Study of the sigmoidal swelling kinetics of carboxymethylchitosan-g-poly(acrylic acid) hydrogels intended for colon-specific drug delivery

The swelling kinetics of a series of carboxymethylchitosan-g-poly(acrylic acid) hydrogels pretreated under acidic buffer media has been studied. Sigmoidal swelling curves are exhibited when these pretreated hydrogels are subsequently immersed in the buffer solutions of pH 6.0–7.4. This phenomenon may be attributed to the disruption of a cooperative physical cross-linking (i.e. the hydrogen bonding and the ionic cross-linking) on the networks, which was proved by the change of FT-IR spectra of hydrogels during swelling. The buffer pH, the pretreating pH and the composition of hydrogels have an obvious influence on the sigmoidal effect. The profile of drug release at pH 7.0 from the hydrogel which was prepared in the pH 2.2 buffer containing 5-aminosalicylic acid (5-ASA) exhibits a sigmoidal release curve, namely, an initial slow release followed by a burst release. The swelling kinetics shows the potential in the design of the colon-specific drug delivery system.