Water-absorbent polymers or hydrogels, sometimes defined as superabsorbent polymers (SAP) in the literature, are water-insoluble hydrophilic polymers, able to swell and absorb amounts of water, saline solutions, or physiological fluids as high as 10-1000 times their own weight.
They consist of polyelectrolytes or other highly hydrophilic polymeric matrices, usually bearing crosslinking sites along the macromolecular chains in order to avoid dissolution. These polymers generally contain carboxylic groups that are in equilibrium with their dissociated form in the presence of water or carboxylate groups. The polymer coils extend themselves and widen in consequence of the electrostatic repulsion of negative charges. Carboxylate groups are also able to interact through hydrogen bonding with additional quantities of water. The presence of crosslinking allows swelling of the three-dimensional network and gel formation without polymer dissolution.