View source for Chemistry:Hydrogel

{{Short description|Soft water-rich polymer gel}}
[[File:Gelatine.png|thumb|right|Gelatin, here in sheets for cooking, is a hydrogel.]]
[[File:Picture of peptide hydrogel formation shown by the inverted vial method.jpg|thumb|Peptide hydrogel formation shown by the inverted vial method.]]<!-- Definition -->
A '''hydrogel''' is a biphasic material, a mixture of [[Earth:Porosity|porous]], [[Physics:Permeation|permeable]] solids and at least 10% by weight or volume of interstitial fluid composed completely or mainly by water.<ref>{{Cite journal |last1=Wichterle |first1=O. |last2=Lím |first2=D. |date=1960-01-01 |title=Hydrophilic Gels for Biological Use |url=https://ui.adsabs.harvard.edu/abs/1960Natur.185..117W |journal=Nature |volume=185 |issue=4706 |pages=117–118 |doi=10.1038/185117a0 |bibcode=1960Natur.185..117W |s2cid=4211987 |issn=0028-0836}}</ref><ref name=":7">{{Cite journal |last=Ghosh |first=Shampa |last2=Ghosh |first2=Soumya |last3=Sharma |first3=Hitaishi |last4=Bhaskar |first4=Rakesh |last5=Han |first5=Sung Soo |last6=Sinha |first6=Jitendra Kumar |date=2024-01-01 |title=Harnessing the power of biological macromolecules in hydrogels for controlled drug release in the central nervous system: A review |url=https://www.sciencedirect.com/science/article/pii/S0141813023046068 |journal=International Journal of Biological Macromolecules |volume=254 |pages=127708 |doi=10.1016/j.ijbiomac.2023.127708 |issn=0141-8130}}</ref> In hydrogels the porous permeable solid is a water [[Chemistry:Solubility|insoluble]] three dimensional network of natural or synthetic polymers and a fluid, having absorbed a large amount of water or biological fluids.<ref name=":7" /><ref>{{Citation |last1=Shrivastava |first1=Priya |title=Magnetically responsive polymeric gels and elastomeric system(s) for drug delivery |date=2023 |url=http://dx.doi.org/10.1016/b978-0-323-91248-8.00012-x |work=Smart Polymeric Nano-Constructs in Drug Delivery |pages=129–150 |publisher=Elsevier |isbn=978-0-323-91248-8 |access-date=2023-01-16 |last2=Vishwakarma |first2=Nikhar |last3=Gautam |first3=Laxmikant |last4=Vyas |first4=Suresh P.|doi=10.1016/b978-0-323-91248-8.00012-x }}</ref><ref>{{Cite book |date=2018 |title=Fundamental Biomaterials: Polymers |url=http://dx.doi.org/10.1016/c2016-0-03544-1 |doi=10.1016/c2016-0-03544-1|isbn=9780081021941 }}</ref><ref>{{Cite book |url=http://dx.doi.org/10.1016/c2009-1-28406-1 |title=Polymer Science: A Comprehensive Reference |date=2012 |publisher=Elsevier |doi=10.1016/c2009-1-28406-1 |isbn=978-0-08-087862-1}}</ref> These properties underpin several applications, especially in the biomedical area. Many hydrogels are synthetic, but some are derived from nature.<ref name="KO">{{cite book |doi=10.1002/0471238961.0825041807211620.a01.pub2|chapter=Hydrogels|title=Kirk-Othmer Encyclopedia of Chemical Technology|year=2012| vauthors = Cai W, Gupta RB |pages=1–20 |isbn=978-0471238966}}</ref><ref name=":2" /> The term 'hydrogel' was coined in 1894.<ref>{{cite journal | vauthors = Bemmelen JM |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 |s2cid=197928622 |doi=10.1007/BF01830147}}</ref>

==Chemistry==
=== Classification ===
The crosslinks which bond the polymers of a hydrogel fall under two general categories: physical hydrogels  and chemical hydrogels. Chemical hydrogels have [[Chemistry:Covalent bond|covalent cross-linking bonds]], whereas physical hydrogels have non-covalent bonds.{{Cn|date=February 2023}} Chemical hydrogels  can result in strong reversible or irreversible gels due to the covalent bonding.<ref>{{Citation |last1=Nikolić |first1=Ljubiša B. |title=Synthetic Hydrogels and Their Impact on Health and Environment |date=2018 |url=https://doi.org/10.1007/978-3-319-76573-0_61-1 |work=Cellulose-Based Superabsorbent Hydrogels |pages=1–29 |editor-last=Mondal |editor-first=Md. Ibrahim H. |place=Cham |publisher=Springer International Publishing |language=en |doi=10.1007/978-3-319-76573-0_61-1 |isbn=978-3-319-76573-0 |access-date=2023-01-17 |last2=Zdravković |first2=Aleksandar S. |last3=Nikolić |first3=Vesna D. |last4=Ilić-Stojanović |first4=Snežana S.}}</ref> Chemical hydrogels that contain reversible covalent cross-linking bonds such as hydrogels of [[Physics:Thiomer|thiomer]]s being cross-linked via disulfide bonds are non-toxic and are used in numerous medicinal products.<ref>{{cite journal |last1=Summonte |first1=S |last2=Racaniello |first2=GF |last3=Lopedota |first3=A |last4=Denora |first4=N |last5=Bernkop-Schnürch |first5=A |title=Thiolated polymeric hydrogels for biomedical application: Cross-linking mechanisms |journal= Journal of Controlled Release|date=2021 |volume=330 |pages=470–482 |doi=10.1016/j.jconrel.2020.12.037 |pmid=33359581|s2cid=229694027 |doi-access=free }}</ref><ref>{{cite journal |last1=Federer |first1=C |last2=Kurpiers |first2=M |last3=Bernkop-Schnürch |first3=A |title=Thiolated Chitosans: A Multi-talented Class of Polymers for Various Applications |journal=Biomacromolecules |date=2021 |volume=22 |issue=1 |pages=24–56 |doi=10.1021/acs.biomac.0c00663 |pmid=32567846|pmc=7805012 }}</ref><ref>{{cite journal|last1=Leichner|first1=C|last2=Jelkmann|first2=M|last3=Bernkop-Schnürch|first3=A|title=Thiolated polymers: Bioinspired polymers utilizing one of the most important bridging structures in nature|journal= Advanced Drug Delivery Reviews|date=2019|volume=151-152|pages=191–221|doi=10.1016/j.addr.2019.04.007|pmid=31028759|s2cid=135464452}}</ref>  Physical hydrogels usually have high biocompatibility, are not toxic, and are also easily reversible by simply changing an external stimulus such as pH, ion concentration ([[Chemistry:Alginic acid|alginate]]) or temperature ([[Biology:Gelatin|gelatin]]e); they are also used for medical applications.<ref name=":6">{{Cite journal |last1=Rosales |first1=Adrianne M. |last2=Anseth |first2=Kristi S. |date=2016-02-02 |title=The design of reversible hydrogels to capture extracellular matrix dynamics |journal=Nature Reviews Materials |language=en |volume=1 |issue=2 |page=15012 |doi=10.1038/natrevmats.2015.12 |pmid=29214058 |pmc=5714327 |bibcode=2016NatRM...115012R |issn=2058-8437}}</ref><ref>{{Cite journal |last1=Jeong |first1=Byeongmoon |last2=Kim |first2=Sung Wan |last3=Bae |first3=You Han |date=2002-01-17 |title=Thermosensitive sol-gel reversible hydrogels |url=https://pubmed.ncbi.nlm.nih.gov/11755705/ |journal=Advanced Drug Delivery Reviews |volume=54 |issue=1 |pages=37–51 |doi=10.1016/s0169-409x(01)00242-3 |issn=0169-409X |pmid=11755705}}</ref><ref>{{Cite journal |last1=Yan |first1=Yonggan |last2=Xu |first2=Shulei |last3=Liu |first3=Huanxi |last4=Cui |first4=Xin |last5=Shao |first5=Jinlong |last6=Yao |first6=Peng |last7=Huang |first7=Jun |last8=Qiu |first8=Xiaoyong |last9=Huang |first9=Chuanzhen |date=2020-05-20 |title=A multi-functional reversible hydrogel adhesive |url=https://www.sciencedirect.com/science/article/pii/S0927775720302156 |journal=Colloids and Surfaces A: Physicochemical and Engineering Aspects |language=en |volume=593 |pages=124622 |doi=10.1016/j.colsurfa.2020.124622 |s2cid=213116098 |issn=0927-7757}}</ref><ref>{{Cite journal |last1=Monteiro |first1=O. A. |last2=Airoldi |first2=C. |date=November 1999 |title=Some studies of crosslinking chitosan-glutaraldehyde interaction in a homogeneous system |url=https://pubmed.ncbi.nlm.nih.gov/10517518/ |journal=International Journal of Biological Macromolecules |volume=26 |issue=2–3 |pages=119–128 |doi=10.1016/s0141-8130(99)00068-9 |issn=0141-8130 |pmid=10517518}}</ref><ref>{{Cite journal |last1=Zhang |first1=Zhen |last2=He |first2=Chaoliang |last3=Chen |first3=Xuesi |date=2018-09-27 |title=Hydrogels based on pH-responsive reversible carbon–nitrogen double-bond linkages for biomedical applications |url=https://pubs.rsc.org/en/content/articlelanding/2018/qm/c8qm00317c |journal=Materials Chemistry Frontiers |language=en |volume=2 |issue=10 |pages=1765–1778 |doi=10.1039/C8QM00317C |issn=2052-1537}}</ref> Physical crosslinks consist of [[Chemistry:Hydrogen bond|hydrogen bond]]s, [[Chemistry:Hydrophobic effect|hydrophobic interactions]], and chain entanglements (among others). A hydrogel generated through the use of physical crosslinks is sometimes called a 'reversible' hydrogel.<ref name=":6" /> Chemical crosslinks consist of covalent bonds between polymer strands. Hydrogels generated in this manner are sometimes called 'permanent' hydrogels.

