DRUG-RELEASE WITH SIMULTANEOUS DIMENSIONAL CHANGES FROM A NEW COPOLYMERIC HYDROGEL

The diffusional behaviour of sodium salicylate (SSA) from a new chemically crosslinked copolymeric hydrogel of 2-hydroxyethyl methacrylate with N,N'-dimethyl-N-methacryloyloxyethyl-N-(3-sulfopropyl)ammonium betaine is reported. The drug release into water was effected under sink conditions and the kinetics at 310 K were followed by continuous measurement of conductivity. A modified form of Fick's second law has been derived to take into account the dimensional changes of copolymer during drug release, which were measured photographically. The procedure was used to evaluate the diffusion coefficient D(s) for transport of SSA into water. The value of D(s) = 4.4 x 10(-11) m2 s-1 yielded over a range of drug loading is smaller than the load-dependent values obtained when allowance is not made for dimensional changes. The procedure has also been used to determine the diffusion coefficient for water into polymer.