Array ( [0] => {{Short description|Soft water-rich polymer gel}} [1] => {{TopicTOC-Polymer}} [2] => [[File:Gelatine.png|thumb|right|Gelatin, here in sheets for cooking, is a hydrogel.]] [3] => [[File:Picture of peptide hydrogel formation shown by the inverted vial method.jpg|thumb|Peptide hydrogel formation shown by the inverted vial method.]] [4] => A '''hydrogel''' is a biphasic material, a mixture of [[Porosity|porous]], [[Permeation|permeable]] solids and at least 10% by weight or volume of interstitial fluid composed completely or mainly by water.{{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}}{{Cite journal |last1=Ghosh |first1=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 |s2cid=264944892 |issn=0141-8130}} In hydrogels the porous permeable solid is a water [[Solubility|insoluble]] three dimensional network of natural or synthetic polymers and a fluid, having absorbed a large amount of water or biological fluids.{{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 }}{{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 }}{{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}} These properties underpin several applications, especially in the biomedical area. Many hydrogels are synthetic, but some are derived from nature.{{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}} The term 'hydrogel' was coined in 1894.{{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}} [5] => [6] => [[File:IUPAC definition for a hydrogel.png|thumb|right|550px|link=https://doi.org/10.1351/goldbook.HT07519|IUPAC definition for a hydrogel]] [7] => [8] => ==Chemistry== [9] => === Classification === [10] => The crosslinks which bond the polymers of a hydrogel fall under two general categories: physical hydrogels and chemical hydrogels. Chemical hydrogels have [[Covalent bond|covalent cross-linking bonds]], whereas physical hydrogels have [[non-covalent bond]]s.{{Cn|date=February 2023}} Chemical hydrogels can result in strong reversible or irreversible gels due to the covalent bonding.{{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.}} Chemical hydrogels that contain reversible covalent cross-linking bonds such as hydrogels of [[thiomer]]s being cross-linked via disulfide bonds are non-toxic and are used in numerous medicinal products.{{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 }}{{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 }}{{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}} 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 ([[Alginic acid|alginate]]) or temperature ([[gelatin]]e); they are also used for medical applications.{{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}}{{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}}{{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}}{{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}}{{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}} Physical crosslinks consist of [[hydrogen bond]]s, [[Hydrophobic effect|hydrophobic interactions]], and chain entanglements (among others). A hydrogel generated through the use of physical crosslinks is sometimes called a 'reversible' hydrogel. Chemical crosslinks consist of covalent bonds between polymer strands. Hydrogels generated in this manner are sometimes called 'permanent' hydrogels. [11] => [12] => Hydrogels are prepared using a variety of [[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 [[hyaluronic acid]], [[chitosan]], [[heparin]], [[alginate]], [[gelatin]] and [[fibrin]].{{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 }} Common synthetic polymers include [[polyvinyl alcohol]], [[polyethylene glycol]], [[sodium polyacrylate]], [[acrylate polymer]]s and [[copolymer]]s thereof. Whereas natural hydrogels are usually non-toxic, and often provides other advantages for medical use, such as [[biocompatibility]], [[Biodegradation|biodegradability]], [[antibiotic]]/[[antifungal]] effect and improve [[Regeneration (biology)|regeneration]] of nearby tissue, their [[Chemical stability|stability]] and [[Strength of materials|strength]] is usually much lower than synthetic hydrogels.{{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}} There are also synthetic hydrogels than can be used for medical applications, such as [[Polyethylene glycol|polyethylene glycol (PEG)]], [[Acrylate polymer|polyacrylate]], and [[Polyvinylpyrrolidone|polyvinylpyrrolidone (PVP)]].{{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 }} [13] => [14] => === Preparation === [15] => [[File:Peptide hydrogel formation simplified scheme.png|thumb|Simplified scheme to show the self-assembly process involved in hydrogel formation.]] [16] => [17] => There are two suggested mechanisms behind physical hydrogel formation, the first one being the gelation of nanofibrous [[peptide]] assemblies, usually observed for [[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.