Hydrogels are prepared using a variety of [[Chemistry:Polymer|polymeric materials]], which can be divided broadly into two categories according to their origin: natural or synthetic polymers. Natural polymers for hydrogel preparation include [[Chemistry:Hyaluronic acid|hyaluronic acid]], [[Chemistry:Chitosan|chitosan]], [[Chemistry:Heparin|heparin]], alginate, [[Biology:Gelatin|gelatin]] and [[Biology:Fibrin|fibrin]].<ref>{{cite journal | vauthors = Kharkar PM, Kiick KL, Kloxin AM | title = Designing degradable hydrogels for orthogonal control of cell microenvironments | journal = Chemical Society Reviews | volume = 42 | issue = 17 | pages = 7335–7372 | date = September 2013 | pmid = 23609001 | pmc = 3762890 | doi = 10.1039/C3CS60040H }}</ref> Common synthetic polymers include [[Chemistry:Polyvinyl alcohol|polyvinyl alcohol]], [[Chemistry:Polyethylene glycol|polyethylene glycol]], [[Chemistry:Sodium polyacrylate|sodium polyacrylate]], [[Physics:Acrylate polymer|acrylate polymer]]s and [[Chemistry:Copolymer|copolymer]]s thereof.<ref name="KO" /> Whereas natural hydrogels are usually non-toxic, and often provides other advantages for medical use, such as [[Chemistry:Biocompatibility|biocompatibility]], biodegradability, [[Chemistry:Antibiotic|antibiotic]]/[[Chemistry:Antifungal|antifungal]] effect and improve [[Biology:Regeneration|regeneration]] of nearby tissue, their [[Chemistry:Chemical stability|stability]] and [[Physics:Strength of materials|strength]] is usually much lower than synthetic hydrogels.<ref>{{Cite journal |last1=Jeong |first1=Kwang-Hun |last2=Park |first2=Duckshin |last3=Lee |first3=Young-Chul |date=July 2017 |title=Polymer-based hydrogel scaffolds for skin tissue engineering applications: a mini-review |url=http://link.springer.com/10.1007/s10965-017-1278-4 |journal=Journal of Polymer Research |language=en |volume=24 |issue=7 |pages=112 |doi=10.1007/s10965-017-1278-4 |s2cid=136085690 |issn=1022-9760}}</ref> There are also synthetic hydrogels than can be used for medical applications, such as [[Chemistry:Polyethylene glycol|polyethylene glycol (PEG)]], [[Physics:Acrylate polymer|polyacrylate]], and [[Chemistry:Polyvinylpyrrolidone|polyvinylpyrrolidone (PVP)]].<ref>{{Cite journal |last1=Gdansk University of Technology, Chemical Faculty, Polymer Technology Department, 80-233 Gdansk, ul Narutowicza 11/12 |last2=Gibas |first2=Iwona |last3=Janik |first3=Helena |date=2010-12-15 |title=Review: Synthetic Polymer Hydrogels for Biomedical Applications |url=http://science2016.lp.edu.ua/chcht/review-synthetic-polymer-hydrogels-biomedical-applications |journal=Chemistry & Chemical Technology |volume=4 |issue=4 |pages=297–304 |doi=10.23939/chcht04.04.297|doi-access=free }}</ref>

=== Preparation ===
[[File:Peptide hydrogel formation simplified scheme.png|thumb|Simplified scheme to show the self-assembly process involved in hydrogel formation.]]

There are two suggested mechanisms behind physical hydrogel formation, the first one being the gelation of nanofibrous [[Chemistry:Peptide|peptide]] assemblies, usually observed for [[Chemistry:Oligopeptide|oligopeptide]] precursors. The precursors self-assemble into fibers, tapes, tubes, or ribbons that entangle to form non-covalent cross-links. The second mechanism involves non-covalent interactions of cross-linked domains that are separated by water-soluble linkers, and this is usually observed in longer multi-domain structures.<ref>{{cite book | vauthors = Dooling LJ, Tirrell DA | date = 2013 | chapter = Peptide and Protein Hydrogels. | title = Polymeric and self assembled hydrogels: from fundamental understanding to applications. | series = Monographs in supramolecular chemistry.| volume = 11 | publisher = Royal Society of Chemistry | location = Cambridge, UK | pages = 93–124 | isbn = 978-1-84973-561-2 | chapter-url = https://authors.library.caltech.edu/38546/ }}</ref> Tuning of the supramolecular interactions to produce a self-supporting network that does not precipitate, and is also able to immobilize water which is vital for to gel formation. Most oligopeptide hydrogels have a β-sheet structure, and assemble to form fibers, although α-helical peptides have also been reported.<ref>{{cite journal | vauthors = Mehrban N, Zhu B, Tamagnini F, Young FI, Wasmuth A, Hudson KL, Thomson AR, Birchall MA, Randall AD, Song B, Woolfson DN | display-authors = 3 | title = Functionalized α-Helical Peptide Hydrogels for Neural Tissue Engineering | journal = ACS Biomaterials Science & Engineering | volume = 1 | issue = 6 | pages = 431–439 | date = June 2015 | pmid = 26240838 | pmc = 4517957 | doi = 10.1021/acsbiomaterials.5b00051 }}</ref><ref>{{cite journal | vauthors = Banwell EF, Abelardo ES, Adams DJ, Birchall MA, Corrigan A, Donald AM, Kirkland M, Serpell LC, Butler MF, Woolfson DN | display-authors = 3 | title = Rational design and application of responsive alpha-helical peptide hydrogels | journal = Nature Materials | volume = 8 | issue = 7 | pages = 596–600 | date = July 2009 | pmid = 19543314 | pmc = 2869032 | doi = 10.1038/nmat2479 | bibcode = 2009NatMa...8..596B }}</ref> The typical mechanism of gelation involves the oligopeptide precursors self-assemble into fibers that become elongated, and entangle to form cross-linked gels.

