180
Bacterial Cellulose
Another.possibility.to.enhance.the.adhesion.of.cells.is.the.coating.of.BNC.
surfaces. with. arginine-glycine-aspartic. acid. (RGD). peptides. (Bodin. et. al..
2007a)..To.mimic.ECM.binding.sites,.xyloglycan.bearing.a.peptide.containing.
the.RGD.sequence.was.bound.to.BNC.pellicles..Initial.cell.adhesion.studies. .
showed.that.the.adhesion.of.endothelial.cells.was.significantly.higher.on.the.
RGD-modified. samples.. The. RGD. sequence. has. been. found. in. proteins. of.
the.extracellular.matrix,.especially.in.fibronectin.and.vitronectin,.and.acts.
as.a.binding.site.of.specific.cell.receptors.like.integrins..The.RGD-related.cell.
adhesion. leads. to. a. mechanical. fixation. of. cells. and. generates. cell-specific.
signals..This.result.was.confirmed.by.Andrade.et.al..(2010),.who.showed.that.
an. RGD. peptide,. but. not. a. glycine-arginine-glycine-aspartic. acid-tyrosine.
(GRGDY).control.peptide.enhanced.the.adhesion.of.cultured.fibroblasts.on.
the.BNC.surface.
As. an. example. for. a. composite. structure. of. the. biomaterial,. nanocomposites.of.BNC.and.hydroxyapatite.(HA).have.been.designed.to.mimic.the.
natural. composition. of. organic. and. inorganic. compounds. of. bone. (Jiang.
et al..2007;.Wan.et.al..2007),.which.showed.excellent.biocompatibility.and.a.
. complete.lack.of.cytotoxicity.(Grande.et.al..2009).
Biodegradation
Hydrolytic. degradation. of. cellulose. can. be. catalyzed. either. chemically.
(e.g., by.the.action.of.acids).or.enzymatically..Cellulose-degrading.enzymes,.
called.cellulases,.are.common.in.certain.species.of.fungi.and.bacteria.and.
enable. them. to. convert. cellulose. to. glucose. monomers.. The. human. body.
does.not.contain.enzymes.capable.of.catalyzing.the.hydrolysis.of.the.β-1,4.
linkages.of.cellulose.chains,.rendering.plant.cellulose.or.BNC.biostable.and.
nonresorbable,. at. least. inside. the. human. body.. In. vivo. and. in. vitro. investigations. have. been. performed. to. evaluate. the. degradability. of. BNC. with.
respect. to. medical. applications.. Since. glucose. as. the. potential. degradation. product. is. not. detectable. in. vivo,. recent. studies. have. investigated. the.
macro. scopic. and. physical. composition. of. the. material. after. implantation.
in.. animals..No changes.in.the.structure.of.BNC.membranes.were.detected.
after.implantation.into.Swiss.albino.mice.for.90.days.(Mendes.et.al..2009)..
The.same.was.reported.by.Helenius.et.al..(2006).after.subcutaneous.implantation. of. BNC. membranes. in. rats. for. 12. weeks.. In. this. study,. no. evidence.
for.in.vivo.degradation.was.observed,.but.the.authors.pointed.out.that.the.
implantation.time.may.have.been.too.short.for.detectable.structural.changes.
caused.by.degradation..In.addition.to.the.action.of.cellulases,.cellulose.could.
be.degraded.on.the.basis.of.an.alteration.of.the.highly.ordered.structure.of.
tion.of.amorphous.structures.may.cause.interactions.with.chemically.reactive.substances.and.cleavage.of.the.β-1,4.glycosidic.bonds..Such.degradation.
has.been.verified.by.measuring.the.concentration.of.reducing.sugars.in.the.
supernatant.of.BNC.by.in.vitro.testing.(Chen.et.al..2009)..BNC.samples.were.
Bacterial Cellulose
Another.possibility.to.enhance.the.adhesion.of.cells.is.the.coating.of.BNC.
surfaces. with. arginine-glycine-aspartic. acid. (RGD). peptides. (Bodin. et. al..
2007a)..To.mimic.ECM.binding.sites,.xyloglycan.bearing.a.peptide.containing.
the.RGD.sequence.was.bound.to.BNC.pellicles..Initial.cell.adhesion.studies. .
showed.that.the.adhesion.of.endothelial.cells.was.significantly.higher.on.the.
RGD-modified. samples.. The. RGD. sequence. has. been. found. in. proteins. of.
the.extracellular.matrix,.especially.in.fibronectin.and.vitronectin,.and.acts.
as.a.binding.site.of.specific.cell.receptors.like.integrins..The.RGD-related.cell.
adhesion. leads. to. a. mechanical. fixation. of. cells. and. generates. cell-specific.
signals..This.result.was.confirmed.by.Andrade.et.al..(2010),.who.showed.that.
an. RGD. peptide,. but. not. a. glycine-arginine-glycine-aspartic. acid-tyrosine.
(GRGDY).control.peptide.enhanced.the.adhesion.of.cultured.fibroblasts.on.
the.BNC.surface.
As. an. example. for. a. composite. structure. of. the. biomaterial,. nanocomposites.of.BNC.and.hydroxyapatite.(HA).have.been.designed.to.mimic.the.
natural. composition. of. organic. and. inorganic. compounds. of. bone. (Jiang.
et al..2007;.Wan.et.al..2007),.which.showed.excellent.biocompatibility.and.a.
. complete.lack.of.cytotoxicity.(Grande.et.al..2009).
Biodegradation
Hydrolytic. degradation. of. cellulose. can. be. catalyzed. either. chemically.
(e.g., by.the.action.of.acids).or.enzymatically..Cellulose-degrading.enzymes,.
called.cellulases,.are.common.in.certain.species.of.fungi.and.bacteria.and.
enable. them. to. convert. cellulose. to. glucose. monomers.. The. human. body.
does.not.contain.enzymes.capable.of.catalyzing.the.hydrolysis.of.the.β-1,4.
linkages.of.cellulose.chains,.rendering.plant.cellulose.or.BNC.biostable.and.
nonresorbable,. at. least. inside. the. human. body.. In. vivo. and. in. vitro. investigations. have. been. performed. to. evaluate. the. degradability. of. BNC. with.
respect. to. medical. applications.. Since. glucose. as. the. potential. degradation. product. is. not. detectable. in. vivo,. recent. studies. have. investigated. the.
macro. scopic. and. physical. composition. of. the. material. after. implantation.
in.. animals..No changes.in.the.structure.of.BNC.membranes.were.detected.
after.implantation.into.Swiss.albino.mice.for.90.days.(Mendes.et.al..2009)..
The.same.was.reported.by.Helenius.et.al..(2006).after.subcutaneous.implantation. of. BNC. membranes. in. rats. for. 12. weeks.. In. this. study,. no. evidence.
for.in.vivo.degradation.was.observed,.but.the.authors.pointed.out.that.the.
implantation.time.may.have.been.too.short.for.detectable.structural.changes.
caused.by.degradation..In.addition.to.the.action.of.cellulases,.cellulose.could.
be.degraded.on.the.basis.of.an.alteration.of.the.highly.ordered.structure.of.
tion.of.amorphous.structures.may.cause.interactions.with.chemically.reactive.substances.and.cleavage.of.the.β-1,4.glycosidic.bonds..Such.degradation.
has.been.verified.by.measuring.the.concentration.of.reducing.sugars.in.the.
supernatant.of.BNC.by.in.vitro.testing.(Chen.et.al..2009)..BNC.samples.were.