Thermo-sensitive IPN hydrogels composed of PNIPAAm gels supported on alginate-Ca2+ with LCST tailored close to human body temperature

Transition temperature of interpenetrating polymer networks (IPN) hydrogels, composed of PNIPAAm gels supported on alginate-Ca2+, was investigated. Plots of partition coefficient, k, vs. temperature of Orange II converge to a maximum k value, attributed to the hydrophilic-to-hydrophobic transition in IPN hydrogels. This was associated with the lower critical solution temperature (LCST) of PNIPAAm in these IPN hydrogels. Shifts in the LCST values were related to the PNIPAAm and alginate-Ca2+ contents in the hydrogel, which affects the movements of water molecules outward and PNIPAAm chains inward at the point in time of the shrinking process. Bound and free water contents in the IPN hydrogels were investigated by differential scanning calorimetry (DSC). The higher the PNIPAAm and alginate-Ca2+ contents in the IPN hydrogel, the lower is the relative amount of free water and the higher is the relative amount of bound water. The resultant tighter polymer structure of the hydrogels was evaluated by average molar mass between two adjacent cross-links (Mc). The LCST of these hydrogels may be tailored to close to human body temperature by controlling the amount of NIPAAm and alginate-Ca2+ during synthesis. This characteristic makes IPN hydrogels of alginate-Ca2+/PNIPAAm attractive for potential uses in biomaterial applications.