1EE C245Microfluidics: Electrokinetics and FabricationDr. Thara SrinivasanLecture 182U. Srinivasan ©EE C245Lecture Outline: Part I• Reading• From S. Senturia, Microsystem Design, Chapter 13, “Fluids,” p.339-349.• Today’s lecture, Part I• Electrolytes and Electrokinetic Effects• Electroosmosis• Electrophoresis23U. Srinivasan ©EE C245Electrolytes• Electrolytes, solutions of ionic species, have unique flow possibilities because electric fields can be used to direct the flow.• Total charge density of solution and charge density far from bounding surfaces• Charge density near a wall()02=∇∑=φρandCqziieieερφe−=∇2∑−=∇−−iTBkoeqiziieeCzq)(02φφεφ)(yφφ=y4U. Srinivasan ©EE C245Ionic Double Layers• Electrolyte in contact with insulating solid surface• Inner and outer Helmholtz layers are compact layers of adsorbed ions• Diffuse layer of ions has compensating net charge+- - - - - - - - - -+++++++++---++++--+Bulk solution (+ = -)Diffuse layer (+ > -)Compact layer (+ < -)potentialzetaewDLyw,ˆςφφφ==−35U. Srinivasan ©EE C245Potential• For small potential variations, expand exponential • Reference location is in neutral region• Variation in potential of ionic solution has exponential dependence =∑==∇≈∇−=∑−=∇−−φεφφφφφεφφˆˆˆˆ,ˆ0221220ˆ02iiiBediTBkeqizieieCzTkqLeCqzq6U. Srinivasan ©EE C245Debye Length• Debye length (LD) is decay length ~ back to neutral 3 LDfrom wall• Ionic strength ↑ LD↓• LDin pure water _____, in 1 mM KCl _____, in 1M KCl _____.• MEMS channels typically much wider than LD: ______.∑=iiiBedCzTkqL0221ε47U. Srinivasan ©EE C245Electroosmosis• Diffuse double layer has net charge• Electrostatic body force applied tangentially to diffuse double layer drags fluid along wall • Glass channels have negative charge (Si-OH-), diffuse layer has net positive charge and shows net flow towards cathode• Surface-mediated flow+- --- - - - - --+++++++++---++++--+Bulk solutionDiffuse layerCompact layerCathode (-)Anode (+)Electric FieldglassDiffuse layer(-)(+)8U. Srinivasan ©EE C245xexxEUdxdPDtDUρηρ+∇+−=2Electroosmotic Flow• E field (up to 1 kV/cm)• Exis applied voltage divided by fluidic path length• Does not disrupt double layer3LDU0DLyDxwxDLyDweeLEdyUdeL−−=−=ησσρ2259U. Srinivasan ©EE C245Electroosmotic Flow• No-slip at channel walls• Plug flow profile for 99.9% of cross section• Typical experiment, double layer thickness δ = _____.• No shear throughout most of volume3LDU0xEOFDxwDLyDxwxEUandLEUeLEUµησησ==−=−maxmax110U. Srinivasan ©EE C245Electrophoresis• Ionic species such as DNA, protein segments and amino acids migrate under E-field• For particle with electrophoretic mobility µEP steady-state speed UEP reached when accelerating force is balanced by frictional force from medium• Generally µEOF > µEP (for SiO21.25 -3×)• Since different chemical species have different µEP it is possible to separate them while they are being carried by electroosmoticflow rqrfandUfqEEUEPEPxxEPEPπηµπηµ66====q = r =η =611U. Srinivasan ©EE C245Chip Electrophoresis Set-Up12U. Srinivasan ©EE C245Chip Electrophoresis• Sample injectionSample well V1 (+)Sample Waste V2 = 0Buffer Well V3 > VJBuffer wasteV4 > VJJunctionSeparation channel1111212ALRVRRRVetoJJσ=+=713U. Srinivasan ©EE C245Chip Electrophoresis• Sample flowSample well V1 < VJSample Waste V2 < VJBuffer Well V3 (+)Buffer wasteV4 = 0U014U. Srinivasan ©EE C245Chip Electrophoresis• Sample flow and separationSample well V1 < VJSample Waste V2 < VJBuffer Well V3 (+)Buffer wasteV4 = 0U08EE C245Fabrication Techniques for MicrofluidicsPicture credit: Fluidigm16U. Srinivasan ©EE C245Lecture Outline: Part II• From reader:• Becker, H and Locascio, L., “Polymer Microfluidic Devices,” Talanta, Vol. 56, 2002, pp. 267-87.• Boone, T. et al., “Plastic Advances in Microfluidic Devices,” Analytical Chemistry A, February 2002, pp 79-86.• Today’s lecture, Part II• Replication vs. Direct Techniques• Polymer Replication Techniques• Master Fabrication• Electroplating• Back-End Processes917U. Srinivasan ©EE C245Materials for Microfluidics• Important material properties for microfluidic chips• Micromachinability• Surface charge • Molecular adsorption• Optical properties• Why polymers?• Polymer granules: _____ cheaper than Si, glass or quartz• Replication processes suitable for automated production• Injection-molded CD costs: ___________.• Relatively low capital cost compared to silicon IC equipment• Benefits• Biocompatible, plastics molding is well-developed technology, may not need coating to suppress EOF, chips may be cheap enough to be disposable (better, recyclable)18U. Srinivasan ©EE C245Replication vs. Direct Techniques• Direct fabrication: each part must be made separately+ Can create certain mechanical structures difficult to create with molding– Expensive in materials and labor– Routine access to cleanroom for fab and bonding± For economical use, re-use chip • Replicated microsystems:+ Cost-effective in materials, continuous processing possible with higher yields and reproducibility± Back-end processes required for each product; may be able to do wafer level assembly± Chips may be single-use– Moving parts difficult to create1019U. Srinivasan ©EE C245Choosing a Chip Material• Silicon ~ for special research applications• Glass ~ for research, or if you can use existing chip, or for organic solvents or high temperatures• Thermoplastics ~ general industry trend for high-volume, disposable chips• Elastomers ~ for research applications, quick turnaround with rapid prototyping20U. Srinivasan ©EE C245Direct Fabrication• Typical glass microfabricationprocess• Etch mask deposition (Cr or Au thin film) and patterning• Etch mask etching (KI/I2for Au or K3Fe(CN)6/NaOH for Cr)• Substrate etching with dilute HF/NH4F• Ultrasonic drilling of access holes into the cover plate before bonding• Thermal bonding of glass cover plate to seal microchannels • Attachment of “sipper” capillaries into the chip reservoirs• Coating of glass channel walls to suppress EOFCaliper Technologies1121U. Srinivasan ©EE C245Direct Fabrication• Jensen group, MIT • DRIE on silicon substrates• Mastrangelo group, U. Michigan• Sacrificial etching process to fabricate parylene-C polymer channels•