Biomedical Engineering Reference
In-Depth Information
the.Taylor.cone.to.the.collector.
10,40
.Additionally,.morphological.changes.can.occur.upon.
decreasing.the.distance.between.the.syringe.needle.and.the.substrate.by.forming.beaded.
as-spun.ibers,.which.can.be.attributed.to.inadequate.drying.of.the.polymer.iber.prior.to.
reaching.the.collector.
38
23.2.2 System Parameters
In.addition.to.the.process.parameters,.a.number.of.system.parameters.play.an.important.
role.in.iber.formation.and.structure..System.parameters.involve.molecular.weight,.molec-
ular.weight.distribution,.the.architecture.of.the.polymer,.and.solution.properties..Solution.
properties. play. especially. an. important. role. in. the. electrospinning. process.. In. relative.
order.of.their.impact.on.the.electrospinning.process,.these.include.polymer.concentration,.
solvent.volatility,.and.solution.conductivity.
23.2.2.1 Polymer Concentration
Polymer.concentration.inluences.both.the.viscosity.and.the.surface.tension.of.the.solution..
The.formation.of.nanoibers.through.electrospinning.is.based.on.the.uniaxial.stretching.of.
a.viscoelastic.solution.
5
.For.instance,.the.polymer.solution.must.have.a.concentration.high.
enough. to. cause. polymer. entanglements.. However,. the. viscosity. should. not. be. so. high.
as.to.prevent.polymer.motion.induced.by.the.electric.ield,.and.not.so.low.as.to.prevent.
the.charged.jet.from.collapsing.into.droplets.before.the.solvent.has.evaporated.
41
.Further.
investigations.on.polymer.concentration.and.viscosity.have.observed.that.ibers.become.
more.uniform.and.assume.a.cylindrical.shape.with.increasing.polymer.concentration.in.
solution,.and.iber.diameters.also.increase.signiicantly.with.increasing.polymer.concen-
tration.. At. lower. concentrations,. increasingly. thinner. ibers. are. formed,. with. additional.
beads.along.the.iber.axis..However,.at.very.high.dilution,.iber.formation.no.longer.takes.
place.
42
.It.is.not.possible.to.make.a.general.recommendation.for.particular.concentrations.
and.the.resulting.viscosities,.because.the.ideal.values.of.these.parameters.vary.considerably.
with.the.polymer-solvent.system.
23.2.2.2 Solvent Volatility
The.choice.of.solvent.is.also.critical.as.to.whether.ibers.are.capable.of.forming,.as.well.as.
determining,.surface.topography.
36,39
.If.the.luid.jet.is.collected.prior.to.complete.solvent.
evaporation,. the. deformable. ibers. may. either. latten. upon. impact. with. the. surface. of. a.
collector.or.adhere.to.other.ibers..If.the.arriving.jet.or.iber.lands.on.previously.collected.
ibers,.the.still.luid.material.can.merge.and.coalesce.at.a.crossing.point.to.create.a.conglu-
tinated.network,.as.shown.in.Figure 23.2..Conglutinated.networks.appear.to.be.useful.in.
some.situations.where.a.well-established.network.is.desirable.
23.2.2.3 Solution Conductivity
Solution. conductivity. can. inluence. iber. size. within. one. to. two. orders. of. magnitude.
36
.
Solutions.with.high.conductivity.will.have.a.greater.charge-carrying.capacity.than.solu-
tions.with.low.conductivity..Therefore,.the.iber.jet.of.highly.conductive.solutions.will.be.
subjected.to.a.greater.tensile.force.in.the.presence.of.an.electric.ield.than.will.a.iber.jet.
from.a.solution.with.low.conductivity..Highly.conductive.solutions.were.extremely.unstable.
in.the.presence.of.strong.electric.ields,.which.led.to.a.dramatic.bending.instability.as.well.
