Hydrogels as Controlled Drug Delivery Systems: Hydrogels as Controlled Drug Delivery Systems: Hydrogels as Controlled Drug Delivery Systems: Hydrogels as Controlled Drug Delivery Systems: Hydrogels as Controlled Drug Delivery Systems: Synthesis, Crosslinking, W Synthesis, Crosslinking, W Synthesis, Crosslinking, W Synthesis, Crosslinking, W Synthesis, Crosslinking, Water and Drug T

Hydrogels are presently under investigation as a delivery system for bioactive molecules, because of their similar physical properties as that of living tissue, which is due to their high water content, soft and rubbery consistency, and low interfacial tension with water or biological fluids. Anionic hydrogels are used in the design of intelligent controlled release devices for site-specific drug delivery of therapeutic proteins to the large intestine, where the biological activity of the proteins are prolonged, and cationic hydrogels are studied for the development of self­ regulated insulin delivery system, which releases the insulin in response to changing glucose concentration. The different methods of preparation of hydrogels, novel methods of crosslinking used in the preparation of hydrogels, the mechanism of water transport through the ionic hydrogels, and the release mechanism of the solute from the hydrogels, are discussed in the present article. The hydrogels, since their discovery by Wichterle and Lim in 1960 of poly(2-hydroxyethyl methacrylate) 1 , have been of great interest to biomedical scientists. Hydrogels are three dimensional hydrophilic polymer networks capable of swelling in water or biological fluids, and retaining a large amount of fluids in the swollen state 2 . Their ability to absorb water is due to the presence of hydrophilic groups such as –OH, –CONH–, –CONH 2 , – COOH, and –SO 3 H 3 . The water content in the hydrogels affect different properties like permeability, mechanical properties, surface properties, and biocompatibility. Hydrogels have similar physical properties as that of living tissue, and this similarity is due to the high water content, soft and rubbery consistency, and low interfacial tension with water or biological fluids 4 . The ability of molecules of different size to diffuse into (drug loading), and out (release drug) of hydrogels, permit the use of hydrogels as delivery systems. Since hydrogels have high permeability for water soluble drugs and proteins, the most common mechanism of drug release in the hydrogel system, is diffusion. Factors like polymer composition, water content, crosslinking density, and crystallinity, can be used to control the release rate and release mechanism from hydrogels 5 .