One notable method of initiating a polymerization fuving involves the use of light as a stimulus. In this method, [[Earth:Photoinitiator|photoinitiator]]s, compounds that cleave from the absorption of photons, are added to the precursor solution which will become the hydrogel. When the precursor solution is exposed to a concentrated source of light, usually [[Physics:Ultraviolet|ultraviolet]] irradiation, the photoinitiators will cleave and form free radicals, which will begin a polymerization reaction that forms crosslinks between polymer strands. This reaction will cease if the light source is removed, allowing the amount of crosslinks formed in the hydrogel to be controlled.<ref name="Choi2019">{{cite journal | vauthors = Choi JR, Yong KW, Choi JY, Cowie AC | title = Recent advances in photo-crosslinkable hydrogels for biomedical applications | journal = BioTechniques | volume = 66 | issue = 1 | pages = 40–53 | date = January 2019 | pmid = 30730212 | doi = 10.2144/btn-2018-0083 | doi-access = free }}</ref> The properties of a hydrogel are highly dependent on the type and quantity of its crosslinks, making photopolymerization a popular choice for fine-tuning hydrogels. This technique has seen considerable use in cell and tissue engineering applications due to the ability to inject or mold a precursor solution loaded with cells into a wound site, then solidify it in situ.<ref name="Calo2015">{{cite journal| vauthors = Caló E, Khutoryanskiy VV |year=2015|title=Biomedical applications of hydrogels: A review of patents and commercial products|journal=[[Physics:European Polymer Journal|European Polymer Journal]]|volume=65|pages=252–267|doi=10.1016/j.eurpolymj.2014.11.024|doi-access=free}}</ref><ref name="Choi2019" />

Physically crosslinked hydrogels can be prepared by different methods depending on the nature of the crosslink involved. [[Chemistry:Polyvinyl alcohol|Polyvinyl alcohol]] hydrogels are usually produced by the freeze-thawed technique. In this, the solution is frozen for a few hours, then thawed at room temperature, and the cycle is repeated until a strong and stable hydrogel is formed.<ref>{{Cite journal |last1=Adelnia |first1=Hossein |last2=Ensandoost |first2=Reza |last3=Shebbrin Moonshi |first3=Shehzahdi |last4=Gavgani |first4=Jaber Nasrollah |last5=Vasafi |first5=Emad Izadi |last6=Ta |first6=Hang Thu |date=2022-02-05 |title=Freeze/thawed polyvinyl alcohol hydrogels: Present, past and future |url=https://www.sciencedirect.com/science/article/pii/S0014305721007084 |journal=European Polymer Journal |language=en |volume=164 |pages=110974 |doi=10.1016/j.eurpolymj.2021.110974 |s2cid=245576810 |issn=0014-3057}}</ref> [[Chemistry:Alginic acid|Alginate]] hydrogels are formed by ionic interactions between alginate and double-charged cations. A salt, usually [[Chemistry:Calcium chloride|calcium chloride]], is dissolved into an aqueous sodium alginate solution, that causes the calcium ions to create ionic bonds between alginate chains.<ref>{{Cite journal |last1=Augst |first1=Alexander D. |last2=Kong |first2=Hyun Joon |last3=Mooney |first3=David J. |date=2006-08-07 |title=Alginate Hydrogels as Biomaterials |url=https://onlinelibrary.wiley.com/doi/10.1002/mabi.200600069 |journal=Macromolecular Bioscience |language=en |volume=6 |issue=8 |pages=623–633 |doi=10.1002/mabi.200600069 |pmid=16881042 |issn=1616-5187}}</ref> [[Biology:Gelatin|Gelatin]] hydrogels are formed by temperature change. A water solution of gelatin forms an hydrogel at temperatures below 37–35&nbsp;°C, as Van der Waals interactions between collagen fibers become stronger than thermal molecular vibrations.<ref>{{Cite journal |last1=Jaipan |first1=Panupong |last2=Nguyen |first2=Alexander |last3=Narayan |first3=Roger J. |date=2017-09-01 |title=Gelatin-based hydrogels for biomedical applications |journal=MRS Communications |language=en |volume=7 |issue=3 |pages=416–426 |doi=10.1557/mrc.2017.92 |bibcode=2017MRSCo...7..416J |issn=2159-6867|doi-access=free }}</ref>

=== Peptides based hydrogels ===
Peptides based hydrogels possess exceptional [[Chemistry:Biocompatibility|biocompatibility]] and biodegradability qualities, giving rise to their wide use of applications, particularly in biomedicine;<ref name=":7" /> as such, their physical properties can be fine-tuned in order to maximise their use.<ref name=":7" /> Methods to do this are: modulation of the [[Chemistry:Amino acid|amino acid]] sequence, [[Chemistry:PH|pH]], [[Biology:Chirality|chirality]], and increasing the number of [[Chemistry:Aromatic compound|aromatic]] residues.<ref>{{cite journal | vauthors = Fichman G, Gazit E | title = Self-assembly of short peptides to form hydrogels: design of building blocks, physical properties and technological applications | journal = Acta Biomaterialia | volume = 10 | issue = 4 | pages = 1671–1682 | date = April 2014 | pmid = 23958781 | doi = 10.1016/j.actbio.2013.08.013 }}</ref> The order of amino acids within the sequence is crucial for gelation, as has been shown many times. In one example, a short peptide sequence Fmoc-Phe-Gly readily formed a hydrogel, whereas Fmoc-Gly-Phe failed to do so as a result of the two adjacent aromatic moieties being moved, hindering the aromatic interactions.<ref>{{Cite journal | vauthors = Jayawarna V, Ali M, Jowitt TA, Miller AF, Saiani A, Gough JE, Ulijn RV |display-authors=3|date=2006-03-03|title=Nanostructured Hydrogels for Three-Dimensional Cell Culture Through Self-Assembly of Fluorenylmethoxycarbonyl–Dipeptides|url=https://onlinelibrary.wiley.com/doi/10.1002/adma.200501522|journal=Advanced Materials|language=en|volume=18|issue=5|pages=611–614|doi=10.1002/adma.200501522|bibcode=2006AdM....18..611J|s2cid=136880479 |issn=0935-9648}}</ref><ref>{{cite journal | vauthors = Orbach R, Adler-Abramovich L, Zigerson S, Mironi-Harpaz I, Seliktar D, Gazit E |display-authors=3| title = Self-assembled Fmoc-peptides as a platform for the formation of nanostructures and hydrogels | journal = Biomacromolecules | volume = 10 | issue = 9 | pages = 2646–2651 | date = September 2009 | pmid = 19705843 | doi = 10.1021/bm900584m }}</ref> Altering the pH can also have similar effects, an example involved the use of the naphthalene (Nap) modified dipeptides Nap-Gly-Ala, and Nap- Ala-Gly, where a drop in pH induced gelation of the former, but led to crystallisation of the latter.<ref>{{Cite journal| vauthors = Adams DJ, Morris K, Chen L, Serpell LC, Bacsa J, Day GM |display-authors=3 |date=2010|title=The delicate balance between gelation and crystallisation: structural and computational investigations|url=http://xlink.rsc.org/?DOI=c0sm00409j|journal=Soft Matter|language=en|volume=6|issue=17|pages=4144|doi=10.1039/c0sm00409j|bibcode=2010SMat....6.4144A|issn=1744-683X}}</ref> A controlled pH decrease method using glucono-δ-lactone (GdL), where the GdL is hydrolysed to gluconic acid in water is a recent strategy that has been developed as a way to form homogeneous and reproducible hydrogels.<ref name=":02">{{cite journal | vauthors = Chen L, Morris K, Laybourn A, Elias D, Hicks MR, Rodger A, Serpell L, Adams DJ | display-authors = 3 | title = Self-assembly mechanism for a naphthalene-dipeptide leading to hydrogelation | journal = Langmuir | volume = 26 | issue = 7 | pages = 5232–5242 | date = April 2010 | pmid = 19921840 | doi = 10.1021/la903694a }}</ref><ref name=":12">{{Cite journal| vauthors = Adams DJ, Mullen LM, Berta M, Chen L, Frith WJ |display-authors=3|date=2010|title=Relationship between molecular structure, gelation behaviour and gel properties of Fmoc-dipeptides|url=http://xlink.rsc.org/?DOI=b921863g|journal=Soft Matter |volume=6|issue=9|pages=1971|doi=10.1039/b921863g|bibcode=2010SMat....6.1971A|issn=1744-683X}}</ref> The hydrolysis is slow, which allows for a uniform pH change, and thus resulting in reproducible homogenous gels. In addition to this, the desired pH can be achieved by altering the amount of GdL added. The use of GdL has been used various times for the hydrogelation of Fmoc and Nap-dipeptides.<ref name=":02" /><ref name=":12" /> In another direction, Morris et al reported the use of GdL as a ‘molecular trigger’ to predict and control the order of gelation.<ref>{{cite journal | vauthors = Morris KL, Chen L, Raeburn J, Sellick OR, Cotanda P, Paul A, Griffiths PC, King SM, O'Reilly RK, Serpell LC, Adams DJ | display-authors = 3 | title = Chemically programmed self-sorting of gelator networks | journal = Nature Communications | volume = 4 | issue = 1 | pages = 1480 | date = June 2013 | pmid = 23403581 | doi = 10.1038/ncomms2499 | bibcode = 2013NatCo...4.1480M | doi-access = free }}</ref> Chirality also plays an essential role in gel formation, and even changing the chirality of a single amino acid from its natural L-amino acid to its unnatural D-amino acid can significantly impact the gelation properties, with the natural forms not forming gels.<ref>{{cite journal | vauthors = Marchesan S, Waddington L, Easton CD, Winkler DA, Goodall L, Forsythe J, Hartley PG |display-authors=3| title = Unzipping the role of chirality in nanoscale self-assembly of tripeptide hydrogels | journal = Nanoscale | volume = 4 | issue = 21 | pages = 6752–6760 | date = November 2012 | pmid = 22955637 | doi = 10.1039/c2nr32006a | bibcode = 2012Nanos...4.6752M |hdl=11368/2841344}}</ref> Furthermore, aromatic interactions play a key role in hydrogel formation as a result of π- π stacking driving gelation, shown by many studies.<ref>{{Cite journal| vauthors = Birchall LS, Roy S, Jayawarna V, Hughes M, Irvine E, Okorogheye GT, Saudi N, De Santis E, Tuttle T, Edwards AA, Ulijn RV | display-authors = 3 |date=2011|title=Exploiting CH-π interactions in supramolecular hydrogels of aromatic carbohydrate amphiphiles|url=http://xlink.rsc.org/?DOI=c0sc00621a|journal=Chemical Science|language=en|volume=2|issue=7|pages=1349|doi=10.1039/c0sc00621a|issn=2041-6520}}</ref><ref>{{cite journal | vauthors = Ma M, Kuang Y, Gao Y, Zhang Y, Gao P, Xu B |display-authors=3| title = Aromatic-aromatic interactions induce the self-assembly of pentapeptidic derivatives in water to form nanofibers and supramolecular hydrogels | journal = Journal of the American Chemical Society | volume = 132 | issue = 8 | pages = 2719–2728 | date = March 2010 | pmid = 20131781 | doi = 10.1021/ja9088764 }}</ref>

