Editing Gel (section)

==Types==

===Colloidal gels===

A [[colloid]]al gel consists of a [[Percolation theory|percolated]] network of particles in a fluid medium,<ref>{{cite journal | vauthors=((Zaccarelli, E.)) | journal=Journal of Physics: Condensed Matter | title=Colloidal gels: equilibrium and non-equilibrium routes | volume=19 | issue=32 | pages=323101 | date=15 August 2007 |  doi=10.1088/0953-8984/19/32/323101| arxiv=0705.3418 | bibcode=2007JPCM...19F3101Z | s2cid=17294391 }}</ref> providing [[Rheology|mechanical properties]],<ref>{{cite journal | vauthors=((Tsurusawa, H.)), ((Leocmach, M.)), ((Russo, J.)), ((Tanaka, H.)) | journal=Science Advances | title=Direct link between mechanical stability in gels and percolation of isostatic particles | volume=5 | issue=5 | pages=eaav6090 | date= May 2019 | doi=10.1126/sciadv.aav6090| pmid=31172025 | pmc=6544450 | arxiv=1804.04370 | bibcode=2019SciA....5.6090T }}</ref> in particular the emergence of elastic behaviour.<ref>{{cite journal | vauthors=((Whitaker, K.)), ((Varga, Z.)), ((Hsiao, L.)), ((Solomon, M.)), ((Swan, J.)), ((Furst, E.))  | journal=Nature Communications | title=Colloidal gel elasticity arises from the packing of locally glassy clusters | volume= 10 | date= May 2019 | issue=1 | page=2237 | doi=10.1038/s41467-019-10039-w | pmid=31110184 | pmc=6527676 | bibcode=2019NatCo..10.2237W }}</ref> The particles can show attractive interactions through [[Depletion force|osmotic depletion]] or through polymeric links.<ref>{{cite journal | vauthors=((Howard, M. P.)), ((Jadrich, R. B.)), ((Lindquist, B. A.)), ((Khabaz, F.)), ((Bonnecaze, R. T.)), ((Milliron, D. J.)), ((Truskett, T. M.)) | journal=The Journal of Chemical Physics | title=Structure and phase behavior of polymer-linked colloidal gels | volume=151 | issue=12 | pages=124901 | date=28 September 2019 | doi=10.1063/1.5119359| pmid=31575167 | arxiv=1907.04874 | bibcode=2019JChPh.151l4901H | s2cid=195886583 }}
</ref>

Colloidal gels have three phases in their lifespan: gelation, aging and collapse.<ref>{{cite journal | vauthors=((Lu, P. J.)), ((Zaccarelli, E.)), ((Ciulla, F.)), ((Schofield, A. B.)), ((Sciortino, F.)), ((Weitz, D. A.)) | journal=Nature | title=Gelation of particles with short-range attraction | volume=453 | issue=7194 | pages=499–503 | date= May 2008 | doi=10.1038/nature06931| pmid=18497820 | bibcode=2008Natur.453..499L | s2cid=4409873 }}</ref><ref>{{cite journal | vauthors=((Zia, R. N.)), ((Landrum, B. J.)), ((Russel, W. B.)) | journal=Journal of Rheology | title=A micro-mechanical study of coarsening and rheology of colloidal gels: Cage building, cage hopping, and Smoluchowski's ratchet | volume=58 | issue=5 | pages=1121–1157 | date= September 2014  | doi=10.1122/1.4892115| bibcode=2014JRheo..58.1121Z }}
</ref> The gel is initially formed by the assembly of particles into a space-spanning network, leading to a phase arrest. In the aging phase, the particles slowly rearrange to form thicker strands, increasing the elasticity of the material. Gels can also be collapsed and separated by external fields such as gravity.<ref>{{cite journal | vauthors=((Manley, S.)), ((Skotheim, J. M.)), ((Mahadevan, L.)), ((Weitz, D. A.)) | journal=Physical Review Letters | title=Gravitational Collapse of Colloidal Gels | volume=94 | issue=21 | pages=218302 | date=3 June 2005 | doi=10.1103/PhysRevLett.94.218302| pmid=16090356 | bibcode=2005PhRvL..94u8302M | s2cid=903595 | url=http://nrs.harvard.edu/urn-3:HUL.InstRepos:41417294 }}</ref> Colloidal gels show linear response rheology at low amplitudes.<ref>{{cite journal | vauthors=((Johnson, L. C.)), ((Zia, R. N.)), ((Moghimi, E.)), ((Petekidis, G.)) | journal=Journal of Rheology | title=Influence of structure on the linear response rheology of colloidal gels | volume=63 | issue=4 | pages=583–608 | date= July 2019 | doi=10.1122/1.5082796| bibcode=2019JRheo..63..583J | s2cid=189985243 | doi-access= }}</ref> These materials have been explored as candidates for a drug release matrix.<ref>{{cite journal | vauthors=((Meidia, H.)), ((Irfachsyad, D.)), ((Gunawan, D.)) | journal=IOP Conference Series: Materials Science and Engineering | title=Brownian Dynamics Simulation of Colloidal Gels as Matrix for Controlled Release Application | volume=553 | pages=012011 | date=12 November 2019 | issue=1 | doi=10.1088/1757-899X/553/1/012011| bibcode=2019MS&E..553a2011M | s2cid=210251780 | doi-access=free }}</ref>

