06-AutumnAmSciCVR Gecko Feet

8/4/2019 06-AutumnAmSciCVR Gecko Feet

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How Gecko Toes StickThe pow erfu l, fantastic adhesive used by geckos ism ad e o f na no sc ale h airs th at en ga ge tin y fo rces,inspiring envy am ong hum an im ita tors

Kellar Autumn

Geckos can run up a wall or acrossa ceilingwith easebecause oftheirremarkable toes. But gecko toes aren'tsticky in the usual way, like duct tapeor Post-it notes. Instead, geckotoes beara hierarchy of structures that acts to-gether as a smarter adhesive.

The pad of a gecko toe is crossedby ridges covered with hair-like stalkscalledsetae, which branch into hundredsof tiny endings. Gecko toes stick tonearly every material under nearly anyconditions (evenunderwater or in a vac-uum), and neither stay dirty nor stick toone another. Geckos can attach and de-tach their adhesive toes in millisecondswhile running on smooth vertical andinverted surfaces,a feat no conventionaladhesive can match. And unlike stickypressure-sensitive adhesives, gecko toesdon't degrade, foul or attach accidental-ly to the wrong spot. MycolleaguesandI have been studying these remarkableanimals for over a decade, and these aresome ofour latest results.Sticky FingersThe ability ofgeckos to stick to surfaceshas attracted scientificscrutiny since thetime of Aristotle, but the microscopicsetae ongecko toepads were onlydocu-Ke ll ar Au tumn is a n a ss oc ia te p ro fe ss or o f b io lo gya t L ew is & C la rk C olle ge i n P or tla nd , O re go n.After receivin g his Ph .D. in 1995 from th e Un i-v er si ty o f C a li fo rn ia , B e rk el ey , h e h e ld p o st do ct or alp osition s a t the O ffice o f N aval Resea rch an d theM useu m o f Verteb ra te Zo olo gy at U.c. B erkeley.H is r es ea rc h fo cu se s o n th e p hy sio lo gy , b io me ch an -ic s a nd e vo lu tio n o f a nim al lo co mo tio n. A utu mna nd h is c olle ag ue s h old th e p ate nt fo r sy nth etica dh esiv es in sp ir ed b y g eck o fe et. H e is current lyc olla bo ra tin g w ith e ng in ee rs to d es ig n le gg edr ob ots th at c an ru n u p w alls. A dd re ss: D ep ar t-m en t o f B io lo gy, L ew is & C la rk C o ll eg e , 0 61 5 SWP ala tin e H il l R oa d, P or tla nd , OR 97129. Internet:[email protected]

124 American Scientist, Volume 94

mented in the 1870s.The underside ofa gecko toe typically bears a series ofridges, or scansors, which are coveredwith uniform ranks ofsetae.Bythe early19OOs,cientistsusing light microscopesobserved that the setae themselves hadbranches. It took the development ofelectron microscopy in the 1950sto re-veal hundreds ofsplit ends and flattipscalled spatulae on each seta.

A single seta of the tokay gecko(Gekko gecko) is roughly 110 microm-eters long and 4.2 micrometers wide.Each of a seta's branches ends in athin, triangular spatula connected atits apex. The end is about 0.2microm-eter long and 0.2micrometer wide.

Although the tokay gecko is thebest studied (and one of the biggest)gecko species, more than a thousandspecies of geckos encompass a varietyof sizes and shapes of spatulae, setae,scansors and toes. Some geckos evenhave setae on their tails. Remarkably,similar structures have evolved inde-pendently in certain iguanian lizards(genus Anolis) and scincid lizards (ge-nus Prasinohaema).

In the laboratory, a tokay's two frontfeet with a pad area of 227 square mil-limeters (smaller than a dime) wereable to withstand 20.1newtons (about4.5pounds) offorceparallel to the sur-face, according to the work of DuncanJ . Irschick at TulaneUniversity and hiscolleagues. There are about 14,400se-tae per square millimeter on the footof a tokay gecko. However, in isola-tion, single setae proved to be muchless-and much more-sticky thanpredicted, depending on test condi-tions. The fact that their stickiness canbe so variable led me and my cowork-ers to conclude that control of attach-ment and detachment is mechanicalrather than chemical.