===Other===
Hydrogels also possess a degree of flexibility very similar to natural tissue due to their significant water content. As responsive "[[Chemistry:Smart material|smart material]]s", 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 [[Physics:Sol–gel process|gel–sol transition]] to the liquid state. Chemomechanical polymers are mostly also hydrogels, which upon stimulation change their volume and can serve as actuators or [[Engineering:Sensor|sensor]]s.

<gallery>
File:Hydrogel micropump.webm|A [[Physics:Micropump|micropump]] based on a hydrogel bar (4×0.3×0.05&nbsp;mm size) actuated by applied voltage. This pump can be continuously operated with a 1.5 V battery for at least 6 months.<ref>{{cite journal | vauthors = Kwon GH, Jeong GS, Park JY, Moon JH, Lee SH |display-authors=3| title = A low-energy-consumption electroactive valveless hydrogel micropump for long-term biomedical applications | journal = Lab on a Chip | volume = 11 | issue = 17 | pages = 2910–2915 | date = September 2011 | pmid = 21761057 | doi = 10.1039/C1LC20288J }}</ref>
File:Short-peptide-based hydrogel, electron microscope image.jpg|A short-peptide-based hydrogel matrix, capable of holding about one hundred times its own weight in water. Developed as a medical dressing.
File:Crosslinked ultrashort peptide hydrogel.jpg|Photo of the same short-peptide-based hydrogel, held in forceps to demonstrate its stiffness and transparency.
</gallery>

==Mechanical properties==
Hydrogels have been investigated for diverse applications. By modifying the polymer concentration of a hydrogel (or conversely, the water concentration), the [[Physics:Young's modulus|Young's modulus]], [[Physics:Shear modulus|shear modulus]], and storage modulus can vary from 10 Pa to 3 MPa, a range of about five orders of magnitude.<ref name=":0">{{cite journal | vauthors = Oyen ML |date=January 2014 |title=Mechanical characterisation of hydrogel materials |journal=International Materials Reviews |language=en |volume=59 |issue=1 |pages=44–59 |doi=10.1179/1743280413Y.0000000022 |bibcode=2014IMRv...59...44O |s2cid=136844625 |issn=0950-6608}}</ref> A similar effect can be seen by altering the crosslinking concentration.<ref name=":0" /> This much variability of the mechanical stiffness is why hydrogels are so appealing for biomedical applications, where it is vital for implants to match the mechanical properties of the surrounding tissues.<ref>{{Cite book | vauthors = Los MJ, Hudecki A, Wiechec E |url=https://books.google.com/books?id=B293DwAAQBAJ&q=Matching%20the%20modulus%20with%20the%20surrounding%20tissue&pg=PA95 |title=Stem Cells and Biomaterials for Regenerative Medicine |date=2018-11-07 |publisher=Academic Press |isbn=978-0-12-812278-5 |language=en}}</ref> Characterizing the mechanical properties of hydrogels can be difficult especially due to the differences in mechanical behavior that hydrogels have in comparison to other traditional engineering materials. In addition to its rubber [[Physics:Elasticity|elasticity]] and [[Physics:Viscoelasticity|viscoelasticity]], hydrogels have an additional time dependent deformation mechanism which is dependent on fluid flow called [[Physics:Poroelasticity|poroelasticity]]. These properties are extremely important to consider while performing mechanical experiments. Some common mechanical testing experiments for hydrogels are [[Physics:Tension|tension]], [[Engineering:Compressive strength|compression]] (confined or unconfined), indentation, [[Physics:Shear stress|shear]] [[Physics:Rheometry|rheometry]] or [[Dynamic mechanical analysis|dynamic mechanical analysis]].<ref name=":0" />

Hydrogels have two main regimes of mechanical properties: [[Physics:Rubber elasticity|rubber elasticity]] and [[Physics:Viscoelasticity|viscoelasticity]]:

===Rubber elasticity===
In the unswollen state, hydrogels can be modelled as highly crosslinked chemical gels, in which the system can be described as one continuous polymer network. In this case:

<math>G=N_{p}kT={\rho RT \over \overline{M}_{c}}</math>

where ''G'' is the [[Physics:Shear modulus|shear modulus]], ''k'' is the Boltzmann constant, ''T'' is temperature, ''N<sub>p</sub>'' is the number of polymer chains per unit volume, ''ρ'' is the density, ''R'' is the ideal gas constant, and <math>\overline{M}_{c}</math>&nbsp;is the (number) average molecular weight between two adjacent cross-linking points. <math>\overline{M}_{c}</math> can be calculated from the swell ratio, ''Q'', which is relatively easy to test and measure.<ref name=":0" />

For the swollen state, a perfect gel network can be modeled as:<ref name=":0" />

<math>G_{\textrm{swollen}}=GQ^{-1/3}</math>

In a simple uniaxial extension or compression test, the true stress, <math>\sigma _{t}</math>, and engineering stress, <math>\sigma _{e}</math>, can be calculated as:

<math>\sigma _{t}=G_{\textrm{swollen}}\left ( \lambda ^{2}-\lambda ^{-1} \right )</math>

<math>\sigma _{e}=G_{\textrm{swollen}}\left ( \lambda -\lambda ^{-2} \right )</math>

where <math>\lambda =l_{\textrm{current}}/l_{\textrm{original}}</math>&nbsp;is the stretch.<ref name=":0" />

===Viscoelasticity===
For hydrogels, their elasticity comes from the solid polymer matrix while the viscosity originates from the polymer network mobility and the water and other components that make up the aqueous phase.<ref>{{cite journal | vauthors = Tirella A, Mattei G, Ahluwalia A | title = Strain rate viscoelastic analysis of soft and highly hydrated biomaterials | journal = Journal of Biomedical Materials Research. Part A | volume = 102 | issue = 10 | pages = 3352–3360 | date = October 2014 | pmid = 23946054 | pmc = 4304325 | doi = 10.1002/jbm.a.34914 }}</ref> Viscoelastic properties of a hydrogel is highly dependent on the nature of the applied mechanical motion. Thus, the time dependence of these applied forces is extremely important for evaluating the viscoelasticity of the material.<ref name="mech" />

