Bacterial Nanocellulose Hydrogels Designed as Bioartificial Medical Implants 183
microporous.and.interconnected.scaffold.adequate.for.3D.migration.of.cells..
This. material. allowed. a. homogeneous. distribution. of. smooth. muscle. cells.
(Baeckdahl.et.al..2008),.osteoprogenitor.cells.(Zaborowska.et.al..2010),.human.
chondrocytes. (Andersson. et. al.. 2010),. and. urine-derived. stem. cells. (Bodin.
et al..2010).in.the.porous.structure..Alternatively,.a.porous.foam.of.BNC.fiber.
aggregates. was. generated. by. disintegration. of. the. BNC. bodies. and. addition.of.a.composite.substance.(e.g.,.polyethylene.glycol.[PEG].400).(Gao.et.al..
2011)..However,.future.studies.have.to.address.the.delicate.balance.between.
enlarged.pore.size.and.sufficient.mechanical.strength.
Mechanical Properties
The.modulus.of.elasticity,.ultimate.tensile.strength,.elongation.to.failure,.and.
fracture.toughness.are.important.features.describing.the.mechanical.properties.of.biomaterials.(Teoh.2004)..In.comparison.to.stiff.materials.like.metals.
or.ceramics,.BNC.in.its.native,.hydrated.state.is.a.hydrogel.with.low.modulus.
of. elasticity. and. fracture. toughness.. While. the. single. BNC. fibers. have. the.
mechanical.strength.of.steel.or.Kevlar.(Klemm.et.al..2011),.the.mechanical.
properties.of.the.whole.hydrogel.are.partially.defined.by.its.water.content.
and.the.discharge.of.water.during.compression..The.water.retention.capacity.of.a.BNC.hydrogel.is.influenced.by.the.structural.composition.of.the.fiber.
network.. Because. of. the. anisotropic. architecture. of. the. fleeces,. there. are.
regions.with.different.mechanical.properties..The.wide-meshed.network.of.
the.bottom.layer.leads.to.lower.water.binding.and.reduced.mechanical.properties.. Sufficient. mechanical. properties. of. the. BNC. biomaterial. are. pivotal.
for.surgical.fixation.and.the.compressive.and.tribological.stress.experienced.
by.the.biomaterial.at.the.site.of.implantation..Current.studies.therefore.focus.
on.comparing.the.biomechanical.properties.of.BNC.and.body.tissues.or.the.
formation.of.composite.(e.g.,.double-network).gels.
When.comparing.the.mechanical.characteristics.of.BNC.with.those.of.pig.
meniscus.and.a.collagen.implant,.BNC.had.a.higher.Young’s.modulus.and.
compression.modulus.than.the.collagen.matrix,.but.the.meniscus.material.
was.stronger.than.BNC.(Bodin.et.al..2007b).
By.soaking.BNC.hydrogels.in.gelatinous.solution.for.an.adapted.period,.
the. water. is. replaced. by. the. composite. substance. and. an. additional.
cross-linking.leads.to.a.double-network.gel.with.improved.compression.and.
tensile.strength.(Nakayama.et.al..2004)..Other.network.components,.such.as.
the.polysaccharides.sodium.alginate.or.i-Carrageenan,.also.lead.to.the.formation.of.double-network.gels..A.prominent.feature.of.such.double-network.
gels.is.their.considerably.improved.mechanical.stability.
Sterilization
Cellulose.is.thermostable.up.to.at.least.200°C..Similar.to.cellulose.of.plant.
origin,.the.thermal.degradation.of.BNC.starts.in.the.range.of.200°C–400°C.