Use of impulse excitation technique for the characterization of the elastic anisotropy of pharmaceutical tablets

Capping and lamination are common defects occurring during the manufacturing of pharmaceutical tablets. Several studies showed that tablet anisotropy can play a role in the occurrence of such defects. In this work, we propose a new and easy methodology to characterize the anisotropy of flat-faced cylindrical tablets, which are considered as transversally isotropic due to the process, through the study of their elastic properties using impulse excitation technique and finite-element method (FEM) simulations. The study was performed for tablets with a thickness-to-diameter-ratio between 0.160 and 0.222. FEM simulations showed that it was possible to determine three out of the five elastic constants of the tablet using the first three natural vibration modes. An anisotropic index was then built as the ratio of the two apparent shear moduli. Moreover, in order to simplify the estimation of tablet anisotropy and to avoid the systematic use of FEM simulations, an analytical model was also developed. It only requires the measurement of the tablet dimensions and of the first three natural frequencies. Using this technique, experimental measurements on tablets made of classical pharmaceutical excipients were done and found coherent with the existing literature. This indicates thus that this methodology is a quick, easy and reliable characterization method in order to access tablet anisotropy.