{{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/ }} 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 [[Beta sheet|β-sheet structure]], and assemble to form fibers, although [[Alpha helix|α-helical]] peptides have also been reported.{{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 }}{{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 }} The typical mechanism of gelation involves the oligopeptide precursors self-assemble into fibers that become elongated, and entangle to form cross-linked gels. [18] => [19] => One notable method of initiating a polymerization fuving involves the use of light as a stimulus. In this method, [[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 [[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.{{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 }} 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.{{cite journal| vauthors = Caló E, Khutoryanskiy VV |year=2015|title=Biomedical applications of hydrogels: A review of patents and commercial products|journal=[[European Polymer Journal]]|volume=65|pages=252–267|doi=10.1016/j.eurpolymj.2014.11.024|doi-access=free}} [20] => [21] => Physically crosslinked hydrogels can be prepared by different methods depending on the nature of the crosslink involved. [[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.{{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 |hdl=10072/417476 |s2cid=245576810 |issn=0014-3057|hdl-access=free }} [[Alginic acid|Alginate]] hydrogels are formed by ionic interactions between alginate and double-charged cations. A salt, usually [[calcium chloride]], is dissolved into an aqueous sodium alginate solution, that causes the calcium ions to create ionic bonds between alginate chains.{{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}} [[Gelatin]] hydrogels are formed by temperature change. A water solution of gelatin forms an hydrogel at temperatures below 37–35 °C, as Van der Waals interactions between collagen fibers become stronger than thermal molecular vibrations.{{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 }} [22] => [23] => === Peptides based hydrogels === [24] => Peptides based hydrogels possess exceptional [[biocompatibility]] and [[biodegradability]] qualities, giving rise to their wide use of applications, particularly in biomedicine; as such, their physical properties can be fine-tuned in order to maximise their use. Methods to do this are: modulation of the [[amino acid]] sequence, [[pH]], [[Chirality (chemistry)|chirality]], and increasing the number of [[Aromatic compound|aromatic]] residues.{{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 }} 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.{{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}}{{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 }} 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.{{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}} 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.{{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 }}{{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}} 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. In another direction, Morris et al reported the use of GdL as a 'molecular trigger' to predict and control the order of gelation.{{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 }} 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.{{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}} Furthermore, aromatic interactions play a key role in hydrogel formation as a result of π- π stacking driving gelation, shown by many studies.{{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}}{{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 }} [25] => [26] => ===Other=== [27] => Hydrogels also possess a degree of flexibility very similar to natural tissue due to their significant water content. As responsive "[[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 [[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 [[sensor]]s. [28] => [29] => [30] => File:Hydrogel micropump.webm|A [[micropump]] based on a hydrogel bar (4×0.3×0.05 mm size) actuated by applied voltage. This pump can be continuously operated with a 1.5 V battery for at least 6 months.{{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 }} [31] => 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. [32] => File:Crosslinked ultrashort peptide hydrogel.jpg|Photo of the same short-peptide-based hydrogel, held in forceps to demonstrate its stiffness and transparency. [33] => [34] => [35] => ==Mechanical properties== [36] => Hydrogels have been investigated for diverse applications. By modifying the polymer concentration of a hydrogel (or conversely, the water concentration), the [[Young's modulus]], [[shear modulus]], and [[storage modulus]] can vary from 10 Pa to 3 MPa, a range of about five orders of magnitude.{{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}} A similar effect can be seen by altering the crosslinking concentration. 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.