Figure 1. Gecko toe pads operate under per-haps the most severe conditions of any adhe-sive. The underside of a gecko toe is striped

In 2000, I published the results of acollaborative study with Robert J . Fulland Ronald Fearing at the Universityof California, Berkeley, and ThomasW.Kenny at Stanford University. Thestudy used a newly developed micro-sensor to measure the adhesive force(which resists pulling) and shear force
mailto:[email protected]:[email protected]


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Andrew Syred / Science Photo Librarywith ridges covered with rows of microscopic, hair-like stalks, as shown in this colored scanning electron micrograph. The stalks end in hundredsof tips, each just 200nanometers wide, which make intimate contact with the surface. The functional properties of gecko feet are as extraordinaryas their structure, enabling geckos to run up walls and across ceilings with seeming indifference to gravity. Shown at 30x magnification.(which resists sliding) of an isolatedgecko seta. When we first tried tomeasure these forces we kept com-ing up with very small numbers-theresistance to sliding was no more thanwhat we expected from plain friction.Itwasn't until we oriented the setacorrectly that we discovered the im-

portance ofspecificmotions in gettingthe seta to stick. The best mechanicsmimicked the way gecko legs moveduring climbing. Slightlypressing theseta against the surface (what we callanormal preload force)yielded a shearforce of about 40 micronewtons-sixtimes the force predicted by whole-

animal measurements. Combining thepreload with 5 micrometers of rear-ward displacement (drag) gave aneven larger shear forceof200microne-wtons-32 times more than whole-animal measurements and 100 timesmore than the friction of backward-facing spatulae.

www.americanscientist.org 2006 March-April 125
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Figure 2.The structural hierarchy of the gecko adhesive system reveals different features at each scale. At the macro-level, a naked-eye view from behinda vertical glass window shows the tokay gecko (Gekko gecko) navigating that smooth surface with ease. A closer view of the bottom of the foot showsmany ridges crossing each toe. The microstructure of a ridge reveals that it is covered with densely packed projections called setae, which are orderedin a neat, grid-like pattern. Each diamond-shaped structure is the branched end of a cluster of four setae. The fine microstructure of a single gecko setashows individual fibrils of [3-keratin, which comprise the shaft, and extensive branching at the end. The branched filaments form a nano-scale array ofhundreds of flattened tips. (Photographs courtesy ofMark Moffett; electron micrographs courtesy of Stas Gorb and the author.)

sensor

pullperpendiculartosurtaceof sensor

Figure 3. The author used a micro-electromechanical system (MEMS) cantilever to measurethe shear force (parallel to the surface) generated by a single seta (left). A thin wire mea-sured the attachment force of a single seta. The angle between the setal stalk and the wire, a,is critical to determining this force. Arrows show the pulling direction for each experiment.(Photographs courtesy of the author.)


Theoretically, the 6.5 million setaeon a tokay gecko could generate 1,300newtons of shear force-enough tosupport the weight of two medium-sized people-based onmeasurementsfrom single setae. Thesenumbers sug-gest that a gecko is only attaching 3percent of its setae in generating thestrongest force (20newtons) measuredin whole-animal experiments. Evenmore surprising, a 50-gram geckoneeds less than 0.04 percent of its se-tae (attached maximally) to support itsmass (which requires half a newton offorce)on a wall. At first glance, geckofeet seem to be enormously overbuiltby virtue of a safetymargin of at least3,900percent.

In real life the safety factor is prob-ably not this high because the setaecannot all orient in the same direc-tion at the same time. Moreover,manyspatulae are unable to contact thesubstrate on uneven, dusty or flak-ing surfaces, particularly those with


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roughness on the same scale as thespatulae. But the excesscapacity isun-likely to go towaste: Geckoscould useit to withstand tropical storms, resistpredator attack or recover their gripafter a drop.

