Fabrication of microfluidic devices

Lecture 10 Part II:Microfabrication forMicrofluidics andMicrofluidics DevicesSilicon EtchingPolymer-basedMicromachiningAssembly and PackagingBiocompartibility

Techniques involved:• Wet etching of channels in Si and glass(isotropic, anysotropic)• Dry etching• Resist lithography• PDMS soft lithography• Hot embossing• Other machining techniques in plastics,glass etc.• Bonding

Wet etching of (100) Silicon

Wet etching of other orientation ofSilicon

Isotropic etching of Silicon andGlassSilicon:Etchant: 66% HNO3 and 34% HFEtching rate: 5um/minGlass:Etchant: HF (or BHF)

Chemical dry etching• Deep trenches with highaspect ratios can bemade in Si, glass orplastic• Gases used:– Fluorine chemistry (CHF 3 ,SF 6 , CF 4 )– Chlorine chemistry (HCl(HCl,Cl 2 )– Oxygen

Bulk micromachined channels

Silicon surface micromachiningPECVD oxideoxidePECVD oxidewet etching

Polymer based micromachining• Thick resist lithography• Polymeric based micromachining• Soft lithography• Microstereo lithography• Micromolding

SU-8 resist• Negative photoresist for NUV exposureReally Reallythick thicklayers layersin in one onespin!L/S10/30 20/60 30/90

Example of SU-8 structuresFabrication of open channelsFabrication of covered channelsembeddedmaskselectiveproton writingsacrif.layer

Soft lithography• Uses elastomeric stamp, usually PDMS(Polydimethylsiloxane)) to transfer the pattern.– Microcontact printing– Micromolding

Fabrication of microchannels usingsoft lithography• Advantages of PDMS:– Low cost– Transparency in VIS and NUV– Chemically inert• Technology:– Mix prepolymer and curing agent 10:1 – 5:1– Pour into solid master made in SU-8 with inlets defined byglass posts– Cure at 60 – 80 oC for couple of hours– Peel off– treat with ozon or Oxygen plasma and attach to cleanglass, silicon or another PDMS

Microstereo lithographySingle photon adsorbtionTwo photon absorbtionLayer-by-Layer photolithography

Micromolding• Injection molding: high pressure injection of moltenPMMA, PC (polycarbonate(polycarbonate), PSU (polysulfone(polysulfone) ) etc.• Compression molding (hot embossing)

Other micromachining techniques• Laser micromachinig(usuallyusing an excimer lasers,Nd:YAG or CO 2 lasers)• Focused ion beam• Microelectro discharge• Ultrasonic micromachining

Assembly and packaging• Anodic bonding(T=400 o C, , V=1kV)• Silicon direct bondingreaction of OH groups on Si surfaces at T=300 – 1000 o C• Glass direct bonding(T=600 o C for 6-8h) 6• Polymer direct bonding• Adhesive bonding (lowmelting glass (400-600o C, photoresists,UV curable epoxies, epoxies etc.)• Eutectic bonding (e.g.. gold/siliconeutectic at 363 o C)

Other microfabrication issues:Biocompartibility• Material responce to biologicalenvironments (swelling, corrosion etc.)• Tissue and cellular responce to thematerial

Microfluidics: Devices forFlow Control•Valves•Pumps•Micromixers

Microvalve consist of closure element driven by an actuatorNormally openActive MicrovalvesNormally closedBistableActive MicrovalvesAnalogDigitalAnalogDigitalPulse-width modulationFluidic DAC

Active MicrovalvesPneumaticThermopneumaticThermomechanicalPiezoelectricElectrostaticElectromagneticElectrochemicalCapillary forcelActuation principle

Valve specification:• leakage• valve capacityLvalveQ=Q• power consumption – total power consumption in active state• closing force (pressure range) – pressure generated by the valve• temperature range• responce time• reliability• biocompartibility• chemical compartibilityclosedopenC valve=∆pQmaxmax/( ρg)

Design considerations• Specification required• Materials to be used• Cost• Suitable type of actuators• Optimal valve spring and valve seat

Design consideration: Actuators• Moving function (enoughforce, displacement andcontrollability)• Holding function (shouldkeep valve in a setposition)• Dynamic function (requiredresponce time)Energy density:E ='aFasVaa

Design consideration: Valve spring• For normally closed valves (NC) – large spring constant to resist thepressure• For normally open valves (NO) – soft spring constant, optimized foractuator closing force

Design consideration: Valve seatMain requirements:• Zero leakage• Resistance against particles trapped

Design consideration: PressurecompensationAim: Maintain closing force when the inletpressure varyPassive flow controller

Stack of 3 directly bonded Si wafers, Simembrane 25 um thickSilicon rubber membrane 25 um thick preparedby spin coating, hole drilled with laserSilicon rubber membrane 30 um thick preparedby surface micromachinig, photoresist used asa sacrificial layer.Boron implantationTypical parameters of pneumatic valves:

