Bacterial Nanocellulose Hydrogels Designed as Bioartificial Medical Implants 181
stored.in.phosphate.buffered.saline.(PBS).at.37°C.for.a.period.of.12.weeks,.
during.which.the.structure.built.fuzzy.aggregates.or.formed.fragments.via.
slow.degradation..In.contrast,.Li.et.al..(2009).measured.the.mass.loss.of.the.
samples.and.observed.no.degradation.of.BNC.in.vitro..Indeed,.in.contrast.to.
biodegradable.materials.like.polyglycolic.acid,.polylactic.acid.or.copolymers.
of.the.two,.BNC.showed.only.negligible.degradation.
It.is.presently.unclear,.whether.bioimplants.or.scaffolds.for.tissue.engineering.should.be.degradable.or.not..Potential.problems.of.degradable.materials.
include.nonsynchronized.resorption.and.regeneration.of.new.extracellular.
matrix.resulting.in.compromised.biofunctionality.of.the.implant,.and.induction.of.subchronic.inflammation.in.the.surrounding.tissue.by.an.altered.pH.
value.or.the.release.of.solid.particles..In.contrast,.nondegradable.. materials.
such. as. BNC. may. support. adequate. regeneration. of. defects. by. guiding.
. cellular.migration.and.providing.mechanical.stability..The.result.should.be.
a. composite. of. nonresorbable,. highly. porous. BNC. and. the. newly. formed.
extracellular.matrix..On.the.other.hand,.the.degradation.properties.of.BNC.
can.be.changed.by.oxidation.to.so-called.2,3-dialdehyde.cellulose.(Li.et.al..
2009),. leading. to. its. rapid. degradation. in. water,. PBS,. and. simulated. body.
fluid.(SBF).at.37°C,.subsequent.collapse.of.its.porous.structure,.and.loss.of.its.
mechanical.stability.
Porosity
Often. the. migration. of. cells. and. the. 3D. distribution. inside. a. tissue. engineering.scaffold.or.a.cell-free.implant.are.essential.for.regenerative.aspects..
Cells. must. have. the. possibility. to. enter. the. biomaterial. structure. in. order.
to.build.a.homogeneous.extracellular.matrix..Especially.for.nondegradable.
bio. materials,.porosity.is.an.important.feature,.because.it.allows.cell.immigration. and. offers. a. 3D. scaffold. during. the. entire. regeneration. process..
In addition,.nutrition.and.oxygen.delivery.are.essential.for.growth,.proliferation,.and.matrix.biosynthesis..Particularly.at.implantation.sites.with.no.
vascularization.or.angiogenesis.during.the.regeneration.process.(e.g.,.cartilage),.the.porous.structure.must.support.diffusion.of.nutrients.over.distances.
of.more.than.1.mm.
In.its.native.hydrated.state,.BNC.consists.of.more.than.99%.water.and.less.
than.1%.solid.cellulose.(Klemm.et.al..2011)..Due.to.the.extremely.small.. volume.
fraction.of.the.cellulose.nanofibers,.the.porosity.of.BNC.ranges.from.92%.to.
94%. (void. volume. in. relation. to. total. volume). (Sokolnicki. et. al.. 2006;. Tang.
et al..2010)..Nevertheless,.the.meshes.of.the.texture.vary.between.1.and.20 μm.
and.are.therefore.mostly.smaller.than.most.of.the.cells..The.dimensions.of.
the. meshes. vary. within. the. BNC. hydrogel. because. of. its. layered,. nonhomogeneous.structure,.preconditioned.by.the.BNC.biosynthesis..The.middle.
layer.of.the.BNC.hydrogel.body.generated.by.static.culture.is.relatively.homogeneous.and.the.dimensions.of.the.meshes.range.from.1.to.10 μm..In.contrast,.the.bottom.part.of.the.BNC.hydrogel.shows.a.wide-meshed.structure.