A novel method for measuring gel strength of controlled release hydrogel tablets using a cone/plate rheometer

A novel method for measuring gel strength of hydrogel controlled release (CR) tablets is presented. CR tablets containing hydroxypropyl methylcellulose (HPMC) were fixed to a glass plate, placed in a dissolution apparatus, and subjected to dissolution conditions corresponding to 50% drug release. Tablets were then removed from the dissolution apparatus without disturbing the swollen (gel) layer. Tablet gel strength (gamma) was determined by shearing the external gelled layer of the tablet by a cone/plate rheometer recording E, energy/volume, as a function of rotational speed of the cone, RPM. The RPM is increased to the point of "Shear Failure," RPMsf, i.e., until there is no further increase in E. Plots of E vs. RPM were fit to a bi-exponential equation, where epsilon is the zero RPM intercept of the plot. gamma is calculated from gamma = ERPMsf - epsilon. gamma is defined as the minimum energy input/unit volume needed to cause shear failure of the tablet gel layer. Mean gamma for tablets containing 50% theophylline and different ratios of HPMC to Fast Flo lactose were determined. Tablets containing 30%, 15%, and 8% HPMC K100MP have gamma means of 6600, 4700, and 1500 ergs/cm3, and percent relative standard deviations of 9, 13, 33%, respectively. Tablets made entirely of HPMC K100MP (no theophylline or lactose) have an gamma of 15,500 ergs/cm3. gamma values are significantly different (p < 0.0001) between the various formulations. gamma values also show a strong inverse correlation with the cumulative percent of drug released in vitro. Results obtained demonstrate the application of this novel method for measuring gel strength of different CR hydrogel tablets and using this strength to characterize drug diffusion in vitro.