Polymer gels are swollen (in a solvent) long polymer chains cross-linked with each other by covalent bonds (crosslinks) to form a single spatial grid. If a change in the external environment leads to effective attraction between the polymer links (hydrophobic interactions, hydrogen bonds), then the gel collapses.

The density of an uncharged gel is determined by the interaction of monomer links with each other and with the solvent. Polyelectrolyte gels contain charged groups that are formed simultaneously counterions in dissociation of monomer links. The density of the polyelectrolyte gel in the swollen state is determined by the competition between the expanding pressure of mobile counterions and elastic forces arising from the stretching of polymer chains.

The volume of such a gel in the swollen and collapsed states can vary by hundreds of times; some hydrogels are capable of holding up to 2 kg of water per 1 g of dry polymer.

Thus, polymer gels can be classified as smart materials capable of responding to small changes in the environment in a known manner. To date, research has been made into the phenomenon of gels collapse under the influence of various factors, such as temperature, light radiation, solvent composition, pH (pH) of the environment.