Thermopneumatic valves• Relies on the change in volume of sealedliquid or solid under thermal loading. Usuallyutilize solid/liquidliquid and liquid/gasphasetransition for maximum performance

Example• Thermopneumatic valve with air.Height of the expansion cylinder500um.• Assuming the volume constant:T1 p1p2109.67= → T2= T1= 300 = 329K=T p p 10022156C

Example

Design examplesHeater is placed on SiN membrane, 50umSilicon rubber used for membrane9um paraffin used as actuation material2um deflection with 50mW heating

Thermomechanical valves• Solid expansion• Bimetallic• Shape-memoryalloys

Solid expansion valves• Generated force:F ∝ γ∆Twhere γ is the thermal expansion coefficient

Bimetallic valves• Uses difference in thermal expansion coefficient of two metalsF ∝ γ 2−γ) ∆T(1

Bimetallic valves: : Design examples

Shape memory Alloy valves• Shape memory alloys (SMA) are materials that have propertyto return to their original undeformed shape upon a change oftemperature• Advantages: high force and large stroke• Disadvantages: low efficiency, low frequency (bandwidth)

Piezolelectric valves• generate small strain (0.1%) and high stresses (MPa(MPa),therefore suitable for applications with high force and lowdisplacementϕ = ϕ = dlvtϕ = d33Eel31Eel

Example:

Design examples

Electrostatic valve• Based on attractive force between two oppositely chargedplatesVF 1 2−= εrε A() , 8.854*10120ε0=F / m2 d• Advantages: : fast responce• Disadvantages: high voltage and small discplacement

Design examples

Electromagnetic valves• Uses solenoid actuator with a magnetic core and a coil• Advantage: : large deflection• Disadvantage: low efficiency

Design examples

Electrochemical valves• Actuated by a bubble created by water electrolysisConsumes 4.3uW (10000 smaller that thermopneumatic !!!)2.5 V operational voltage

Capillary force valvesCapacity of double layer.• ElectrocapillaryMaximum value of surface tension at V 0.

Thermocapillary effect• Caused by temperature dependence of surfacetension: viscosity and surface tension both dropwith the temperature increase

Passive capillary effectExample: a bubble valve is designed with a vapour bubble betweenchannel sections 50um and 200um sections. What pressure can itwithstand?σ w=72*10 -3 N/m

Micropumps• micropumpsmechanicalNon-mechanical

Mechanical pumps• Require an electromechanical actuator, eitherexternal or integrated• External actuators: drawback: : large size,advantages: : large force and displacement.• Integrated actuators: : fast responce and reliability

Parameters of micropumps• Maximum flow rate (determined(at 0 back pressure)• Maximum back pressure (at which flow becomes 0),also described as pump head• Pump efficiency

Check-valvepump• Function under conditions of small compression ration andhigh pump pressure– Compression ratio– High pump pressureDesign rules• Minimize critical pressure• Maximize stroke volume• Minimize the dead volume• Maximize the pump pressure using large force actuators

Design examples• Typical micro check valves

Peristalstic pumps• Do not require passive valves to set flow direction• Require 3 or more chambers (actuallyjust valvesconnected in seria)• Drawbacks: leakage and small pressure difference,require a one check valve to prevent back flow• Design rules: : large stroke and large compressionratio.

Example• A peristaltic pump has 3 chambers and 3 circularunimorph piezodiscs, membrane diameter 4mm,frequency 100Hz, maximum deflection 40um.

Design examples

Valvless rectification pumps• Diffusers/nozzlesused instead of check valves forflow rectification∆p= ξpu2Fluidic diodicity:2ξ - pressure loss coefficientξηF=ξnegativepositiveFlow rate:Q=2∆VfηηFF−1+ 1χ - rectification efficiency

Design examplesTesla pump:χ=0.045, η=1.2

Micromixers• mixing in microscale relies mainly ondiffusion due to laminar flow at lowReynolds numbersτ =2d2D

Lamination in mixer• parallel lamination• sequential lamination

Focusing in mixer• geometric and hydrodynamic focusing

Mixing by twisting theflowStroockA. D., DertingerS. K. W., AjdariA., MezicI., Stone H. A., Whitesides G. M.“Chaotic Mixer for Microchannels” Science 295, 647-651 (2002)

Electrohydrodynamicmixing• Difference in liquids conductivities and permittivitiescreated transversal flows across the interface anddestabilized it, promoting mixing.

Magnetoelectrodynamicmixing• Alternating electric fields were applied to the chamber with twoindependent center electrodes in the presence of a magnetic field

Mechanical mixing• Stirring• Oscillating walls• Magnetic particles etc.