Abstract:
Flow properties contribute to the stability of disperse systems and the performance of a variety of pharmaceutical products. While the viscosity value is itself important, the rheologic character of the product, both in the bottle and during utilization, must also be considered. Many polymers are commercially available to produce the desired rheologic character. Selection of a polymer is based on biological compatibility and absence of undesired interactions with other ingredients as well as rheology. The role of structure-building additives in reducing sedimentation/separation in disperse systems is well known. We have found a semi-logarithmic relationship between sedimentation rate and polymer concentration. Another property sometimes exhibited by polymers is flocculation of dispersed particles in certain systems. This may be a benefit (e.g. to prevent caking in fluid suspensions) or an undesired effect. In any case, the formulator must be aware of the possibility that viscosity builders may induce changes in other physical properties. Several important applications depend on the development of rheological structure in situ. An example is in the formulation of matrix tablets, which develop a viscous shell during dissolution in the gastro-intestinal tract. In another instance, we have developed an approach for producing sustained release oral suspensions based on gelation of polymers in the stomach. Recent work has focused on systems that do not rely on an acid pH to induce gelation. Single phase gels, frequently used to deliver medication topically, have an open structure that restricts movement of particles and macromolecules, but allows free passage of ordinary drug molecules. This is an exception to the usual situation in that drug release and absorption are relatively insensitive to wide changes in macroscopic rheology. The examples cited can be applied to fit the needs and challenges of other applications.