Indeed, when geckos fall, they canarrest themselves by re-attaching theirtoes to passing leaves orbranches, a re-covery that requires much ofa gecko'sadhesive safety margin. Consider theexample of a 50-gram gecko fallingfrom rest. If the gecko falls10centime-ters before attaching a footto a verticalsurface, then it will be moving at 1.4meters per second (neglecting air re-sistance). If the foot is able to produce5 newtons of friction, the gecko willcome to a stop in 15milliseconds aftersliding 1.1centimeters. Inthis theoreti-cal example, recovering from a mod-est fall of 10 centimeters requires 50percent of the shear capacity of onefoot based on whole animal measure-ments (but still less than 4 percent ofthe theoretical maximum calculatedfrom single setae).

The surprisingly large forces gener-ated by single setae made us wonderhow geckos manage to lift their feetso quickly-in just 15 milliseconds-with no measurable detachmentforces. A few years ago, we observedthat simply increasing the angle be-

tween the setal shaft and the substrateto 30 degrees causes detachment. Asthis angle increases, we think that in-creased stress at the trailing edge ofthe seta causes the bonds between setaand substrate to break. The seta thenreturns to an unloaded default state.Thus, gecko adhesive can be thoughtof as the first known programmableadhesive: Preload and drag steps turnon and modulate stickiness; increas-ing the shaft angle to 30degrees turnsoffstickiness.F oo t F e t i s hAlthough scientists have spent manyyears documenting the setal structuresof geckos, finding out how they stickhas been harder. In1900,Anton HaaseofSchleswig,Germany first suggestedthat geckos stick by intermolecularforces (adhiision). It now appears hewas right, but at least seven possiblemechanisms for gecko adhesion havebeen discussed over the past 175years.Scientists have eliminated glue, fric-tion, suction, electrostatics and micro-interlocking as candidates.

Geckos lack glandular tissue ontheir toes, so sticky secretions wereruled out early in the study of geckoadhesion. The friction hypothesis wasalso dismissed quickly because, bydefinition, friction only acts in shear;

therefore, it cannot in itself explainthe adhesive capabilities of geckos oninverted surfaces.

The hypothesis that toe pads act assuction cups proved harder to dispel,despite abundant evidence. Some ex-perts remained convinced of a suctionmechanism through the late 1800sandearly 1900s,although they had no evi-dence to prove it. Then in 1934,Wolf-Dietrich Dellit of Gotha, Germanypublished a paper that described ex-periments carried out in a vacuum.Thegecko toes remained stuck, strong-ly refuting the suction hypothesis. Butthe idea has been surprisingly tena-cious in the popular literature: Itwasadvocated as recently as 1969 in anarticle in Natural History magazine.

Other scientists proffered electro-staticattraction as a mechanism for ad-hesion. Dellit also dashed this claimbyshowing that geckos could still adhereevenwhen the build-up ofelectrostaticchargewas impossible. (Heconductedthis experiment on a metal surface inair ionized by a stream of x rays.)

Dellit himself came to favor themechanism of microinterlocking toexplain gecko adhesion. The curvedtips of setae could act as micro- ornanoscale hooks that catch on surfaceirregularities-much like organic, mi-croscale Velcro or the crampons on

sliding on sliding not 60attaching suriace off end touching\ \ \ \ ~ 50 -~[ ~ r~ . . . . .J250 O J~ small force, big angle big force, big angleif) C 40s 200

1 1O J~ ' 1 E IIJ:: J l' o .~.150 C IS B I I e f : o ~j 30 .oA.l -0 ~1 1100 C ii It.

O J maximum O J2 force . , ' 1 g > 20 Ii,Q 50 C IS ~]! C ii~ 0 Qj(f)C IS . ... 10 - ' -I-50 small force, small angle big force, small angle-100 00 2 4 6 8 10 0 5 10 15 20time (seconds) perpendicular force at detachment (micronewtons)Figure 4. The author measured the shear force (left) of a single seta by pressing it against a microsensor, then pulling perpendicular to the surfaceof the sensor. These data report the resulting force as a function of time. Inset diagrams show the relative positions of seta and sensor at differentpoints in the experiment, with arrows to indicate the direction of force applied to the seta. The maximum observed force of 200 micronewtons was32 times greater than the predicted value from animal experiments. At right, the author plotted attachment forces exerted by single setae as a func-tion of the angle between the setal shaft and the surface. The results of two different types of experiments are shown: Filled symbols representsetae pulled away from the surface until they released; open symbols indicate setae held at a constant force as the angle increased. Each symbolshape represents a different seta. The data reveal a consistent angle of detachment-about 30degrees=-over the entire range of pulling forces.



