Biomedical Engineering Reference
In-Depth Information
electrode. system,. Yasukawa. et. al.. successfully. directed. particles. of. different. sizes. into.
various.locations.of.the.low.stream,.as.shown.in.Figure.17.8b.(Yasukawa.et.al..2009).
17.4.2 DeP Trapping for Particle Positioning
Besides.sorting.particles.of.a.large.population,.trapping.and.further.positioning.single.or.
a.few.particles.by.DEP.is.also.favored.on.some.occasions..The.quadrupole.microelectrode.
and.its.electrodeless.alternatives.are.often.used.in.these.applications.for.their.allowance.of.
conductive.liquid.medium.(e.g.,.the.physiological.luids.and.the.culture.growth.medium)..
In.quadrupole.microelectrode.DEP,.four.electrodes.are.usually.arranged.at.the.corners.of.
a.cross.geometry..When.an.electric.signal.is.applied,.a.potential.well.is.naturally.created.
at. the. center.. By. choosing. appropriate. DEP. conditions,. colloids. with. sizes. varying. from.
650. to. 14.nm. have. been. successfully. captured. either. at. electrode. edges. (with. high. ield.
strength). or. the. cross. center. (with. low. ield. strength),. as. shown. in. Figure. 17.9a. and. b..
Various. particles. could. also. be. captured. simultaneously.. As. illustrated. in. Figure. 17.9c,.
the.herpes.simplex.virus.was.trapped.under.n-DEP.force.at.the.center.of.the.polynomial.
quadrupole. microelectrodes. while. tobacco. mosaic. virus. was. collected. at. the. electrode.
edge.surface.(Morgan.et.al..1999)..The.minimum.size.of.particles.with.effective.trapping.
was.found.to.be.proportional.to.one-third.of.the.trap.width.and.the.electric.ield.gradient.
(Hughes. and. Morgan. 1998).. Sometimes,. a. single. quadrupole. electrode. set. cannot. create.
closed.trapping.as.the.gravity.tends.to.pull.particles.down.to.the.substrate.surface.slowly..
If.this.occurs,.octopole.electrode.structure.may.be.used,.in.which.one.set.of.polynomial.
electrodes.is.placed.in.the.planar.and.the.other.set.on.its.top,.together.with.other.electrode.
structure.deviates..As.particles.can.be.caged.in.the.ield.minimum.region.(e.g.,.the.center.
of.the.cross),.highly.conductive.liquid.medium.(e.g.,.physiological.solution).are.allowed.in.
these.DEP.trapping.without.signiicant.impacts.from.water.electrolysis.or.Joule.heating..
This.makes.them.a.valuable.manipulation.tool.in.applications.involving.cells.or.proteins..
As.shown.in.Figure.17.9d.through.f,.a.single.yeast.cell.was.initially.captured.at.the.center.
of. a. planar. quadrupole. microelectrode. set. and. it. grew. into. a. large. cell. aggregate. when.
the. trapping. electric. signal. (i.e.,. 1.4.
V
rms
. at. 7.7.MHz). was. maintained. for. 2. days.. Similar.
strategies. were. also. adopted. to. trap. single. particles. or. cells. to. accomplish. biochemical.
assays.(Kuo.and.Hsieh.2009;.Voldman.et.al..2002)..With.strong.Brownian.motions,.single.
nanoparticles. are. generally. more. dificult. to. get. trapped. (Zheng. et. al.. 2003).. However,.
with.a.slightly.different.geometry.like.bead's.beak,.an.electrodeless.quadrupole.electrode.
design.was.used.as.electronic.tweezers.to.capture.single.molecules.(Cohen.and.Moerner.
2006)..As.shown.in.Figure.17.9g.through.i,.single.proteins,.lipid.vehicles,.and.virus.have.
been.successfully.conined.at.the.center.stagnation.point.the.four.electrodes.point.toward.
with.their.Brownian.motions.largely.suppressed.
17.4.3 DeP Levitation for Particle Deflection
Besides.trapping,.the.DEP.force.can.also.be.used.as.a.delector.to.levitate.particles.to.different.
directions.based.on.their.various.sizes.and.dielectric.properties..As.shown.in.Figure.17.10a,.
with.appropriate.DEP.signals.(e.g.,.a.speciic.frequency.around.the.crossover.frequency.of.
a.certain.population.of.particles),.strong.n-DEP.forces.from.the.delector.electrode.levitate.
small.particles.into.the.top.channel.while.the.force.on.the.large.particles.is.near.zero.(because.
the. frequency. is. near. their. crossover. frequency).. Therefore,. the. large. particles. are. nearly.
unaffected.and.continuously.travel.with.the.luid.into.the.bottom.channel..More.delectors.
can.be.combined,.one.after.another.or.side.by.side,.to.achieve.a.high.delecting.resolution..
