Biomedical Engineering Reference
In-Depth Information
by. modifying. the. surface. with. hydrophilic. polymers. including. poly(ethylene. glycol).
(PEG),.poly(vinyl.alcohol).(PVA),.and.glycol.chitosan..Stealth.liposomes.have.a.couple.of.
advantages.such.as.circumventing.the.reticuloendothelial.system.(RES).and.extending.the.
blood.circulation.time,.and.reducing.toxicity.originating.from.the.charged.vesicle.(Allen.
et.al..2002;.Harris.1992)..Doxil/Caelyx,.which.is.a.PEGylated.doxorubicin.liposome.to.be.
applied.to.metastatic.ovarian.cancer.and.AIDS-related.Kaposis's.sarcoma,.is.available.for.
clinical.applications.
Among. the. nanoliposomes,. niosome. has. emerged. as. a. new. platform. technology..
Niosome,. a. nonionic. surfactant. vesicle,. is. similar. to. typical. liposomes. in. structure,. but.
different.in.bilayer.composition..Surfactants.commonly.used.in.niosomes.are.derivatives.
of. polysorbates.. The. advantages. of. niosomes. over. liposomes. are. increased. stability,. low.
cost.of.production,.and.no.need.to.handle.variable.purity.of.phospholipids..Moreover,.nio-
somes.also.show.a.slower.drug.release.as.a.depot.system.as.compared.with.conventional.
liposomes..Oral.bioavailability.of.acyclovir.has.been.known.to.be.very.poor.(around.15%-
20%),.so.it.needs.a.high.dose.of.the.drug.or.frequent.dosing..To.overcome.this.phenom-
enon,. niosome. and. liposome. systems. were. introduced. to. incorporate. the. acyclovir,. and.
drug.release.proiles.were.compared..In.vitro.cumulative.drug.release.showed.that.90%.
of.the.drug.was.detected.from.the.media.containing.the.liposome.vesicle.within.150.min;.
however,.in.the.case.of.niosomal.formulation,.only.50%.of.the.drug.was.released.even.after.
200.min.(Mukherjee.et.al..2007;.Blazek-Welsh.and.Rhodes.2001).
23.2.3 . Solid Lipid Nanoparticles
Solid. lipid. nanoparticles. (SLNs). have. many. beneits. as. a. drug. delivery. system.. The.
production. step. of. SLNs. can. avoid. using. organic. solvents.. Because. of. this. fabrication.
process,.SLNs.could.possibly.prevent.toxic.effects.from.organic.solvents.commonly.used.
for.making.other.polymeric.nanoparticles..In.addition,.it.is.much.easier.to.scale.up.SLN.
production. to. meet. the. need. of. industrial. scale.. One. of. the. methods. of. making. SLNs. is.
using. high-pressure. homogenization,. which. can. fabricate. a. large. amount. of. even-sized.
nanoparticles..SLNs.combine.important.other.proits,.such.as.protection.of.incorporated.
drugs. from. enzymatic. degradation,. sustained. release,. increased. biocompatibility,. and.
enhanced.bioavailability.(Barratt.2000,.2003;.Gasco.1993).
SLNs.are.made.by.several.methods.(Table.23.1)..As.mentioned,.the.one.shown.is.using.
a.high-pressure.homogenization.technique,.which.includes.hot.and.cold.homogenization.
means..High.pressure.through.a.ine.pore.pushes.the.lipids.into.shear.stress.and.cavita-
tion,. which. can. downsize. the. particle.. Generally,. the. concentration. of. the. lipid. contents.
to. be. optimized. ranges. from. 5%. to. 10%. (Saupe. and. Rades. 2006).. Melted. lipid. solutions.
loaded.with.drugs.should.be.prepared.with.an.increased.temperature.around.60°C-70°C.
to.prevent.crystallization.of.the.lipid..In.the.case.of.hot.homogenization.method,.molten.
lipid. loaded. with. drug. is. transferred. to. an. aqueous. solution. that. contains. surfactant. to.
attain.pre-emulsion.mix..Typically,.SLNs.can.be.stabilized.and.can.maintain.their.particle.
size. by. a. small. portion. of. surfactant. and/or. co-surfactant.. Then,. high-pressure. homog-
enization. is. applied. to. the. pre-emulsion. at. the. pressure. of. 100-200. bar. while. retaining.
the. temperature. high. enough. for. the. lipid. to. remain. melted.. The. hot. nanoemulsion. is.
cooled.down.to.around.room.temperature.and.then.the.lipid.starts.to.solidify.with.drugs.
by.crystallization..While.doing.this.process,.the.drug.can.be.located.in.the.internal.phase.
of.the.lipid,.exposed.to.the.surface.of.SLN.or.lost.from. the.particles;. basically,.this.phe-
nomenon.comes.from.the.physical.property.of.the.encapsulated.drug..It.is.believed.that.
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