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a mountaineer's boot. However, theability of geckos to adhere upside-down on polished glass challengedthis proposition. Microinterlockingcould playa secondary role undersome conditions, but the fact thatgeckos generate large adhesive forceson the molecularly smooth surfaceof a silicon dioxide-coated wafer (afinding that we published a few yearsago) shows that a rough surface isnotnecessary for adhesion.

The hypothesis that geckos adhereby intermolecular forceswas proposedby Rodolfo Ruibal and Valerie Ernstat the University of California, River-side, who first described the spatularstructures at the tips of setae in 1965using electron microscopy. They con-cluded that spatulae were unlikely to

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function like the spikes on climbingboots and postulated that the spatulaelie flat against the substrate-therebyincreasingcontact area-when the setais engaged. Thus, gecko adhesion wasalmost certainly the result of molecu-lar interactions rather than mechanicalinterlocking.

The turning point in the study ofgecko adhesion came with a series ofexperiments by Uwe Hiller at the Uni-versity ofMunster in the late 1960sandearly 1970s.Hiller concluded that thechemical properties of the substrate,rather than its texture, determined thestrength of attachment. These obser-vations provided the first direct evi-dence that intermolecular forces areresponsible for attachment in geckos.Butwhich forces?

I .1 centimeters10 centimeters

+ 9 = 9.8 metersper secondsquared

Figure 5. Geckos can recover from a fall by slapping a foot against a passing leaf or branch.This recovery takes advantage of the large adhesive forces that gecko toes are capable of gen-erating. Consider the example of a 50-gram gecko that falls 10 centimeters before attaching afoot to a nearby leaf. During the fall, the gecko accelerates at 9.8 meters per second squared;at the instant it touches the leaf below, itwill be moving at 1.4 meters per second. If the footproduces 5 newtons of friction, the gecko will come to a sudden stop (0.015 seconds) aftersliding only 1.1centimeters. This arrest uses 50percent of the maximum shear capacity of onefoot based on whole-animal measurements but less than 4 percent of the theoretical maximumcalculated from single setae.


Several forces exist between mol-ecules. Many insects, amphibians andmammals take advantage of inter-molecular capillary forces (which existbetween solids and liquids) to stickto surfaces. And although geckos lacksecretoryglands on their feet,capillaryadhesion could still exist because wa-ter molecules are commonly presenton polar, hydrophilic surfaces at ambi-ent humidities. Hiller's observationthat geckos cannot adhere to polytet-rafluoroethylene (PTFE, also knownas Teflon) could be explained by thecapillary hypothesis, since Teflon isstrongly nonpolar and hydrophobic,and would not harbor stray water mol-ecules. Indeed, the inverse correlationbetween adhesive force and hydro-phobicity suggested that the surfacepolarity might determine the strengthof adhesion.

Another possibility is the van derWaals force, named after Dutchman Jo-hannes Diderik van der Waals whowon the 1910 Nobel Prize in phys-ics. The force that carries his name isstrongly dependent on the distancebe-tween surfaces; it also increases withthe polarizability of the two surfaces(in other words, the ease with whichtheir electron clouds are temporarilydistorted) but is not related directly tosurface polarity (the intrinsic, unequalsharing of electrons in a molecule thatgenerates tiny poles of positive andnegative charge). Teflon,for example,is not very polarizable. It is also ex-tremely nonpolar. Thus, the gecko'sinability to cling to Tefloncould be ex-plained either by the van der Waals orcapillary hypotheses.

Hiller's experiments were ground-breaking because they provided thefirst direct evidence foradhesion in thestrictest sense, but the precise natureof the adhesion remained in questionuntil recently. In 2002, I published apaper with Anne M. Peattie, an un-dergraduate student (nowa Ph.D. stu-dent at Ll.C. Berkeley), in which wereanalyzed Hiller's data to test thehypothesis that van der Waals forcesare sufficient for gecko adhesion. Weprimarily focused on hydrophobicityand a theoretical quantity called adhe-sion energy, which takes geometry andsurface chemistry into account whencalculatingattractive forces.Aquantitycalled water contact angle, or 8, is usedto measure hydrophobicity, based onthe premise that the more hydropho-bic a surface is, the more water will


