A hydrogel is a three-dimensional polymer network made of a hydrophilic polymer or a mixture of polymers. In general, at least 10-20% of the total weight of a hydrogel is water. When a hydrogel is dried, it is called xerogel, or simply a dried hydrogel. When a dried hydrogel is placed in an aqueous environment, it can absorb a large amount of water and swell isotropically to maintain its original shape. Swollen hydrogels maintain their shape without dissolving even in abundant water because of the presence of chemical or physical cross-linking of polymer chains. The extent of swelling depends inversely on the crosslinking density. When more than 95% of the total weight is water, the hydrogel is also called superabsorbent. It is not unusual to see hydrogels with more than 99% of water.