A hydrogel is a network of crosslinked polymer chains that are hydrophilic, sometimes found as a colloid in which water is the dispersion medium. A three-dimensional solid results from the hydrophilic polymer chains being held together by cross-links. The crosslinks which bond the polymers of a hydrogel fall under two general categories: physical and chemical. Physical crosslinks consist of hydrogen bonds, hydrophobic interactions, and chain entanglements. Because of the inherent cross-links, the structural integrity of the hydrogel network does not dissolve from the high concentration of water. Hydrogels are highly absorbent natural or synthetic polymeric networks.