In vitro degradation of pH-sensitive hydrogels containing aromatic azo bonds

Biodegradable and pH-sensitive hydrogels containing azoaromatic moieties were synthesized from the same polymeric precursors by two synthetic methods, namely a polymer-polymer reaction and cross-linking of polymeric precursors. The effect of the synthetic route employed and the detailed network structure on in vitro degradation of hydrogels was studied. Regardless of the synthetic method used, two patterns of degradation were observed. Hydrogels with lower cross-linking density underwent a surface erosion process and degraded at a faster rate. Hydrogels with higher cross-linking densities degraded at a slower rate by a process where a colourless degradation front moved inward to the yellow core. It appears that hydrogels synthesized by a polymer-polymer reaction degraded at a slightly faster rate than their analogues synthesized by cross-linking of polymeric precursors. The degradation rate of a hydrogel was compared with those of a linear azopolymer and a low-molecular-weight azosubstrate (methyl orange) respectively. The degradation rates were in the order of hydrogel < linear azopolymer < low-molecular-weight azosubstrate.