forces between the layers, so layers
came apart and liquid stopped flowing
through the channel
– This was also solved with applied force from a few
fingers in some cases
– Could have also been too much pressure applied
with syringe
Device Design: Stage 3
(Pressure Actuated Valve Test Design)
Device Objective
– To integrate an active control element into a basic microchannel
design based on Stage 2
Modifications from Stage 2
– Removed all microchannels except for T-shaped section
– Added a completely top layer microchannel
– Incorporated negative pressure gas valves in design (Figure _)
Device Logic
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–
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Two distinct fluid paths
Five I/O
Two channel levels
One gas channel level
One thin flex layer
One top cover layer
Device Design: Stage 3
(Pressure Actuated Valve Test Design)
Device Objective
– To integrate an active control element into a basic microchannel
design based on Stage 2
Modifications from Stage 2
– Removed all microchannels except for T-shaped section
– Added a completely top layer microchannel
– Incorporated negative pressure gas valves in design (Figure _)
Device Logic
–
–
–
–
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Two distinct fluid paths
Five I/O
Two channel levels
One gas channel level
One thin flex layer
One top cover layer
Device Design: Stage 3
(Pressure Actuated Valve Test Design)
Device Geometry
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Made for feasibility
4 gas control sites
1 fluid interconnect
Thin PDMS flex layer
Critical Dimension
Value
SU-8 Layer Height
100 µm
PDMS Layer Height
100 µm
PDMS Flex Layer
50 µm
Micro-channel
Width
500 µm
Interconnect Width
1000 µm
Interconnect Depth
1000 µm
Reservoir Diameter
0.4 cm
Materials
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SU-8 for rigid portions in valve design (gate)
SU-8 for fluid layers
PDMS for gas control layer
PDMS used for flexible gas/fluid membrane
2 substrates required (Si, Pyrex)
Device Design: Stage 3
(Pressure Actuated Valve Design)
Fluid Flow Modeling
– Assumed Fluid Flow Rate based on Fluid velocity
• Based on literature search: 1500 cm/minute= 2.5 E5 μm/sec
• 1.25 E 10 μm3/sec= 0.0125 cm3/sec
– Fluidic Resistance: R= ΔP/Q [(N*s)/m5]
• R(circular cross section)= 8μL/(πr4)
– μ= Fluid Viscosity= 0.01 g/sec*cm
– L= Length of channel
– r= Radius of channel
• R(Rectangular cross section)~ 12μL/(wh3)
– w= Width of the channel
– h= Height of the Channel
• Total Fluidic Resistance = Rr + Rc + Ri + Rv
Rr + Rc + Ri + Rv
RTotal
Device Design: Stage 3
(Pressure Actuated Valve Design)
Fluid Flow Modeling
– Determined the velocity, fluidic resistance,
Reynolds number, and pressure gradient in each
section of the fluids path and found the relevant
total fluid path properties
Sample output for the two valve fluid path
– Total Pressure Gradient
• ~15330Pa~115 Torr
– Pressure Gradient at the Valve
• 3750 Pa~28 Torr
– Fluid Flow Rate
• 1.25 E 10 μm3/sec= 0.0125 cm3/sec
– Total Cycle Time
• ~21.2 seconds
Processing Problems
• Substantial amount of cracks in SU-8 layer
• Layer alignment problems
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Razor blade/ tweezers method
Layer thickness
Wrinkles
Air pockets
Cracks in reservoir
region of SU-8 mold
Processing Problems
Continued
• Feature alignment
– Extremely difficult
– Inaccurate
• Interconnect layer
– No connection provided
– Problem with layer thickness
Alternative Designs
• Phase Change Bubble Valve
– Design Elements
• Isolated fluid chamber
• Membrane division between chamber and fluid
channel
• Stopper to aid in the control of the fluid
– Principles of Actuation
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Volatile liquid (cyclopentane)
Resistive heaters
Heater cause fluid to change from liquid to gas
Expansion form gas pressure deflects membrane
Alternative Designs
• Phase Change Bubble Valve
– Associated Problems
• How much pressure is needed to deflect
membrane
• How much power is needed to create enough
heat
• How quick is the reverse reaction
Alternative Designs
(Piezoelectric)
• Piezoelectric Valve
– Design Elements
• Isolated liquid chamber
• Membrane division between
chamber and fluid channel
• Partial gate to aid in closing
of valve
–Principles of Actuation
•Piezoelectric material electrically activated
•Expansion causes compression in liquid chamber
•Compression translated to membrane deformation with larger
amplitude
Future Work
Here are a few material changes we would
pursue if time allowed:
• Replace Pyrex with acrylic for the top substrate
• Promote adhesion or a seal between the PDMS
layers
• Alter surface chemistry of the channels to be
hydrophilic
Summary