Biomedical Engineering Reference
In-Depth Information
Drawing. is. a. method. to. produce. single. nanoibers.. It. requires. a. minimum. amount. of.
equipment.and.is.a.discontinuous.process..A.micropipette.is.dipped.into.a.droplet.near.
the.solution-solid.surface.contact.line.via.a.micromanipulator,.and.then.the.micropipette.
is. withdrawn. from. the. liquid. at. a. certain. speed,. yielding. a. nanoiber.. These. steps. are.
repeated.many.times,.each.time.with.a.different.droplet..The.solution.viscosity,.however,.
increases. with. solvent. evaporation,. and. some. iber. breaking. occurs. due. to. instabilities.
that.occur.during.the.process.(Ondarcuhu.and.Joachim,.1998)..The.main.limitation.of.this.
method.is.that.it.can.only.use.solutions.of.viscoelastic.materials,.which.can.undergo.strong.
deformations. that. are. cohesive. enough. to. withstand. the. stresses. developed. during. the.
pull..Again,.the.iber.diameter.is.dependent.on.the.oriice.size.used..It.is.dificult.to.obtain.
iber.diameters.of.less.than.100.nm.
Phase.separation.is.another.process.utilized.in.nanoiber.production..The.main.principle.
of.this.method.is.that.a.polymer.dissolved.in.an.appropriate.solvent.is.stirred.at.a.certain.
gelation.temperature.for.a.period.of.time.until.a.homogeneous.solution.is.obtained..Then,.
the.formed.gel.is.immersed.in.water.several.times.for.solvent.exchange.to.occur..The.inal.
gel. is. lyophilized.. Phase. separation. occurs. due. to. chemical. incompatibility. and. results.
in.nanoibers,.but.the.problem.is.that.a.long.time.is.needed.to.convert.the.polymer.into.
a.nanoporous.foam..The.iber.diameter.ranges.from.50.to.500.nm.with.a.length.of.a.few.
micrometers.(Zhang.et.al.,.2005),.which.is.a.disadvantage.for.some.applications..Moreover,.
only.polymers.that.have.gelation.capability.can.be.used.to.obtain.nanoibrous.structures.
Wet.spinning.is.a.process.similar.to.drawing,.with.the.difference.being.that.a.polymeric.
solution.is.precipitated.or.coagulated.by.dilution,.adding.into.a.nonsolvent.or.a.chemical.
reaction.. Fibers. produced. by. this. method. are. continuous. and. in. microscale. (from. 10. to.
100. µm).. Polymer. solution. concentration. and. the. spinneret. diameter. are. the. two. crucial.
parameters.that.inluence.the.resulting.iber.diameter.
The.last,.and.one.of.the.most.preferred,.methods.used.to.prepare.ibers.in.nano-.and.low.
microscale. is. the. electrospinning. process.. High. electrostatic. forces. are. applied. to. draw.
continuous.ibers.from.a.polymeric.solution.through.a.syringe..Basically,.four.components.
are.required.to.accomplish.the.process:.a.capillary.tube.ending.in.a.needle.of.small.diam-
eter,.a.high-voltage.supplier,.a.syringe.pump,.and.a.metallic.component..One.of.the.elec-
trodes. is. connected. to. the. needle. of. the. syringe. containing. the. polymer,. and. the. other.
one.is.connected.to.the.collector..Leakage.of.the.polymer.solution.is.prevented.by.surface.
tension;.however,.as.the.applied.potential.increases,.the.surface.tension.contribution.starts.
to.decrease.as.a.result.of.charge.repulsion,.and.the.luid.shape.at.the.needle.tip.becomes.
like.a.cone,.what.is.called.the.Taylor.cone..The.voltage.is.increased.until.the.repulsive.elec-
trostatic.forces.overcome.the.inluence.of.solution.surface.tension..Meanwhile,.the.solvent.
evaporates.and.the.charged.jet.of.the.solution.is.ejected.from.the.needle.tip.in.the.shape.
of.a.long,.continuous,.thin.iber..Therefore,.ibers.with.various.diameters.are.collected.on.
either. a. stationary. or. rotating. collector.. Rotating. collectors. are. used. to. produce. aligned.
nanoibers,.which.are.more.preferable.in.certain.applications..A.variety.of.polymers.have.
been.used.until.now.to.produce.ibers.from.tens.of.nanometers.to.a.few.micrometers.(Jin.
et.al.,.2004;.Li.et.al.,.2005;.Ma.et.al.,.2005;.Ndreu.et.al.,.2008;.Erisken.et.al.,.2008;.Jeong.et.al.,.
2009;.Kenar.et.al.,.2009;.Yucel.et.al.,.2010).
Certain.properties.are.expected.from.a.polymer.that.is.proven.to.be.spinnable..The.ibers.
should.be.consistent,.defect-free,.and.have.controllable.diameters..The.main.parameters.
to.control.are.process.conditions.such.as.polymer.type,.its.concentration,.solvents,.applied.
potential,.distance.between.the.needle.tip.and.the.collector,.and.the.collector.type..These.
parameters. are. modiied. to.control. iber. thickness,. porosity. and. pore. size. (or. rather. the.
spacing. between. the. ibers. or. iber. density),. surface. chemistry,. and. surface. topography.
