Rheological Behavior and Improved Swelling of Porous Semi-IPN Hydrogels Based on Poly(isopropylacrylamide-co-itaconic Acid)/Sodium Alginate – In Vitro Theophylline Release Analysis

A series of semi-interpenetrating networks (semi-IPN) hydrogels composed of temperature sensitive poly(N-isopropylacrylamide) (PNIPAAm) and pH sensitive itaconic acid (IA) and sodium alginate (SA) were prepared by radical copolymerization/crosslinking reaction. The structures, morphology, thermal, rheological and swelling behaviors of the hydrogels were studied. The evidence for successful synthesis was confirmed by infrared spectroscopy. In thermal analysis, all prepared semi-IPN samples showed a clear endotherm corresponding to a volume phase transition temperature. Morphological aspects of the samples displayed highly porous structure and expanded network depending on alginate content. Rheological analysis showed that all measured viscoelastic properties were influenced by gel composition and temperature; a sharp transition to higher values of the storage modulus G' was observed above the transition temperature. The swelling behavior revealed the high sensitivity of samples for temperature and pH: higher swelling was observed in simulated intestinal fluid (SIF) than in gastric one (SGF). Moreover, the swelling drops radically as the temperature rises up to 37 °C. In both fluids at 20 °C and 37 °C, the swelling is diffusion-controlled mechanism with a Fickian transport. From in vitro degradation study, the hydrogels were degradable in pancreatin-containing SIF solution at 37 °C and samples with higher alginate ratio showed high degradation rate. The high cumulative release of theophylline observed in SIF provides a significant improvement for drug delivery from these hydrogels to intestinal regions; the release profile displays close fitting to Korsmeyer-Peppas model with Fickian transport.