===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]]

===Organogels===
{{See also|Organogels}}

An '''organogel''' is a [[crystallinity|non-crystalline]], [[glass|non-glassy]] thermoreversible ([[thermoplastic]]) solid material composed of a [[liquid]] [[organic compound|organic]] phase entrapped in a three-dimensionally cross-linked network. The liquid can be, for example, an [[organic solvent]], [[mineral oil]], or [[vegetable oil]]. The [[solubility]] and [[wikt:Particle|particle]] dimensions of the structurant are important characteristics for the [[Elasticity (physics)|elastic]] properties and firmness of the organogel. Often, these systems are based on [[self-assembly]] of the structurant molecules.<ref>Terech P. (1997) "Low-molecular weight organogelators", pp. 208–268 in: Robb I.D. (ed.) ''Specialist surfactants''. Glasgow: Blackie Academic and Professional, {{ISBN|0751403407}}.</ref><ref>{{cite book | vauthors = Van Esch J, Schoonbeek F, De Loos M, Veen EM, Kellogg RM, Feringa BL | date =  1999 | chapter = Low molecular weight gelators for organic solvents | pages = 233–259 | veditors = Ungaro R, Dalcanale E | title = Supramolecular science: where it is and where it is going | publisher = Kluwer Academic Publishers | isbn = 079235656X}}</ref> (An example of formation of an undesired thermoreversible network is the occurrence of wax crystallization in [[petroleum]].<ref>{{cite journal | vauthors = Visintin RF, Lapasin R, Vignati E, D'Antona P, Lockhart TP | title = Rheological behavior and structural interpretation of waxy crude oil gels | journal = Langmuir | volume = 21 | issue = 14 | pages = 6240–9 | date = July 2005 | pmid = 15982026 | doi = 10.1021/la050705k }}</ref>)

Organogels have potential for use in a number of applications, such as in [[pharmaceutics|pharmaceuticals]],<ref>{{cite journal | vauthors = Kumar R, Katare OP | title = Lecithin organogels as a potential phospholipid-structured system for topical drug delivery: a review | journal = AAPS PharmSciTech | volume = 6 | issue = 2 | pages = E298-310 | date = October 2005 | pmid = 16353989 | pmc = 2750543 | doi = 10.1208/pt060240 }}</ref> cosmetics, art conservation,<ref>{{cite journal | vauthors = Carretti E, Dei L, Weiss RG |doi=10.1039/B501033K|title=Soft matter and art conservation. Rheoreversible gels and beyond|year=2005|journal=Soft Matter|volume=1|issue=1|pages=17 |bibcode = 2005SMat....1...17C }}</ref> and food.<ref>{{cite journal|vauthors=Pernetti M, van Malssen KF, Flöter E, Bot A |doi=10.1016/j.cocis.2007.07.002|title=Structuring of edible oils by alternatives to crystalline fat|year=2007|journal=Current Opinion in Colloid & Interface Science|volume=12|issue=4–5|pages=221–231}}</ref>

===Xerogels===

[[File:IUPAC definition for a xerogel.png|550px|thumb|right|alt=IUPAC definition for a xerogel|link=https://doi.org/10.1351/goldbook.X0670|https://doi.org/10.1351/goldbook.X06700.]]

