Biomedical Engineering Reference
In-Depth Information
Chitosan
Cross-linking with genipin
pH 5.0 and pH 7.4
pH 9.0
pH 13.6
1~4 Heterocyclic amine units
7~88 Genipin monomer units
Figure 4.8
Conformations of the network segments of genipin-cross-linked chitosan gels consisting of short cross-linking
units of cyclic bridges and long cross-linking units of polymerized genipin. (From Mi, F. L., Shyu, S. S., and
Peng, C. K. 2005. J Polym Sci B-Polym Chem 43: 1985-2000. With permission.)
protonated. Genipin cross-linkers may form wasted cross-links as well as polymerize,
leading to cross-links formed from not just one genipin molecule but various dimers, trim-
ers, oligomers, and possibly polymers of genipin [106].
The degradation rate of the genipin-cross-linked chitosan network is much lower than
the GA-cross-linked counterparts, because the genipin-cross-linked network may have a
higher stereohindrance for the penetration of lysozyme than the GA-cross-linked network
due to the bulky heterocyclic structure of genipin [101].
4.2.3.3 Diisocyanate-Cross-Linked Chitosan
Welsh and Price [107] prepared a water-soluble, blocked diisocyanate as a cross-linking
agent for chitosan network formation. With increased pH or temperature, the adduct read-
ily reacts with amines of chitosan, forming a urea linkage, at a rate much greater than that
characteristic of competing reactions with alcohols or water. The cross-linker could tolerate
high levels of hydration to achieve greater reaction efficiency and alternative applications,
such as hydrogels. With greater control over cross-linking efficiency, material properties
(such as physical, mechanical, and biological) of diisocyanate-cross-linked chitosan can be
tailored to a given application [108].
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