An examination of the thermorheological and drug release properties of zinc tetraphenylporphyrin-containing thermoresponsive hydrogels, designed as light activated antimicrobial implants

This study describes the thermorheological, mechanical and drug release properties of novel, light-activated antimicrobial implants. Hydrogels, based on N-isopropylacrylamide (NIPAA) and hydroxyethylmethacrylate (HEMA) and either devoid of or containing zinc tetraphenylporphyrin, were prepared by free radical polymerisation and characterised using oscillatory rheometry and texture profile analysis. Drug release was studied at both 20 and 37 °C. Hydrogels containing NIPAA exhibited a sol–gel temperature (Tm  ), which increased as the proportion of HEMA increased and was <37∘C. The viscoelastic properties (storage modulus G′G′, loss modulus G″G″, loss tangent and dynamic viscosity η′η′) were affected by hydrogel composition and temperature, with the copolymers exhibiting lower values of G′,G″G′,G″ and η′η′ values than either homopolymer. Similar relationships between the composition of the hydrogels and the textural/mechanical properties were observed. At 37 °C rheological structuring (increased G″,G″,η′G″,G″,η′ and reduced loss tangent) occurred for all NIPAA-containing polymers and increased as the NIPAA content increased. At 20 °C drug release was diffusion controlled, the rate of which was similar for all NIPAA-containing polymers and was lower than drug release from p(HEMA), despite the greater elasticity of this homopolymer. At 37 °C drug release from the NIPAA-containing hydrogels was initially non-diffusion controlled, following which drug release levelled. Drug release decreased as the NIPAA content increased and correlated to hydrogel elasticity. It is suggested that the ability to engineer the release of Zn-TPP from these hydrogels, in conjunction with their acceptable mechanical properties, may be clinically advantageous for the treatment of infection.