Thermo-responsive hydrogels based on poly(N-isopropyl- acrylamide) and hyaluronic acid cross-linked with nanoclays

Semi-interpenetrating polymer networks (SIPN) based on thermo-responsive poly(N-isopropylacrylamide) (PNIPA) and water-soluble sodium salts of linear hyaluronic acid (Na-HA) were physically cross-linked with synthetic nanoclay (laponite XLG). PNIPA hydrogels with different cross-linking densities and Na-HA concentrations were synthesized by in situ free-radical redox polymerization. The structure and heterogeneity of the semi-IPN hydrogels were examined by SEM and XRD. The content of clay incorporated in the gel was determined by TGA. DSC measurements showed that volume phase transition temperature and its enthalpy varied with the clay and hyaluronic acid content. SIPN hydrogels containing negatively charged polyelectrolyte, Na-HA, exhibited higher Q(e) and faster deswelling rates than the corresponding PNIPA NC hydrogels. The presence of the anionic Na-HA polymer reduced the storage modulus, indicating a weakening of the hydrogel network structure, especially at lower clay contents. The nanocomposite hydrogels exhibited high tan delta values, which increased with increasing Na-HA content.