formation of hydrogels with a permanent network structure because irreversible
chemical links are formed. This type of linkage allows absorption of water and/or
bioactive compounds without dissolution, and permits drug release by diffusion
under pH-controlled conditions. Ionic crosslinked CTS hydrogels exhibit a higher
swelling sensitivity to pH changes than covalently crosslinked CTS hydrogels. This
fact extends the potential application of ionic crosslinked CTS since dissolution can
occur in extreme acidic or basic pH conditions [132].
Wan et al. [116] have investigated the interactions of chitin with calcium species.
The strategy for the dispersion of HAp in chitin to produce intimately blended
materials has also been reported. Preliminary mechanical tests revealed a reduction
in strength for the more highly filled composites, but retention of the plastic
properties of the polymer was observed that might be favorable for bone-substitute
applications [133]. Chitin/HAp composites with 25, 50, and 75% (w/w) of HAp
fractions have also been investigated by Ge et al. [134] and processed into air- and
freeze-dried materials. The cell culture study and implantation into the intramusculature of a rat model revealed that these materials are non-cytotoxic and degradable in in vivo systems. The presence of the HAp filler enhanced calcification as well
as accelerated degradation of the chitin matrix. CTS/HAp composites with various
ratios were obtained by Yamaguchi and coworkers [74] using the co-precipitation
method. In these composite materials, calcium phosphate formed crystalline HAp in
an acetic acid–lactic acid solvent system for CTS. The amorphous calcium phosphate was formed in organic acids containing more than two carboxyl groups, which
resulted in aggregations (length 230 nm) of HAp nanocrystals aligned along the
CTS molecules. This is possibly due to the fact that the calcium of HAp nanocrystals
formed a complex with the amino groups in CTS. The composites were found to be
mechanically flexible and could easily be formed into any desired shape. The
mechanical strength could be enhanced by heat treatment in a saturated steam,
and this effect was ascribed to the formation of hydrogen bonds between CTS
macromolecules. Biodegradable CTS/Gel/HAp composites were prepared as 3D
biomimetic scaffolds using phase separation by Zhao et al. [135] and obtained
similar compositions to that of normal human bone. The pore size distribution and
density of these composite materials was controlled by changing the content and the
composition of the variables. Histological and immunohistochemical staining and
SEM observations indicated that the osteoblasts attached to and proliferated on the
scaffolds. The presence of HAp in the CTS/Gel composite also promoted initial
adhesion of human mesenchymal stem cells (hMSC) and supported long-term
growth in 3D porous CTS/Gel/HAp scaffolds [136].
A series of CTS/Gel/nanoHAp composites have been fabricated by Li et al.
[137] by depositing nanoHAp on the surface of CTS/Gel network films. The
amount of polymers in the CTS/Gel networks greatly influenced nucleation and
the development of the nanoHAp crystalline phase. Organic functionalities such as
–COOH, ═C═O, or –NH 2 groups can be active sites for the coordination of calcium
ions, leading to the formation of complexes that initiate and control formation of
nanoHAp nuclei. Therefore, the size of nanoHAp can be controlled by changing the
ratio of CTS and Gel to imitate the structure of natural bone.
158
A. Bhowmick et al.
chemical links are formed. This type of linkage allows absorption of water and/or
bioactive compounds without dissolution, and permits drug release by diffusion
under pH-controlled conditions. Ionic crosslinked CTS hydrogels exhibit a higher
swelling sensitivity to pH changes than covalently crosslinked CTS hydrogels. This
fact extends the potential application of ionic crosslinked CTS since dissolution can
occur in extreme acidic or basic pH conditions [132].
Wan et al. [116] have investigated the interactions of chitin with calcium species.
The strategy for the dispersion of HAp in chitin to produce intimately blended
materials has also been reported. Preliminary mechanical tests revealed a reduction
in strength for the more highly filled composites, but retention of the plastic
properties of the polymer was observed that might be favorable for bone-substitute
applications [133]. Chitin/HAp composites with 25, 50, and 75% (w/w) of HAp
fractions have also been investigated by Ge et al. [134] and processed into air- and
freeze-dried materials. The cell culture study and implantation into the intramusculature of a rat model revealed that these materials are non-cytotoxic and degradable in in vivo systems. The presence of the HAp filler enhanced calcification as well
as accelerated degradation of the chitin matrix. CTS/HAp composites with various
ratios were obtained by Yamaguchi and coworkers [74] using the co-precipitation
method. In these composite materials, calcium phosphate formed crystalline HAp in
an acetic acid–lactic acid solvent system for CTS. The amorphous calcium phosphate was formed in organic acids containing more than two carboxyl groups, which
resulted in aggregations (length 230 nm) of HAp nanocrystals aligned along the
CTS molecules. This is possibly due to the fact that the calcium of HAp nanocrystals
formed a complex with the amino groups in CTS. The composites were found to be
mechanically flexible and could easily be formed into any desired shape. The
mechanical strength could be enhanced by heat treatment in a saturated steam,
and this effect was ascribed to the formation of hydrogen bonds between CTS
macromolecules. Biodegradable CTS/Gel/HAp composites were prepared as 3D
biomimetic scaffolds using phase separation by Zhao et al. [135] and obtained
similar compositions to that of normal human bone. The pore size distribution and
density of these composite materials was controlled by changing the content and the
composition of the variables. Histological and immunohistochemical staining and
SEM observations indicated that the osteoblasts attached to and proliferated on the
scaffolds. The presence of HAp in the CTS/Gel composite also promoted initial
adhesion of human mesenchymal stem cells (hMSC) and supported long-term
growth in 3D porous CTS/Gel/HAp scaffolds [136].
A series of CTS/Gel/nanoHAp composites have been fabricated by Li et al.
[137] by depositing nanoHAp on the surface of CTS/Gel network films. The
amount of polymers in the CTS/Gel networks greatly influenced nucleation and
the development of the nanoHAp crystalline phase. Organic functionalities such as
–COOH, ═C═O, or –NH 2 groups can be active sites for the coordination of calcium
ions, leading to the formation of complexes that initiate and control formation of
nanoHAp nuclei. Therefore, the size of nanoHAp can be controlled by changing the
ratio of CTS and Gel to imitate the structure of natural bone.
158
A. Bhowmick et al.