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 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 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) 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 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 100 m Micro-channel Width 500 m Interconnect Width 1000 m Interconnect Depth 1000 m Reservoir Diameter 0.4 cm Device Design: Stage 2 (Modified Microchannel Design) Process Sequence and Mask Design 1.Begin with four polished Si wafers 2.Spin SU-8 (negative photoresist) on the Si wafers and pre- bake at 95C 3.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 95C 4.Develop the SU8 so that the unexposed areas are removed Results in four distinct SU8 molds Micro-Channel Layer 1 Interconnect Layer Micro-Channel Layer 2 Top Cover Layer Device Design: Stage 2 (Modified Microchannel Design) 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 70C 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 Micro-Channel Layer 1 Interconnect Layer Micro-Channel Layer 2 Top Cover Layer Device Design: Stage 2 (Modified Microchannel Design) Final Expected Result: Stage st Trial Problems Thickness of PDMS layers Interconnects Delamination Air bubbles Stage nd 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 Phase nd Trial Successes & Problems Liquid to flow in all channels all the way through 2 out of 5 channels Tracked fluid flow using bright food coloring Tested the effects of vertical interconnects No capillary action had to use pressure from syringe problems Pressure caused delamination Effects of vertical interconnects Phase 2 Experimental Procedure Phase 2 Channel Layout Processing Problems Substantial amount of cracks in SU-8 layer Layer alignment problems 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 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 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 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 Design) Using the membrane dimensions for Stage 3 and a projected maximum deformation between micrometers a pressure difference of torr is needed. Deflection Equation w =0.0318(ab) 2 (1- v)/(E*t 3 ) P: pressure E: elastic modulus v: poissons ration a and b: width and length of membrane w: maximum deflection openclosed 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 Height100 m PDMS Layer Height 100 m PDMS Flex Layer50 m Micro-channel Width 500 m Interconnect Width1000 m Interconnect Depth1000 m Reservoir Diameter 0.4 cm 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 m/sec 1.25 E 10 m 3 /sec= cm 3 /sec Fluidic Resistance: R= P/Q [(N*s)/m 5 ] R(circular cross section)= 8L/(r 4 ) = Fluid Viscosity= 0.01 g/sec*cm L= Length of channel r= Radius of channel R(Rectangular cross section)~ 12L/(wh 3 ) w= Width of the channel h= Height of the Channel Total Fluidic Resistance = R r + R c + R i + R v R r + R c + R i + R v R Total 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 m 3 /sec= cm 3 /sec Total Cycle Time ~21.2 seconds Processing Problems Substantial amount of cracks in SU-8 layer Layer alignment problems 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 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