Effect of divalent cations on the volume of a maleic acid copolymer gel examined by incorporating lysozyme

An ionic gel of poly(maleic acid-alt-methyl vinyl ether) cross-linked by 1,6-hexanediol was prepared with molar ratio of 1/0.5 at 50 °C. The gels with different molar ratios at different temperatures were also prepared for comparison. Their swelling–shrinkage profiles were examined by measuring the ratio of initial to final weights at various pHs and CaCl2 concentrations in buffer solution. The gel squeezed remarkably in the presence of Ca2+ for pH>6.3, corresponding to the solution properties of maleic acid copolymer. No appreciable change was observed below pH 6.3. To estimate free space in the gel in more detail, lysozyme was incorporated into the free space, and its release rate was measured under several CaCl2 concentrations. The time course of the release was either simple or complicated, depending on pH and CaCl2 concentration. The control mechanism of the release of lysozyme from the gel network may be widely applicable to the drug delivery system.