Editing Gecko (section)

===Adhesion ability===
{{See also|Synthetic setae|Gecko feet}}
[[File:Gecko foot on glass.JPG|thumb|Close-up of the underside of a gecko's foot as it walks on vertical glass|left]]
About 60% of gecko species have adhesive toepads which allow them to [[Adhesion|adhere]] to most surfaces without the use of liquids or [[surface tension]]. Such pads have been gained and lost repeatedly over the course of gecko evolution.<ref name="plosone.org">{{Cite journal|last1=Gamble|first1=Tony|last2=Greenbaum|first2=Eli|last3=Jackman|first3=Todd R.|last4=Russell|first4=Anthony P.|last5=Bauer|first5=Aaron M.|date=27 June 2012|title=Repeated Origin and Loss of Adhesive Toepads in Geckos|journal=PLOS ONE|volume=7|issue=6|pages=e39429|doi=10.1371/journal.pone.0039429|pmid=22761794|pmc=3384654|bibcode=2012PLoSO...739429G|doi-access=free}}</ref> Adhesive toepads evolved independently in about eleven different gecko lineages, and were lost in at least nine lineages.<ref name="plosone.org"/>

It was previously thought that the spatula-shaped [[seta]]e arranged in [[Lamella (anatomy)|lamellae]] on gecko footpads enable attractive [[van der Waals' force]]s (the weakest of the weak chemical forces) between the [[Beta-keratin|β-keratin]] lamellae / setae / spatulae structures and the surface.<ref>{{Cite web|url=http://www.nisenet.org/scientific-images/gecko_toe|title=Scientific image – gecko toe|website=www.NISEnet.org|publisher=NISE Network|access-date=2022-02-01|archive-date=2013-05-09|archive-url=https://web.archive.org/web/20130509025409/http://www.nisenet.org/scientific-images/gecko_toe|url-status=live}}</ref><ref name=Santos2007>{{Cite journal|last1=Santos|first1=Daniel|first2=Matthew|last2=Spenko|first3=Aaron|last3=Parness|first4=Kim|last4=Sangbae|first5=Mark|last5=Cutkosky|date=2007|url=http://www.brill.nl/journal-adhesion-science-and-technology|title=Directional adhesion for climbing: Theoretical and practical considerations|journal=[[Journal of Adhesion Science and Technology]]|volume=21|issue=12–13|pages=1317–1341|quote=''Gecko "feet and toes are a hierarchical system of complex structures consisting of lamellae, setae, and spatulae. The distinguishing characteristics of the gecko adhesion system have been described [as] (1) anisotropic attachment, (2) high pulloff force to preload ratio, (3) low detachment force, (4) material independence, (5) self-cleaning, (6) antiself sticking and (7) nonsticky default state. ... The gecko's adhesive structures are made from ß-keratin (modulus of elasticity [about] 2 GPa). Such a stiff material is not inherently sticky; however, because of the gecko adhesive's hierarchical nature and extremely small distal features (spatulae are [about] 200 nm in size), the gecko's foot is able to intimately conform to the surface and generate significant attraction using [[van der Waals force]]s.''|doi=10.1163/156856107782328399|s2cid=53470787|access-date=2012-02-04|archive-date=2012-01-15|archive-url=https://web.archive.org/web/20120115143737/http://www.brill.nl/journal-adhesion-science-and-technology|url-status=live|url-access=subscription}}</ref> These van der Waals interactions involve no fluids; in theory, a boot made of [[synthetic setae]] would adhere as easily to the surface of the [[International Space Station]] as it would to a living-room wall, although adhesion varies with humidity.<ref name="Puthoff"/><ref name="Prowse"/> However, a 2014 study suggests that gecko adhesion is in fact mainly determined by electrostatic interaction (caused by contact electrification), not van der Waals or capillary forces.<ref name=RSI-2014>{{Cite journal|last1=Izadi|first1=H.|last2=Stewart|first2=K.M.E.|last3=Penlidis|first3=A.|date=9 July 2014|title=Role of contact electrification and electrostatic interactions in gecko adhesion|journal=[[Journal of the Royal Society Interface]]|volume=11|issue=98|pages=20140371|doi=10.1098/rsif.2014.0371|pmid=25008078|pmc=4233685|quote=We have demonstrated that it is the CE-driven electrostatic interactions which dictate the strength of gecko adhesion, and not the van der Waals or capillary forces which are conventionally considered as the main source of gecko adhesion.}}</ref>

