Absorbing water is one of the vital properties of hydrogels, because it makes them biocompatible (1). The water-absorbing capacity of hydrogels was determined in terms of percent equilibrium water content (% EWC) or swelling ratio (SR). The dry hydrogels were weighed (W _d ) and then immersed in distilled water for 24 h at 37 ± 2 °C. The hydrogels then were reweighed (W _s ) after removing the excess water by lightly soaking the swollen hydrogels using filter paper. The swelling ratio of the hydrogels was determined using the formula:

Water diffusion study. Release of the drug from a matrix system generally occurs by a diffusion mechanism. The drug diffuses out, along with water, in the form of a solution at pH 7.0 of the release medium. The water-diffusion study, therefore, is indirectly a measure of solute diffusion. It is assumed that diffusion from the matrices follows Fick's law of drug dissolution and diffusion in the medium, both during the sorption and desorption phenomenon (23). In the present study, the diffusion property of the hydrogels was studied using desorption phenomenon. The swollen hydrogels were dried gradually under air at room temperature and weighed after 15-min intervals until a constant weight was reached. The diffusion coefficients of the hydrogels were calculated using the equation:

in which, D is the diffusion coefficient of the hydrogel, δm_t is the weight loss in t time, δm_∞ is the weight loss at infinity, l is the thickness of the dried hydrogel, and t is the time of diffusion of water from the hydrogels during drying.

Drug loading . The drug can be incorporated in the hydrogels by in situ drug-loading during polymerization and by drug-loading by incubation after polymerization (5). In the present experiment, diclofenac sodium was loaded by incubating a known weight of the hydrogel in a 10% w/v aqueous solution of the drug for 24 h at room temperature. The drug-loaded hydrogels then were dried and stored in a well-closed container for further use. The drug remaining in the incubating solution was determined spectrophotometrically at 276 nm after appropriate dilution. The entrapment efficiency of the hydrogel was calculated as follows:

in which, A _o is the initial amount of drug in the incubation medium, A _f is the final amount of drug in the incubation medium, and W _h is the dry weight of the hydrogel incubated for drug loading.

Release kinetics and porosity. Release studies were carried out in distilled water as receptor medium, maintained at 37 ± 2 °C. Sink condition was continuously maintained. The amount of drug released in the dissolution medium was determined spectrophotometrically at 276 nm after every 1-h interval. The study was continued for 72 h. The data of release profiles for each formulation were analyzed further by model fitting with the help of software PCP DissoV2.5 (Pune College of Pharmacy, Pune, India).