Dv. Moe et Eg. Rippie, NONDESTRUCTIVE VISCOELASTIC ANALYSIS OF ANISOTROPY IN COMPRESSED TABLETS, Journal of pharmaceutical sciences, 86(1), 1997, pp. 26-32
A nondestructive procedure for determining anisotropy levels in compre
ssed 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 opera
ting over the frequency range 1-64 Hz. Tablets of 11 pharmaceutical ex
cipients and drugs, representing both plastic and brittle materials, w
ere found to behave as three-parameter viscoelastic solids in both the
axial and radial directions. Elastic and viscous parameters were comp
uted, based on frequency dependence over the range studied, and were f
ound 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 though
t 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 o
f the internal structure of compressed tablets.