{{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}} 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 [[Elasticity (physics)|elasticity]] and [[viscoelasticity]], hydrogels have an additional time dependent deformation mechanism which is dependent on fluid flow called [[poroelasticity]]. These properties are extremely important to consider while performing mechanical experiments. Some common mechanical testing experiments for hydrogels are [[Tension (physics)|tension]], [[Compressive strength|compression]] (confined or unconfined), indentation, [[Shear stress|shear]] [[rheometry]] or [[dynamic mechanical analysis]]. [37] => [38] => Hydrogels have two main regimes of mechanical properties: [[rubber elasticity]] and [[viscoelasticity]]: [39] => [40] => ===Rubber elasticity=== [41] => 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: [42] => [43] => G=N_{p}kT={\rho RT \over \overline{M}_{c}} [44] => [45] => where ''G'' is the [[shear modulus]], ''k'' is the Boltzmann constant, ''T'' is temperature, ''Np'' is the number of polymer chains per unit volume, ''ρ'' is the density, ''R'' is the ideal gas constant, and \overline{M}_{c} is the (number) average molecular weight between two adjacent cross-linking points. \overline{M}_{c} can be calculated from the swell ratio, ''Q'', which is relatively easy to test and measure. [46] => [47] => For the swollen state, a perfect gel network can be modeled as: [48] => [49] => G_{\textrm{swollen}}=GQ^{-1/3} [50] => [51] => In a simple uniaxial extension or compression test, the true stress, \sigma _{t}, and engineering stress, \sigma _{e}, can be calculated as: [52] => [53] => \sigma _{t}=G_{\textrm{swollen}}\left ( \lambda ^{2}-\lambda ^{-1} \right ) [54] => [55] => \sigma _{e}=G_{\textrm{swollen}}\left ( \lambda -\lambda ^{-2} \right ) [56] => [57] => where \lambda =l_{\textrm{current}}/l_{\textrm{original}} is the stretch. [58] => [59] => ===Viscoelasticity=== [60] => 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.{{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 }} 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. [61] => [62] => 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 [[Maxwell material]]. Another physical model used is called the Kelvin-Voigt Model and a material that follow this model is called a [[Kelvin–Voigt material]].{{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}} In order to describe the time-dependent creep and stress-relaxation behavior of hydrogel, a variety of physical lumped parameter models can be used. 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. [63] => [64] => In order to measure the time-dependent viscoelastic behavior of polymers [[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.{{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 }} One notation used to model the sinusoidal response to the periodic stress or strain is: [65] => :G = G' + iG'' [66] => in which G' is the real (elastic or storage) modulus, G" is the imaginary (viscous or loss) modulus. [67] => [68] => ===Poroelasticity=== [69] => [[Poroelasticity]] is a characteristic of materials related to the migration of solvent through a porous material and the concurrent deformation that occurs. 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.{{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 }} 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. [70] => [71] => 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. [72] => [73] => === Toughness and Hysteresis === [74] => The [[toughness]] of a hydrogel refers to the ability of the hydrogel to withstand deformation or mechanical stress without fracturing or breaking apart. A hydrogel with high toughness can maintain its structural integrity and functionality under higher stress. Several factors contribute to the toughness of a hydrogel including composition, crosslink density, polymer chain structure, and hydration level. The toughness of a hydrogel is highly dependent on what polymer(s) and crosslinker(s) make up its matrix as certain polymers possess higher toughness and certain crosslinking covalent bonds are inherently stronger.{{Cite journal |last1=Kuang |first1=Xiao |last2=Arıcan |first2=Mehmet Onur |last3=Zhou |first3=Tao |last4=Zhao |first4=Xuanhe |last5=Zhang |first5=Yu Shrike |date=2023-02-24 |title=Functional Tough Hydrogels: Design, Processing, and Biomedical Applications |url=https://pubs.acs.org/doi/10.1021/accountsmr.2c00026 |journal=Accounts of Materials Research |language=en |volume=4 |issue=2 |pages=101–114 |doi=10.1021/accountsmr.2c00026 |issn=2643-6728}} Additionally, higher crosslinking density generally leads to increased toughness by restricting polymer chain mobility and enhancing resistance to deformation. The structure of the polymer chains is also a factor in that, longer chain lengths and higher molecular weight leads to a greater number of entanglements and higher toughness.