Swelling-shrinking behavior of chemically cross-linked polypeptide gels from poly(α-l-lysine), poly(α-dl-lysine), poly(ɛ-l-lysine) and thermally prepared poly(lysine): Effects of pH, temperature and additives in the solution

► Hydrogels from four kinds of poly(lysine)s (PLs) cross-linked with glutaraldehyde. ► We studied the pH dependence of the equilibrium swelling ratio. ► Also studied the temperature dependence at pH 11.6 in the absence and presence of additives that affect the secondary structure of PLs. ► The swelling behavior is well described by a force balance approach. ► The repulsive and attractive interactions among the cross-linked polymer segments were considered based on the conformational properties of PLs in aqueous solutions.