Multilayer Microfluidics

ENMA490Fall 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 DefinitionMotivation

– 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 ScopeDesign Requirements

– Two-level microfluidic network– Active control elements

Material Requirements– Ease of patterning and use in microfabrication– Chemically inert– Low Cost / Obtainable– Optically transparent– Specific Elastic modulus (flexible, rigid)

Constraints– Assume external fluid control– Neglect biochemical reactions in channels– Keep design feasible for manufacturing

Initial Material Choices

Substrate Material• Silicon

• Relatively inexpensive• Commonly used in microelectronics

• Well known properties and processing techniques

• Pyrex• Transparent to visible light

• Allows visual monitoring of micro channels

• More expensive than silicon

Initial Material Choices

Microchannel Material• Poly(dimethylsiloxane) or PDMS

• Inexpensive• Poor surface adhesion – releasable from mold• Highly flexible

• modulus of 2.5 MPa

• SU-8• Is a photoresist

• High aspect ratios obtainable

• Good surface adhesion to silicon and pyrex• Very rigid – complementary to PDMS

• modulus of 4000 MPa

Project DevelopmentDefined Problem

Divided into research groups (BioMEMS, Materials, Devices, and Circuits)

Developed Stage 1(Initial Microchannel Design Concept)

Developed and tested Stage 2(Modified Microchannel Design)

Modified design to integrate vertical vias

for multilevel fluid flow

Developed and tested Stage 3 (Final Design: Pressure Actuated Valve Design)

Developed fluid control device to manipulate fluid flow

Summarized manufacturing and experimental results

of final 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

Bottom layer

Middle layer

Top layer

I/O

I/O

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)

Device 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

Reservoir (I/O)

Interconnect

Device Design: Stage 2(Modified Microchannel Design)

Device Geometry– Chosen for process

compatibility– Rectangular micro-

channels– Square interconnects– Circular reservoirs

Materials– SU-8 used as a mold for the PDMS layers– All PDMS layers stacked on a Silicon substrate

Critical Dimension Value

PDMS Layer Height

100m

Micro-channel Width

500m

Interconnect Width 1000m

Interconnect Depth 1000m

Reservoir Diameter 0.4 cm

Device Design: Stage 2(Modified Microchannel Design)

Process Sequence 1. Begin with four polished Si wafers2. Spin SU-8 (negative photoresist) on the Si wafers and pre-

bake at 95°C3. Align each of the four wafers with one of four masks and

expose the SU-8 to ultraviolet light, then post-bake at 95°C 4. Develop the SU8 so that the unexposed areas are removed

– Results in four distinct SU8 molds 5. Spin PDMS on the SU8 molds less than the vertical

dimension of the SU-8 protrusions– 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

Device Design: Stage 2(Modified Microchannel Design)

6. Delaminate and stack all four PDMS layers in the following order: Micro-channel Layer 1, Interconnect Layer, Micro-channel layer 2, Top Cover Layer

Processing Problems

• Substantial amount of cracking in SU-8 layer• Layer assembly problems

– Razor blade/ tweezers method– Layer thickness– Wrinkles– Air pockets

• Feature alignment– Extremely difficult– Inaccurate

Cracks in reservoir region of SU-8 mold

Stage 2 (Experimental Results: Trial 1)

Problems• Thickness of PDMS

layers• Interconnects • Delamination• Air bubbles

Stage 2 (Experimental Results: Trial 2)

Improvements• 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 (Test Results: Trial 2)

Problems• No capillary action

– had to use pressure from syringe

• Pressure caused delamination

• Functionality of vertical interconnects

Successes• Liquid flow in all

channels– Completely through 2

out of 5 channels

• Tracked fluid flow using bright food coloring

• Tested the effects of vertical interconnects

Stage 2 (Test Procedure)

Stage 2 (Channel Layout)

Reservoir (I/O)

Interconnect

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

Device Design: Stage 3(Pressure Actuated Valve Test Design)

Device Logic– 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– Made for feasibility– 4 gas control sites– 1 fluid interconnect– Thin PDMS flex layer

Materials– 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)

Critical Dimension Value

SU-8 Layer Height 100 µm

PDMS Layer Height 100 µm

PDMS Flex Layer Height

50 µm

Micro-channel Width 500 µm

Valve Width 500 µm

Valve Length 500 µm

Device Design: Stage 3(Pressure Actuated Valve Design)

What we need• Deformation between

30-60 µm • Pressure difference

between fluid and gas of 24 - 41.6 torr

Deflection Equation

w =0.0318P(ab)2 (1-)/(Et3)

• P: pressure

• E: elastic modulus : Poisson’s ratio

• a & b: width and length of membrane

• w: maximum deflection

• t: thickness

open closed

Liquid

Gas

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

• Fluid flow rate: 1.25 E 10 μm3/sec = 0.0125 cm3/sec– Used the fluid flow rate calculated to determine the

following properties for the fluid flow path:• Fluidic resistance and pressure gradient:

R = ΔP/Q [(N*s)/m5]• Reynolds number:

Re= (vDh)/μ• Velocity:

v = Q/A • Cycle time

t = Length/v

Device Design: Stage 3(Pressure Actuated Valve Design)

Fluid Flow Modeling Results– 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 + RM + RI + RV

RR + RM + RI + RV RTotal

Path Region

Fluidic Resistance

(g/sec*cm4)Pressure

Gradient (Torr)Reynolds Number

Velocity (cm/sec) Time (sec)

Reservoir 33 0.00031 4.0 0.1 20.7Micro-channel 9264000 86.9 41.7 25 0.15Interconnect 24 0.00023 12.5 1.3 0.016

Valve 3000000 28.13 48.1 125 0.00008Total Path 12264057 115.03054 20.86608

Stage 3 (Fabrication Results)

Alternative Valve Designs • Design Elements

– Isolated fluid chamber– Membrane division between chamber and fluid channel– Stopper to aid in the control of the fluid

• Phase Change Bubble Valve– Principles of Actuation

• Volatile liquid (cyclopentane)• Resistive heaters• Heater cause fluid to change from liquid to gas• Expansion from gas pressure deflects membrane

SU-8

PDMS Flex Layer

PDMS Fluid Layer

SU-8 Bottom Layer

Heater

Alternative Valve DesignsElectrolytic Bubble Valve

– Principles of actuation• Water• Two electrodes• Application of current causes

electrochemical reaction• Creation of bubbles increase

pressure in chamber

Piezoelectric Valve– Principles of actuation

• Piezoelectric material electrically activated

• Expansion causes compression in liquid chamber

• Compression translated to membrane deformation with larger amplitude

SU-8

PDMS Flex Layer

PDMS Fluid Layer

SU-8 Bottom Layer

Electrodes

Future Work

Design:• Improve scaling to accommodate additional

layers

Materials:• Replace Pyrex with acrylic as top substrate• Promote adhesion/seal between PDMS layers• Alter surface chemistry of channels to be

hydrophilic

Summary• Technology for multilevel microfluidic devices

has the potential to increase design flexibility• We succeeded in fabricating two-level

microfluidic circuits with vertical interconnects and valves

• We experienced the design, fabrication, and testing phases of a multistage project

• Modeling and experimental feedback are essential to evolution of design

• We learned that project organization and management are critical to meeting project goals

We learned to work as a team!