In this study, pH- and temperature-responsive hydrogels based on linear sodium alginate (SA) and crosslinked poly(N-isopropylacrylamide) (PNIPAAm) were prepared by semi-interpenetrating network (semi-IPN) technique. The dually responsive hydrogels were characterized by FTIR, DSC, and SEM, and their temperature- and pH-responsive behaviors were investigated by measuring equilibrium swelling ratios and pulsatile swelling experiments. The results showed that these hydrogels underwent volume phase transition at around 33 degrees C irrespective of the pH value of the medium, but their pH sensitivity was evident only below their volume phase transition temperature. Under basic conditions, the swelling ratios of SA/PNIPAAm semi-IPN hydrogels were greater than that of pure PNIPAAm hydrogel and increased with increasing SA content incorporated into the hydrogels, but the case was inverse under acidic conditions. The pulsatile swelling experiments indicated that the higher the SA content in SA/PNIPAAm semi-IPN hydrogels, the faster the response rate to both pH and temperature change. (C) 2005 Wiley Periodicals, Inc.