Biomedical Engineering Reference
In-Depth Information
epoxy. resin,. followed. by. sectioning. with. a. microtome. after. curing. to. yield. membranes.
∼5.μm.thick.and.6.mm.in.diameter..The.MWCNT.membranes.were.further.treated.with.
electrochemical. oxidation. to. selectively. etch. CNTs. into. the. polymer. matrix,. leaving.
behind.a.polymer.well.with.the.outer.diameter.of.the.CNT.(∼40.nm.in.this.case)..Lys.and.
BSA.were.then.electrophoretically.transported.across.the.membrane..Lysozyme.exhibited.
a. pH-dependent. and. voltage-dependent. transport. with. the. highest. luxes. at. pH. 4.7. (net.
charge.+10).and.−2.0.V..At.pH.4.7,.the.pI.of.BSA,.BSA.showed.negligible.transport.across.
the. membrane. at. a. potential. difference. of. −2.0.V.. Even. at. a. pH. of. 3.0. where. BSA. would.
be. positively. charged,. it. showed. no. net. lux. at. −2.0.V. allowing. for. complete. separation.
between.Lys.and.BSA..In.addition,.they.observed.electrophoretic.mobilities.of.lysozyme.
between. 0.33. and. 1.4.×.10
−9
.m
2
. V
−1
. s
−1
,. approximately. 10-fold. faster. than. comparable.
porous. gold. nanotube. membranes,. which. they. attributed. to. minimal. interactions. with.
the.CNT.graphitic.surface..Streimer.and.coworkers.recently.reported.an.ultrathin.porous.
nanocrystalline. silicon. (pnc-Si). membrane. that. provided. control. over. average. pore.
sizes. from. approximately. 5-25.nm. with. ∼10.nm. membrane. thickness.. Their. fabrication.
technique.uses.precision.silicon.deposition.and.etching.techniques.to.create.the.ultrathin.
membrane..However,.instead.of.directly.patterning.pores,.they.found.that.voids.are.formed.
spontaneously.as.nanocrystals.nucleate.and.grow.in.a.15.nm.thick.amorphous.silicon.ilm.
during.a.rapid.thermal.annealing.step..These.voids.then.serve.to.create.the.pores..They.
were.able.to.control.the.pore.size.by.adjusting.the.annealing.temperature,.observing.that.
pore.size.and.density.increase.monotonically.with.temperature,.with.samples.annealed.at.
715°C,.729°C,.and.753°C.having.average.pore.sizes.of.7.3,.13.9,.and.21.3.nm,.respectively..They.
subsequently. explored. diffusive. transport. across. the. membranes. using. small. molecule.
dyes,.BSA,.and.immunoglobulins.(IgG)..Depending.on.the.membrane.pore.size,.they.were.
able.to.exclude.BSA,.while.having.the.small.molecule.dye.effectively.transported.or.show.
a. substantial. difference. in. transport. rates. between. BSA. and. IgG.. They. also. compared.
the. diffusion. rate. of. the. small. molecule. dye. across. their. membranes. with. 50.K. dialysis.
membranes.and.observed.more.than.ninefold.increase.in.diffusion.rate.in.their.system.
Membrane. afinity. chromatography. has. been. widely. proposed. as. an. alternative. to.
column-based. chromatographic. methods.. Membrane. separations. are. faster. as. they. are.
not. subject. to. the. diffusional. limitations. of. bead-based. separations.. However,. for. afin-
ity.separations,.membranes.suffer.from.the.disadvantage.of.having.much.lower.binding.
capacity.than.traditional.chromatography.resins..To.alleviate.this.disadvantage,.a.variety.
of.surface.modiications.have.been.proposed.to.use.nanoporous.alumina.membranes.for.
afinity. separations.. Sun. et. al.. (2006). used. atom. transfer. radical. polymerization. (ATRP).
to. grow. poly(2-hydroxyethyl. methacrylate). (PHEMA). from. initiators. bound. to. a. porous.
alumina.surface..This.method.allows.relatively.ine.control.over.polymer.molecule.weight,.
increasing. the. surface. area. available. for. functionalization. without. clogging. the. pores..
After.polymerization.to.create.the.polymer.brushes,.they.functionalized.the.PHEMA.with.
nitrilotriacetate-Cu
2+
. complexes. to. create. a. protein-binding. membrane,. using. metal-ion.
afinity.interactions..Using.this.technology,.they.measured.a.binding.capacity.of.0.9.mg.of.
BSA/cm
2
. of. external. membrane. surface. (150.mg/cm
3
. of. membrane),. in. comparison. with.
previously. reported. membrane. protein. capacities. of. 4-20.mg/cm
3
.. In. a. quite. different,.
more.biomimetic.approach,.Lazzara.et.al..(2012).coated.porous.alumina.membranes.with.
multifunctional. lipid. monolayers. using. small. unilamellar. vesicles.. They. explored. two.
different.biologically.relevant.functionalizations,.biotin.and.Ni
2+
,.demonstrating.afinity.
interactions. with. streptavidin. and. His-tagged. proteins,. respectively.. Lipid. functional-
ization.provided.an.added.advantage.over.covalent.modiication.of.lateral.mobility,.per-
mitting.them.to.reproduce. receptor-doped. membranes.in.a.3D.arrangement..One.could.