Physical models for viscoelasticity attempt to capture the elastic and viscous material properties of a material. In an elastic material, the stress is proportional to the strain while in a viscous material, the stress is proportional to the strain rate. The Maxwell model is one developed mathematical model for linear viscoelastic response. In this model, viscoelasticity is modeled analogous to an electrical circuit with a Hookean spring, that represents the Young's modulus, and a Newtonian dashpot that represents the viscosity. A material that exhibit properties described in this model is a [[Physics:Maxwell material|Maxwell material]]. Another physical model used is called the Kelvin-Voigt Model and a material that follow this model is called a [[Physics:Kelvin–Voigt material|Kelvin–Voigt material]].<ref>{{cite web | vauthors = Roylance D |title="Engineering viscoelasticity" |url=http://web.mit.edu/course/3/3.11/www/modules/visco.pdf |website=Modules in Mechanics of Materials |publisher=Massachusetts Institute of Technology |access-date=11 May 2021}}</ref> In order to describe the time-dependent creep and stress-relaxation behavior of hydrogel, a variety of physical lumped parameter models can be used.<ref name=":0" /> These modeling methods vary greatly and are extremely complex, so the empirical [[Prony's method|Prony Series]] description is commonly used to describe the viscoelastic behavior in hydrogels.<ref name=":0" />

In order to measure the time-dependent viscoelastic behavior of polymers [[Dynamic mechanical analysis|dynamic mechanical analysis]] is often performed. Typically, in these measurements the one side of the hydrogel is subjected to a sinusoidal load in shear mode while the applied stress is measured with a stress transducer and the change in sample length is measured with a strain transducer.<ref name="mech">{{cite journal | vauthors = Anseth KS, Bowman CN, Brannon-Peppas L | title = Mechanical properties of hydrogels and their experimental determination | journal = Biomaterials | volume = 17 | issue = 17 | pages = 1647–1657 | date = September 1996 | pmid = 8866026 | doi = 10.1016/0142-9612(96)87644-7 }}</ref> One notation used to model the sinusoidal response to the periodic stress or strain is:
:<math>G = G' + iG''</math>
in which G' is the real (elastic or storage) modulus, G" is the imaginary (viscous or loss) modulus.

===Poroelasticity===
[[Physics:Poroelasticity|Poroelasticity]] is a characteristic of materials related to the migration of solvent through a porous material and the concurrent deformation that occurs.<ref name=":0" /> Poroelasticity in hydrated materials such as hydrogels occurs due to friction between the polymer and water as the water moves through the porous matrix upon compression. This causes a decrease in water pressure, which adds additional stress upon compression. Similar to viscoelasticity, this behavior is time dependent, thus poroelasticity is dependent on compression rate: a hydrogel shows softness upon slow compression, but fast compression makes the hydrogel stiffer. This phenomenon is due to the friction between the water and the porous matrix is proportional to the flow of water, which in turn is dependent on compression rate. Thus, a common way to measure poroelasticity is to do compression tests at varying compression rates.<ref name="Poro">{{cite journal | vauthors = Isobe N, Kimura S, Wada M, Deguchi S |title=Poroelasticity of cellulose hydrogel |journal=Journal of the Taiwan Institute of Chemical Engineers |date=November 2018 |volume=92 |pages=118–122 |doi=10.1016/j.jtice.2018.02.017|s2cid=103246330 }}</ref> Pore size is an important factor in influencing poroelasticity. The [[Kozeny–Carman equation]] has been used to predict pore size by relating the pressure drop to the difference in stress between two compression rates.<ref name="Poro"/>

Poroelasticity is described by several coupled equations, thus there are few mechanical tests that relate directly to the poroelastic behavior of the material, thus more complicated tests such as indentation testing, numerical or computational models are utilized. Numerical or computational methods attempt to simulate the three dimensional permeability of the hydrogel network.

===Environmental response===
The most commonly seen environmental sensitivity in hydrogels is a response to temperature.<ref name=":1">{{cite journal | vauthors = Qiu Y, Park K | title = Environment-sensitive hydrogels for drug delivery | journal = Advanced Drug Delivery Reviews | volume = 53 | issue = 3 | pages = 321–339 | date = December 2001 | pmid = 11744175 | doi = 10.1016/S0169-409X(01)00203-4 }}</ref> Many polymers/hydrogels exhibit a temperature dependent phase transition, which can be classified as either an [[Physics:Upper critical solution temperature|upper critical solution temperature]] (UCST) or [[Physics:Lower critical solution temperature|lower critical solution temperature]] (LCST). UCST polymers increase in their water-solubility at higher temperatures, which lead to UCST hydrogels transitioning from a gel (solid) to a solution (liquid) as the temperature is increased (similar to the melting point behavior of pure materials). This phenomenon also causes UCST hydrogels to expand (increase their swell ratio) as temperature increases while they are below their UCST.<ref name=":1" /> However, polymers with LCSTs display an inverse (or negative) temperature-dependence, where their water-solubility decreases at higher temperatures. LCST hydrogels transition from a liquid solution to a solid gel as the temperature is increased, and they also shrink (decrease their swell ratio) as the temperature increases while they are above their LCST.<ref name=":1" />

Applications can dictate for diverse thermal responses. For example, in the biomedical field, LCST hydrogels are being investigated as drug delivery systems due to being injectable (liquid) at room temp and then solidifying into a rigid gel upon exposure to the higher temperatures of the human body.<ref name=":1" /> There are many other stimuli that hydrogels can be responsive to, including: pH, glucose, electrical signals, [[Physics:Photopolymer|light]], [[Chemistry:Pressure-sensitive adhesive|pressure]], ions, [[Biology:Antigen|antigen]]s, and more.<ref name=":1" />

===Additives===
The mechanical properties of hydrogels can be fine-tuned in many ways beginning with attention to their hydrophobic properties.<ref name=":1" /><ref>{{Cite journal| vauthors = Zaragoza J, Chang A, Asuri P |date=January 2017|title=Effect of crosslinker length on the elastic and compression modulus of poly(acrylamide) nanocomposite hydrogels|journal=Journal of Physics: Conference Series|language=en|volume=790|issue=1|pages=012037|doi=10.1088/1742-6596/790/1/012037|bibcode=2017JPhCS.790a2037Z|issn=1742-6588|doi-access=free}}</ref> Another method of modifying the strength or elasticity of hydrogels is to graft or surface coat them onto a stronger/stiffer support, or by making superporous hydrogel (SPH) composites, in which a cross-linkable matrix swelling additive is added.<ref name=":2">{{cite journal | vauthors = Ahmed EM | title = Hydrogel: Preparation, characterization, and applications: A review | journal = Journal of Advanced Research | volume = 6 | issue = 2 | pages = 105–121 | date = March 2015 | pmid = 25750745 | pmc = 4348459 | doi = 10.1016/j.jare.2013.07.006 }}</ref> Other additives, such as [[Physics:Nanoparticle|nanoparticle]]s and [[Medicine:Microparticle|microparticle]]s, have been shown to significantly modify the stiffness and gelation temperature of certain hydrogels used in biomedical applications.<ref>{{cite journal | vauthors = Cidade MT, Ramos DJ, Santos J, Carrelo H, Calero N, Borges JP |display-authors=3| title = Injectable Hydrogels Based on Pluronic/Water Systems Filled with Alginate Microparticles for Biomedical Applications | journal = Materials | volume = 12 | issue = 7 | pages = 1083 | date = April 2019 | pmid = 30986948 | pmc = 6479463 | doi = 10.3390/ma12071083 | doi-access = free | bibcode = 2019Mate...12.1083C }}</ref><ref>{{cite journal | vauthors = Rose S, Prevoteau A, Elzière P, Hourdet D, Marcellan A, Leibler L |display-authors=3| title = Nanoparticle solutions as adhesives for gels and biological tissues | journal = Nature | volume = 505 | issue = 7483 | pages = 382–385 | date = January 2014 | pmid = 24336207 | doi = 10.1038/nature12806 | s2cid = 205236639 | bibcode = 2014Natur.505..382R }}</ref><ref>{{cite journal | vauthors = Zaragoza J, Fukuoka S, Kraus M, Thomin J, Asuri P |display-authors=3| title = Exploring the Role of Nanoparticles in Enhancing Mechanical Properties of Hydrogel Nanocomposites | journal = Nanomaterials | volume = 8 | issue = 11 | pages = 882 | date = October 2018 | pmid = 30380606 | pmc = 6265757 | doi = 10.3390/nano8110882 | doi-access = free }}</ref>

