Editing Gel (section)

===Hydrogels===
{{Main|Hydrogel}}
{{see also|Superabsorbent polymer|Self-healing hydrogels|Hydrogel agriculture}}
[[File:Superabsorber Hydrogel KSG 2917 pK.jpg|thumb|Hydrogel of a superabsorbent polymer]]

A [[hydrogel]] is a network of polymer chains that are hydrophilic, sometimes found as a [[colloid]]al gel in which water is the dispersion medium. A three-dimensional solid results from the hydrophilic polymer chains being held together by cross-links.{{clarify|reason=What is the nature of the cross-links? Covalent? Hydrogen bonds?|date=March 2019}} Because of the inherent cross-links, the structural integrity of the hydrogel network does not dissolve from the high concentration of water.<ref>{{Cite journal| vauthors = Warren DS, Sutherland SP, Kao JY, Weal GR, Mackay SM |date=2017-04-20|title=The Preparation and Simple Analysis of a Clay Nanoparticle Composite Hydrogel|journal=Journal of Chemical Education|language=EN|volume=94|issue=11|pages=1772–1779|doi=10.1021/acs.jchemed.6b00389|issn=0021-9584|bibcode=2017JChEd..94.1772W}}</ref> Hydrogels are highly [[absorption (chemistry)|absorbent]] (they can contain over 90% water) natural or synthetic polymeric networks.
Hydrogels also possess a degree of flexibility very similar to natural tissue, due to their significant water content. As responsive "[[smart materials]]," hydrogels can encapsulate chemical systems which upon stimulation by external factors such as a change of pH may cause specific compounds such as glucose to be liberated to the environment, in most cases by a [[sol-gel|gel-sol transition]] to the liquid state.<ref>{{cite journal | vauthors = Bordbar-Khiabani A, Gasik M | title = Smart hydrogels for advanced drug delivery systems | journal = International Journal of Molecular Sciences | date = 2022 | volume = 23 | issue = 7 | pages = 3665 | doi = 10.3390/ijms23073665 | pmid = 35409025 | pmc = 8998863 | doi-access = free }}</ref> Chemomechanical polymers are mostly also hydrogels, which upon stimulation change their volume and can serve as [[actuators]] or [[sensors]]. The first appearance of the term 'hydrogel' in the literature was in 1894.<ref>{{cite journal | vauthors = Bemmelen JM |s2cid=197928622|doi=10.1007/BF01830147|title=Der Hydrogel und das kristallinische Hydrat des Kupferoxydes|journal=Zeitschrift für Chemie und Industrie der Kolloide |volume=1 |issue=7 |pages=213–214 |year=1907 }}</ref>

[[File:IUPAC definition for a polymer gel.png|thumb|right|550px|link=https://doi.org/10.1351/goldbook.PT07187|IUPAC definition for a polymer gel]]