Suspensions - Introduction to Suspensions
A suspension is a two phased system in which a finely divided solid is dispersed in a continuous phase of solid, liquid, or gas. The undissolved solid exists in equilibrium with a saturated solution of the solid in the continuous phase.
A pharmaceutical suspension is a coarse dispersion in which insoluble solid particles are dispersed in a liquid medium.
Size Classification of Suspensions
A pharmaceutical suspension is a coarse dispersion in which insoluble solid particles are suspended in a liquid medium.
A coarse suspension is a dispersion of finely divided, insoluble solid particles (the disperse phase) in a liquid (the dispersion medium). Most pharmaceutical suspensions consist of an aqueous dispersion medium although in some instances it may be an organic or oily liquid. A disperse phase with a mean diameter of up to 1 mm is usually termed a colloidal dispersion and include such examples as aluminum hydroxide and magnesium hydroxide suspensions.
A solid in liquid dispersion in which the particles are above colloidal size is termed a coarse suspension.
Routes of Administration
- Topical (lotions)
Forces In a Suspension
Properties of a Well-Formulated Suspension
The following animation introduces the properties of a well-formulated suspension.
In Summary, Properties of an Ideal Suspension are:
- Uniform dispersion
- Pleasing odour and colour
- No grittiness
- Easy to pour yet not watery
- No cap-lock
- Temperature insensitive
Uses of Suspensions
- To administer an insoluble compound as a liquid.
- To lessen the unpleasant taste of an insoluble compound by:
- formulating a vehicle in which the drug is not soluble
- using an insoluble form of the drug (ie. salt form or prodrug)
- adsorbing the drug onto an insoluble carrier
- To modify the release rate of the drug.
- To improve the stability by reducing the fraction of drug in solution.
Suspensions as a Dosage Form
Comparing suspensions with solutions:
The solution is considered to be the most bioavilable liquid dosage form. Although also a liquid, the suspension has a dissolution step. However, relative to solid dosage forms, the suspension offers two advantages:
- Drug particle size must be small to avoid a gritty mouth-feel.
- All solid particles must be wetted to achieve physical stability.