229
Biomimetic Mineralization of Apatite on Bacterial Cellulose
ranging.from.60.to.140.nm.long.(Nge.and.Sugiyama.2007)..The.mineralized.
apatite. globules. firmly. adhered. on. the. microfibril. surfaces. and. formed. a.
uniform. Ca-P. layer,. followed. by. further. crystal. growth. upon. the. existing.
layer..The.variation.in.size.of.the.apatite.globules.under.the.same.soaking.
condition. revealed. the. influence. of. the. microfibril. surface. structure. at. the.
initial.nucleation.process.of.apatite.formation.(Sato.et.al..2001;.Uchida.et.al..
2003)..The.globule.size.formed.on.the.microfibril.surfaces.of.the.BC-TEMPO.
template.was.about.1.5.μm..The.presence.of.carboxylate.groups.on.the.BC.
microfibril. surface. (BC-TEMPO). and. further. ion. exchange. treatment. with.
calcium.(BC-TEMPO-Ca).significantly.affect.the.rate.of.apatite.nucleation.by.
a.rapid.consumption.of.calcium.and.phosphate.ions.from.the.surrounding.
supersaturated.fluid..A.large.number.of.nuclei.thus.grew.competitively.on.
BC-TEMPO-Ca,.leading.to.a.smaller.average.size.than.for.the.BC-TEMPO.and.
native.BC.templates..In.other.words,.an.increasing.amount.of.mineral.phase.
or.more.crystals.may.occur.principally.by.multiplication.in.BC-TEMPO-Ca,.
whereas.progressive.mineralization.with.an.increase.in.size.of.the.crystals.
or.crystal.growth.may.occur.in.native.BC,.like.in.bone.matrix.mineralization.
and.calcification.of.enamel.(Rey.et.al..1991a).
The.profiles.of.relative.weight.gain.of.mineralized.Ca-P.on.BC.templates.
as. a. function. of. soaking. time. also. reflect. the. growth. behavior. of. apatite.
crystal. with. respective. BC. templates.. The. BC-TEMPO-Ca. samples,. which.
exhibited. the. smallest. apatite. globules,. showed. the. highest. deposited.
weight,. whereas. the. gradual. increase. of. mineral. deposition. that. occurred.
in.native.BC.was.characterized.by.the.lowest.deposited.weight.throughout.
the.soaking.time,.and.the.profile.of.BC-TEMPO.samples.displayed.between.
BC-TEMPO-Ca.and.native.BC..Because.calcium.ion.is.one.of.the.main.mineral. components. of. the. apatite. crystal,. a. preexisting. ionic. complex. like.
BC-COOCa + . in. BC-TEMPO-Ca. makes. it. easier. and. faster. for. subsequent.
Ca-P. phase. formation. than. with. other. BC. templates.. The. Ca:P. molar. ratio.
analyzed.by.ICP-AES.was.1.52.±.0.09,.1.50.±.0.08,.and.1.45.±.0.04.for.native.
BC,. BC-TEMPO,. and. BC-TEMPO-Ca,. respectively,. after. 21. days. soaking. in.
1.5.SBF,.while.it.was.only.1.37.±.0.08,.1.39.±.0.02,.and.1.31.±.0.05.for.a.soaking.
time.of.2.days..The.Ca:P.ratio.of.about.1.3.at.day.2.indicated.that.the.Ca-P.
phase. nucleated. on. the. surface. of. BC. microfibrils. was. octacalcium. phosphate.(OCP;.Ca 8 H 2 (PO 4 ) 6 ·5H 2 O).or.OCP-like.Ca-P.phase..Although.the.Ca:P.
ratio. increased. with. the. soaking. time,. the. values. for. all. the. BC. templates.
were. lower. than. the. theoretical. value. of. 1.67. for. synthetic. hydroxyapatite.
(Ca 10 (PO 4 ) 6 (OH) 2 )..The.mineralized.Ca-P.phase.thus.formed.on.BC.templates.
was.calcium-deficient.hydroxyapatite..The.lower.Ca:P.ratio.was.due.to.the.
substitution.of.the.trivalent.PO 4
3− .ions.by.divalent.ions.such.as.acid.phosphate.groups.(HPO 4
2− ).and.carbonate.groups.(CO 3
2− )..As.a.result,.vacancies.
at.Ca 2+ .cationic.sites.in.the.apatite.structure.occurred.during.the.formation.
of.the.Ca-P.phase.(Elliott.et.al..1985;.Rey.et.al..1989).
A. number. of. hypotheses. have. been. proposed. stating. that. the. nature. of.
the. initial. deposits,. a. solid. phase. of. Ca-P. in. bone. tissue. and. enamel,. are.
