Formation of Chitosan-Based Hydrogels Network - Chitosan Based Hydrogels: Functions and Applications - page 206

support for the attachment, proliferation, and differentiation of MSCs into osteoblasts and
chondrocytes, respectively. The chitosan/HAp scaffolds show promising biological behav-
ior and may therefore find applications in bone tissue engineering and osteochondral
defects.
A biomimetic HAp/chitosan/gelatin network composite scaffold is developed through
phase separation of HAp powder suspension in an acid aqueous solution of chitosan-
gelatin and GA under freeze-drying conditions. The pore size of scaffolds ranges from
several microns to ca. 500 μm, and the porosity can be adjusted by changing the solid
content of the original component ratio. Typical data of porosity is 90% and the pore size
is ca. 300-500 μm. Rat caldaria osteoblasts seeding onto the scaffold exhibit excellent
bioactivity and can facilitate bone formation [182,183]. And this scaffold leads to enhance-
ment of ECM proteins and calcium, which favors the adhesion and osteogenic diff-
erentiation of MSCs [184].
4.4.2 Mineralization Composites
Bone tissue is extremely strong and dense, which makes it well suited for providing load-
bearing support and protection to the body. It is mainly composed of mineral of HAp set
onto collagen fiber. Bone is characterized by hierarchical organization. Here, the mineral
structure is incorporated into the ECM, where a network of collagen fiber-holding cells
together serves as a template for biomineralization [185]. HAp crystal is sedimented in situ
on a chitosan-based matrix in order to mimic the structure of bone. Coprecipitation and
surface coating are the main approaches to prepare chitosan-HAp mineralization
composites.
4.4.2.1 Mineralization through Coprecipitation
Chitosan can also serve as a template for mineralization due to its special functional
groups, such as −NH 2 and C=O. The process of chitosan mineralization in the case
when a stepwise coprecipitation approach is used is shown in Figure 4.15 [186]. Small HAp
crystallites are able to align along the chitosan molecule upon aggregation through the
interaction between the calcium ions on the HAp surface and the amino groups of the
chitosan molecule. In other words, the self-assembly phenomenon for the c -axis of HAp
nanocrystals parallel to the chitosan molecules can be described by the formation of HAp
nucleation centers on the amino groups in chitosan, resulting in the subsequent crystal
growth of HAp nanocrystals [187].
Porous Hap-chitosan-alginate composite scaffolds are fabricated by in situ coprecipita-
tion methods; eventually, the pore structure is locally collapsed and appears to be agglom-
erated. The size of HAp crystals can be controlled via changing the contents of chitosan in
the chitosan-alginate complex. A higher amount of chitosan leads to a smaller average
crystallite size of HAp [188]. In general, the average size of HAp crystals formed via min-
eralization is ca. 15-100 nm and the crystallinity is lower when compared with those
formed in solution. The maximum compressive strength of the HAp/chitosan composite is
about 120 MPa corresponding to the 30:70 chitosan/HAp composite, while the compres-
sive strength of 20:70 chitosan/HAp composites is ca. 100 MPa. This may be caused by
chemical and mechanical interlocking between HAp and chitosan that accounts for the
efficient stress transfer in the composite system. Besides, the interactions such as hydrogen
bonding and chelation between the two phases also contribute to the good mechanical
properties of the chitosan/HAp composite [189].