NMR MICROSCOPY OF HYDRATING HYDROPHILIC MATRIX PHARMACEUTICAL TABLETS

NMR microscopy has been used to monitor the formation of the gel layer in hydrating hydrophilic polymer tablets. Such tablets are used in th e controlled delivery of drugs, where it has been found that the rate and extent of the swelling of the outer gel layer critically influence s the kinetics of drug release. Tablets were hydrated in distilled wat er at 37 degrees C and then imaged at discrete time intervals using a 500 MHz microscope. The growth of the gel layer was clearly observed i n time sequences of radial and axial sections. Axial images showed som e interesting dimensional changes, with the gel at the flat surface of the tablet developing a concave shape. This is probably a reflection of the occurrence of uni-axial stress relaxation as hydration proceeds . Diffusion- and T-2-weighted images provided evidence that the water in the gel layer is more strongly bound close to the dry core of the t ablet than at the more fully hydrated outer surface. In images of tabl ets containing diclofenac, disruption of the gel layer was shown to oc cur primarily from the flat surfaces of the tablet, whilst the distrib ution of particles could be seen in tablets doped with insoluble calci um phosphate.