Ionic gels - Physical Gels from Biological and Synthetic Polymers - page 152

The effectiveness of the deacetylation process can be followed by infrared or NMR
spectroscopy. The main parameters in

uencing the characteristics of chitosan are its
molecular mass and its degree of deacetylation. The amino group has a pK of 6.6, which
means it maintains a positive (cationic) charge in acidic to neutral solution, dependent on
both pH and its degree of deacetylation.
However, as pointed out earlier, chitosan is not, on its own, a gelling polysaccharide,
even though many biomedical papers still refer to chitosan gels, when these are simply
high-viscosity entangled solutions. Chitosan receives a great deal of attention in medical
and pharmaceutical applications, and one of the reasons is this charge effect. For example,
every other polysaccharide described in this chapter is anionic, and indeed chitosan remains
one of the few cationic natural polymers. However, allied to this, chitosan is known for its
biocompatibility, allowing its use inmedical applications such as topical ocular application,
implantation or injection. Moreover, chitosan is metabolized by certain human enzymes,
e.g. lysozyme, and can be considered as biodegradable. Owing to these positive charges at
physiological pH, chitosan is also bioadhesive, which increases retention at the site of
application. Many researchers and others claim that chitosan also promotes wound-healing
and has bacteriostatic effects (Ueno et al., 2001 ). When chitosan is dispersed as particles
with sizes in the range 75

220 μm (Lima and Airoldi, 2004 ; Monteiro and Airoldi, 2005 )in
aqueous solution, it may also

-

find industrial application, such as cation removal from

- -

wastewater or industrial ef uents through biopolymer cation interactions at the solid

liquid interface, with a preference for copper cations.
More detailed applications of chitosan are described in Chapter 11 , in particular its use
in biomedical applications and, together with a variety of anionic polyelectrolytes, in so-
called polyelectrolyte complexes (PECs).

5.8 Conclusions

In spite of efforts by many research groups to clarify the precise mechanisms of gelation
of ionic polysaccharide gels, few if any have been established unambiguously because of
the absence of ideal samples, i.e. samples with a narrow distribution of molecular mass
and completely converted into a single ionic salt form. Such improved samples, such as
those used in the Japanese collaborative research group on gellan, could be distributed to
laboratories in various disciplines, and the results compared and discussed. Non-perturbatory
techniques such as SAXS combined with rheological studies should be performed in
parallel to explain any unique gel properties. Despite these dif culties, this is an extremely

active area with many innovative applications, some of which reappear in Chapter 11 .

References

Austen, K. R. J., Goodall, D. M., Norton, I. T., 1988. Biopolymers 27 , 139 - 155.

Axelos, M. A. V., Thibault, J.-F., 1991. Int. J. Biol. Macromol. 13 ,77 - 82.
Bayley, S. T., 1955. Biochim. Biophys. Acta 17 , 194 - 205.