Microfluidics: Electrokinetics and Fabrication

1 1EE C245 Microfluidics: Electrokinetics and FabricationDr. Thara SrinivasanLecture 182U. Srinivasan EE C245 Lecture Outline: Part I Reading From S. Senturia,Microsystem Design, Chapter 13, Fluids, Today s lecture, Part I Electrolytes and Electrokinetic Effects Electroosmosis Electrophoresis23U. Srinivasan EE C245 Electrolytes 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= = andCqziieie e = 2 = iTBkoeqiziieeCzq)(02 )(y =y4U. Srinivasan EE C245 Ionic 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, == 35U.

2 Srinivasan EE C245 Potential For small potential variations, expand exponential Reference location is in neutral region Variation in potential of ionic solution has exponential dependence = == = = , 0221220 02iiiBediTBkeqizieieCzTkqLeCqzq6U. Srinivasan EE C245 Debye 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: _____. =iiiBedCzTkqL0221 47U. Srinivasan EE C245 Electroosmosis 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.

3 Srinivasan EE C245xexxEUdxdPDtDU + + =2 Electroosmotic Flow Efield (up to 1 kV/cm) Exis applied voltage divided by fluidic path length Does not disrupt double layer3 LDU0 DLyDxwxDLyDweeLEdyUdeL = = 2259U. Srinivasan EE C245 Electroosmotic Flow No-slip at channel walls Plug flow profile for of cross section Typical experiment, double layer thickness = _____. No shear throughout most of volume3 LDU0xEOFDxwDLyDxwxEUandLEUeLEU == = maxmax110U. Srinivasan EE C245 Electrophoresis Ionic species such as DNA, protein segments and amino acids migrate under E-field For particle with electrophoretic mobility EP steady-state speed UEP reached when accelerating force is balanced by frictional force from medium Generally EOF > EP (for -3 ) Since different chemical species have different EP it is possible to separate them while they are being carried by electroosmoticflow rqrfandUfqEEUEPEPxxEPEP 66====q= r= =611U.

4 Srinivasan EE C245 Chip Electrophoresis Set-Up12U. Srinivasan EE C245 Chip Electrophoresis Sample injectionSample well V1 (+)Sample Waste V2 = 0 Buffer Well V3 > VJBuffer wasteV4 > VJJunctionSeparation channel1111212 ALRVRRRVetoJJ =+=713U. Srinivasan EE C245 Chip Electrophoresis Sample flowSample well V1 < VJSample Waste V2 < VJBuffer Well V3 (+)Buffer wasteV4 = 0U014U. Srinivasan EE C245 Chip Electrophoresis Sample flow and separationSample well V1 < VJSample Waste V2 < VJBuffer Well V3 (+)Buffer wasteV4 = 0U08EE C245 Fabrication Techniques for MicrofluidicsPicture credit: Fluidigm16U. Srinivasan EE C245 Lecture 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.

5 Direct Techniques Polymer Replication Techniques Master Fabrication Electroplating Back-End Processes917U. Srinivasan EE C245 Materials 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 C245 Replication 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.

6 Srinivasan EE C245 Choosing 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 C245 Direct 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 EOFC aliper Technologies1121U. Srinivasan EE C245 Direct Fabrication Jensen group, MIT DRIE on silicon substrates Mastrangelo group, U.

7 Michigan Sacrificial etching process to fabricate parylene-C polymer channels Rossieret al., cole Polytechnique, Lausanne Plasma etching of channels into polyimide substrate Prof. Luke Lee, UCB Low energy ion beam etching (Ar+) to make high aspect ratio Teflon structures Micronics, Redmond, WA Laser cutting of channels on thin film laminate22U. Srinivasan EE C245 Fabrication Techniques for Microfluidics Replication vs. Direct Techniques Polymer Replication Techniques Master Fabrication Electroplating Back-End Processes1223U. Srinivasan EE C245 Polymer Replication Molding process Fabricate a mold insert Fill the mold with polymer Harden the polymer Demold the piece Polymer microfluidic chips Credit card size to 10 20 cm2, mm thick Microchannels: Reservoirs: 1-3 mm in diameter, drilled through card ( L) Types of polymers Thermoplastics ~ polystyrene, polycarbonate Elastomers ~ silicone Thermosets ~ epoxy, acrylicAclara BiosciencesGyros24U.

8 Srinivasan EE C245 Casting Unpolymerized compound is poured into mold and allowed to polymerize Hot embossing Molding compound is introduced into an open mold and formed under heat and pressure Injection molding Heat-softened plastic resin is forced into a mold cavity by high pressurePolymer Molding Techniques mic1325U. Srinivasan EE C245 Important parameters in polymer transition temperatureTemperature range at which polymer changes from hard glassy material to soft (not melted) materialMelt temperatureThermal expansion coefficientChange in length or volume accompanying change in temperatureHardnessMeasured using indentation at a specific applied forceElasticityAbility of polymer to return to original shape after deformationTemperature at which polymer melt flows, generally much higher than Tg. Some polymers do not melt but remain soft until they Srinivasan EE C245 Casting Soft lithography Dow polydimethylsiloxane (PDMS) is elastomer of choice PDMS is poured into silicon stamp and allowed to cure at 40-70 C Once cured, polymer is peeled off stamp Bonding may be done using simple conformal contact to plastics, glass, or reaction between two PDMS layers Multilayer structures possibleFluidigm1427U.

9 Srinivasan EE C245 Multilayer Soft Lithography Rapid turnaround time possible good for academic research Demonstration at Fluidigm websiteWhitesides group, #28U. Srinivasan EE C245 Hot Embossing Imprinting tool: silicon stamp or metal electroform Hard plastic sheet is stamped in a hydraulic press P maintained for minutes, T just above Tgto soften plasticMildendo, Germany1529U. Srinivasan EE C245 Injection Molding Process Metal electroform mold made from silicon or glass master Electroform mounted in mold insert Molten thermoplastic polymer injected into warm mold at high T and P Once part cools and solidifies, mold is opened and part is ejected Small features and low aspect ratios easy; high aspect ratios harderAclaraSteag Lilliput chip Injection molding vs. Casting Injection molding gives rapid cycle time Casting gives best replication fidelity30U. Srinivasan EE C245 CD Fabrication +Standardized handling/automation+Superior flatness, low thickness variations, high precision in multilevel depths+Multiple designs on single tool Final part usually obtained by dicing Limitations due to circular format CD features Injection-molding of PC: After molding, disc is metallized and lacquered Pits of 9 lengths, smallest is: 2 billion pits per disc micGyros1631U.

10 Srinivasan EE C245 Important parameters in polymer chip pumpingControllable surface charge, good insulator for HV operation (dielectric strength, electrical resistance)Joule heatingThermal conductivityChemical propertiesChemical resistance, permeability,analyte adsorption and biocompatibilityOptical detectionHigh transmission and low background fluorescence32U. Srinivasan EE C245 Plastics Pro and Con Varying surface charge and charge density Depends on polymer type and Fabrication process For example, laser-ablation gives higher EOF than hot embossing Location of charge in PMMA different for casting vs. hot embossed channels Can control using surface treatments or may not be necessary Background fluorescence Biggest detection concern working with plastics and short-wavelength excitation (488 nm) Confocal epifluoresence rejects background Can use red or near IR-absorbing fluorophores1733U.