The setae on the feet of geckos are also self-cleaning, and usually remove any clogging dirt within a few steps.<ref name="Hansen"/><ref>{{Cite web|url=http://www.lclark.edu/~autumn/dept/geckostory.html|title=How geckos stick to walls|website=www.lclark.edu|access-date=2007-09-22|archive-date=2007-09-25|archive-url=https://web.archive.org/web/20070925041306/http://www.lclark.edu/~autumn/dept/geckostory.html|url-status=dead}}</ref><ref name=SpatulaeNature>{{Cite journal|last1=Xu|first1=Quan|last2=Wan|first2=Yiyang|last3=Hu|first3=Travis Shihao|last4=Liu|first4=Tony X.|last5=Tao|first5=Dashuai|last6=Niewiarowski|first6=Peter H.|last7=Tian|first7=Yu|last8=Liu|first8=Yue|last9=Dai|first9=Liming|last10=Yang|first10=Yanqing|last11=Xia|first11=Zhenhai|date=20 November 2015|title=Robust self-cleaning and micromanipulation capabilities of gecko spatulae and their bio-mimics|journal=[[Nature Communications]]|volume=6|page=8949|doi=10.1038/ncomms9949|pmid=26584513|pmc=4673831|bibcode=2015NatCo...6.8949X}}</ref> [[Polytetrafluoroethylene]] (PTFE), which has very low surface energy,<ref>{{Cite web|url=http://www.JustAnswer.com/questions/bwl6-feet-gecko-lizard-not-stick|title=Why do the gecko's feet not stick to a teflon surface?|website=www.JustAnswer.com}}{{Unreliable source?|date=February 2022}}</ref> is more difficult for geckos to adhere to than many other surfaces.

Gecko adhesion is typically improved by higher humidity,<ref name="Puthoff">{{Cite journal|last1=Puthoff|first1=J.B.|last2=Prowse|first2=M.|last3=Wilkinson|first3=M.|last4=Autumn|first4=K.|date=2010|title=Changes in materials properties explain the effects of humidity on gecko adhesion|journal=[[Journal of Experimental Biology]]|volume=213|pages=3699–3704|doi=10.1242/jeb.047654|issue=21|pmid=20952618|doi-access=free|bibcode=2010JExpB.213.3699P }}</ref><ref name="Prowse">{{Cite journal|last1=Prowse|first1=M.S.|last2=Wilkinson|first2=Matt|last3=Puthoff|first3=Jonathan B.|last4=Mayer|first4=George|last5=Autumn|first5=Kellar|date=2011|title=Effects of humidity on the mechanical properties of gecko setae|journal=[[Acta Biomaterialia]]|volume=7|pages=733–738|doi=10.1016/j.actbio.2010.09.036|pmid=20920615|issue=2}}</ref><ref name="Huber, G., et al. 2005 16293–6">{{Cite journal|last1=Huber|first1=G.|last2=Mantz|first2=H.|last3=Spolenak|first3=R.|last4=Mecke|first4=K.|last5=Jacobs|first5=K.|last6=Gorb|first6=S.N.|last7=Arzt|first7=E.|date=2005|title=Evidence for capillarity contributions to gecko adhesion from single spatula nanomechanical measurements|journal=[[Proceedings of the National Academy of Sciences]]|volume=102|pages=16293–6|doi=10.1073/pnas.0506328102|pmid=16260737|pmc=1283435|issue=45|bibcode=2005PNAS..10216293H|doi-access=free}}</ref><ref name="Chen, B. 2010 1–9">{{Cite journal|last1=Chen|first1=B.|last2=Gao|first2=H.|date=2010|title=An alternative explanation of the effect of humidity in gecko adhesion: stiffness reduction enhances adhesion on a rough surface|journal=[[International Journal of Applied Mechanics]]|volume=2|issue=1|pages=1–9|doi=10.1142/s1758825110000433|bibcode=2010IJAM....2....1C}}</ref><ref name="Loskill, P. 20120587">{{Cite journal|last1=Loskill|first1=P.|last2=Puthoff|first2=J.|last3=Wilkinson|first3=M.|last4=Mecke|first4=K.|last5=Jacobs|first5=K.|last6=Autumn|first6=K.|date=September 2012|title=Macroscale adhesion of gecko setae reflects nanoscale differences in subsurface composition|journal=[[Journal of the Royal Society Interface]]|volume=10|issue=78|pages=20120587|doi=10.1098/rsif.2012.0587|pmid=22993246|pmc=3565786}}</ref> even on hydrophobic surfaces, yet is reduced under conditions of complete immersion in water. The role of water in that system is under discussion, yet recent experiments agree that the presence of molecular water layers (water molecules carry a very large dipole moment) on the setae, as well as on the surface, increase the surface energy of both, therefore the energy gain in getting these surfaces in contact is enlarged, which results in an increased gecko adhesion force.<ref name="Puthoff"/><ref name="Prowse"/><ref name="Huber, G., et al. 2005 16293–6"/><ref name="Chen, B. 2010 1–9"/><ref name="Loskill, P. 20120587"/> Moreover, the elastic properties of the b-keratin change with water uptake.<ref name="Puthoff"/><ref name="Prowse"/><ref name="Huber, G., et al. 2005 16293–6"/>

