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Bacterial Nanocellulose Biomaterials with Controlled Architecture
Several.research.groups.have.developed.prototypes.of.BNC.tubes.in.the.
required.diameter.range.and.with.a.length.of.5–25.cm.or.more.for.vascular.
bypass.surgery..The.material.properties.of.these.vessels.depend.strongly.on.
the.cultivation.conditions..The.wall.of.the.tubes.is.formed.into.the.typical.
transparent.BNC.hydrogel.and.is.also.characterized.by.a.stable.inner.lumen,.
good.stability.of.sutures,.essential.mechanical.strength,.and.the.important.
feature.of.being.permeable.to.water,.other.liquids,.ions,.and.small.molecules..
Moreover,. the. tubes. show. very. good. surgical. handling. and. can. be. sterilized.in.standard.ways.(Klemm.et.al..2001)..In.animal.experiments.with.rats,.
pigs,. and. sheep. (implantation. in. the. carotid. artery). good. biocompatibility.
and. performance. for. at. least. 13. months. were. demonstrated. (Malm. et al..
2012).. Histological. examination. of. rats. after. 1. week,. 1. month,. and. 1 year.
showed.that.the.tubular.BNC.implants.were.integrated.by.endothelization.
of.the.inner.surface,.colonized.on.the.outer.wall.by.connective.tissue,.and.
characterized.by.the.in-growth.of.vital.collagen-forming.fibroblasts.(Klemm.
et.al..2001,.2006;.Wippermann.et.al..2009)..Confocal.laser.scanning.microscopy.showed.that.the.inner.surface.of.the.BNC.tubes.was.smooth.and.not.
structured. from. its. preparation. (Brackmann. et. al.. 2010).. The. low. surface.
roughness. appears. to. support. good. endothelization. and. to. be. at. low. risk.
of. thrombosis. or. aneurysm. (Schumann. et. al.. 2009).. Commonly. used. artificial. vascular. grafts. formed. from. synthetic. polymers. such. as. polytetrafluoroethylene. (PTFE). or. polyesters. are. prone. to. thrombosis. when. used.
as. small-diameter. vessels,. which. are. essential. for. coronary. artery. bypass.
grafts.. A. study. in. which. BNC. tubes. were. compared. with. synthetic. grafts.
over.a.period.of.2.weeks.also.demonstrated.good.biocompatibility.of.BNC.
and.its.incorporation.in.the.body..Evaluation.of.stents.(endoluminal.vascular.
prostheses). covered. with. BNC. in. a. rabbit. iliac. artery. model. demonstrated.
that.the.implantation.was.safe.and.no.acute.or.late.vessel.occlusion.due.to.
stent.thrombosis.occurred.(Bueno.et.al..2009).
BNC,. hydrogels. with. a. uniaxially. oriented. fibril. structure. and. excellent.
mechanical.properties,.can.be.prepared.by.the.cultivation.of.Gluconacetobacter
xylinus. strains. on. ridged. oxygen-permeable. silicon. substrates.. Cultivation.
inside. silicone. tubes. produced. BNC. tubes. with. uniaxially. oriented. fibrils.
along. the. longitudinal. axis. of. the. silicon. template. (Putra. et. al.. 2008).. The.
inability.of.BNC.to.degrade.in.vivo.has.been.suggested.as.a.limitation.to.its.
use..There.are.no.enzymes.in.mammalian.cells.or.the.extracellular.matrix.
that.use.cellulose.as.a.substrate..Limited.periodate.oxidation.of.BNC.leads.
to. a. modified. BNC. (containing. 2,3-dialdehyde. cellulose. structural. units).
that.can.break.down.at.physiological.pH.values,.although.the.BNC.retains.
its.hydrogel.structure,.crystallinity,.and,.for.example,.its.ability.to.form.biomimetically. calcium-deficient. hydroxyapatite. nanocrystallites. within. the.
. oxidized.matrix.(Evans.et.al..2008)..The.resorbable.BNC.hydrogel.has.a.great.
capacity.for.bone.tissue.repair.(Hutchens.et.al..2009).
The.large.amount.of.water.inside.BNC.hydrogels.acts.as.a.spacer.and.stabilizing.element.for.the.nanofibril.network..In.the.case.of.slow.dewatering.
