Hydrogel is a water-based substance that can retain large amounts of water. It is a biphasic material, a mixture of porous, permeable solids and at least 10% by weight or volume of interstitial fluid composed completely or mainly by water. Hydrogels are three-dimensional networks of polymer chains capable of holding large amounts of water. They are synthesized by crosslinking water-soluble polymers. Hydrogels have a soft consistency and moisturizing properties, making them useful for promoting wound care. They are also used in disposable diapers, sanitary napkins, and dressings for healing of burn or other hard-to-heal wounds. Hydrogels are biocompatible with most cells and have viscoelastic characteristics caused by the chemical or physical cross-linking of individual hydrophilic polymer chains. They can swell when exposed to increased temperatures, electric or magnetic fields, light, pressure, pH, ionic strength, solvent compositions, or molecular species. Some of the growing applications for todays chemical hydrogels include water absorbers, implants, and drug delivery systems.