Synthesis of a novel pH-sensitive hydrogel based on poly(γ-glutamic acid) crosslinked with urea

Chemical hydrogel based on poly(γ-glutamic acid) obtained from Bacillus licheniformis (ATCC-9945a) and using urea as a crosslinking agent was synthesized. The hydrogel was characterized by infrared spectroscopy, scanning electron microscopy, thermal analysis and swelling capacity. The crosslinking of the biopolymer was evidenced by changes in the intensities of signals in the infrared spectrum corresponding to amide groups, with respect to the spectrum of the pure biopolymer. Moreover, a porous structure, characteristic of hydrogels, with average pore size of 80 ± 31 μm was confirmed. The hydrogel showed a glass transition temperature of 144.5 °C and a decomposition temperature of 219 °C. The swelling ratio of the hydrogel increased with the increase of contact time with the swelling medium and pH, presenting values at equilibrium of 6.6, 13.4 and 15.3 at pH 3.6, 7.4 and 10, respectively; meanwhile, little influence of temperature on the swelling of the material was observed. Moreover, through the use of mathematical models, it is deduced that the swelling of the hydrogel in the buffer solutions occurs through lower Fickian and Fickian type mechanisms, and in deionized water an anomalous mechanism predominates. Graphical abstract: [Figure not available: see fulltext.]