Reversible response of poly(aspartic acid) hydrogels to external redox and pH stimuli

A redox- and pH-responsive poly(aspartic acid) (PASP) hydrogel is synthesised by cross-linking cysteamine modified polysuccinimide (PSI) with non-cleavable cross-linker 1,4-diaminobutane followed by the hydrolysis of the resultant PSI gel to a PASP hydrogel. The hydrogel showed a remarkable change in its degree of swelling as a function of external pH because of the polyelectrolyte character of the network. In addition to pH-dependent swelling, the thiolation of the polymer rendered redox responsive behaviour to poly(aspartic acid). The degree of swelling could be tuned in a reversible manner by immersing the hydrogel in reducing or oxidising media. The elastic modulus of the hydrogel was also strongly affected by the presence of reducing or oxidising agents. Moreover, the gel strength could be improved via oxidation. The reversible response of the degree of swelling and the elastic modulus was induced through the thiol-disulphide transformation inside the hydrogel proven using a new analytical method. Consequently, a dual responsive hydrogel is obtained with redox- tunable cross-link density.