===Processing techniques===
While a hydrogel's mechanical properties can be tuned and modified through crosslink concentration and additives, these properties can also be enhanced or optimized for various applications through specific processing techniques. These techniques include [[Physics:Electrospinning|electro-spinning]], [[3D printing|3D]]/[[Engineering:4D printing|4D printing]], [[Biology:Self-assembly|self-assembly]], and [[Earth:Freeze-casting|freeze-casting]]. One unique processing technique is through the formation of multi-layered hydrogels to create a spatially-varying matrix composition and by extension, mechanical properties. This can be done by polymerizing the hydrogel matrixes in a layer by layer fashion via UV polymerization. This technique can be useful in creating hydrogels that mimic articular cartilage, enabling a material with three separate zones of distinct mechanical properties.<ref>{{cite journal | vauthors = Nguyen LH, Kudva AK, Saxena NS, Roy K | title = Engineering articular cartilage with spatially-varying matrix composition and mechanical properties from a single stem cell population using a multi-layered hydrogel | journal = Biomaterials | volume = 32 | issue = 29 | pages = 6946–6952 | date = October 2011 | pmid = 21723599 | doi = 10.1016/j.biomaterials.2011.06.014 }}</ref>

Another emerging technique to optimize hydrogel mechanical properties is by taking advantage of the [[Hofmeister series]]. Due to this phenomenon, through the addition of salt solution, the polymer chains of a hydrogel aggregate and crystallize, which increases the toughness of the hydrogel. This method, called "[[Salting out|salting out]]", has been applied to poly(vinyl alcohol) hydrogels by adding a [[Chemistry:Sodium sulfate|sodium sulfate]] salt solution.<ref>{{cite journal | vauthors = Hua M, Wu D, Wu S, Ma Y, Alsaid Y, He X |display-authors=3| title = 4D Printable Tough and Thermoresponsive Hydrogels | journal = ACS Applied Materials & Interfaces | volume = 13 | issue = 11 | pages = 12689–12697 | date = March 2021 | pmid = 33263991 | doi = 10.1021/acsami.0c17532 | s2cid = 227258845 }}</ref> Some of these processing techniques can be used synergistically with each other to yield optimal mechanical properties. [[Physics:Directional freezing|Directional freezing]] or [[Earth:Freeze-casting|freeze-casting]] is another method in which a directional temperature gradient is applied to the hydrogel is another way to form materials with anisotropic mechanical properties. Utilizing both the freeze-casting and salting-out processing techniques on poly(vinyl alcohol) hydrogels to induce hierarchical morphologies and anisotropic mechanical properties.<ref name="Strong">{{cite journal | vauthors = Hua M, Wu S, Ma Y, Zhao Y, Chen Z, Frenkel I, Strzalka J, Zhou H, Zhu X, He X | display-authors = 3 | title = Strong tough hydrogels via the synergy of freeze-casting and salting out | journal = Nature | volume = 590 | issue = 7847 | pages = 594–599 | date = February 2021 | pmid = 33627812 | doi = 10.1038/s41586-021-03212-z | s2cid = 232048202 | bibcode = 2021Natur.590..594H | osti = 1774154 }}</ref> Directional freezing of the hydrogels helps to align and coalesce the polymer chains, creating anisotropic array honeycomb tube-like structures while salting out the hydrogel yielded out a nano-fibril network on the surface of these honeycomb tube-like structures. While maintaining a water content of over 70%, these hydrogels' toughness values are well above those of water-free polymers such as [[Chemistry:Polydimethylsiloxane|polydimethylsiloxane]] (PDMS), [[Chemistry:Kevlar|Kevlar]], and [[Chemistry:Synthetic rubber|synthetic rubber]]. The values also surpass the toughness of natural [[Biology:Tendon|tendon]] and [[Physics:Spider silk|spider silk]].<ref name="Strong"/>

== Applications ==
===Soft contact lenses===
[[File:Siliconehydrogel.svg|thumb|Molecular structure of silicone hydrogel used in flexible, oxygen-permeable contact lenses.<ref>{{cite book| vauthors = Lai YC, Wilson AC, Zantos SG |title=Kirk‐Othmer Encyclopedia of Chemical Technology|year=2000|isbn=9780471484943| publisher =John Wiley & Sons, Inc|chapter=Contact Lenses|doi=10.1002/0471238961}}</ref>]]

The dominant material for contact lenses are acrylate-[[Chemistry:Siloxane|siloxane]] hydrogels. They have replaced hard contact lenses. One of their most attractive properties is oxygen permeability, which is required since the cornea lacks vasculature.

===Research ===
[[File:Hydrogel MSC Nanolive.gif|thumb|Human mesenchymal stem cell interacting with 3D hydrogel - imaged with label-free live cell imaging]]
[[File:Blasenpflaster.jpg|thumb|An adhesive bandage with a hydrogel pad, used for blisters and burns. The central gel is clear, the adhesive waterproof plastic film is clear, the backing is white and blue.]]

