Osmotic delivery systems, rate controlled

Osmotic pressure, a colligative property, depends on the concentration of solute (neutral molecule or ionic species) that contributes to the osmotic pressure. Solutions of different concentrations having the same solute and solvent system exhibit an osmotic pressure proportional to their concentrations. Thus a constant osmotic pressure, and thereby a constant influx of water, can be achieved by an osmotic delivery system that results in a constant release rate of drug. Therefore, zero-order release, which is important for a controlled release delivery system when indicated, is possible to achieve using these platforms. In 1974,... 

The elementary osmotic delivery system consists of an osmotic core containing drug and, as necessary, an osmogen surrounded by a semiper-meable membrane with an aperture (Fig. 7.1). A system with constant internal volume delivers a volume of saturated solution equal to the volume of solvent uptake in any given time interval. Excess solids present inside a system ensure a constant delivery rate of solute. The rate of delivery generally follows zero-order kinetics and declines after the solute concentration falls below saturation. The solute delivery rate from the system is controlled by solvent influx through the semiper-meable membrane. 

For drugs that possess low water solubility a number of formulation strategies have been examined to enhance their solubility and hence the rate of release from osmotically controlled drug delivery systems. Examples of these approaches include ... 

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