Semi-interpenetrating polymer network (semi-IPN) and fully interpenetrating polymer network (full-IPN) hydrogels composed of alginate and poly(N-isopropylacrylamide) were prepared with gamma-ray irradiation. The semi-IPN hydrogels were prepared through the irradiation of a mixed solution composed of alginate and N-isopropylacrylamide (NIPAAm) monomer to simultaneously achieve the polymerization and self-crosslinking of NIPAAm. The full-IPN hydrogels were formed through the immersion of the semi-IPN film in a calcium-ion solution. The results for the swelling and deswelling behaviors showed that the swelling ratio of semi-IPN hydrogels was higher than that of full-IPN hydrogels. A semi-IPN hydrogel containing more alginate exhibited relatively rapid swelling and deswelling rates, whereas a full-IPN hydrogel showed an adverse tendency. All the hydrogels with NIPAAm exhibited a change in the swelling ratio around 30-40 degrees C, and full-IPN hydrogels showed more sensitive and reversible behavior than semi-IPN hydrogels under a step-wise stimulus. In addition, the swelling ratio of the hydrogels continuously increased with the pH values, and the swelling processes were proven to be repeatable with pH changes. (c) 2006 Wiley Periodicals, Inc.