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