Hydrophobically modified hydrogels containing azoaromatic cross-links: swelling properties, degradation in vivo and application in drug delivery

Hydrogels based on n-alkyl methacrylate esters (n-AMA) of various chain lengths, acrylic acid, and acrylamide cross-linked with 4,4-di(methacryloylamino)azobenzene were synthesized. The equilibrium swelling degree of the hydrogels in buffered solutions at pH 7.4 was shown to be very low in the pH range of the stomach. The entire swelling processes of the gels in the gastrointestinal tract were mainly dependent on those in the small intestine. In the buffered solution of pH 7.4 the diffusion of water into the gel slabs was discussed on the stress relaxation model of polymer chains. The results obtained are in good agreement with Schott's second-order diffusion kinetics. The biodegradability in vivo of their azobenzene cross-linking groups as well as the mechanism of degradation by cecal bacteria was studied. The gels are stable in the stomach but degradable by ananerobes present in the colon. The extent of degradation was considerably related to the equilibrium degree of swelling. The factors influencing the swelling degree were shown to influence the in vivo degradation of the gels. By changing these factors such as the degree of cross-linking, the length and content of the n-AMA side chains, it is possible to control both the degree of swelling and the degradation of the hydrogels.