Synthesis and swelling properties of novel pH-sensitive poly(aspartic acid) gels

Chemically cross-linked poly(aspartic acid) (PASP) gels were prepared by the hydrolysis of poly(succinimide) (PSI). The latter was prepared by thermal polycondensation of aspartic acid. The PSI chains were cross-linked by natural amines and amino acid derivatives such as putrescin, spermine, spermidine, lysine and cystamine to obtain biodegradable, biocompatible, amino acid-based hydrogels. The volume of the synthesized unhydrolyzed PSI gels changes abruptly at a well-defined pH that results in ring opening, while the hydrolyzed gels show a volume phase transition around the pK values of PASP. The unidirectional stress–strain behavior of the gels as well as the dependence of equilibrium swelling degree on the pH was carefully studied and the most important network parameters were determined by a modified version of the Brannon–Peppas–Peppas theory.