{{Cite journal |last1=Nian |first1=Guodong |last2=Kim |first2=Junsoo |last3=Bao |first3=Xianyang |last4=Suo |first4=Zhigang |date=2022-09-20 |title=Making Highly Elastic and Tough Hydrogels from Doughs |url=https://onlinelibrary.wiley.com/doi/10.1002/adma.202206577 |journal=Advanced Materials |language=en |volume=34 |issue=50 |doi=10.1002/adma.202206577 |issn=0935-9648 |via=Wiley Online Library}} A good balance (equilibrium) in the hydration of a hydrogel leads is important because too low hydration causes poor flexibility and toughness within the hydrogel, but too high of water content can cause excessive swelling, weakening the mechanical properties of the hydrogel.{{Cite journal |last1=Xu |first1=Shuai |last2=Zhou |first2=Zidi |last3=Liu |first3=Zishun |last4=Sharma |first4=Pradeep |date=2023-01-06 |title=Concurrent stiffening and softening in hydrogels under dehydration |journal=Science Advances |language=en |volume=9 |issue=1 |pages=eade3240 |doi=10.1126/sciadv.ade3240 |issn=2375-2548 |pmc=9812377 |pmid=36598986}}{{Cite journal |last1=Kessler |first1=Michael |last2=Yuan |first2=Tianyu |last3=Kolinski |first3=John M. |last4=Amstad |first4=Esther |date=2023-02-21 |title=Influence of the Degree of Swelling on the Stiffness and Toughness of Microgel-Reinforced Hydrogels |url=https://onlinelibrary.wiley.com/doi/10.1002/marc.202200864 |journal=Macromolecular Rapid Communications |language=en |volume=44 |issue=16 |doi=10.1002/marc.202200864 |pmid=36809684 |issn=1022-1336 |via=Wiley Online Library}} [75] => [[File:R. V. Lapshin, Model of hysteresis loop, Fig.16.png|thumb|Model of Hysteresis Loop]] [76] => The [[hysteresis]] of a hydrogel refers to the phenomenon where there is a delay in the deformation and recovery of a hydrogel when it is subjected to mechanical stress and relieved of that stress. This occurs because the polymer chains within a hydrogel rearrange, and the water molecules are displaced, and energy is stored as it deforms in mechanical extension or compression.{{Cite journal |last1=Bai |first1=Ruobing |last2=Yang |first2=Jiawei |last3=Morelle |first3=Xavier P. |last4=Yang |first4=Canhui |last5=Suo |first5=Zhigang |date=2018-03-20 |title=Fatigue Fracture of Self-Recovery Hydrogels |url=https://pubs.acs.org/doi/10.1021/acsmacrolett.8b00045 |journal=ACS Macro Letters |language=en |volume=7 |issue=3 |pages=312–317 |doi=10.1021/acsmacrolett.8b00045 |pmid=35632906 |issn=2161-1653}} When the mechanical stress is removed, the hydrogel begins to recover its original shape, but there may be a delay in the recovery process due to factors like viscoelasticity, internal friction, etc.{{Cite journal |last1=Zhu |first1=Ruixin |last2=Zhu |first2=Dandan |last3=Zheng |first3=Zhen |last4=Wang |first4=Xinling |date=2024-02-13 |title=Tough double network hydrogels with rapid self-reinforcement and low hysteresis based on highly entangled networks |journal=Nature Communications |language=en |volume=15 |issue=1 |pages=1344 |doi=10.1038/s41467-024-45485-8 |pmid=38350981 |pmc=10864390 |issn=2041-1723}} This leads to a difference between the stress-strain curve during loading and unloading. Hysteresis within a hydrogel is influenced by several factors including composition, crosslink density, polymer chain structure, and [[temperature]]. [77] => [78] => The toughness and hysteresis of a hydrogel are especially important in the context of biomedical applications such as [[tissue engineering]] and [[drug delivery]], as the hydrogel may need to withstand mechanical forces within the body, but also maintain mechanical performance and stability over time.{{Cite journal |last1=Zhang |first1=Guogao |last2=Steck |first2=Jason |last3=Kim |first3=Junsoo |last4=Ahn |first4=Christine Heera |last5=Suo |first5=Zhigang |date=2023-06-30 |title=Hydrogels of arrested phase separation simultaneously achieve high strength and low hysteresis |journal=Science Advances |language=en |volume=9 |issue=26 |pages=eadh7742 |doi=10.1126/sciadv.adh7742 |issn=2375-2548 |pmc=10313164 |pmid=37390216}} Most typical hydrogels, both natural and synthetic, have a positive correlation between toughness and hysteresis, meaning that the higher the toughness, the longer the hydrogel takes to recover its original shape and vice versa. This is largely due to sacrificial bonds being the source of toughness within many of these hydrogels. Sacrificial bonds are non-covalent interactions such as [[Hydrogen bond|hydrogen bonds]], [[Ionic bonding|ionic interactions]], and [[Hydrophobic effect|hydrophobic interactions]], that can break and reform under mechanical stress.