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OH
CH
2
OH
O
CH
2
OH
H H
2
O
H
H
OH
H
H
OH
CH
2
OH
C
O
O
OH
O
OCH
3
OH
H
C
H
H
O
H
H
CH
2
OH
NH
O
NH
CH
2
OH
H
C
O
C
O
H
H
H
OCH
3
H
OH
O
OH
OH
H
H
O
H
O
H
CH
2
OH
NH
HOH
2
C
OH
O
C
O
HOH
2
C
OH
OH
CH
2
OH
N
CH
2
OH
H
O
H
H
C
H
H
OH
NH
O
O
OH
H
OH
H
H
O
H
CH
2
OH
NH
O
C
N
HOH
2
C
OH
Figure 4.7
Reaction mechanism for the synthesis of genipin-cross-linked chitosan network. (From Mi, F. L., Sung, H. W.,
and Shyu, S. S. 2000.
J Polym Sci A-Polym Chem
38: 2804-2814. With permission.)
Genipin-cross-linking-produced chitosan networks are insoluble in acidic and alkaline
solutions but are able to swell in these aqueous media. The swelling characteristics of the
films exhibit sensitivity to the environmental pH and temperature [105], whereas at pH 1.0
and pH 13.0, the swelling ratio increase to 315% and 240%, respectively, while at pH 7 the
swelling ratio is about 62%. The increase of swelling of the cross-linked hydrogel at pH
lower than 3 and higher than 11 may be ascribed to the hydrolysis of amide linkage in the
cr-CS network by acid or alkaline and the regeneration of carboxyl acid and amine groups
in networks. The swelling behavior is dictated by their polyelectrolyte nature. An increase
in swelling is observed below the p
K
a
of the chitosan amine groups as they become
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