Gecko toes seem to be [[double jointed|double-jointed]], but this is a misnomer, and is properly called digital hyperextension.<ref name="Russell, A.P. 1975">{{Cite journal|last=Russell|first=A.P.|date=1975|title=A contribution to the functional analysis of the foot of the Tokay, ''Gekko gecko'' (Reptilia: Gekkonidae)|journal=[[Journal of Zoology]]|volume=176|issue=4|pages=437–476|doi=10.1111/j.1469-7998.1975.tb03215.x}}</ref> Gecko toes can hyperextend in the opposite direction from human fingers and toes. This allows them to overcome the van der Waals force by peeling their toes off surfaces from the tips inward. In essence, by this peeling action, the gecko separates spatula by spatula from the surface, so for each spatula separation, only some force necessary. (The process is similar to removing [[Scotch Tape]] from a surface.)

Gecko toes operate well below their full attractive capabilities most of the time, because the margin for error is great depending upon the [[surface roughness]], and therefore the number of setae in contact with that surface.

Use of small van der Waals force requires very large surface areas; every square millimetre of a gecko's footpad contains about 14,000&nbsp;hair-like setae. Each seta has a diameter of 5&nbsp;[[micrometer (unit)|μm]]. Human hair varies from 18 to 180&nbsp;μm, so the cross-sectional area of a human hair is equivalent to 12 to 1300 setae. Each seta is in turn tipped with between 100 and 1,000 spatulae.<ref name="Hansen">{{Cite journal|last1=Hansen|first1=W.R.|last2=Autumn|first2=K.|date=2005|title=Evidence for self-cleaning in gecko setae|journal=[[Proceedings of the National Academy of Sciences]]|volume=102|issue=2|pages=385–389|doi=10.1073/pnas.0408304102|quote=Setae occur in uniform arrays on overlapping lamellar pads at a density of 14,400 per mm<sup>2</sup>|pmid=15630086|pmc=544316|bibcode=2005PNAS..102..385H|doi-access=free}}</ref> Each spatula is 0.2&nbsp;μm long<ref name=Hansen/> (one five-millionth of a metre), or just below the wavelength of visible light.<ref name="Autumn">{{Cite journal|last1=Autumn|first1=Kellar|last2=Sitti|first2=M.|last3=Liang|first3=Y.A.|last4=Peattie|first4=A.M.|last5=Hansen|first5=W.R.|last6=Sponberg|first6=S.|last7=Kenny|first7=T.W.|last8=Fearing|first8=R.|last9=Israelachvili|first9=J.N.|last10=Full|first10=R.J.|date=2002|title=Evidence for van der Waals adhesion in gecko setae|journal=[[Proceedings of the National Academy of Sciences]]|volume=99|issue=19|pages=12252–12256|doi=10.1073/pnas.192252799|pmid=12198184|pmc=129431|bibcode=2002PNAS...9912252A|doi-access=free}}</ref>

