Biomedical Engineering Reference
In-Depth Information
system,.low.toxicity,.high.biocompatibility,.and.linking.ability.to.biologically.active.mate-
rial. (e.g.,. nucleic. acid,. proteins). are. principal. properties. of. suitable. MR. contrast. agents..
Magnetic. nanoparticles. consist. of. a. core. of. superparamagnetic. iron. oxide. and. polymer.
coating,.to.have.high.magnetic.moment.and.functionality.to.be.linked.to.biologically.active.
species..Moreover,.their.provision.of.the.maximum.change.in.signal.(albeit.hypointensity).
per.unit.of.metal,.in.particular.on.T2*-weighted.images,.their.easy.detection.by.light.and.
electron. microscopy,. their. changing. of. magnetic. properties. according. to. their. size,. and.
their. biocompatibility. due. to. their. iron-made. structure. make. SPIO. nanoparticles. a. pre-
ferred.contrast.agent.in.MRI..Ferumoxides.(Feridex
®
,.Bayer.Healthcare).were.approved.by.
the.U.S..Food.and.Drug.Administration.in.1996.for.clinical.cell.tracking.
2.16 SuperparamagneticNanoparticles
Superparamagnetic. (SPIO). nanoparticles. are. being. used. as. MRI. contrast. agents.. They.
can.dephase.neighboring.protons.in.a.region.equivalent.to.many.times.the.size.of.metal.
itself,.in.particular.on.T2*-weighted.images,.which.is.caused.by.a.crystal.ordering.(spinels)..
SPIOs.consist.of.two.components,.a.superparamagnetic.magnetite.(Fe
3
O
4
).or.maghemite.
(γFe
2
O
3
).core,.which.is.biocompatible.and.recycled.in.normal.iron.metabolism.pathways,.
and. a. hydrophilic. coating,. which. allows. the. chemical. linkage. of. functional. groups. and.
ligands..SPIO.crystals,.which.are.smaller.than.a.magnetic.domain.(approximately.30.nm),.
have.random.orientation.in.the.absence.of.an.external.magnetic.ield..However,.external.
magnetic. ields. can.reorient. the.crystal.magnetic. dipoles. to.induce. a.net.magnetic. ield,.
which.is.larger.than.the.sum.total.of.individual.unpaired.magnetic.dipoles.
Jun.et.al..demonstrated.that.magnetic.moments.of.nanoparticles.are.size.dependent.
200
.
By. controlling. the. synthesis. conditions,. they. prepared. magnetic. Fe
3
O
4
. nanoparticles. in.
different. sizes. through. thermal. decomposition. of. Fe(acac)3. (acac. =. acetylacetonate). in. a.
hot.organic.solvent..The.T2-weighted.spin-echo.MR.images.at.1.5.T.changed.from.white.to.
black.via.gray.colors.as.the.dimension.of.magnetic.nanoparticles.increased.from.4.to.12.nm..
Such.an.effect.makes.them.suitable.to.identify.a.variety.of.biological.events..Nanoparticles.
larger.than.30.nm.are.taken.up.by.phagocytes.(dark.cells),.so.the.tumor.cells.that.cannot.
phagocyte.these.particles.(white.cells).can.be.distinguished.from.other.cells.by.their.white.
color.
201,202
.Smaller.magnetic.nanoparticles.can.be.conjugated.with.target-speciic.biomol-
ecules. to. detect. target. cells. via. interaction. between. conjugated. particles. and. expressed.
markers.on.target.cells.
203
Numerous.chemical.methods.have.been.used.for.magnetic.nanoparticle.synthesis..The.
solution.method.is.the.simplest.and.most.eficient.technique.to.produce.magnetic.nano-
particles.with.a.monodisperse.population.of.suitable.size..Iron.oxides.are.prepared.by.the.
injection.of.precursors.(e.g.,.ferrous.and.ferric.salts).into.an.aqueous.medium.containing.
surfactants.at.high.temperature,.followed.by.aging.and.a.size.control.process..Use.of.neu-
tral. organometallic. precursors. in. coordinating. alkyl. solutions. with. a. high. boiling. point.
in.the.hot-injection.solvothermal.process.represents.a.promising.method,.compared.with.
conventional.solution.techniques.in.which.ionic.precursors.in.water.or.another.polar.sol-
vent.are.used.
204
Although.the.conventional.synthesis.techniques.control.the.mean.size.of.nanoparticles.
by.pH.adjustment,.ionic.strength,.temperature,.or.the.Fe(II)/Fe(III).ratio,.their.novel.electri-
cal,.chemical,.magnetic,.and.optical.properties.need.extensive.efforts.to.develop.uniform.
