Application of PGSTE-NMR technique to characterize the porous structure of pharmaceutical tablets

Direct compaction of pharmaceutical tablets is a complex process that results in a heterogeneous density distribution inside the compact. In the present study, we have used a non-invasive and non-destructive technique: the pulsed-gradient stimulated-echo (PGSTE) NMR method to access to topological information (connectivity, tortuosity) about the porous structure of the tablets obtained with three different pharmaceutical excipients: the microcrystalline cellulose, the lactose and the anhydrous calcium phosphate. These materials were chosen since their mechanical properties under pressure are highly differentiated. To probe the pore space with the PGSTE-NMR technique, the tablets were initially impregnated with silicone oil that is NMR sensitive (1H NMR). The time-dependent apparent self-diffusion coefficient was measured over a suitable range of diffusion time in the directions perpendicular and parallel to the compression axis, from which the tortuosity factor and the anisotropy of the porous structure can be studied. These results show that the porous structure varies with pressure and depends on the excipient behaviour under pressure. Then, this work demonstrates that PGSTE-NMR could be an alternative and a very interesting technique to obtain useful information on the structural properties of such compacted materials.