Environment–Stimuli Response of Chitosan-Based Hydrogels - Chitosan Based Hydrogels: Functions and Applications - page 240

10

8

6

4

2

0 1234567
pH

8910 11 12 13

Figure 5.11
Swelling behavior of the chitosan-gelatin hybrid network specimen with a—CHO/-NH 2 molar ratio of 10 in
solutions of different pH values with ionic strength I = 0.1 at 37°C.

5.2.2.3 Chitosan-Based HPN Hydrogels

The uncross-linked polymer of the chitosan-based semi-IPN hydrogels is always subject to
diffusion in the solvent, especially in swelled condition. HPN hydrogels may overcome the
problem via cocross-linking.
Gelatin is a denatured form of collagen, composed of glycine, praline, hydroxyproline,
arginine, and other amino acids. The amphiphilic protein (p I = 4.96) can provide amino
groups for cocross-linking with chitosan to prepare a chitosan/gelatin hybrid polymer
network (HPN). The pH-sensitive swelling behavior of HPN gel is displayed in Figure 5.11
[24]. The data show that the degree of swelling declines sharply at pH 7.0; this can be
explained by the fact that the hydrogen bonds within the chitosan/gelatin HPN dissociate
in an acidic medium. The elastic modulus of chitosan/gelatin HPN hydrogels in basic
medium is higher than that in acidic medium due to the reassociation of hydrogen bond-
ings between networks [25]. An increase in cross-link density induced a decrease in swell-
ing and pH sensitivity. The pH-sensitive chitosan hydrogel properties can be tuned by
preparatory conditions and the inclusion of gelatin [26].

5.3 Temperature Response
It is well known that thermosensitive behaviors in a polymer solution can generally be
considered as a change in intermolecular interactions in response to temperature. These
polymer chains contain either moderately hydrophobic groups (if too hydrophobic, the
polymer chains would not dissolve in water at all) or a mixture of hydrophilic and hydro-
phobic segments. When a polymer is dissolved in water, there are three types of interac-
tions that take place: between polymer molecules, polymer and water, and between water
molecules [27]. These interactions are the main drive force underlying the formation of
thermosensitive hydrogels. At low temperature, water molecules are presumed to form
enclosed structures that surround the polymer chains. At high temperature, water is
released from polymer chains due to the high rotation energy of water, and the dewatered