202
Bacterial Cellulose
(i.e.,.under.mild.drying.conditions),.the.fibril.network.is.stable,.and.aerogels.
are. formed.. Such. aerogels. are. characterized. by. a. high. reswelling. capacity.
(i.e.,.the.aerogels.can.absorb.nearly.the.same.large.amount.of.water.as.was.
removed.by.the.drying.step)..By.drying.under.pressure,.very.flat.foils.(loss.of.
99%.water).of.high.density.and.high.mechanical.stability.can.be.obtained..In.
such.materials,.the.nanofibril.network.collapses,.resulting.in.a.dense.physical. cross-linking. of. the. cellulose. chains. (“hornification”).. Such. foils. are. of.
interest.for.technical.(e.g.,.membranes.for.audio.technologies).and.medical.
(wound. dressings). applications.. In. the. latter. context,. partially. dewatered.
ties.of.the.native.BNC.gels.with.the.better.mechanical.stability.of.the.compressed.materials..The.water.uptake.of.the.compressed.and.completely.dried.
BNC. materials. is. low.. Before. drying,. dissolved. or. suspended. substances,.
such.as.proteins,.pharmaceutics,.colorants,.and.nanosilica,.can.be.incorporated.into.BNC.by.simple.dipping.and.insertion.technologies..After.drying,.
these.additives.are.fixed.inside.the.nanocellulose.materials.
The.placement.of.different.agents.into.the.BNC.also.can.be.carried.out.by.
impregnating. freeze-dried. BNC. under. adsorption. conditions. in. the. fibers.
and/or.by.filling.the.network.meshes..That.is,.the.nanofibers.can.act.as.templates.for.nanoparticles.or.organic.polymer.layers..A.very.important.part.of.
the.development.of.BNC.materials.concerns.the.design.of.BNC.composites.by.
either.in.situ.or.later.modification..In.the.case.of.additives.with.hydroxyl.or.
similar.groups,.the.in.situ.insertion.is.affected.by.hydrogen.bridge.formation..
In.the.case.of.nanoparticles,.the.BNC.fibers.can.act.as.nanofiber.. templates..
A. recent. example. is. the. impregnation. of. BNC. with. silver. nitrate. and. the.
subsequent. reduction. with. water-soluble. sodium. borohydride. (a. technical.
reducing. agent),. which. forms. silver. metal. deposited. as. nanoparticles. on.
the. supporting. BNC. nanofibers.. Nanofibers. of. BNC. material,. when. modified.with.carboxylic.functionalities,.have.shown.the.unique.ability.to.induce.
biomimetic.crystallization.of.calcium-deficient.hydroxyapatite.(Bodin.et.al..
2006)..The.size.of.the.crystals.can.be.controlled.by.selecting.the.fibril.density.
of. the. BNC. network.. The. presence. of. nanosize. hydroxyapatite. crystals. on.
the.surfaces.of.cellulose.nanofibrils.promotes.osteoprogenitor.cell.adhesion.
and.differentiation.(Zimmermann.et.al..2011)..The.ability.to.control.macroporosity.combined.with.surface.decoration.with.hydroxyapatite.makes.BNC.
materials.promising.candidates.for.bone.grafts.
Scale-Dependent Properties of BNC
Control. of. the. 3D. morphology. of. cellulose. networks. from. the. nanoscale.
through. the. mesoscale. to. the. macroscale. is. a. critical. aspect. in. developing.
Bacterial Cellulose
(i.e.,.under.mild.drying.conditions),.the.fibril.network.is.stable,.and.aerogels.
are. formed.. Such. aerogels. are. characterized. by. a. high. reswelling. capacity.
(i.e.,.the.aerogels.can.absorb.nearly.the.same.large.amount.of.water.as.was.
removed.by.the.drying.step)..By.drying.under.pressure,.very.flat.foils.(loss.of.
99%.water).of.high.density.and.high.mechanical.stability.can.be.obtained..In.
such.materials,.the.nanofibril.network.collapses,.resulting.in.a.dense.physical. cross-linking. of. the. cellulose. chains. (“hornification”).. Such. foils. are. of.
interest.for.technical.(e.g.,.membranes.for.audio.technologies).and.medical.
(wound. dressings). applications.. In. the. latter. context,. partially. dewatered.
ties.of.the.native.BNC.gels.with.the.better.mechanical.stability.of.the.compressed.materials..The.water.uptake.of.the.compressed.and.completely.dried.
BNC. materials. is. low.. Before. drying,. dissolved. or. suspended. substances,.
such.as.proteins,.pharmaceutics,.colorants,.and.nanosilica,.can.be.incorporated.into.BNC.by.simple.dipping.and.insertion.technologies..After.drying,.
these.additives.are.fixed.inside.the.nanocellulose.materials.
The.placement.of.different.agents.into.the.BNC.also.can.be.carried.out.by.
impregnating. freeze-dried. BNC. under. adsorption. conditions. in. the. fibers.
and/or.by.filling.the.network.meshes..That.is,.the.nanofibers.can.act.as.templates.for.nanoparticles.or.organic.polymer.layers..A.very.important.part.of.
the.development.of.BNC.materials.concerns.the.design.of.BNC.composites.by.
either.in.situ.or.later.modification..In.the.case.of.additives.with.hydroxyl.or.
similar.groups,.the.in.situ.insertion.is.affected.by.hydrogen.bridge.formation..
In.the.case.of.nanoparticles,.the.BNC.fibers.can.act.as.nanofiber.. templates..
A. recent. example. is. the. impregnation. of. BNC. with. silver. nitrate. and. the.
subsequent. reduction. with. water-soluble. sodium. borohydride. (a. technical.
reducing. agent),. which. forms. silver. metal. deposited. as. nanoparticles. on.
the. supporting. BNC. nanofibers.. Nanofibers. of. BNC. material,. when. modified.with.carboxylic.functionalities,.have.shown.the.unique.ability.to.induce.
biomimetic.crystallization.of.calcium-deficient.hydroxyapatite.(Bodin.et.al..
2006)..The.size.of.the.crystals.can.be.controlled.by.selecting.the.fibril.density.
of. the. BNC. network.. The. presence. of. nanosize. hydroxyapatite. crystals. on.
the.surfaces.of.cellulose.nanofibrils.promotes.osteoprogenitor.cell.adhesion.
and.differentiation.(Zimmermann.et.al..2011)..The.ability.to.control.macroporosity.combined.with.surface.decoration.with.hydroxyapatite.makes.BNC.
materials.promising.candidates.for.bone.grafts.
Scale-Dependent Properties of BNC
Control. of. the. 3D. morphology. of. cellulose. networks. from. the. nanoscale.
through. the. mesoscale. to. the. macroscale. is. a. critical. aspect. in. developing.