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Figure 6. The strong adhesion between thehydrophobic surface of tokay gecko toes andthe molecularly smooth, hydrophobic surfaceof a wafer of gallium arsenide demonstratesthat the van der Waals force is a sufficientmechanism of gecko adhesion. (Photographcourtesy of the author.)bead up on it. Values of 8greater than90 degrees are considered hydropho-bic. Using an approach developed bymy colleague Jacob Israelachvili at theUniversity of California, Santa Barba-ra, Anne and I linearized the relationbetween hydrophobicity and adhe-sion energy and showed that adhesiveforces do not increase on hydrophilicsurfaces, consistent with the van derWaalshypothesis.

Sincethat time,my labhas made sev-eral direct tests to determine whethercapillary adhesion or van der Waalsforce is sufficient to explain adhe-sion in geckos. In measuring the hy-drophobicity of the setal surface, wediscovered that tokay gecko setae areultrahydrophobic (8 = 160.9 degrees),probably because of the hydropho-bic amino acids that make up the ~-keratin protein of which they are con-

structed.Thisproperty suggests that se-tae interact primarily via van der Waalsforceswhether water ispresent ornot.

In a collaboration with Bob Full,Ron Fearing, Tom Kenny and JacobIsraelachvili, we also measured adhe-sion and friction of setae on two po-larizable surfaces: gallium arsenide,GaAs, which is strongly hydropho-bic, and silicon dioxide, Si02, whichis hydrophilic. If capillary adhesionwas the dominant force, then geckotoes should stick to the hydrophilicSi02 but not to the hydrophobic GaAs.However, ifvan der Waals forcesweresufficient, then geckos should be ableto adhere to both GaAs and Si02 sur-faces.Weobserved the latter tobe true.Using live geckos,we found no signifi-cant differencebetween shear stress onGaAs (8= 110 degrees) and Si02 (8= 0degrees). In separate experiments witha single gecko seta, the adhesion onSi02 differed by only 2 percent fromadhesion on a hydrophobic sensormade ofsilicon alone.

Because the van der Waals force isthe only mechanism that can causetwo hydrophobic surfaces to adherein air, the experiments with GaAs andsilicon provide direct evidence thatvan der Waals force is sufficient foradhesion in gecko setae. Water-basedcapillary forces are not required. Setaladhesion is strong on polar and non-polar surfaces, perhaps because setaeare so hydrophobic themselves, andcertainly because of the very largecontact area of the spatular nanoarray.Gecko setae thus have the propertyof material independence: They can

prediction hydrophilic hydrophobica capillaryadhesionprediction b

adhere strongly to a wide range ofmaterials with little regard for surfacechemistry.

This independence does not pre-clude an effect of water on gecko ad-hesion under some conditions. Waterprobably alters contact geometry andadhesion energies between hydropho-bic and hydrophilic surfaces (for ex-ample, between spatulae and glass).But it is exceedingly difficult to predictwhat the effect of water would be be-cause the system is so complex.

Last year a group of scientists ledby Pavel Neuzil from the Institute ofBioengineering and Nanotechnologyin Singapore published results show-ing that gecko spatulae adhered morestrongly in a wet atmosphere than ina dry one. The authors concluded thatcapillary adhesion must dominate un-der most conditions. However, thisinterpretation is difficult to reconcilewith the extreme hydrophobicity ofgecko setae.

New results from the research teamheaded by Eduard Arzt at the MaxPlanck Institute in Stuttgart resolvethis dilemma. They found strongadhesion at very low humidity and,like the Singapore group, measuredincreased adhesion with increasinghumidity. However, they found thatcapillary bridges, which mediatecapillary forces, do not form. Even atvery high humidity, no more than twolayers of water molecules were pres-ent-far too few to form a capillarybridge. Thus, Arzt's group rejected"true" capillary forces under all con-ditions and surmised that the water

test IvanderWaals

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Si02 SiFigure 7. The difference between polar surfaces and polarizable surfaces can be used to test the capillary and van der Waals hypotheses ofgecko adhesion. For highly polarizable surfaces such as gallium arsenide (GaAs) and silicon dioxide (Si02), the capillary hypothesis (a) pre-dicts that geckos will adhere strongly to the hydrophilic (polar) Si02 but not the hydrophobic (nonpolar) GaAs. The van der Waals hypothesis(b) predicts that the adhesive forces will be similarly large for both. Experiments that tested the adhesive force with whole animals on GaAsand Si02 surfaces (c)and with single setae on Si02 and silicon micro sensors (d) showed comparable adhesion forces for both types of surfaces.These data match the predictions of the van der Waals hypothesis.



