Biomedical Engineering Reference
In-Depth Information
viscosity. creates. higher. initial. forces. on. the. modeled. cell. and. a. lower. ejection. velocity.
due.to.viscous.energy.dissipation..A.greater.distance.from.the.pressurized.gas.bubble.to.
the.cell.creates.a.lower.and.delayed.initial.acceleration,.although.this.has.little.effect.on.
ultimate.ejection.velocity..Finally,.a.higher.initial.bubble.pressure,.which.corresponds.to.
higher. laser. luence,. yields. increased. pressure. and. acceleration. on. the. cell. (Wang. et. al..
2009)..All.of.these.lend.support.to.the.advantages.of.a.thick.support.layer.for.cells.on.the.
ribbon;.that.said,.coatings.should.possess.a.low.viscosity,.and.the.laser.luence.should.be.
reduced.as.far.as.transfer.conditions.will.permit..This.model.predicts.very.short.duration.
accelerations. upward. of. 10
9
. m/s
2
,. which. is. supported. by. other. time-resolved. studies.
(Hopp. et. al.. 2005).. That. these. accelerations. are. sustained. and. survived. by. cells. is. likely.
attributable.to.their.extremely.short.durations.combined.with.the.viscoelastic.nature.of.the.
cell.mechanical.response.
13.3.2 Transfer
Following. the. irst. stage. of. excitation. by. the. laser. and. droplet. or. jet. formation,. material.
transfer.proceeds..Ejection.velocity.is.the.result.of.droplet.formation.dynamics.and.will.
determine.travel.time.and.impact.parameters.that.are.critical.in.cell-speciic.printing.oper-
ations.. The. magnitudes. of. effective. stress,. deceleration,. and. maximum. shear. strains. all.
increase.with.increasing.ejection.velocity..This.implies.that,.for.force.sensitive.materials.
like.cells,.the.lowest.luence.that.consistently.produces.ejection.of.the.transfer.material.will.
be.desired.
Transfer.of.a.luid.in.the.LIFT.and.AFA-LIFT.process.occurs.through.a.jetting.mecha-
nism..The.travel.process.for.a.luid.develops.in.four.stages..The.initial.gas.pocket.forma-
tion,.the.collapse.of.the.gas.pocket.and.development.of.the.jet,.the.jet.extension,.and.inally.
the.jet.breaking.up.into.multiple.droplets..Jet.breakup.is.not.useful.for.the.creation.of.single.
precise.spots..Hence,.the.separation.distance.from.ribbon.to.receiving.substrate.should.be.
minimized..Jet.formation.is.hypothesized.to.occur.due.to.inertial.effects.at.the.gas.balloon.
tip. (Duocastella. et. al.. 2008).. This. breakup. distance. is. dependent. on. material. properties.
and.the.speciics.of.transfer..It.is.vital.that.the.separation.of.ribbon.receiving.substrate.be.
less.than.the.breakup.distance.for.consistent.creation.of.a.single.deposited.drop.
13.3.3 receiving Substrate
Impact. experienced. during. landing. of. the. droplet. can. also. apply. signiicant. forces. to.
transferred.materials.(Wang.et.al..2007,.2008)..These.effects.are.largely.irrelevant.for.the.
transfer. of. metals,. but. potentially. damaging. for. printing. with. cells. and. other. delicate.
materials.(Lin.et.al..2009)..The.violence.of.impact.should.be.minimized.to.limit.mechanical.
damage.to.biological.materials..Culture.media.and.hydrogels.provide.elastic.cushioning.
for.deceleration.of.living.materials,.allowing.them.to.survive.very.high.accelerations.(Hopp.
et.al..2005)..FEM.simulations.with.hydrogels.have.indicated.that.there.is.a.critical.thickness,.
below. which. the. impact. is. not. fully. absorbed. by. the. receiving. hydrogel. layer,. and. the.
forces.experienced.by.the.cell.increase.due.to.a.secondary.impact.with.the.stiff.underlying.
substrate..Above.this.critical.value,.thicker.cushioning.layers.provide.negligible.additional.
beneit.. The. same. simulations. also. showed. that. even. thin. hydrogel. coatings. provide. a.
substantial.reduction.in.impact.stresses.compared.to.a.bare.rigid.receiving.substrate.
It. is. important. to. control. the. dynamics. of. the. transfer. process. as. described. earlier..
They.are.critical.to.accurate.printing,.thus.indicating.that.an.appropriate.luence.is.a.vital.
