ABSTRACT
Nuclear magnetic resonance (NMR) microscopy was used to monitor the formation of the gel layer in hydrating hydrophilic polymer tablets. Such tablets were used in the controlled delivery of drugs, for which the rate and extent of the swelling of the outer gel layer critically influenced the kinetics of drug release. Tablets were hydrated in distilled water at 37°C and then imaged at discrete time intervals using a 500 MHz microscope. The growth of the gel layer was clearly observed in time sequences of radial and axial sections. Axial images showed some interesting dimensional changes, with the gel at the flat surface of the tablet becoming concave. This was probably a reflection of the occurrence of uniaxial stress relaxation as hydration proceeds. Diffusion-and T2-weighted images provided evidence that the water in the gel layer was more strongly bound close to the dry core of the tablet than at the more fully hydrated outer surface. In images of tablets containing