A '''xerogel''' {{IPAc-en|ˈ|z|ɪər|oʊ-|ˌ|dʒ|ɛ|l}} is a solid formed from a gel by drying with unhindered shrinkage. Xerogels usually retain high porosity (15–50%) and enormous surface area (150–900 m<sup>2</sup>/g), along with very small [[Porosity|pore]] size (1–10&nbsp;nm). When [[solvent]] removal occurs under [[supercritical fluid|supercritical]] conditions, the network does not shrink and a highly porous, low-density material known as an ''[[aerogel]]'' is produced. Heat treatment of a xerogel at elevated temperature produces viscous [[sintering]] (shrinkage of the xerogel due to a small amount of viscous flow) which results in a denser and more robust solid, the density and porosity achieved depend on the sintering conditions.

=== Nanocomposite hydrogels ===
[[Nanocomposite hydrogels]]<ref name="ReferenceA">{{cite journal | vauthors = Gaharwar AK, Peppas NA, Khademhosseini A | title = Nanocomposite hydrogels for biomedical applications | journal = Biotechnology and Bioengineering | volume = 111 | issue = 3 | pages = 441–53 | date = March 2014 | pmid = 24264728 | pmc = 3924876 | doi = 10.1002/bit.25160 }}</ref><ref>{{cite journal|last1=Carrow|first1=James K.|last2=Gaharwar|first2=Akhilesh K. | name-list-style = vanc |title=Bioinspired Polymeric Nanocomposites for Regenerative Medicine|journal=Macromolecular Chemistry and Physics|volume=216|issue=3|pages=248–264|date=November 2014|doi=10.1002/macp.201400427}}</ref> or hybrid hydrogels, are highly hydrated polymeric networks, either physically or covalently crosslinked with each other and/or with nanoparticles or nanostructures.<ref>{{cite journal | vauthors = Kutvonen A, Rossi G, Puisto SR, Rostedt NK, Ala-Nissila T | s2cid = 26096794 | title = Influence of nanoparticle size, loading, and shape on the mechanical properties of polymer nanocomposites | journal = The Journal of Chemical Physics | volume = 137 | issue = 21 | pages = 214901 | date = December 2012 | pmid = 23231257 | doi = 10.1063/1.4767517 | arxiv = 1212.4335 | bibcode = 2012JChPh.137u4901K }}</ref> Nanocomposite hydrogels can mimic native tissue properties, structure and microenvironment due to their hydrated and interconnected porous structure. A wide range of nanoparticles, such as carbon-based, polymeric, ceramic, and metallic [[nanomaterials]] can be incorporated within the hydrogel structure to obtain nanocomposites with tailored functionality. Nanocomposite hydrogels can be engineered to possess superior physical, chemical, electrical, thermal, and biological properties.<ref name="ReferenceA"/><ref>{{cite journal | vauthors = Zaragoza J, Babhadiashar N, O'Brien V, Chang A, Blanco M, Zabalegui A, Lee H, Asuri P | display-authors = 6 | title = Experimental Investigation of Mechanical and Thermal Properties of Silica Nanoparticle-Reinforced Poly(acrylamide) Nanocomposite Hydrogels | journal = PLOS ONE | volume = 10 | issue = 8 | pages = e0136293 | date = 2015-08-24 | pmid = 26301505 | pmc = 4547727 | doi = 10.1371/journal.pone.0136293 | bibcode = 2015PLoSO..1036293Z | doi-access = free }}</ref>