A new temperature-sensitive hydrogel with α-amino acid group as side chain of polymer

Abstract

New thermo-responsive hydrogels with α-amino acid groups as side chains, e.g. methacryloylglycine methyl ester, methacryloyl- l-proline [MA-( l)Pro] and methacryloyl- l-phenylalanine methyl ester, were synthesized by copolymerizing a hydrophobic monomer viz. 2-hydroxypropyl methacrylate (HPMA) and a crosslinking monomer viz. polyethylene glycol No. 600 dimethacrylate (14G), using γ-rays from a 60Co source. Of these hydrogels, the MA-( l)Pro-containing copolymer gels gave the strongest reversible thermo-response; the swelling-deswelling mechanisms can be classified into two types according to the kind of copolymer, viz. the surface regulating system for the MA-( l)Pro/HPMA copolymer and matrix pumping system for the MA-( l)Pro/14G copolymer. Thus, when raising the temperature, the copoly[MA-( l)Pro/HPMA] gel in the swollen state undergoes initial rapid shrinkage of the outer layer of the sample in contact with water, followed by a gradual deswelling. From microscopic observation, it was found that there are many micropores in the swollen gel but they disappear immediately after treating at high temperature, especially at the surface of the gel. In contrast, the swollen copoly[MA-( l)Pro/14G] gel undergoes rapid shrinkage of the whole sample without the disappearance of micropores. In addition, the control of drug release from such thermo-responsive copolymer formulations was investigated.