Degradability of hydrogels containing azoaromatic crosslinks

A novel type of biodegradable, pH‐sensitive hydrogels was synthesized by crosslinking of N , N ‐dimethylacrylamide copolymer precursors. These hydrogels contained azoaromatic crosslinks which may be degraded by the azoreductase activities in the colon. Hydrogel degradation experiments were performed both in vitro and in vivo , using rat cecum contents and implantation into the rat cecum, respectively. In order to evaluate the influence of the detailed network structure on the degradation properties, the degradability of hydrogels synthesized by two different methods, i.e., crosslinking of polymeric precursors and crosslinking copolymerization, were compared. Two different patterns of the degradation of hydrogels were observed, i. e., a surface erosion process and a bulk‐degradation‐like process. The azo bond cleavage rate of hydrogels prepared by crosslinking of polymeric precursors was faster than that of hydrogels prepared by crosslinking copolymerization. The differences in the gel degradation pattern and the cleavage rate of azo bonds were attributed to the differences in the structure of the hydrogel network, i. e., the molecular weight of primary chains and the formation of chain entanglements. Under the experimental conditions used, the degradation rate in vivo was 3 to 5 times faster than that in vitro. It appears that these hydrogels have a potential for colon‐specific drug delivery.