Pharmaceutical applications of rheology

Pharmaceutical applications of rheology

Thixotropy is a desirable property in liquid pharmaceutical preparations. A well-formulated thixotropic suspension does not settle out readily in the container and becomes fluid on shaking, with increasing duration of shaking resulting in greater reduction of viscosity or increase in flow. A similar pattern of behavior is desirable with emulsions, lotions, creams, ointments, and parenteral suspensions to be used for intramuscular depot therapy. With regard to suspension stability, there is a relationship between the degree of thixotropy and the rate of sedimentation—the greater the thixotropy, the lower the rate of settling. Importantly, the degree of thixot-ropy of a system may change over time (e.g., storage during shelf life) and result in an inadequate formulation.

Examples of pharmaceutical applications of rheology can be exemplified by the following:

·           Surfactants, poly(oxyethylene)-poly(oxypropylene)-poly(oxyethylene) block copolymers, also known as Pluronic® and poloxamers, exhibit Newtonian behavior in the liquid state, at low concentrations and low temperatures. Poloxamer vehicles are used in dermatological bases or topical ophthalmics, since they are nontoxic and form clear water-based gels.

·           Polymer solutions are used as wetting agents for contact lenses or tear substitutes for the dry eye syndrome. These solutions should ideally exhibit pseudoplastic behavior; that is, low viscosity at high shear rates produces lubrication during blinking, and high viscosity at low or zero shear rate prevents fluid from flowing away from the cornea when eyelids are not blinking. Both natural (e.g., dextran) and synthetic (e.g., polyvinyl alcohol) ones are used with the addition of various preservatives. High-molecular-weight sodium hyaluronate with a concentration of 0.1%–0.2% is used for the dry eye condition.

·           Rheologic properties of suppositories at rectal temperatures can influence the release and bioabsorption of drugs from suppositories, particularly those having a fatty base. Some fatty acid bases exhibit either Newtonian or plastic flow at rectal temperatures.

·           When polymer, opacifier, and surfactant suspensions are prepared for tablet coating, the rheologic behavior of the suspension determines the ability to spray the suspension through a thin nozzle for tablet coating.

·           Stability and pourability of liquid preparations is determined by their viscosity and thixotropic behavior.