* Coatings for gas evolution reaction electrodes for efficient bubble detachment <ref>{{Cite journal |last1=Jeon |first1=Dasom |last2=Park |first2=Jinwoo |last3=Shin |first3=Changhwan |last4=Kim |first4=Hyunwoo |last5=Jang |first5=Ji-Wook |last6=Lee |first6=Dong Woog |last7=Ryu |first7=Jungki |date=2020-04-10 |title=Superaerophobic hydrogels for enhanced electrochemical and photoelectrochemical hydrogen production |journal=Science Advances |language=en |volume=6 |issue=15 |pages=eaaz3944 |doi=10.1126/sciadv.aaz3944 |issn=2375-2548 |pmc=7148083 |pmid=32300656|bibcode=2020SciA....6.3944J }}</ref><ref>{{Cite journal |last1=Bae |first1=Misol |last2=Kang |first2=Yunseok |last3=Lee |first3=Dong Woog |last4=Jeon |first4=Dasom |last5=Ryu |first5=Jungki |date=August 2022 |title=Superaerophobic Polyethyleneimine Hydrogels for Improving Electrochemical Hydrogen Production by Promoting Bubble Detachment |journal=Advanced Energy Materials |language=en |volume=12 |issue=29 |pages=2201452 |doi=10.1002/aenm.202201452 |s2cid=249355500 |issn=1614-6832|doi-access=free }}</ref><ref>{{Cite journal |last1=Park |first1=Jinwoo |last2=Jeon |first2=Dasom |last3=Kang |first3=Yunseok |last4=Ryu |first4=Jungki |last5=Lee |first5=Dong Woog |date=2023-01-24 |title=Nanofibrillar hydrogels outperform Pt/C for hydrogen evolution reactions under high-current conditions |url=https://pubs.rsc.org/en/content/articlelanding/2023/ta/d2ta08775h |journal=Journal of Materials Chemistry A |language=en |volume=11 |issue=4 |pages=1658–1665 |doi=10.1039/D2TA08775H |s2cid=254387206 |issn=2050-7496}}</ref>
* [[Unsolved:Breast implant|Breast implant]]s
* [[Engineering:Contact lens|Contact lens]]es ([[Chemistry:Silicone|silicone]] hydrogels, [[Chemistry:Polyacrylamide|polyacrylamide]]s, [[Chemistry:Polymacon|polymacon]])
* Water sustainability: Hydrogels have emerged as promising materials platforms for solar-powered water purification,<ref>{{cite journal |author1=Youhong Guo |author2=H. Lu |author3=F. Zhao |author4=X. Zhou |author5=W. Shi |author6=Guihua Yu | title=Biomass-Derived Hybrid Hydrogel Evaporators for Cost-Effective Solar Water Purification | journal=Advanced Materials| year=2020 |volume= 32 |issue=11 |pages= 1907061 | doi=10.1002/adma.201907061 |pmid= 32022974 |bibcode=2020AdM....3207061G |s2cid=211036014 }}</ref> water disinfection,<ref>{{cite journal |author1=Youhong Guo |author2=C. M. Dundas |author3=X. Zhou |author4=K. P. Johnston |author5=Guihua Yu | title=Molecular Engineering of Hydrogels for Rapid Water Disinfection and Sustainable Solar Vapor Generation | journal=Advanced Materials| year=2021 |volume= 33 |issue=35 |pages= 2102994 | doi=10.1002/adma.202102994 |pmid= 34292641 |bibcode=2021AdM....3302994G |s2cid=236174198 }}</ref> and [[Earth:Atmospheric water generator|Atmospheric water generator]].<ref>{{cite journal |author1=Youhong Guo |author2=W. Guan |author3=C. Lei |author4=H. Lu |author5=W. Shi |author6=Guihua Yu | title=Scalable super hygroscopic polymer films for sustainable moisture harvesting in arid environments | journal=Nature Communications| year=2022 |volume= 13 |issue=1 |pages= 2761 | doi=10.1038/s41467-022-30505-2 |pmid= 35589809 |pmc=9120194 |bibcode=2022NatCo..13.2761G }}</ref>
* Disposable diapers where they absorb [[Biology:Urine|urine]], or in sanitary napkins<ref name="Calo2015" />
* Dressings for healing of burn or other hard-to-heal [[Medicine:Wound|wound]]s. Wound gels are excellent for helping to create or maintain a moist environment.
* [[Biology:Electroencephalography#Method|EEG]] and ECG medical electrodes using hydrogels composed of [[Chemistry:Cross-link|cross-link]]ed polymers (polyethylene oxide, [[Chemistry:PolyAMPS|polyAMPS]] and [[Chemistry:Polyvinylpyrrolidone|polyvinylpyrrolidone]])
* Encapsulation of quantum dots
* Environmentally sensitive hydrogels (also known as 'smart gels' or 'intelligent gels'). These hydrogels have the ability to sense changes of pH, temperature, or the concentration of metabolite and release their load as result of such a change.<ref>{{cite journal | vauthors = Brudno Y, Mooney DJ | title = On-demand drug delivery from local depots | journal = Journal of Controlled Release | volume = 219 | pages = 8–17 | date = December 2015 | pmid = 26374941 | doi = 10.1016/j.jconrel.2015.09.011 }}</ref><ref>{{cite journal | vauthors = Blacklow SO, Li J, Freedman BR, Zeidi M, Chen C, Mooney DJ |display-authors=3| title = Bioinspired mechanically active adhesive dressings to accelerate wound closure | journal = Science Advances | volume = 5 | issue = 7 | pages = eaaw3963 | date = July 2019 | pmid = 31355332 | pmc = 6656537 | doi = 10.1126/sciadv.aaw3963 | doi-access = free | bibcode = 2019SciA....5.3963B }}</ref><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> 
*[[Chemistry:Hydrogel fiber|Fibers]]
* [[Chemistry:Adhesive|Glue]]
* Granules for holding [[Chemistry:Soil|soil]] moisture in arid areas
* Air bubble-repellent (superaerophobicity). Can improve the performance and stability of electrodes for water [[Chemistry:Electrolysis|electrolysis]].<ref>{{cite journal | vauthors = Jeon D, Park J, Shin C, Kim H, Jang JW, Lee DW, Ryu J |display-authors=3| title = Superaerophobic hydrogels for enhanced electrochemical and photoelectrochemical hydrogen production | journal = Science Advances | volume = 6 | issue = 15 | pages = eaaz3944 | date = April 2020 | pmid = 32300656 | pmc = 7148083 | doi = 10.1126/sciadv.aaz3944 |bibcode=2020SciA....6.3944J}}</ref>
* Culturing cells: Hydrogel-coated wells have been used for cell culture.<ref>{{cite journal | vauthors = Discher DE, Janmey P, Wang YL | title = Tissue cells feel and respond to the stiffness of their substrate | journal = Science | volume = 310 | issue = 5751 | pages = 1139–1143 | date = November 2005 | pmid = 16293750 | doi = 10.1126/science.1116995 | s2cid = 9036803 | citeseerx = 10.1.1.318.690 | bibcode = 2005Sci...310.1139D }}</ref>
* Biosensors: Hydrogels that are responsive to specific molecules,<ref>{{cite book | title = Chemoresponsive Materials | veditors = Schneider HJ | publisher = Royal Society of Chemistry | location = Cambridge | date = 2015 | isbn = 978-1-78262-242-0 | url = https://pubs.rsc.org/en/content/ebook/ }}</ref> such as glucose or antigens, can be used as biosensors, as well as in DDS.<ref>{{cite journal | vauthors = Yetisen AK, [[Biography:Izabela Naydenova|Naydenova I]], da Cruz Vasconcellos F, Blyth J, Lowe CR |display-authors=3| title = Holographic sensors: three-dimensional analyte-sensitive nanostructures and their applications | journal = Chemical Reviews | volume = 114 | issue = 20 | pages = 10654–10696 | date = October 2014 | pmid = 25211200 | doi = 10.1021/cr500116a | doi-access = free }}</ref>
*Cell carrier: Injectable hydrogels can be used to carry drugs or cells for applications in tissue regeneration or [[Biology:3D bioprinting|3D bioprinting]].<ref name=":3">{{cite journal | vauthors = Lee JH | title = Injectable hydrogels delivering therapeutic agents for disease treatment and tissue engineering | journal = Biomaterials Research | volume = 22 | issue = 1 | pages = 27 | date = December 2018 | pmid = 30275970 | pmc = 6158836 | doi = 10.1186/s40824-018-0138-6 | doi-access = free }}</ref><ref name=":4">{{cite journal | vauthors = Liu M, Zeng X, Ma C, Yi H, Ali Z, Mou X, Li S, Deng Y, He N | display-authors = 3 | title = Injectable hydrogels for cartilage and bone tissue engineering | journal = Bone Research | volume = 5 | issue = 1 | pages = 17014 | date = December 2017 | pmid = 28584674 | pmc = 5448314 | doi = 10.1038/boneres.2017.14 }}</ref><ref name=":5">{{cite journal | vauthors = Pupkaite J, Rosenquist J, Hilborn J, Samanta A | title = Injectable Shape-Holding Collagen Hydrogel for Cell Encapsulation and Delivery Cross-linked Using Thiol-Michael Addition Click Reaction | journal = Biomacromolecules | volume = 20 | issue = 9 | pages = 3475–3484 | date = September 2019 | pmid = 31408340 | doi = 10.1021/acs.biomac.9b00769 | s2cid = 199574808 }}</ref> Hydrogels with reversible chemistry are required to allow for fluidization during injection/printing followed by [[Physics:Self-healing material|self-healing]] of the original hydrogel structure.<ref name="Bertsch2023">{{cite journal |last1=Bertsch |first1=Pascal |last2=Diba |first2=Mani |last3=Mooney |first3=David J. |last4=Leeuwenburgh |first4=Sander C. G. |title=Self-Healing Injectable Hydrogels for Tissue Regeneration |journal=Chemical Reviews |date=25 January 2023 |volume=123 |issue=2 |pages=834–873 |doi=10.1021/acs.chemrev.2c00179 |pmid=35930422 |pmc=9881015 }}</ref>
*Investigate cell biomechanical functions combined with [[Holotomography|holotomography]] microscopy
* Provide absorption, desloughing and debriding of necrotic and fibrotic tissue
* [[Engineering:Tissue engineering|Tissue engineering]] scaffolds. When used as scaffolds, hydrogels may contain human cells to repair tissue. They mimic 3D microenvironment of cells.<ref>{{cite journal| vauthors = Mellati A, Dai S, Bi J, Jin B, Zhang H |display-authors=3|year=2014|title=A biodegradable thermosensitive hydrogel with tuneable properties for mimicking three-dimensional microenvironments of stem cells|journal=RSC Adv.|volume=4|issue=109|pages=63951–63961|bibcode=2014RSCAd...463951M|doi=10.1039/C4RA12215A|issn=2046-2069}}</ref> Materials include [[Chemistry:Agarose|agarose]], methylcellulose, hyaluronan, [[Chemistry:Elastin-like polypeptides|elastin-like polypeptides]], and other naturally derived polymers. 
* Sustained-release [[Medicine:Drug delivery|drug delivery]] systems. Ionic strength, pH and temperature can be used as a triggering factor to control the release of the drug.<ref>{{cite journal| vauthors = Malmsten M, Bysell H, Hansson P |date=2010-12-01|title=Biomacromolecules in microgels — Opportunities and challenges for drug delivery|journal=Current Opinion in Colloid & Interface Science|volume=15|issue=6|pages=435–444|doi=10.1016/j.cocis.2010.05.016|issn=1359-0294}}</ref>
* Window coating/replacement: Hydrogels are under consideration for reducing infrared light absorption by 75%.<ref>{{Cite web |last=Irving |first=Michael |date=2022-08-31 |title=Hydrogel glass windows let in more light and less heat |url=https://newatlas.com/materials/hydrogel-glass-windows-more-light-less-heat/ |access-date=2022-09-26 |website=New Atlas |language=en-US}}</ref> Another approach reduced interior temperature using a [[Chemistry:Smart glass|temperature-responsive hydrogel]].<ref>{{cite journal |last1=Miller |first1=Brittney J. |title=How smart windows save energy |journal=Knowable Magazine |date=8 June 2022 |doi=10.1146/knowable-060822-3 |url=https://knowablemagazine.org/article/technology/2022/how-smart-windows-save-energy |doi-access=free |access-date=15 July 2022}}</ref>
* Thermodynamic electricity generation: When combined with ions allows for heat dissipation for electronic devices and batteries and converting the heat exchange to an electrical charge.<ref>{{Cite web|date=April 22, 2020|title=A new way to cool down electronic devices, recover waste heat|url=https://phys.org/news/2020-04-cool-electronic-devices-recover.html|access-date=April 23, 2020|website=Phys.org}}</ref>
* Water gel explosives
* Controlled release of agrochemicals (pesticides and fertilizer)
* [[Biology:Talin (protein)|Talin]] Shock Absorbing Materials - protein-based hydrogels that can absorb supersonic impacts<ref>{{Cite web |last=Lavars |first=Nick |date=2022-12-15 |title=New protein-based armor material can withstand supersonic impacts |url=https://newatlas.com/materials/protein-based-armor-material-withstand-supersonic-impacts/ |access-date=2022-12-25 |website=New Atlas |language=en-US}}</ref>