{{Cite book |url=https://onlinelibrary.wiley.com/doi/book/10.1002/9783527815562 |title=Macromolecular Engineering: From Precise Synthesis to Macroscopic Materials and Applications |date=2022-03-07 |publisher=Wiley |isbn=978-3-527-34455-0 |editor-last=Hadjichristidis |editor-first=Nikos |edition=1 |language=en |doi=10.1002/9783527815562.mme0043 |editor-last2=Gnanou |editor-first2=Yves |editor-last3=Matyjaszewski |editor-first3=Krzysztof |editor-last4=Muthukumar |editor-first4=Murugappan}} The reforming of these bonds takes time, especially when there are more of them, which leads to an increase in hysteresis. However, there is currently research focused on the development of highly entangled hydrogels, which instead rely on the long chain length of the polymers and their entanglement to limit the deformation of the hydrogel, thereby increasing the toughness without increasing hysteresis as there is no need for the reformation of the bonds. [79] => [80] => ===Environmental response=== [81] => The most commonly seen environmental sensitivity in hydrogels is a response to temperature.{{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 }} Many polymers/hydrogels exhibit a temperature dependent phase transition, which can be classified as either an [[upper critical solution temperature]] (UCST) or [[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. 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. [82] => [83] => 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. There are many other stimuli that hydrogels can be responsive to, including: pH, glucose, electrical signals, [[photopolymer|light]], [[pressure-sensitive adhesive|pressure]], ions, [[antigen]]s, and more. [84] => [85] => ===Additives=== [86] => The mechanical properties of hydrogels can be fine-tuned in many ways beginning with attention to their hydrophobic properties.{{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}} 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.{{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 }} Other additives, such as [[nanoparticle]]s and [[microparticle]]s, have been shown to significantly modify the stiffness and gelation temperature of certain hydrogels used in biomedical applications.{{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 }}{{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 }}{{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 }} [87] => [88] => ===Processing techniques=== [89] => 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 [[Electrospinning|electro-spinning]], [[3D printing|3D]]/[[4D printing]], [[self-assembly]], and [[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.{{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 }} [90] => [91] => 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]]", has been applied to poly(vinyl alcohol) hydrogels by adding a [[sodium sulfate]] salt solution.{{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 }} Some of these processing techniques can be used synergistically with each other to yield optimal mechanical properties. [[Directional freezing]] or [[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.{{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 }} 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 [[polydimethylsiloxane]] (PDMS), [[Kevlar]], and [[synthetic rubber]]. The values also surpass the toughness of natural [[tendon]] and [[spider silk]]. [92] => [93] => == Applications == [94] => ===Soft contact lenses=== [95] => [[File:Siliconehydrogel.svg|thumb|Molecular structure of silicone hydrogel used in flexible, oxygen-permeable contact lenses.{{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}}]] [96] => [97] => The dominant material for contact lenses are acrylate-[[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]]. [98] => [99] => ===Research === [100] => [[File:Hydrogel MSC Nanolive.gif|thumb|Human mesenchymal stem cell interacting with 3D hydrogel - imaged with label-free live cell imaging]] [101] => [[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.]] [102] => [103] => * Coatings for gas evolution reaction electrodes for efficient bubble detachment {{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 }}{{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 |bibcode=2022AdEnM..1201452B }}{{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}} [104] => * [[Breast implant]]s [105] => * [[Contact lens]]es ([[silicone]] hydrogels, [[polyacrylamide]]s, [[polymacon]]) [106] => * Water sustainability: Hydrogels have emerged as promising materials platforms for solar-powered water purification,{{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 }} water disinfection,{{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 }} and [[Atmospheric water generator]].{{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 }} [107] => * Disposable [[diapers]] where they absorb [[urine]], or in [[sanitary napkin]]s [108] => * Dressings for healing of [[burn (injury)|burn]] or other hard-to-heal [[wound]]s. [[Wound gel]]s are excellent for helping to create or maintain a moist environment. [109] => * [[Electroencephalography#Method|EEG]] and [[Precordial leads|ECG]] medical electrodes using hydrogels composed of [[cross-link]]ed polymers ([[polyethylene oxide]], [[polyAMPS]] and [[polyvinylpyrrolidone]]) [110] => * [[Hydrogel Encapsulation of Quantum Dots|Encapsulation of quantum dots]] [111] => * 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.