Gels are interesting materials from many perspectives. Their ability to absorb large amounts of solvent results in a range of attractive material behaviors. Hydrogels are characterized by the fact that they absorb water, in contrast with organogels, which gelate organic solvents. Hydrogels can further be classified as natural or synthetic, based on the gelator (high molecular weight, i.e. polymeric, or low molecular weight, i.e. small molecule), on their physical structure (i.e. amorphous, semicrystalline or crystalline) or on the nature of the network (permanently or physically crosslinked).