Dielectrophoresis - The Nanobiotechnology - page 404

and. microscale. post. arrays. inside. the. low. channel. (Figure. 17.6d. through. f).. Unlike. in.
traditional.DEP,.where.metallic.electrodes.are.used,.the.electric.ield.gradient.in.EDEP.is.
preserved.and.may.reach.very.high.levels.without.the.occurrence.of.electrolysis.reactions..
Therefore,. luids. with. very. high. ion. concentration. (e.g.,. physiological. buffer. solutions).
and/or. a. high-voltage. AC. or. DC. signal. are. allowed. to. be. used. in. EDEP-based. particle.
manipulation.. This. greatly. breaks. the. limited. use. of. DEP. in. many. biological. systems,.
particularly.those.involving.live.cells.
As.shown.in.Figure.17.6a.through.c,.the.irst.type.of.EDEP.has.a.nonuniform.electric.ield.
created.from.spatial.contraction.and/or.the.expansion.of.the.entire.low.channel.geometry..
Similar. to. the. velocity. component. in. hydrodynamic. lows,. the. electric. ield. strength. in.
EDEP.also.varies.with.the.cross.section.area.of.the.low.channel.between.two.electrodes.
(according.to.the.Maxwell's.equations)..Therefore,.the.nonuniform.shape.of.the.low.chan-
nel. generates. a. high. electric. ield. gradient. locally,. without. direct. involvement. of. metal-
lic. electrodes. in. the. DEP. working. zone. (Chou. et. al.. 2002).. In. the. second. type. of. EDEP,.
the.insulator.post.arrays.are.embedded.in.the.low..The.shape.of.individual.posts.and.the.
created.post.array.patterns.generate.zones.with.high.and.low.electric.ield.strength..The.
availability.of.microfabrication.technologies.enables.precise.control.of.the.dimensions.of.
the.low.channel.and.post.arrays.at.micrometer.or.even.nanometer.scale..This.enables.the.
convenient.acquisition.of.the.desired.electric.ield.strength.and.successful.DEP.trapping.
for.nanoparticles.or.individual.macromolecules.
In.the.presence.of.an.electric.ield.in.low,.electrokinetics.(EP.and.EO).also.exist.simul-
taneously.with.EDEP..However,.due.to.its.higher.order.(second.order).dependence.on.the.
ield. strength,. EDEP. effects. usually. overcome. EP. and. EO. (their. dependence. to. the. ield.
strength.follows.a.linear.relationship).and.dominate.the.ield-mediated.particle.motions..
The.transitions.between.different.low.regimes.happen.around.a.ield.strength.threshold,.
namely,. the. DEP. trapping. threshold:. (1). At. low. ield. strength. (e.g.,.∼1.V peak-peak /mm),. EP.
and/or. EO. dominates. the. motions. of. particles. and. DEP. effect. can. be. ignored;. particles.
transport.uniformly.almost.everywhere..(2).At.high.ield.strength.(e.g.,.approximately.sev-
eral. hundred. V peak-peak /mm. to. a. few. kV peak-peak /mm),. DEP. dominates;. the. concentration.
and.trapping.of.particles.occurs.in.zones.with.high.or.low.ield.strength;.particles.can.be.
sorted.or.collected.through.a.trap-and-release.mechanism.(refer.to.Section.17.4)..(3).Near.
the.threshold,.DEP.apparently.is.unable.to.capture.suspended.particles.completely.but.is.
often.suficient.to.concentrate.and.rarefy.them.into.groups.or.ilamentary.along.the.low.
streams..Such.particle.motion.is.called.streaming.DEP..In.streaming.DEP,.the.aggregation.
pattern. of.suspended. particles. largely. relies. on. the.shape.of.the.insulator.posts.and.the.
angle.between.the.post.array.and.the.applied.electric.ield.(Cummings.and.Singh.2003)..
The.value.of.the.DEP.trapping.threshold.ield.strength.is.determined.by.the.ratio.of.the.
particle.mobility.from.EP/EO.to.that.of.DEP.(Cummings.et.al..2000).
17.3.4 Optoelectronic Tweezers
In.both.conventional.DEP.and.EDEP,.metallic.electrodes.are.included,.although.in.EDEP,.
they. are. placed. away. from. the. locations. where. DEP. motions. occur.. The. requirement.
for. physical. electrodes. is. often. inconvenient. and. sometimes. limits. the. lexibility. of. the.
applications.of.DEP.on.many.occasions.(e.g.,.particle.manipulation.and/or.transportation)..
Wu's.group.at.the.University.of.California-Berkeley.recently.used.“virtual.electrodes”.in.
DEP.applications.(Chiou.et.al..2005)..In.their.approach,.namely,.“optoelectronic.tweezers.
(OET),”. the. needed. nonuniform. electric. ield. is. generated. by. selectively. illuminating. a.
photoconductive.electrode.surface..The.basic.OET.setup.is.constructed.with.two.parallel.