Synthesis and characterization of pH-sensitive and biodegradable hydrogels prepared by γ irradiation using microbial poly(γ-glutamic acid) and poly(ε-lysine)

Poly(γ-glutamic acid) (PGA) and poly(e-lysine) (PL) solutions were used as components to prepare mixed hydrogels by γ irradiation. A PGA and PL mixed solution was crosslinked to form a hydrogel with specific water content (weight of absorbed water/weight of dry get) of 10-100 when the 5 wt % solution of mixed polymer was exposed to γ radiation of 87 kGy dosage under N 2 atmosphere. The specific water content increased with increasing PGA content of the PGA/PL mixed get. The influence of pH and salt concentration on equilibrium swelling was studied. A characteristic pH-sensitive swelling behavior was obtained using compositional changes of PGA and PL in the get. PGA/PL 50/50 wt % mixed get swelled in acid (pH 6.0) conditions and was deswelled between pH 4.0 and 6.0 due to the ionic composition changes of the get network. With an increase in the ratio of PGA to PL, the hydrogels showed increasing sensitivity to salt solutions (NaCl, Na 2 SO 4 , and CaCl 2 ). In addition, degradation of PGA/PL get by protease produced from Aspergillus oryzae was investigated at 40°C and pH 7.0. PL get was degraded completely within 2 days. An increase in the ratio of PGA in the PGA/PI, mixed get led to a decrease in the degree of degradation as expected. Some subtle degradation changes were found in the 50/50 and 80/20 wt % (PGA/PL) gels that were degraded by only 3.5 and 3.8% by protease, respectively. © 1995 John Wiley & Sons, Inc.