{{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 }}{{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 }}{{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 }} [112] => *[[Hydrogel fiber|Fibers]] [113] => * [[Adhesive|Glue]] [114] => * Granules for holding [[soil]] moisture in arid areas [115] => * Air bubble-repellent (superaerophobicity). Can improve the performance and stability of electrodes for water [[electrolysis]].{{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}} [116] => * Culturing cells: Hydrogel-coated wells have been used for cell culture.{{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 }} [117] => * [[Biosensors]]: Hydrogels that are responsive to specific molecules,{{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/ }} such as glucose or antigens, can be used as biosensors, as well as in DDS.{{cite journal | vauthors = Yetisen AK, [[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 }} [118] => *Cell carrier: Injectable hydrogels can be used to carry drugs or cells for applications in tissue regeneration or [[3D bioprinting]].{{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 }}{{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 }}{{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 }} Hydrogels with reversible chemistry are required to allow for fluidization during injection/printing followed by [[Self-healing material|self-healing]] of the original hydrogel structure.{{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 }} [119] => *Investigate cell biomechanical functions combined with [[holotomography]] microscopy [120] => * Provide absorption, desloughing and debriding of necrotic and fibrotic tissue [121] => * [[Tissue engineering]] scaffolds. When used as scaffolds, hydrogels may contain human cells to repair tissue. They mimic 3D microenvironment of cells.{{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}} Materials include [[agarose]], [[methylcellulose]], [[hyaluronan]], [[elastin-like polypeptides]], and other naturally derived polymers. [122] => * Sustained-release [[drug delivery]] systems. Ionic strength, pH and temperature can be used as a triggering factor to control the release of the drug.{{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}} [123] => * The swelling behavior exhibited by charged hydrogels can be used as a valuable tool for investigating interactions between charged [[polymer]]s and various species, including multivalent ions, [[peptide]]s, and [[protein]]s.{{Cite journal |last1=Nilsson |first1=Peter |last2=Hansson |first2=Per |date=2005-12-01 |title=Ion-Exchange Controls the Kinetics of Deswelling of Polyelectrolyte Microgels in Solutions of Oppositely Charged Surfactant |url=https://pubs.acs.org/doi/10.1021/jp054835d |journal=The Journal of Physical Chemistry B |language=en |volume=109 |issue=50 |pages=23843–23856 |doi=10.1021/jp054835d |pmid=16375370 |issn=1520-6106}} This response arises due to fluctuating [[Osmosis|osmotic]] swelling forces resulting from the exchange of counterions within the gel matrix. Particularly significant is its application in assessing the binding of peptide drugs to biopolymers within the body, as the swelling response of the gel can provide insights into these interactions.{{Cite journal |last1=Wanselius |first1=Marcus |last2=Rodler |first2=Agnes |last3=Searle |first3=Sean S. |last4=Abrahmsén-Alami |first4=Susanna |last5=Hansson |first5=Per |date=2022-09-15 |title=Responsive Hyaluronic Acid–Ethylacrylamide Microgels Fabricated Using Microfluidics Technique |journal=Gels |volume=8 |issue=9 |pages=588 |doi=10.3390/gels8090588 |doi-access=free |pmid=36135299 |pmc=9498840 |issn=2310-2861}}{{Cite journal |last1=Wanselius |first1=Marcus |last2=Searle |first2=Sean |last3=Rodler |first3=Agnes |last4=Tenje |first4=Maria |last5=Abrahmsén-Alami |first5=Susanna |last6=Hansson |first6=Per |date=June 2022 |title=Microfluidics platform for studies of peptide – polyelectrolyte interaction |url=http://dx.doi.org/10.1016/j.ijpharm.2022.121785 |journal=International Journal of Pharmaceutics |volume=621 |pages=121785 |doi=10.1016/j.ijpharm.2022.121785 |pmid=35500690 |issn=0378-5173}} [124] => * Window coating/replacement: Hydrogels are under consideration for reducing infrared light absorption by 75%.{{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}} Another approach reduced interior temperature using a [[Smart glass|temperature-responsive hydrogel]].{{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}} [125] => * 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.