Biomimetic Mineralization of Hydrogel Biomaterials for Bone Tissue Engineering - Biomimetics: Advancing Nanobiomaterials and Tissue Engineering - page 57

after implantation, they permit faster mineral formation than incubation
in SBF. Additionally, since simulation of in vivo conditions during miner-
alization is not the aim, there is more freedom to vary the experimental
parameters, e.g., temperature, ion concentration, which allows more con-
trol over the type and amount of mineral formed.
Madhumathi et al. [34], induced hydroxyapatite mineralization in a chi-
tosan hydrogel by alternate soaking in solutions of CaCl 2 and Na 2 HPO 4 ,
leading to formation of homogeneously dispersed hydroxyapatite deposits
throughout the hydrogel after fi ve soaking cycles. Du et al. [35] presoaked
collagen matrices in PO 3− and immersed them in Ca 2+ solutions. Under
different experimental conditions, different crystal polymorphs were
formed. Hutchens et al. [36] prepared composites of cellulose and calcium-
defi cient hydroxyapatite by alternate immersion of cellulose hydrogels in
calcium and phosphate solutions. Similarly, Furuichi et al. [37] mineralized
a polyacrylic acid hydrogel by crosslinking a polyacrylic acid polymer in
the presence of (NH 4 ) 2 HPO 4 solution with subsequent immersion in a
Ca 2+ -containing solution. Combination of presoaking in non-physiological
solutions followed by incubation in physiological SBF solution is also
possible. Hong et al. [38] used a cellulose hydrogel that was fi rst presoaked
in CaCl 2 solution and subsequently incubated in SBF, leading to uniform
and dense biomimetic mineralization after 14 days.
It should be noted that mineralization by incubation in solutions of cal-
cium and phosphate ions can be enhanced signifi cantly by incorporation
of biomacromolecules which stimulate mineralization (see Section 3.5.2),
which has been reported for chitosan with added gelatin [39], or pectin [40].

3.3.2 In Situ Synthesis of Hydroxyapatite

The main drawback of soaking in SBF and/or non-physiological Ca 2+ and
PO 3− solutions, is the fact that minerals are deposited predominantly on
the surface of the hydrogel samples but not in their bulk interior. The sec-
ond approach, preforming of homogeneously distributed CaP nanopar-
ticles throughout the entire volume of hydrogels during the hydrogel
formation process, aims to improve homogeneity of mineralization.
Azami employed a double diffusion method whereby CaCl 2 and
Na 2 HPO 4 diffused into a gelatin gel from opposite sides to form amor-
phous CaP and brushite in the hydrogel's central region [41]. Subsequent
incubation in SBF resulted in transformation of the mineral phase into
hydroxyapatite. Chen et al. [42] created hydroxyapatite precipitates in situ
within a chitosan hydrogel network by neutralizing acidic chitosan with
tris-buffer solution containing Ca(NO 3 ) 2 and K 2 HPO 4 . Luo et al. [43] cre-
ated chitosan/collagen hybrid gels with hydroxyapatite nanoparticles by
neutralization of acidic chitosan/collagen solutions with (NH 4 ) 2 HPO 4 and
Ca(NO 3 ) 2 solutions.