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effectivemodulus ~-keratinofgecko setae (bulkmodulus)

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1kilopascal109 1010 1011 101205 106 107 108

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Figure 8.Young's modulus is a quantity that describes how deformable or rigid a substance is. Although the bulk modulus (arelated quantity)for I3-keratin is around 2 gigapascals, the effective modulus for setal arrays is orders of magnitude lower-near 100 kilopascals-which is theupper limit for "tack" or stickiness. This value probably represents a compromise between strong, rapid adhesion and the avoidance of spon-taneous or inappropriate attachment.molecules must increase the numberof van der Waals bonds that are made.The data support our conclusion thatgeckos stick by van der Waals forcesalone-even at high humidity.Nonstick SurfaceParadoxical as it may seem, there isgrowing evidence that gecko setae arethemselves strongly anti-adhesive. Se-tae do not stick spontaneously to sur-faces, but instead require a mechanicalprogram for attachment. And unlikeadhesive tapes, gecko setae do not self-adhere: Pushing the setal surfaces of agecko's feet together does not causethem to stick. Furthermore, gecko se-tae do not seem to stay dirty.

How is it that sticky gecko feet re-main quite clean around everydaycontaminants such as sand, dust, leaflitter, pollen and plant waxes? Insects,which face similar challenges, must

1 megapascal

restore soiled adhesive pads to normalfunction by spending much of theirtime grooming. By contrast, geckos donot groom their feet. Although someplant and animal surfaces self-clean(with water droplets), no self-cleaningadhesive had ever been shown untilwe documented it in geckos in a paperpublished last year.

With undergraduate Wendy Han-sen (who now is also working towardsa Ph.D. at U'.C. Berkeley), I studiedthe phenomenon of self-cleaning byapplying 2.5-micrometer-radius mi-crospheres to the feet of tokay geckos.We found that the microspheres diddiminish a gecko's adhesive capaci-ties, but the animal needed only foursimulated steps on a clean glass sur-face to recover enough setal functionto support its body weight by a singletoe. The debris stuck to the glass, notthe setae.

Figure 9. The author tested the self-cleaning properties of gecko setae by dusting gecko feetwith 2.5-micrometer-radius microspheres. The images from a scanning electron microscopeshow setal arrays after the initial dirtying with microspheres (a). The red arrow indicates a mi-crosphere adhering to several spatulae. After five simulated steps on clean glass, the spatulaehave shed most of the debris (b). The graph at far right shows the average shear stress exertedby clean, dirty and self-cleaned gecko digits. The dotted line indicates the minimum forcerequired to support one gecko's body weight (43grams) by a single toe (an area of 0.19squarecentimeters). (Micrographs courtesy of the author.)


The key to this phenomenon seemsto be adhesion energy. Wendy and I de-veloped mathematical models of self-cleaning that suggested that in order toshed debris, the adhesion energy of allspatulae adhering to a dirt particle mustbe equal to or less than the adhesionenergy between the same particle andthe surface. Perhaps this iswhy spatulaehave to be made of a hydrophobic, anti-adhesive material: Although the adhe-sion energy of each spatula is low, theadhesion of the array as a whole is high-er by maximizing the number of un-contaminated spatulas. If the adhesionenergy were higher (perhaps because ofpolar forces or hydrogen bonding), thenthe self-cleaning and anti-self-adher-ing properties would probably be lost.Thus, the evolution of extremely stickytoes that are also self-cleaning probablyrepresents a sweet spot in the designspace for adhesive nanostructures.