Bacterial Nanocellulose Biomaterials with Controlled Architecture
Several.research.groups.have.developed.prototypes.of.BNC.tubes.in.the.
required.diameter.range.and.with.a.length.of.5–25.cm.or.more.for.vascular.
bypass.surgery..The.material.properties.of.these.vessels.depend.strongly.on.
the.cultivation.conditions..The.wall.of.the.tubes.is.formed.into.the.typical.
transparent.BNC.hydrogel.and.is.also.characterized.by.a.stable.inner.lumen,.
good.stability.of.sutures,.essential.mechanical.strength,.and.the.important.
feature.of.being.permeable.to.water,.other.liquids,.ions,.and.small.molecules..
Moreover,. the. tubes. show. very. good. surgical. handling. and. can. be. sterilized.in.standard.ways.(Klemm.et.al..2001)..In.animal.experiments.with.rats,.
pigs,. and. sheep. (implantation. in. the. carotid. artery). good. biocompatibility.
and. performance. for. at. least. 13. months. were. demonstrated. (Malm. et al..
2012).. Histological. examination. of. rats. after. 1. week,. 1. month,. and. 1 year.
showed.that.the.tubular.BNC.implants.were.integrated.by.endothelization.
of.the.inner.surface,.colonized.on.the.outer.wall.by.connective.tissue,.and.
characterized.by.the.in-growth.of.vital.collagen-forming.fibroblasts.(Klemm.
et.al..2001,.2006;.Wippermann.et.al..2009)..Confocal.laser.scanning.microscopy.showed.that.the.inner.surface.of.the.BNC.tubes.was.smooth.and.not.
structured. from. its. preparation. (Brackmann. et. al.. 2010).. The. low. surface.
roughness. appears. to. support. good. endothelization. and. to. be. at. low. risk.
of. thrombosis. or. aneurysm. (Schumann. et. al.. 2009).. Commonly. used. artificial. vascular. grafts. formed. from. synthetic. polymers. such. as. polytetrafluoroethylene. (PTFE). or. polyesters. are. prone. to. thrombosis. when. used.
as. small-diameter. vessels,. which. are. essential. for. coronary. artery. bypass.
grafts.. A. study. in. which. BNC. tubes. were. compared. with. synthetic. grafts.
over.a.period.of.2.weeks.also.demonstrated.good.biocompatibility.of.BNC.
and.its.incorporation.in.the.body..Evaluation.of.stents.(endoluminal.vascular.
prostheses). covered. with. BNC. in. a. rabbit. iliac. artery. model. demonstrated.
that.the.implantation.was.safe.and.no.acute.or.late.vessel.occlusion.due.to.
stent.thrombosis.occurred.(Bueno.et.al..2009).
BNC,. hydrogels. with. a. uniaxially. oriented. fibril. structure. and. excellent.
mechanical.properties,.can.be.prepared.by.the.cultivation.of.Gluconacetobacter
xylinus. strains. on. ridged. oxygen-permeable. silicon. substrates.. Cultivation.
inside. silicone. tubes. produced. BNC. tubes. with. uniaxially. oriented. fibrils.
along. the. longitudinal. axis. of. the. silicon. template. (Putra. et. al.. 2008).. The.
inability.of.BNC.to.degrade.in.vivo.has.been.suggested.as.a.limitation.to.its.
use..There.are.no.enzymes.in.mammalian.cells.or.the.extracellular.matrix.
that.use.cellulose.as.a.substrate..Limited.periodate.oxidation.of.BNC.leads.
to. a. modified. BNC. (containing. 2,3-dialdehyde. cellulose. structural. units).
that.can.break.down.at.physiological.pH.values,.although.the.BNC.retains.
its.hydrogel.structure,.crystallinity,.and,.for.example,.its.ability.to.form.biomimetically. calcium-deficient. hydroxyapatite. nanocrystallites. within. the.
. oxidized.matrix.(Evans.et.al..2008)..The.resorbable.BNC.hydrogel.has.a.great.
capacity.for.bone.tissue.repair.(Hutchens.et.al..2009).
The.large.amount.of.water.inside.BNC.hydrogels.acts.as.a.spacer.and.stabilizing.element.for.the.nanofibril.network..In.the.case.of.slow.dewatering.