Laser Direct-Write - The Nanobiotechnology - page 277

power. and. short. pulse. length. of. the. laser. ensures. that. it. is. absorbed. nearly. entirely. by.
the.target.matrix.and.minimizes.damage.to.the.cells..The.heated.matrix.rapidly.expands,.
forcing.the.attached.material.to.detach.and.move.to.the.receiving.substrate..As.the.target.
material.is.clear,.and.the.transfer.material.can.be.heterogeneous,.a.coaxial.camera.is.often.
used.with.MAPLE.DW..The.camera.is.used.to.select.speciic.areas.of.the.print.ribbon.for.
transfer.(Spargo.et.al..2001;.Wu.et.al..2003).
13.3 Mechanism of Transfer
Each.form.of.laser.direct-write.explicitly.involves.a.laser.pulse.used.to.excite.the.matrix.
material.that.is.held.on.a.print.ribbon..This.causes.the.transfer.material.to.release.from.
the. print. ribbon. when. excited. and. transfer. to. the. receiving. substrate.. Understanding.
the. dynamics. involved. in. the. transfer. mechanism. is. critical. for. accurate. and. precise.
printing.. Examples. reviewed. in. this. chapter. focus. on. a. particular. variation. of. laser.
direct-write.
The.dynamics.of.laser-material.interactions.are.generalizable,.and.thus.speciic.studies.
can. present. useful. commonalities.. For. each. method. of. laser. direct-writing,. optical.
interaction.between.the.laser.pulse.and.the.target.on.the.print.ribbon.energizes.the.transfer.
material..Laser.energy-dependent.thermal.processes.lead.to.material.ejection.(Miotello.
and. Kelly. 1999;. Garrison. et. al.. 2003;. Paltauf. and. Dyer. 2003;. Zhigilei. 2003;. Vogel. et. al..
2008)..For.luences.on.the.order.of.a.few.hundred.mJ/cm 2 ,.plasma.generation.is.not.likely.
to.occur,.although.it.could.become.an.issue.for.higher.energy.densities..Vaporization.does.
not.occur.for.timescales.less.than.1.ns,.and.normal.boiling.encounters.kinetic.obstacles.
in.nucleation.for.timescales.less.than.100.ns..For.solid.targets,.explosive.sublimation.can.
take.place.and.faster.pulses.can.cause.fracture.via.a.fast.stress.increase.without.causing.
sublimation. (Sigrist. and. Kneubijhl. 1978;. Park. et. al.. 1996;. Hosokawa. et. al.. 2001,. 2002;.
Apitz.and.Vogel.2005)..For.longer.laser.pulses,.different.excitation.mechanisms.may.take.
precedence. over. the. course. of. the. pulse.. The. eficiency. of. material. expulsion. tends. to.
increase.strongly.with.decreased.pulse.duration.for.a.set.total.radiant.energy.exposure..
Relations. describing. the. acoustic. waves. created. in. a. medium. by. laser. pulse. interaction.
have.been.developed.and.serve.to.describe.the.dynamics.of.local.excitation.of.the.bulk..
In.combination.with.the.surface.effects.on.the.target.material,.transfer.material.is.ejected.
from.the.ribbon,.and.travels.to.the.receiving.substrate..Transfer.can.occur.as.a.spray.or.
a.jetted.droplet..Viscous.forces.in.the.transfer.material.are.critical.for.the.formation.of.a.
coherent.jet,.and.help.limit.dispersion.(Brown.et.al..2011)..Precise.understanding.of.this.
phenomenon.is.vital.for.accurate.and.repeatable.printing..Additionally,.precise.knowledge.
of.the.forces.active.in.the.laser.direct-write.process.are.critical.for.ensuring.the.transfer.
and.viability.of.fragile.organic.molecules.and.cells..Because.of.the.vulnerability.of.cells.
and.biological.molecules.both.to.stress,.thermal,.and.optical.damage,.it.is.essential.that.
steps.are.taken.to.ensure.that.delicate.print.materials.are.not.harmed.by.laser.direct-write.
processing.
13.3.1 Material ejection
In. AFA-LIFT,. laser. energy. interaction. with. both. metal. and. polymer. sacriicial. target.
ilms.causes.bubble.formation.on.the.print.ribbon..Fluences.at.the.threshold.of.consistent.