Formation of Chitosan-Based Hydrogels Network - Chitosan Based Hydrogels: Functions and Applications - page 180

the interest in the intrinsic properties of these polymers including biocompatibility, low
toxicity, and susceptibility to enzymatic degradation. Among these polymers, polysaccha-
rides do not suffer some of the disadvantages of other naturally derived materials, such as
immunogenicity and the potential risk of transmitting animal-originated pathogens. One
such polysaccharide is chitosan. These attractive natural polysaccharides share the advan-
tages of other natural polymers (lysozomal degradation, etc.), but do not induce an immune
response [8].
Chitosan hydrogels have been prepared with a variety of different shapes, geometries,
and formulations that include liquid gels, powders, beads, films, tablets, capsules, micro-
spheres, microparticles, sponges (porous scaffolds), nanofibrils, textile fibers, and inor-
ganic composites. In each preparation chitosan is either physically associated or chemically
cross-linked to form the hydrogel [6].
4.2 Chitosan-Based Network
In cross-linked chitosan (cr-CS), polymeric chains are interconnected by cross-linkers,
leading to the formation of a 3D network. A cross-linking network of chitosan can be
formed by complexation with another polymer, generally ionic, or by aggregation after
chitosan grafting [9]. Chitosan-based biomaterials can form hybrid polymer networks
(HPNs) or semi- or full-interpenetrating polymer networks (semi- or full-IPNs) through
the cross-linking reaction [10]. In these cross-linking systems, covalent bonds are the
main interactions to form the networks, but other interactions cannot be excluded.
Indeed, there are secondary interactions including hydrogen bridges and hydrophobic
interactions. In general, the network structures include polyelectrolyte complex (PEC)
networks and covalent cross-linking and ionic cross-linking networks ( cf . Figure 4.2) .
These chitosan networks could modulate the swelling behaviors, mechanical properties,
and some bioactive functions. Desirable chitosan-based network biomaterials need spe-
cial interaction with or mimicry of ECM components, GFs, or cell-surface receptors.
Chitosan is similar to GAGs in structure, but it is absent in vivo . Therefore, the hybrid
networks are formed via introducing some bioactive molecule or polysaccharide. The
chitosan-based network structure is an efficient method for simulating the cell growth
microenvironment [11].
4.2.1 Complex Cross-linking Network
Chitosan is a copolymer of glocosamine and N -acetyl-d D -glycoamine linked together by
β(1-4) glycoside bonds. Due to its unique cationic nature, chitosan is able to form the
PEC with negatively charged polyanions, for example, proteins and GAGs. Chitosan-
based PECs (CS-PECs) are generally obtained through the reaction of chitosan and poly-
anions. In general, the CS-PEC films' formation may be schematically classified into
three main stages: (1) primary complex formation; (2) formation process within intrac-
omplexes; and (3) intercomplex aggregations. Since chitosan has a rigid, stereo-regular
structure containing bulky pyranose rings, the formation of PEC can induce a conforma-
tional change of the other polyelectrolyte if the latter has a nonrigid structure [12].
Various different characteristics of CS-PECs can be obtained by changing the chemical
characteristics of the polymers' components, such as the M W , flexibility, functional group