Biomimetic Mineralization of Apatite on Bacterial Cellulose
ranging.from.60.to.140.nm.long.(Nge.and.Sugiyama.2007)..The.mineralized.
apatite. globules. firmly. adhered. on. the. microfibril. surfaces. and. formed. a.
uniform. Ca-P. layer,. followed. by. further. crystal. growth. upon. the. existing.
layer..The.variation.in.size.of.the.apatite.globules.under.the.same.soaking.
condition. revealed. the. influence. of. the. microfibril. surface. structure. at. the.
initial.nucleation.process.of.apatite.formation.(Sato.et.al..2001;.Uchida.et.al..
2003)..The.globule.size.formed.on.the.microfibril.surfaces.of.the.BC-TEMPO.
template.was.about.1.5.μm..The.presence.of.carboxylate.groups.on.the.BC.
microfibril. surface. (BC-TEMPO). and. further. ion. exchange. treatment. with.
calcium.(BC-TEMPO-Ca).significantly.affect.the.rate.of.apatite.nucleation.by.
a.rapid.consumption.of.calcium.and.phosphate.ions.from.the.surrounding.
supersaturated.fluid..A.large.number.of.nuclei.thus.grew.competitively.on.
BC-TEMPO-Ca,.leading.to.a.smaller.average.size.than.for.the.BC-TEMPO.and.
native.BC.templates..In.other.words,.an.increasing.amount.of.mineral.phase.
or.more.crystals.may.occur.principally.by.multiplication.in.BC-TEMPO-Ca,.
whereas.progressive.mineralization.with.an.increase.in.size.of.the.crystals.
or.crystal.growth.may.occur.in.native.BC,.like.in.bone.matrix.mineralization.
and.calcification.of.enamel.(Rey.et.al..1991a).
The.profiles.of.relative.weight.gain.of.mineralized.Ca-P.on.BC.templates.
as. a. function. of. soaking. time. also. reflect. the. growth. behavior. of. apatite.
crystal. with. respective. BC. templates.. The. BC-TEMPO-Ca. samples,. which.
exhibited. the. smallest. apatite. globules,. showed. the. highest. deposited.
weight,. whereas. the. gradual. increase. of. mineral. deposition. that. occurred.
in.native.BC.was.characterized.by.the.lowest.deposited.weight.throughout.
the.soaking.time,.and.the.profile.of.BC-TEMPO.samples.displayed.between.
BC-TEMPO-Ca.and.native.BC..Because.calcium.ion.is.one.of.the.main.mineral. components. of. the. apatite. crystal,. a. preexisting. ionic. complex. like.
BC-COOCa + . in. BC-TEMPO-Ca. makes. it. easier. and. faster. for. subsequent.
Ca-P. phase. formation. than. with. other. BC. templates.. The. Ca:P. molar. ratio.
analyzed.by.ICP-AES.was.1.52.±.0.09,.1.50.±.0.08,.and.1.45.±.0.04.for.native.
BC,. BC-TEMPO,. and. BC-TEMPO-Ca,. respectively,. after. 21. days. soaking. in.
1.5.SBF,.while.it.was.only.1.37.±.0.08,.1.39.±.0.02,.and.1.31.±.0.05.for.a.soaking.
time.of.2.days..The.Ca:P.ratio.of.about.1.3.at.day.2.indicated.that.the.Ca-P.
phase. nucleated. on. the. surface. of. BC. microfibrils. was. octacalcium. phosphate.(OCP;.Ca 8 H 2 (PO 4 ) 6 ·5H 2 O).or.OCP-like.Ca-P.phase..Although.the.Ca:P.
ratio. increased. with. the. soaking. time,. the. values. for. all. the. BC. templates.
were. lower. than. the. theoretical. value. of. 1.67. for. synthetic. hydroxyapatite.
(Ca 10 (PO 4 ) 6 (OH) 2 )..The.mineralized.Ca-P.phase.thus.formed.on.BC.templates.
was.calcium-deficient.hydroxyapatite..The.lower.Ca:P.ratio.was.due.to.the.
substitution.of.the.trivalent.PO 4
3− .ions.by.divalent.ions.such.as.acid.phosphate.groups.(HPO 4
2− ).and.carbonate.groups.(CO 3
2− )..As.a.result,.vacancies.
at.Ca 2+ .cationic.sites.in.the.apatite.structure.occurred.during.the.formation.
of.the.Ca-P.phase.(Elliott.et.al..1985;.Rey.et.al..1989).
A. number. of. hypotheses. have. been. proposed. stating. that. the. nature. of.
the. initial. deposits,. a. solid. phase. of. Ca-P. in. bone. tissue. and. enamel,. are.