Nondestructive viscoelastic analysis of anisotropy in compressed tablets

A nondestructive procedure for determining anisotropy levels in compressed tablets was developed based on dynamic stress/strain measurements of viscoelasticity in the axial and radial directions. An instrument was designed and built capable of 10 nm resolution of strain and operating over the frequency range 1-64 Hz. Tablets of 11 pharmaceutical excipients and drugs, representing both plastic and brittle materials, were found to behave as three-parameter viscoelastic solids in both the axial and radial directions. Elastic and viscous parameters were computed, based on frequency dependence over the range studied, and were found to differ greatly in the axial and radial directions. Degrees of anisotropy, defined as the ratio of the uncoupled spring parameters in the radial vs the axial direction, were found to range from 2- to 12-fold over the materials studied. These levels of anisotropy are thought to be typical of tablets manufactured in a rotary press under normal conditions. Measurements of this type can be easily made routinely as a further means of quality control and reflect an important feature of the internal structure of compressed tablets.