{{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}} [126] => * [[Water Gel Explosives|Water gel explosives]] [127] => * Controlled release of agrochemicals (pesticides and fertilizer) [128] => * [[Talin (protein)|Talin]] Shock Absorbing Materials - protein-based hydrogels that can absorb supersonic impacts{{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}} [129] => [130] => === Biomaterials === [131] => Implanted or injected hydrogels have the potential to support tissue regeneration by mechanical tissue support, localized drug or cell delivery, local cell recruitement or [[immunomodulation]], or encapsulation of nanoparticles for local [[Photothermal therapy|photothermal]] or [[brachytherapy]]. Polymeric drug delivery systems have overcome challenge due to their biodegradability, biocompatibility, and anti-toxicity.{{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 }}{{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 }} Materials such as [[collagen]], chitosan, [[cellulose]], and poly (lactic-co-glycolic acid) have been implemented extensively for drug delivery to organs such as eye,{{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 }} nose, kidneys,{{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 }} lungs,{{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 }} intestines,{{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 }} skin{{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 }} and brain. Future work is focused on reducing toxicity, improving biocompatibility, expanding assembly techniques{{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}} [132] => [133] => Hydrogels have been considered as vehicles for drug delivery.{{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 }} They can also be made to mimic animal mucosal tissues to be used for testing mucoadhesive properties.{{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 }}{{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}} They have been examined for use as reservoirs in [[topical drug delivery]]; particularly ionic drugs, delivered by [[iontophoresis]]. [134] => [135] => == References == [136] => {{CC-notice|cc=by3|url=https://en.wikipedia.org/wiki/User_talk:Minihaa/Impact_of_Lipidation|author= Jessica Hutchinson}} [137] => {{Reflist}} [138] => [139] => == Further reading == [140] => {{refbegin}} [141] => * {{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}} [142] => {{refend}} [143] => [144] => [[Category:Colloidal chemistry]] [145] => [[Category:Gels]] [146] => [[Category:Water chemistry]] [147] => [[Category:Soft matter]] [] => )
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Hydrogel

Hydrogel is a crosslinked polymer network that can absorb and retain large amounts of water or other liquids. It is composed of hydrophilic polymer chains that can swell in the presence of water, but remain solid and gel-like.

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It is composed of hydrophilic polymer chains that can swell in the presence of water, but remain solid and gel-like. Hydrogels have a wide range of applications in various fields, including medicine, agriculture, and environmental science. In medicine, hydrogels are used as drug delivery systems, wound dressings, and scaffolds for tissue engineering. They can release drugs in a controlled manner, providing sustained release and targeted therapy. Hydrogel dressings are popular for wound healing, as they create a moist environment, promote cell growth, and allow for easy removal without causing trauma to the wound. In agriculture, hydrogels are utilized to improve water retention in soil and enhance crop productivity. They can absorb and store water, then release it slowly to plants, reducing irrigation needs and preventing water loss through evaporation. Hydrogel-based superabsorbent polymers are used in agricultural films, seed coatings, and soil amendments. Additionally, hydrogels have applications in environmental science, such as wastewater treatment, water purification, and pollution control. They can absorb and remove contaminants from water, making it suitable for reuse or discharge. Hydrogel-based sensors and adsorbents are also being developed for environmental monitoring and remediation. The synthesis and properties of hydrogels vary depending on the type of polymers used, crosslinking mechanisms, and environmental conditions. Common polymers used in hydrogel production include polyacrylamide, polyvinyl alcohol, and sodium alginate. Crosslinking methods may involve chemical reactions, physical interactions, or radiation. While hydrogels have numerous advantages, they also present challenges. Some hydrogels can shrink or degrade over time, limiting their long-term applications. Researchers are striving to improve the mechanical properties, stability, and biocompatibility of hydrogels, as well as develop bioactive and stimuli-responsive hydrogels for advanced applications. In conclusion, hydrogels are highly versatile materials with a wide range of practical uses. They continue to be a subject of extensive research and development, with the potential to revolutionize various industries and contribute to the advancement of scientific knowledge.

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