Biomedical Engineering Reference
In-Depth Information
5.2.2 Chemical Cross-linking of Chitosan-based Hydrogels
Chemical cross-linking is an effective method for preparing pH-sensitive chitosan-based
hydrogels. The amino groups of the glucosamine residue within chitosan chains can serve
as cross-linking sites, for example, by reacting with glutaraldehyde, glyoxa, proanthocya-
nidin, genipin, and so on, to form cross-linking between linear chitosan chains that leads
to gel formation. But the pH-sensitivity of pure chitosan cross-linking hydrogels does not
satisfy the application. First, amphoteric cross-linking hydrogels can be formed via intro-
ducing some negative groups into chitosan in order to modulate the pH-sensitive capabil-
ity. In addition, a chitosan-based heterocyclic polymer network (HPN) could be formed via
the addition of other polymers and cross-linked. In this system, chitosan and polymer are
cross-linked by co-cross-linkers and form a covalent network, as shown in Figure 5.6a. On
the other hand, when other polymer chains interpenetrate and interact with chitosan via
hydrogen bonds or physical interactions, the chitosan-based semi-interpenetrating poly-
mer network (semi-IPN) is formed ( cf. Figure 5.6b). The HPN and semi-IPN technique can
be used to improve the properties of chitosan-based gels. Moreover, their pH sensitivity
could be modulated via adjusting the composition and cross-linking degree.
5.2.2.1 Amphoteric Cross-Linking Hydrogels
The density of -NH 2 of chitosan has a very important role in the pH sensitivity of chitosan-
based hydrogels. Therefore, an effective method for modulating pH sensitivity is by chang-
ing the density of -NH 2 of a chitosan network. Introducing negative groups into chitosan
chains could improve the swelling degree of chitosan-based hydrogels in neutral and basic
media. For example, the swelling degree of carboxymethyl chitosan hydrogels is higher
(a)
Chitosan
Cross-linking site
Polymer
(b)
Chitosan
Cross-linking site
Polymer
Figure 5.6
Schematic representation of (a) chitosan-based HPN hydrogels and (b) semi-IPN hydrogels.
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