Due to their reactive ease of production and cost compared to other methods of sustained or controlled delivery, the dissolution and diffusion-controlled systems have classically been of primary importance in oral delivery of medication. Dissolution systems have been some of the oldest and most successful oral systems in early attempts to market sustaining products.
1- Diffusional Systems:
Diffusional systems are characterized by the release rate of drug being dependent on its diffusion through an inert membrane barrier. Usually, this barrier is an insoluble polymer. In general, two types are recognized: reservoir devices and matrix devices.
A) Reservoir Devices
Reservoir devices are characterized by a core of drug, the reservoir, surrounded by a polymeric membrane. The nature of the membrane determines the rate of release of drug from the system.
The process of diffusion is generally described by a series of equations that were first described by Fick and hence retain his name.