Copolymers of acrylic acid with N-vinylpyrrolidinone and 2-hydroxyethyl methacrylate as pH-responsive hydrogels synthesized through a photoinitiator-free photopolymerization technique

pH-sensitive hydrogels for biomedical applications were synthesized using a photoinitiator-free technique involving the initiation of photopolymerization by donor/acceptor pairs. The differential photocalorimetric technique indicated a high polymerization rate for the N-vinylpyrrolidinone (NVP, donor)/acrylic acid (AA, acceptor) pair at a 1:1 molar ratio. However, photopolymerization of larger quantities of these monomers (1:1 molar ratio) produced a water-soluble polymer. Nevertheless, an anionic hydrogel was successfully formed when a small quantity of 2-hydroxyethyl methacrylate (HEMA) was included in the NVP/AA formulation. A mixture of HEMA and AA, although both are classified as acceptors, photopolymerized to produce a copolymer which functioned as an anionic hydrogel. The swelling and drug release of these hydrogels were investigated in acidic, neutral and basic pH environments. Their biocompatibility with HaCaT human epidermal keratinocyte cells was tested and a positive cell growth as evidenced by the 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl tetrazolium bromide (MTT) cell proliferation assay indicated that these hydrogels have no toxic effect on HaCaT.