Effect of solvent mixture on the properties of temperature- and pH-sensitive polysaccharide-based hydrogels

Novel partially biodegradable, temperature- and pH-sensitive polysaccharide-based hydrogels (NDF) were synthesized from modified dextran (dextran-maleic acid, Dex-MA) and N-isopropylacrylamide precursors over a wide range of mixed solvent ratios of dimethyl formamide (DMF) to water. N-Isopropylacrylamide monomers were chosen to impart thermo-responsive capability to Dex-MA, while Dex-MA was chosen to impart pH-responsive capability to N-isopropylacrylamide. The pH-sensitive precursor (Dex-MA) was synthesized by reacting dextran with maleic anhydride in the presence of triethylamine catalyst. To fabricate multi-stimuli hybrid hydrogel networks, both Dex-MA and N-isopropylarylamide precursors were photo-cross-linked via UV at a fixed Dex-MA to N-isopropylarylamide feed ratio over a wide range of DMF to water mixed solvent ratios. The newly synthesized PNIPAAm/Dex-MA hybrid hydrogels (NDF) were characterized by Fourier transform infrared spectroscopy for chemical structure determination, differential scanning calorimetry for thermal analysis and scanning electron microscopy for morphological study. The properties of the hybrid hydrogels, such as thermo-induced deswelling, pH-sensitivity, ionic strength sensitivity and thermo-reversibility, were also examined. The swelling data obtained clearly showed that newly synthesized multi-stimuli NDF hydrogels exhibited multi-responsive capability to external stimuli like temperature and pH. The morphological data obtained showed that this new class of PNIPAAm/Dex-MA hybrid hydrogels had a wide range of unique three-dimensional porous network structures that depended on the composition ratio of the mixed DMF/water solvent during cross-linking reaction. This unique but versatile 3D porous network structures of NDF hydrogels were correlated to the data from thermo-induced swelling behavior, thermo-reversibility, pH-dependent swelling and ionic strength sensitivity.