Biomedical Engineering Reference
In-Depth Information
any.disruption.(e.g.,.a.big,.bright.trap.at.the.center.of.the.view.ield)..One.major.drawback.
of. current. dynamic. holographic. SLM. comes. from. its. relatively. slow. operating. speed.
(varying. from. a. few. hertz. to. several. kilohertz).. In. contrast,. an. EOD. offers. an. operating.
speed. as. high. as. 10.MHz. and. a. switching. time. as. short. as. 100.ns.. This. is. achieved. by.
varying. the. refractive. index. of. the. equipped. crystal. with. programmed. electric. signals..
However,.its.high.cost.prevents.its.wide.application.in.optical.tweezers..As.high.operation.
speed.and.short.response.time.is.generally.involved.in.dynamic.multiple.traps,.QDPs.and.
high-speed.cameras.are.widely.needed.in.the.characterization.and.calibration.of.optical.
tweezers.with.this.function.
19.4 Optical Tweezers Applications in Life Science
Because.of.its.noncontact.feature.for.trapping.and.manipulation.and.its.accuracy.in.molec-
ular.force.measurement,.the.optical.tweezers.technique.has.been.quickly.used.in.life.sci-
ence,. since. its. invention.. Numerous. successful. applications. have. been. reported. on. the.
trapping.and.manipulation.of.single.or.multiple.cells.or.subcellular.components.as.well.
as.single.molecule.level.force.and.interaction.measurements.on.biomolecules.(e.g.,.DNA,.
proteins)..Many.other.techniques,.such.as.Raman.spectroscopy,.confocal.microscopy,.and.
patch-clamp. technology,. have. also. been. integrated. or. combined. with. optical. tweezers.
technologies.to.further.diversify.their.applications.in.both.molecular.and.cell.biology.
19.4.1 Biomechanics
The.major.optical.tweezers.application.in.molecular.biology.lies.in.the.exploitation.of.the.
physical. and. mechanical. properties. of. various. polynucleotide. acids. (e.g.,. DNA,. RNA),.
molecular.motors.(kinesin),.and.other.subcellular.components.(e.g.,.actin.ilaments,.micro-
tubules)..The.optical.trapping.of.a.single.biomolecule.usually.requires.anchoring.one.end.of.
its.molecular.chain.to.an.easy-to-trap.object,.such.as.a.dielectric.microparticle..The.other.end.
of.the.biomolecule.is.either.bound.to.the.surface.of.another.microparticle,.a.coverslip,.or.a.
micropipette.(Figure.19.6)..Upon.the.establishment.of.an.optical.trap,.by.moving.the.attached.
coverslip.or.micropipette.(or.changing.the.relative.displacement.between.two.ends.of.the.
biomolecules),.the.trapped.molecule.or.its.fragments.experience.deformations.(i.e.,.stretch-
ing).. The. resultant. intrinsic. molecular. force. change. leads. to. a. difference. in. the. required.
trapping.force..Optical.tweezers,.therefore,.serve.as.the.force.sensor.to.indicate.the.force.or.
stress.dynamics..Since.the.trapped.microparticle.is.free.to.rotate,.the.tension.or.torque.on.
individual.molecules.may.also.be.attained.if.the.other.end.of.the.molecule.(the.end.attached.
to.a.micropipette.in.Figure.19.6a).is.attached.to.another.microparticle.(so-called.a.rotor.bead).
or.a.micropipette,.which.twists.or.rotates.instead.of.following.a.linear.displacement.
19.4.1.1 DNA and RNA Mechanics
With. optical. tweezers,. the. typical. measured. force. range. is. 0.1-100. pico-Newton. (pN),.
which.satisies.most.force.measuring.needs.for.polynucleotide.acids..The.force-extension.
dynamics. for. a. single. dsDNA. or. ssDNA. can. be. easily. measured. with. an. optical.
trap. coniguration. shown. in. Figure. 19.6a.. For. B-type. DNA. molecules,. four. different.
deformation. stages. exist:. (1). the. entropic. elasticity. regime. when. DNA. align. straight;.
(2). the. intrinsic. elasticity. regime. when. DNA. deform. toward. its. full. contour. length;.