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The shape ofa seta alsohelps it resistindiscriminate sticking. In their restingstate, setal stalks are curved in towardthe body and the spatulae are disorga-nized. In a paper soon to be published,Wendy and I hypothesize that this ar-rangement explains why the default,unloaded state for gecko toes is notsticky. When the foot is planted, thesetae flatten and their tips point out,thereby bringing the spatulae uniform-ly flush with the substrate and maxi-mizing their area ofcontact-and adhe-sion. We conclude that gecko setae areprobably nonsticky by default becauseonly a small contact fraction is possibleunless the setal array is mechanicallydeformed by a preload force.

The deformation ofa substance isdic-tated by its stiffness or elasticity,whichis reflected in a quantity called Young'smodulus, measured in pascals (newtonsper square meter). High values corre-spond to extremely rigid materials suchas diamond (1012 pascals or 1 terapas-cal); fat cells have some of the lowestvalues (100pascals). Bulk ~-keratin isfairly hard, with a Young's modulusranging from 1.3 to 2.5 gigapascals inbird claws and feathers (the values for~-keratin in lizards remain unknown).

Bycontrast, a pressure-sensitive ad-hesive, like that used in masking tape,is made from a soft, viscoelastic ma-terial that is tacky-it spontaneouslydeforms to increase the area of surfacecontact and has a Young's modulusof below 100 kilopascals at 1 hertz,according to the so-called Dahlquistcriterion. (Carl A. Dahlquist was a pi-oneering adhesives scientist at 3M.)Such adhesives can be attached anddetached repeatedly without leavinga residue because they work primarilythrough weak intermolecular forces.However, they are prone to creep, deg-radation, self-adhesion and fouling.

Structures made of ~-keratin---suchas gecko setae---should be too stiff towork likea pressure-sensitive adhesive.How can setae function as an adhesiveif they are made of something so rig-id? The answer lies in the micro- andnanostructure ofthe seta, according to amathematical model developed in RonFearing's laboratory. His model repre-sents setae as tiny cantilever beams thatact as springs with an effective Young'smodulus much lower than the gigapas-cal-hard ~-keratin they are made of.Themost recent experiments from my labobserved an effectivemodulus ofabout100 kilopascals in isolated arrays fromwww.americanscientist.org

tokay geckos-remarkably close to theupper limit of the Dahlquist criterion.The unique hierarchy of structures onthe gecko toe results in a low effectivemodulus, which causes gecko adhesiveto have some of the same properties asproperly tacky materials without thedrawbacks. Thecombination ofstrength(at the level of the keratin protein) andease of deformation (at the level of thespatulae and setae) may enable geckoadhesive to tolerate heavy, repeated usewithout creep or degradation. And be-cause setae have a nonsticky defaultstate and require mechanical deforma-tion in order to adhere, they don't stickto each other orbecome fouled. The ad-hesion of gecko setae isprogrammable,direction-dependent and possesses abuilt-in release mechanism.The Gecko MuseWith such remarkable properties, it isunsurprising that materials scientistsare trying to create artificial, geckolikeadhesives. Using a nanostructure to cre-ate an adhesive is a novel and bizarreconcept. It is possible that had it notevolved, humans never would haveinvented it. For the booming nanotech-nology industry, such products wouldbe valuable for picking up, moving andaligning ultraminiature circuits, sensorsormotors. Forbigger applications, suchas robots that could explore the wreck-age of a fallen building or the surfaceof another planet, artificial gecko setaewould endow the machines with un-precedented freedom ofmovement. Be-

cause a geckolike nanostructure couldbe applied directly to the surface of aproduct, such adhesives could replacescrews, glues and interlocking tabs inmanufactured goods. More whimsi-cally, they might enable fumble-freefootball gloves or revolutionary rock-climbing aids. (This last idea is notnew. Shivaji, a legendary Hindu rulerof 17th-century India, reportedly usedadhesive lizards as grappling devices toscale a sheer cliffand mount a surpriseattack onhis enemies.)

