Effect of chemical crosslinking on the swelling and shrinking properties of thermal and pH-responsive chitosan hydrogels

The ability to form a get through the physical or chemical crosslinking of chitosan has been well documented. In an attempt to mimic biological systems, thermal and pH-sensitive chitosan cylindrical hydrogels were produced by a combination of physical and chemical crosslinking processes. To this end, chitosan hydrogels prepared from alkali chitin were molded in cylinders and, once washed, were further crosslinked with glutaraldehyde at stoichiometric ratios, R (= [-CH=O]/[-NH2]), of 1.61 and 3.2-2 x 10(-2). Variation in swelling as a result of stepwise changes, in temperature between 40 and 2degreesC at pH values of 7.0, 7.6, and 8.0 revealed that the system responds in markedly different manners dependent upon the pH. At pH 7.0, cooling from 40 to 2 degreesC results in contraction of the gel network structure, While raising the temperature from 2 to 40degreesC leads to a rapid swelling response (i.e., ca. a twofold increase in the amount of solvent uptake), Subsequent cooling to 2 degreesC 6 accompanied by a new contraction cycle. At pH greater than or equal to 7.6 the temperature dependence of the swelling-contraction behavior is exactly the opposite of that observed at pH 7.0. Very similar trends were observed for the gels at both degrees of crosslinking. The swelling-shrinking behavior observed in gels of pH greater than or equal to 7.6, is similar in kind to that of uncrosslinked gels and is interpreted in terms of a lower critical solution temperature (LCST) volume phase transition, driven by hydrophobic association, presumably involving residual acetyl groups in the chitin. The results at pH 7.0 suggest that the slight ionization of the -NH3+ groups leads to destruction of the hydrophobic hydration thus effectively reversing the negative thermal shrinking.