Polymeric delivery systems are mainly used to achieve either temporal or spatial control of drug delivery. Functional polymers are designed to modify the pharmaceutical function of the dosage form and to control the release of the active ingredient. it is possible to modulate the release characteristics by optimizing the properties of the drug and polymer coat.The majority of controlled-release dosage forms can be categorized as: matrix, reservoir or osmotic systems. In matrix systems, the drug is embedded in the polymer matrix and release takes place by partitioning of the drug into the matrix and the release medium (mass transport phenomenon).<br />In contrast, reservoir systems have a drug core surrounded by a rate controlling membrane. Factors such as pH and presence of food affect the drug release rate from reservoir devices.<br />An increase in hydrostatic pressure drives osmotic devices, forcing the drug solution or suspension out of the device through a small delivery port. <br />Mechanisms for controlled drug release<br />The most important attribute of a controlled-release device is the ability to maintain a constant rate of drug delivery. There are three common mechanisms of action, namely diffusion, osmotic effects and erosion.<br />1- Diffusion<br />Polymer films that use a diffusion mechanism permit the entry of aqueous fluids from the gastrointestinal (GI) tract into the tablet core. Dissolution of the drug ensues, which is followed by diffusion of the drug solution<br />through the polymeric membrane into the body. The rate of drug diffusion can be determined by the physicochemical properties of the drug and the membrane itself. The properties of the polymer membrane can be altered by (type of polymer, its molecular weight and the inclusion of plasticizers).<br />If the chosen polymer membrane is hydrophilic, the rate of absorption of liquid is very high and the dosage swells. Consequently, there is an associated increase in diffusibility, which enhances the rate of drug release. Conversely, swelling is negligible & the diffusion of the drug out of the polymer matrix is much slower. if the polymer membrane is hydrophobic.<br />2- Osmotic effects<br />Osmotic drug delivery systems suitable for oral drugs that consist of a compressed tablet core coated with a semipermeable membrane . The thickness of the membrane is usually between 200 and 300 μm. Most osmotic devices use water-permeable materials such as cellulosic polymers, particularly cellulose acetate (CA). The permeability of CA films can be tailored by adjusting the degree of acetylation; as the acetyl content increases, the permeability decreases. Ethyl cellulose is also widely used as a membrane for oral osmotic systems. The water permeability of pure ethyl cellulose is low but is enhanced by the incorporation of water-soluble additives such as HPMC. The membrane is non-extensible and the increase in volume caused by the imbibition of water raises the hydrostatic pressure. Osmotic drug delivery systems release drug at a rate that is independent of the pH and hydrodynamics of the external dissolution medium.<br />Osmotic system is also applicable to drugs with a broad range of aqueous solubility. Consequently, the drug can either be released as a solution or as a suspension. If the drug is released as a suspension, it must dissolve in vivo before it becomes systemically available.<br />3- Polymer erosion<br />Biodegradable polymers are used to reduce the need for additional surgical intervention required to remove non-biodegradable matrices. Biodegradation designates the process of polymer chain cleavage, which leads to a loss in molecular weight. Degradation induces the subsequent erosion of the material which is defined by a mass loss. Numerous biodegradable polymers have been synthesized to deliver drugs, cells and enzymes. The properties of these polymers can be modified by incorporating a variety of labile groups such as esters, anhydride and urethane in their backbone. The most widely used systems, poly(lactic acid) (PLA), poly (glycolic acid) (PGA) and their copolymers (PLGA). The biodegradation kinetics can be altered by controlling the proportion of PLA and PGA in the copolymer and altering the molecular weight of the polymer. For PLGA microspheres, low molecular weight and high glycolic acid content resulted in a faster release. PLGA suffers from an increase in local acidity during degradation, which can cause irritation and can also be detrimental to the stability of protein drugs.<br />