Biomedical Engineering Reference
In-Depth Information
devices.are.briely.discussed..Then.in.Section.18.4,.various.nanoluidics-based.biomedi-
cal. applications. such. as. DNA. separation,. DNA. mapping,. and. electroporation. are. thor-
oughly. discussed.. In. Section. 18.5,. we. give. the. summary. and. perspectives. to. complete.
this.chapter.
18.2 Basic Theories
Whether.our.luidic.knowledge.in.the.large.scale.can.be.applied.to.nanoluidics.is.a.fre-
quently.argued.issue..It.is.found.that.the.continuum.theory.still.holds.and.we.can.steadily.
apply.classical.luid.theories.such.as.Navier-Stokes.equations.in.nanoluidics,.as.long.as.
the.minimum.length.scale.is.larger.than.2.nm.(Bocquet.and.Charlaix.2010)..However,.we.
must.incorporate.dominant.driving.forces.and.important.surface.effects.in.order.to.well.
explain.some.phenomena.in.nanoluidics..In.this.section,.we.irst.introduce.some.major.
driving.forces.in.nanoluidics.in.Section.18.2.1..Two.important.phenomena,.i.e.,.electroki-
netics.and.multiphase,.and.their.unique.properties.in.nanoluidics.are.further.discussed.
in.Sections.18.2.2.and.18.2.3.
18.2.1 Driving Forces in Nanofluidics
Due. to. the. extremely. high. surface-to-volume. ratio,. driving. forces. in. nanoluidics. are.
mainly.surface.forces..However,.hydrodynamic.pressure.is.an.exception,.since.it.is.gen-
erally.not.practical.to.use.a.conventional.pump.to.drive.luid.low.in.nanoscale.unless.a.
substantially.high.pressure.difference.can.be.provided..Various.surface.forces.include.the.
electrostatic.force.(either.repulsive.or.attractive).between.charged.surfaces.and.electrolytes.
or.polyelectrolytes,.van.der.Waals.force.at.cases.when.nanoparticles.or.molecules.get.close.
to.each.other.or.to.the.solid.wall,.electroosmotic.force.caused.by.the.ionic.low.due.to.the.
surface.charge.of.a.solid.wall.and.the.applied.electric.ield,.and.adhesive.force.between.
gas,.luid,.and.solid.wall..Note.that.surface.tension,.due.to.the.cohesive.force.in.liquid,.is.a.
special.form.of.van.der.Waals.force.
Conventional.body.forces.such.as.gravity.and.buoyancy.are.almost.always.neglected.due.
to.their.small.magnitudes.at.nanoscale..However,.the.electrophoresis.(EP),.which.might.be.
a.body.force.if.an.object.is.speciied.with.a.net.volume.charge,.is.extremely.important.in.
nanoluidics..Also,.the.magnetic.force,.as.a.body.force,.can.be.used.in.driving.magnetic.
particles.in.nanoluidics.
Concentration-induced. diffusion. can. be. also. used. as. a. driving. force. in. nanoluidics..
However,.the.diffusion.is.much.hindered.in.nanoscale.due.to.the.strong.coninement.effect..
For. example,. the. diffusion. of. glucose. in. a. 1D. Si. nanochannel. array. shows. a. zero-order.
kinetics.(linear.diffusion.with.time),.while.large.proteins.such.as.immunoglobulin.G.(IgG).
are.almost.totally.excluded.from.diffusing.through.(Desai.et.al..1999)..Hindered.diffusion.
in.such.nanoluidic.devices.can.be.used.for.cell.immunoisolation.and.viral.iltration,.since.
the. immunogenic. components. such. as. cytotoxic. cells,. macrophages,. virus,. antibodies,.
and.cytokines.can.be.eficiently.blocked.by.size.exclusion,.while.small.molecules.such.as.
oxygen,.glucose,.and.insulin.are.suficiently.permeable.
Finally,. it. is. worth. noting. that. mechanical. or. ield. forces. generated. by. AFM,. optical.
tweezers,. and. magnetic. tweezers. can. be. combined. with. electrokinetics. or. luid. low.
in. nanoluidics. to. control. the. dynamics,. i.e.,. conformation. change. and. movement.
