Biomedical Engineering Reference
In-Depth Information
genomics,.proteomics,.drug.discovery,.and.POC.medical.diagnostics.can.also.beneit.from.
these.technologies.
1-15
.To.fully.realize.the.potential.of.this.ield,.rapid.prototyping.of.new.
and. intricate. chips. without. compromising. important. material. properties. must. irst. be.
developed.
Microfabrication.of.complex.patterns.into.silicon,.glass,.or.quartz.is.traditionally.resource.
intensive..In.addition,.it.typically.requires.costly.investments.in.large.specialized.equip-
ment,.a.clean.room,.and.consumables.
5
.However,.the.introduction.of.poly(dimethylsiloxane).
(PDMS).has.enabled.the.accelerated.progress.of.rapid.prototyping.in.the.ield.of.microlu-
idics. through. soft. lithography.
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. Soft. lithography. is. generally. used. to. construct. features.
measured.in.the.micro-.to.nanometer.scale,.and.it.drastically.reduces.the.chip.fabrication.
time..PDMS.is.one.of.the.most.popular.materials.used.in.microluidics.because.it.is.opti-
cally. transparent. (from. 240. to. 1,100. nm),. lexible,. gas. permeable. (for. cell-based. assays),.
and.low.cost.
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.However,.the.inherent.material.properties.of.this.polymer.present.signii-
cant. limitations.. The. disadvantages. of. PDMS. include. its. hydrophobicity. and. absorption.
of. organic. solvent. and. small. molecules.. Hydrophobic. and. small. molecules. absorb. eas-
ily. into. the. porous. PDMS. matrix.. The. absorption. of. molecules. could. potentially. affect.
results,. which. is. unacceptable. for. many. analytical. applications.
18
. PDMS. also. swells. in.
certain.organic.solvents.and.has.inherently.unstable.surface.properties.
19
.Due.to.its.high.
lexibility,. channels. fabricated. using. PDMS. tend. to. expand. and. contract. with. different.
pressures..Although.this.property.could.potentially.beneit.some.applications,.it.could.be.
detrimental. to. others.. These. limitations. have. prevented. the. industrial. use. of. PDMS. for.
applications.in.drug.discovery.and.other.sensitive.assays.
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.Thus,.PDMS.has.been.mainly.
used.in.academic.prototyping.
Instead.of.using.PDMS,.industry.prefers.to.work.with.more.rigid.and.chemical-resis-
tant.materials,.such.as.polystyrene.(PS)..PS.is.optically.transparent,.biocompatible,.inert,.
rigid,.and.its.surface.can.be.easily.functionalized..In.addition,.it.is.economical,.recyclable,.
and.can.be.molded.into.desired.shapes..The.applications.of.PS.range.from.daily.items,.
such. as. CD. cases. and. cups,. to. laboratory. items,. such. as. cell. culture. petri. dishes,. test.
tubes,.and.microtiter.plates.
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.Its.hydrophobic.surface.can.be.easily.made.hydrophilic.by.
various.means,.through.corona.discharge,.gas.plasma,.irradiation,.and.surface.chemis-
try.. However,. the. cost. microfabricating. intricate. patterns. on. such. plastics. is. limited. to.
academic.pursuit.
To. overcome. this. persistent. chasm. between. academic. prototyping. and. industry. stan-
dards,.various.approaches.have.been.investigated..For.example,.hot.embossing.is.used.in.
polymer.microfabrication.to.rapidly.produce.identical.devices.using.a.stamp.to.create.pat-
terns.in.polymeric.materials..However,.creating.a.precise.stamp.with.microfeatures.is.often.
time-.and.expense-intensive..To.obviate.this.issue,.an.epoxy.stamp,.which.is.durable,.rela-
tively.inexpensive,.and.rapid.to.produce,.has.been.developed.
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.Although.the.epoxy.stamp.
addresses.the.resource.problem.for.creating.a.stamp,.it.does.not.help.reduce.the.time.and.
expense.required.for.fabricating.the.mold..Others.have.attempted.to.address.this.issue.by.
developing.PS-based.thermoplastic.elastomer.gels.to.create.viscoelastic.three-dimensional.
(3D).stacked.chips.using.a.molding.approach.
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.Perluoropolyether.(PFPE),.which.is.resis-
tant.to.organic.solvents,.was.molded.to.develop.a.DNA.synthesizer.
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.Thermoset.polyester.
was.used.to.develop.microluidic.devices.
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.Shrinkable.PS.ilms.were.also.used.previously.
to.demonstrate.high-aspect-ratio.microstructures;.however,.a.complete.microluidic.chip.
was.not.developed.heretofore.with.this.approach.
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In. this. chapter,. we. will. describe. a. new. manufacturing. process. that. obviates. the. most.
expensive.and.dificult.aspect.of.micro-.and.nanofabrication,.which.is.patterning.at.high.
