Synthesis and characterization of stimuli-sensitive hydrogels having a different length of ethylene glycol chains carrying phosphate groups: loading and release of lysozyme.

In order to prepare a polymer matrix capable of loading protein at high density, anionic hydrogels were synthesized by copolymerizing a monomer carrying a pendant phosphate group, methacryloyl-polyoxyethyl phosphate, with N-isopropylacrylamide and N,N'-methylene-bis-acrylamide, and the stumuli-sensitivity of hydrogels was characterized. The number of repeating ethylene glycol units in the phosphate carrying monomer was 1, 2, 5 or 8. Lysozyme bearing a positive net charge was immobilized in the hydrogel through formation of polyelectrolyte complex. It was shown that the amount of complexed lysozyme reached to 1.7 g/g dry gel, when high content of a phosphate-carrying monomer with 5 ethylene glycol units was incorporated into a hydrogel. It was further found that lysozyme complexed with phosphate-carrying network could be released by immersion of the lysozyme/hydrogel composite in a phosphate buffer solution of pH 7.4 owing to the pH-sensitivity of the hydorgel but no lysozyme was released at pH 1.4. The initial rate of lysozyme release was varied depending on the length of the ethylene glycol chains possessed by a network polymer and the content of the phosphate-carrying monomer unit. Lysozyme released from the phosphate-carrying hydrogel was proved to retain enzymatic activity at a level similar to the activity of lysozyme, which had been kept in buffer solution.