The setae of a typical mature {{Convert|70|g|oz|abbr=off|adj=on|lk=on}} gecko would be capable of supporting a weight of {{Convert|133|kg|lb|abbr=off}}:<ref>{{Cite web|title=Geckos can hang upside down carrying 40kg|url=http://www.physics.org/facts/gecko-really.asp|website=www.Physics.org|access-date=2 November 2012|archive-date=21 May 2008|archive-url=https://web.archive.org/web/20080521015041/http://www.physics.org/facts/gecko-really.asp|url-status=live}}</ref><ref name=Autumn_SciAmer>{{Cite journal|last=Autumn|first=Kellar|date=29 September 2003|url=http://www.scientificamerican.com/article.cfm?id=how-do-gecko-lizards-unst|title=How do gecko lizards unstick themselves as they move across a surface?|journal=[[Scientific American]]|access-date=23 March 2013|archive-date=23 October 2012|archive-url=https://web.archive.org/web/20121023131852/http://www.scientificamerican.com/article.cfm?id=how-do-gecko-lizards-unst|url-status=live}}</ref> each spatula could exert an adhesive force of 5 to 25&nbsp;nN.<ref name="Huber, G., et al. 2005 16293–6"/><ref name="Lee">{{Cite journal|last1=Lee|first1=Haeshin|last2=Lee|first2=Bruce P.|last3=Messersmith|first3=Phillip B.|year=2007|title=A reversible wet / dry adhesive inspired by mussels and geckos|journal=[[Nature (journal)|Nature]]|volume=448|issue=7151|pages=338–341|doi=10.1038/nature05968|pmid=17637666|bibcode=2007Natur.448..338L|s2cid=4407993}}</ref> The exact value of the adhesion force of a spatula varies with the surface energy of the substrate to which it adheres. Recent studies<ref name="Loskill, P. 20120587"/><ref>{{Cite journal|last1=Loskill|first1=P.|last2=Haehl|first2=H.|last3=Grandthyll|first3=S.|last4=Faidt|first4=T.|last5=Mueller|first5=F.|last6=Jacobs|first6=K.|date=November 2012|title=Is adhesion superficial? Silicon wafers as a model system to study van der Waals interactions|journal=[[Advances in Colloid and Interface Science]]|volume=179–182|pages=107–113|doi=10.1016/j.cis.2012.06.006|pmid=22795778|arxiv=1202.6304|s2cid=5406490}}</ref> have moreover shown that the component of the surface energy derived from long-range forces, such as van der Waals forces, depends on the material's structure below the outermost atomic layers (up to 100&nbsp;nm beneath the surface); taking that into account, the adhesive strength can be inferred.

Apart from the [[seta]]e, [[phospholipids]]; fatty substances produced naturally in their bodies, also come into play.<ref name=Hsu_Phospholipids>{{Cite journal|last1=Hsu|first1=P.Y.|last2=Ge|first2=L.|last3=Li|first3=X.|last4=Stark|first4=A.Y.|last5=Wesdemiotis|first5=C.|last6=Niewiarowski|first6=P.H.|last7=Dhinojwala|first7=A.|date=24 August 2011|title=Direct evidence of phospholipids in gecko footprints and spatula-substrate contact interface detected using surface-sensitive spectroscopy|journal=[[Journal of the Royal Society Interface]]|volume=9|issue=69|pages=657–664|doi=10.1098/rsif.2011.0370|pmid=21865250|pmc=3284128}}</ref> These lipids lubricate the setae and allow the gecko to detach its foot before the next step.

The origin of gecko adhesion likely started as simple modifications to the epidermis on the underside of the toes. This was recently discovered in the genus ''[[Gonatodes]]'' from South America.<ref name="Higham, T.E. 2016">{{Cite journal|last1=Higham|first1=T.E.|last2=Gamble|first2=T.|last3=Russell|first3=A.P.|date=2017|title=On the origin of frictional adhesion in geckos: small morphological changes lead to a major biomechanical transition in the genus ''Gonatodes''|journal=[[Biological Journal of the Linnean Society]]|doi=10.1111/bij.12897|volume=120|issue=3|pages=503–517|doi-access=free}}</ref><ref>{{Cite journal|last1=Russell|first1=A.P.|last2=Baskerville|first2=J.|last3=Gamble|first3=T.|last4=Higham|first4=T.|date=November 2015|title=The evolution of digit form in Gonatodes (Gekkota: Sphaerodactylidae) and its bearing on the transition from frictional to adhesive contact in gekkotans|journal=[[Journal of Morphology]]|volume=276|issue=11|pages=1311–1332|pmid=26248497|doi=10.1002/jmor.20420|s2cid=20296012}}</ref> Simple elaborations of the epidermal spinules into setae have enabled ''Gonatodes humeralis'' to climb smooth surfaces and sleep on smooth leaves.

[[Biomimetic]] technologies designed to [[Synthetic setae|mimic gecko adhesion]] could produce reusable self-cleaning dry adhesives with many applications. Development effort is being put into these technologies, but manufacturing synthetic setae is not a trivial material design task.