Viscosity measures the
resistance of a solution to flow when a stress is applied. The viscosity of
a solution is given in poise units. The unit centipoise (cp or the plural cps)
is equal to 0.01 poise and is most often used in pharmaceutical applications.
Compounds used to enhance viscosity are available in various grades such as
15 cps, 100 cps, etc. The grade number refers to the viscosity that results
when a fixed percentage aqueous solution is made. Generally the solutions are
1% or 2% and the viscosity is measured at 20oC.
Viscosity enhancers are
used in ophthalmic solutions to increase their viscosity. This enables the formulation
to remain in the eye longer and gives more time for the drug to exert its therapeutic
activity or undergo absorption. Commonly used viscosity enhancers and their
maximum concentrations are given in the table below.
The most common viscosity
desired in an ophthalmic solution is between 25 and 50 cps. The actual concentration
of the enhancer required to produce that viscosity will depend on the grade
of the enhancer. For example, if methylcelluse 25 cps is used, a 1% solution
will create a viscosity of 25 cps. If methylcellulose 4000 cps is used, a 0.25%
solution provides the desired viscosity. Standard references give tables of
viscosities produced by percentage solutions and grades of ingredients.