Biomedical Engineering Reference
In-Depth Information
when.the.EDL.thickness.of.the.ionic.liquid.is.comparable.to.the.size.of.the.nanostructures..
In. order. to. avoid. such. complicated. EO-EP. interaction. with. the. movement. of. charged.
particles.or.electrolytes,.usually.the.EO.is.suppressed.by.either.increasing.the.ion.strength.
of.solution.or.coating.the.surface.with.neutral.polymer.such.as.polyethylene.glycol.(PEG)..
These.methods.have.been.widely.adapted.in.capillary.EP.
18.2.3 Multiphase Phenomena in Nanofluidics
Surface. tension,. wetting,. and. contact. angles. are. terms. we. usually. use. to. describe. a.
multiphase.system..Surface.tension.is.the.ability.of.a.liquid.to.maintain.its.shape.under.
external.forces..It.is.caused.by.the.net.cohesive.force.of.liquid.molecules.near.the.surface..
Surface.tension.of.a.liquid.is.dependent.on.temperature,.but.it.can.also.be.modiied.with.
an. applied. electric. ield,. and. this. effect. is. termed. as. electrowetting. or. electrowetting-
on-dielectric. (EWOD). (Cho. et. al.. 2003;. Chang. and. Yeo. 2009).. Wetting. is. the. ability. of. a.
liquid.to.stay.in.contact.with.a.solid..It.is.balanced.by.the.adhesive.force.tending.to.spread.
the. liquid. on. the. solid. surface. and. the. surface. tension,. which. resists. the. adhesive. force.
between.liquid.and.solid..Contact.angle.(either.static.or.dynamic).is.the.angle.at.the.liquid,.
solid,.and.gas.three-phase.interfaces.due.to.the.balance.of.phase-phase.interactions.on.a.
droplet..It.can.be.determined.from.the.total.surface.energy,.which.is.dependent.on.surface.
roughness,.i.e.,.micro-.or.nanostructures.of.a.surface,.and.properties.of.three.phases.
Multiphase. phenomena. have. been. widely. used. in. many. applications. such. as. surface.
treatment.(antifog.coatings.or.self-cleaning.surfaces).and.chemical.or.biological.sensors..
For.example,.by.controlling.the.growth.and.alignment.of.polyaniline.nanoibers.on.a.lat.
or. patterned. surface,. superhydrophobicity. (“lotus. effect”). and. superhydrophilicity. can.
be.achieved.(Chiou.et.al..2007)..Such.surface.coating.with.nanostructures.has.been.used.
to.stretch.DNA.molecules.and.might.be.useful.for.biomedical.applications.such.as.DNA.
mapping.
Another.application.that.uses.multiphase.low.is.molecular.or.DNA.combing,.which.uti-
lizes.the.receding.of.meniscus.to.stretch.and.align.DNA.molecules.on.a.lat.or.patterned.
hydrophobic. surface. (Ondarçuhu. and. Joachim. 1999;. Otobe. and. Ohtani. 2001;. Petit. and.
Carbeck.2003)..Recently,.it.has.been.demonstrated.that.by.using.the.patterned.microwell.
or.nanopillar.arrays,.DNA.molecules.can.be.uniformly.stretched.and.well.aligned.to.form.
DNA.nanowire.arrays.(Guan.and.Lee.2005;.Guan.et.al..2007)..These.DNA.nanowires.can.
be.easily.transferred.to.a.lat.surface,.and.thus,.this.process.is.promising.for.large-scale.
DNA.mapping.(see.Section.18.4.2).
The. generation. of. individual. droplets. separated. by. gas. phase. in. microluidic. devices.
has. been. used. in. many. biomedical. applications. such. as. detection. and. quantiication. of.
biological.agents.(Lai.et.al..2004)..However,.due.to.the.extremely.small.pneumatic.pressure.
in.nanovalves,.luid.low.might.not.be.eficient.in.controlling.nanodroplets.in.nanoluidic.
channels..An.alternative.method.might.use.the.aforementioned.electrowetting.technique.
to.move.nanodroplets.on.the.surface.or.inside.nanochannels.
18.3 Fabrication of Nanofluidic Devices
More. details. on. nanofabrication. for. bioapplications. can. be. found. in. “Part. II,.
Nanofabrication”.of.this.topic..Here,.we.just.list.some.fabrication.methods.for.nanoluidic.