=== Biomaterials ===
Implanted or injected hydrogels have the potential to support tissue regeneration by mechanical tissue support, localized drug or cell delivery,<ref name=":7" /> local cell recruitement or [[Biology:Immunomodulation|immunomodulation]], or encapsulation of nanoparticles for local [[Physics:Photothermal therapy|photothermal]] or [[Medicine:Brachytherapy|brachytherapy]].<ref name="Bertsch2023" /> Polymeric drug delivery systems have overcome challenge due to their biodegradability, biocompatibility, and anti-toxicity.<ref>{{cite journal | vauthors = Tang Y, Heaysman CL, Willis S, Lewis AL | title = Physical hydrogels with self-assembled nanostructures as drug delivery systems | journal = Expert Opinion on Drug Delivery | volume = 8 | issue = 9 | pages = 1141–1159 | date = September 2011 | pmid = 21619469 | doi = 10.1517/17425247.2011.588205 | s2cid = 24843309 }}</ref><ref>{{cite journal | vauthors = Aurand ER, Lampe KJ, Bjugstad KB | title = Defining and designing polymers and hydrogels for neural tissue engineering | journal = Neuroscience Research | volume = 72 | issue = 3 | pages = 199–213 | date = March 2012 | pmid = 22192467 | pmc = 3408056 | doi = 10.1016/j.neures.2011.12.005 }}</ref> Materials such as [[Biology:Collagen|collagen]], chitosan, [[Chemistry:Cellulose|cellulose]], and poly (lactic-co-glycolic acid) have been implemented extensively for drug delivery to organs such as eye,<ref>{{cite journal | vauthors = Ozcelik B, Brown KD, Blencowe A, Daniell M, Stevens GW, Qiao GG |display-authors=3| title = Ultrathin chitosan-poly(ethylene glycol) hydrogel films for corneal tissue engineering | journal = Acta Biomaterialia | volume = 9 | issue = 5 | pages = 6594–6605 | date = May 2013 | pmid = 23376126 | doi = 10.1016/j.actbio.2013.01.020 }}</ref> nose, kidneys,<ref>{{cite journal | vauthors = Gao J, Liu R, Wu J, Liu Z, Li J, Zhou J, Hao T, Wang Y, Du Z, Duan C, Wang C | display-authors = 3 | title = The use of chitosan based hydrogel for enhancing the therapeutic benefits of adipose-derived MSCs for acute kidney injury | journal = Biomaterials | volume = 33 | issue = 14 | pages = 3673–3681 | date = May 2012 | pmid = 22361096 | doi = 10.1016/j.biomaterials.2012.01.061 }}</ref> lungs,<ref>{{cite journal | vauthors = Otani Y, Tabata Y, Ikada Y | title = Sealing effect of rapidly curable gelatin-poly (L-glutamic acid) hydrogel glue on lung air leak | journal = The Annals of Thoracic Surgery | volume = 67 | issue = 4 | pages = 922–926 | date = April 1999 | pmid = 10320229 | doi = 10.1016/S0003-4975(99)00153-8 | doi-access = free }}</ref> intestines,<ref>{{cite journal | vauthors = Ramdas M, Dileep KJ, Anitha Y, Paul W, Sharma CP |display-authors=3| title = Alginate encapsulated bioadhesive chitosan microspheres for intestinal drug delivery | journal = Journal of Biomaterials Applications | volume = 13 | issue = 4 | pages = 290–296 | date = April 1999 | pmid = 10340211 | doi = 10.1177/088532829901300402 | s2cid = 31364133 }}</ref> skin<ref>{{Citation | vauthors = Liu X, Ma L, Mao Z, Gao C | veditors = Jayakumar R, Prabaharan M, Muzzarelli RA |title=Chitosan-Based Biomaterials for Tissue Repair and Regeneration |date=2011 |work=Chitosan for Biomaterials II |pages=81–127 |series=Advances in Polymer Science | volume = 244 |publisher=Springer Berlin Heidelberg |language=en |doi=10.1007/12_2011_118 |isbn=978-3-642-24061-4 }}</ref> and brain.<ref name=":7" /> Future work is focused on reducing toxicity, improving biocompatibility, expanding assembly techniques<ref>{{cite journal | vauthors = Wu ZL, Gong JP |date=June 2011 |title=Hydrogels with self-assembling ordered structures and their functions |journal=NPG Asia Materials |language=en |volume=3 |issue=6 |pages=57–64 |doi=10.1038/asiamat.2010.200 |issn=1884-4057 |doi-access=free}}</ref>

Hydrogels have been considered as vehicles for drug delivery.<ref>{{cite journal | vauthors = Kim J, Yaszemski MJ, Lu L | title = Three-dimensional porous biodegradable polymeric scaffolds fabricated with biodegradable hydrogel porogens | journal = Tissue Engineering. Part C, Methods | volume = 15 | issue = 4 | pages = 583–594 | date = December 2009 | pmid = 19216632 | pmc = 2819712 | doi = 10.1089/ten.TEC.2008.0642 }}</ref><ref name=":3" /><ref name=":4" /><ref name=":5" /> They can also be made to mimic animal mucosal tissues to be used for testing mucoadhesive properties.<ref>{{cite journal |vauthors=Cook MT, Smith SL, Khutoryanskiy VV |date=October 2015 |title=Novel glycopolymer hydrogels as mucosa-mimetic materials to reduce animal testing |journal=Chemical Communications |volume=51 |issue=77 |pages=14447–14450 |doi=10.1039/C5CC02428E |pmid=26221632 |doi-access=free|hdl=2299/16512 |hdl-access=free }}</ref><ref>{{cite journal |vauthors=Cook MT, Khutoryanskiy VV |date=November 2015 |title=Mucoadhesion and mucosa-mimetic materials--A mini-review |journal=International Journal of Pharmaceutics |volume=495 |issue=2 |pages=991–998 |doi=10.1016/j.ijpharm.2015.09.064 |pmid=26440734 |hdl-access=free |hdl=2299/16856}}</ref> They have been examined for use as reservoirs in [[Medicine:Topical drug delivery|topical drug delivery]]; particularly ionic drugs, delivered by [[Biology:Iontophoresis|iontophoresis]].

== References ==
{{Reflist}}

== Further reading ==
{{refbegin}}
* {{Cite journal| vauthors = Warren DS, Sutherland SP, Kao JY, Weal GR, Mackay SM |display-authors=3|year=2017|title=The Preparation and Simple Analysis of a Clay Nanoparticle Composite Hydrogel|journal=Journal of Chemical Education|volume=94|issue=11|pages=1772–1779|doi=10.1021/acs.jchemed.6b00389|issn=0021-9584|bibcode=2017JChEd..94.1772W}}
{{refend}}

[[Category:Colloidal chemistry]]
[[Category:Water chemistry]]
[[Category:Soft matter]]

{{Sourceattribution|Hydrogel}}