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 Design 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 Development Defined 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 –90 o 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

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 90 o 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 DimensionValue PDMS Layer Height 100  m Micro-channel Width 500  m Interconnect Width 1000  m Interconnect Depth 1000  m Reservoir Diameter0.4 cm

Device Design: Stage 2 (Modified Microchannel Design) Process Sequence 1.Begin with four polished Si wafers 2.Spin SU-8 (negative photoresist) on the Si wafers and pre- bake at 95°C 3.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 DimensionValue SU-8 Layer Height100 µm PDMS Layer Height100 µm PDMS Flex Layer Height 50 µm Micro-channel Width500 µm Valve Width500 µm Valve Length500 µm

Device Design: Stage 3 (Pressure Actuated Valve Design) What we need Deformation between µm Pressure difference between fluid and gas of torr Deflection Equation w =0.0318P(ab) 2 (1- )/(Et 3 ) P: pressure E: elastic modulus : Poisson’s ratio a & b: width and length of membrane w: maximum deflection t: thickness openclosed 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 μm 3 /sec = cm 3 /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)/m 5 ] Reynolds number: R e = (  vD h )/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/(πr 4 ) μ = fluid viscosity= 0.01 g/sec*cm L = Length of channel r = Radius of channel –R (rectangular cross section) ~ 12μL/(wh 3 ) w = Width of the channel h = Height of the Channel –Total Fluidic Resistance = R R + R M + R I + R V R R + R M + R I + R V R Total

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 Designs Electrolytic 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!