Compositional Influence of Cross-linked Polyion Hydrogels from Poly(L-Lysine) and Poly(L-Glutamic Acid) on Their Properties for Potential Skin Applications

Chemically cross-linked hydrogels are synthesized from a set of protected L-lysine and L-glutamic acid containing homo and copolypetides. Cross-linking of the linear copolypeptides is achieved through the incorporation of L-tryptophan and its reaction with hexamethylene bis(triazolinodione). Upon deprotection, hydrogels with oppositely charged polypeptides of different L-lysine and L-glutamic acid composition form pH-dependent polyion complexes through electrostatic interaction. The obtained networks show distinct pH-dependent differences in their water swelling and rheological properties. Hydrogels at pH 4 display higher strengths compared to pH 8 and their rheological properties scale with L-lysine/L-glutamic acid ratio. At pH 8 results suggests that ionic interactions and presumably secondary structure effects have a significant impact on the hydrogel properties. This is further evident from the influence of the copolypetide structures, random versus block copolypeptides, used in the cross-linked hydrogel. None of the hydrogel shows any significant cytotoxicity against skin cell lines.