STRUCTURE AND BEHAVIOR IN HYDROPHILIC MATRIX SUSTAINED-RELEASE DOSAGEFORMS .4. STUDIES OF WATER MOBILITY AND DIFFUSION-COEFFICIENTS IN THEGEL LAYER OF HPMC TABLETS USING NMR IMAGING

Purpose. The purpose of this study was to characterise the water mobil ity in the gel layer of hydrating HPMC tablets. Water mobility in the gel layer of different HPMCs was studied. Methods. NMR imaging, a non- invasive technique, has been used to measure the spatial distribution of self-diffusion coefficient (SDC) and T-2 relaxation times across th e gel layer. Results. It has been shown that there is a water mobility gradient across the gel layer of HPMC tablets. Although SDC and T-2 r elaxation times in the outer parts of the gel layer approached that of free water, in the inner parts they decreased progressively. Water mo bility and SDC in the gel layer of different HPMCs appeared to vary wi th degree of substitution of the polymer and the lowest values were ob tained across the gel layer of K4M tablets. Conclusions. Water mobilit y varies across the gel layer of hydrating HPMC tablets and it is depe ndent on the degree of substitution of the polymer.