Biomedical Engineering Reference
In-Depth Information
(McDonald.and.Whitesides.2002)..The.low.surface.energy.allows.easy.dissociation.of.ink.
from.the.stamp.during.printing..It.also.plays.critical.roles.in.the.selection.of.ink.materials.
and.solvents,.and.the.formation.of.ink.patterns.on.the.stamp..Fifth,.PDMS.is.transparent.
to.visible.light,.which.is.useful.for.process.control.such.as.aligning.a.stamp.and.a.substrate.
by. the. eye. and. microscopy.. Sixth,. PDMS. stamps. can. be. produced. easily. as. described.
in. the. following. section.. However,. some. of. these. properties. of. PDMS. can. also. restrict.
its. application. in. μCP.. Notably,. the. low. modulus. (compressive. modulus:. 2.0.N/mm
2
). of.
Sylgard.184.PDMS.renders.the.slender.relief.features.of.a.stamp.prone.to.deform,.buckle,.
or.collapse.(Schmid.and.Michel.2000)..Moreover,.the.stamp.tends.to.sag.upon.contact.with.
the.substrate.at.large.noncontact.areas..As.a.result,.it.is.typically.dificult.to.print.features.
smaller.than.1.μm.using.the.Sylgard.184.PDMS.stamp.(Delamarche.et.al..1997;.Hui.et.al..
2002).. To. overcome. this. limitation,. Schmid. and. Michel. (2000). developed. high-modulus.
PDMS.(also.called.hard.PDMS.and.h-PDMS).with.compressive.modulus.of.9.7.N/mm
2
.and.
layered. stamps. composed. of. a. layer. of. the. h-PDMS. with. relief. features. and. one. or. two.
backing.layers.
11.3.1.2 Preparation of PDMS Stamp
To. prepare. a. Sylgard. 184. PDMS. stamp,. prepolymer. (also. called. base). and. cross-linker.
(curing. agent). typically. at. a. 10:1. weight. ratio. are. mixed. and. poured. on. a. master,. which.
is. usually. prepared. by. photolithography. (Xia. and. Whitesides. 1998).. After. air. bubbles.
trapped.in.the.mixture.are.removed,.the.liquid.precursor.is.cured.usually.at.an.elevated.
temperature.(60°C-90°C).to.accelerate.the.cross-linking.reaction..The.cured.PDMS.consists.
of.cross-linked.chains.containing.the.-Si(CH
3
)
2
-O-.structural.unit..It.is.eventually.peeled.
off.from.the.master.and.cut.to.a.proper.size.as.the.stamp.
11.3.1.3 Surface Modiication of PDMS Stamp
The. hydrophobic. nature. of. PDMS. stamps. is. not. suitable. for. printing. many. polar. inks.
because.aqueous.solutions.of.the.inks.do.not.wet.the.stamp.surface..This.problem.can.be.
solved.or.alleviated.by.modifying.the.surface.properties.of.the.PDMS.stamps..Perhaps.the.
most.widely.used.approach.to.increasing.the.hydrophilicity.of.the.PDMS.stamps.is.through.
oxidation.by.either.oxygen.plasma.or.UV.ozone.treatment.(Eimenko.et.al..2002;.Yan.et.al..
1999)..Olander.et.al..(2004).revealed.that.oxidation.led.to.the.formation.of.silica-like.struc-
tures.at.the.PDMS.surface..The.surface.polarity.of.PDMS.can.also.be.increased.by.apply-
ing.a.coating..Delamarche.et.al..(2001).and.Lange.et.al..(2004).respectively.coated.PDMS.
stamps.with.positively.charged.amino-propyltriethoxysilane..Similarly,.Rozkiewicz.et.al..
and. Kim. et. al.. coated. PDMS. stamps. with. a. positively. charged. dendrimer. (Rozkiewicz.
et.al..2007b).and.parylene-C,.respectively.(Kim.et.al..2008a).
11.3.1.4 Alternative Stamp Materials
In.parallel.to.the.development.of.novel.PDMS.materials.and.new.methods.for.modifying.
PDMS. surfaces,. efforts. have. been. made. to. develop. non-PDMS. materials. mainly. for.
overcoming.the.drawbacks.of.Sylgard.184.PDMS.including.the.low.modulus,.hydrophobic.
surface,. and. contamination. caused. by. uncross-linked. siloxane. fragments. in. the. stamp.
(Sharpe. et. al.. 2006).. Csucs. et. al.. (2003a). prepared. stamps. using. commercial. polyolein.
polymers,.which.were.more.rigid.than.Sylgard.184.PDMS.and.free.of.the.contamination..
Features. as. small. as. 100.nm. were. generated. using. the. stamps.. Trimbach. et. al.. (2007).