Several groups of scientists havemade good progress toward fabricatingsynthetic spatulae in the years since ourteam published the first such effort in2002.However, by gecko standards, to-day's best synthetic setae are stillprimi-tive. Two materials, one by Andre K.Geim and colleagues at the Universityof Manchester, the other by Michael T.Northen and Kimberly L. Turner at theUniversity ofCalifornia, Santa Barbara,have adhesion coefficients (a ratio ofadhesive force to preload force)that areabout half a percent and one percent,respectively, of real gecko setae. In late2005,Ali Dhinojwala and others fromthe University ofAkron and RensselaerPolytechnic Institute published theirdescription of a carbon-nanotube car-pet that generated adhesive force evengreater than that of gecko setae. How-ever, the product only works at a nano-meter scale, rather than the centimeterscale of real gecko toes. Clearly, betterdesigns will require deeper explorationof real gecko setae. And as technology

Figure 10. Some artificial nanoscale adhesives closely match the length and thickness ofbiological counterparts. The setae of the knight anole (Anolis equestris, left) are unbranchedwith spatular tips. The polyimide fiber array developed by the author and his collaborators(right) has similar proportions. (Anole micrograph courtesy of Anne Peattie. Polyimide fibermicrograph courtesy of Ronald Fearing.)

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and the science of gecko adhesion ad-vance, itmay even become possible totune the design to create completelynew properties.

Many questions remain for scien-tists who study mechanisms of geckoadhesion. What is the effect of surfaceroughness on friction and adhesion?How can scientists better model thehierarchical contributions of spatulae,setae, scansors, toes and legs? How dospatulae and setae work in more than athousand other geckospecies(assumingthey don't go extinct before scientistscan study them)?What is the molecularstructure of setae? Answers to thesebasic biological questions are key to thedevelopment of bio-inspired adhesivesthat may someday rival their naturalcounterparts. Then maybe we will beable to scamper acrossthe ceiling too.ReferencesArzt, E., S. Gorb and R Spolenak. 2003. From

micro to nano contacts in biological attach-ment devices. Proceedings of theNational Acad-emy of Sciences of the U.S.A. 100:10603-1 0606.

Autumn, K, and W.Hansen. 2006. Ultrahydro-phobicity indicates a nonadhesive defaultstate in gecko setae. Journal of ComparativePhysiology A-Sensory Neural & BehavioralPhysiology. (in press).

Autumn, K, and A. Peattie. 2002. Mechanismsof adhesion in geckos. Integrative and Com-parative Biology 42:1081-1 090.

Autumn, K, Y. A. Liang, S.T.Hsieh, W. Zesch,W.-P. Chan, W. T. Kenny, R. Fearing andR J. Full. 2000. Adhesive force of a singlegecko foot-hair. Nature 405:681-685.

Autumn, K, M. Sitti, A. Peattie, W. Hansen, S.Sponberg, Y. A. Liang, T.Kenny, R Fearing,J.Israelachvili and R. J.Full. 2002. Evidencefor van der Waals adhesion in gecko setae.Proceedings of the National Academy of Sci-ences of the U.S.A. 99:12252-12256.

Autumn, K, S. T.Hsieh, D. M. Dudek, J. Chen,C. Chitaphan and R. J. Full. 2006. Dynam-ics of geckos running vertically. Journal ofExperimental Biology 209: 260-272.

Dellit, W.-D. 1934. Zur Anatomie und Physi-ologie der Geckozehe. Jenaische Zeitschriftfur Naturwissenschaft 68:613-656.

Gao, H. J., x. Wang, H. M. Yao, S. Gorb and E.Arzt. 2005. Mechanics of hierarchical adhe-sion structures of geckos. Mechanics ofMate-rials 37:275-285.

Geim, A. K., S. V. Dubonos, I.V. Grigorieva,K S. Novoselov and A. A. Zhukov. 2003.Microfabricated adhesive mimicking geckofoot-hair. Nature Materials 2:461-463.

Gennaro, J. G. J. 1969. The gecko grip. NaturalHistory 78:36-43.Haase, A. 1900. Untersuchungen tiber den Bauund die Entwicklung der Haftlappen beiden Geckotiden. Archiv fur Naturgeschichte66:321-345.

Hansen, W., and Autumn K 2005. Evidence forself-cleaning in gecko setae. Proceedings ofthe National Academy of Sciences of the U.S.A.102:385-389.

Hiller, U. 1975. Comparative studies on thefunctional morphology of two gekkonidlizards. Journal of the Bombay Natural HistorySociety 73:278-282.

Huber, G., H. Mantz, R Spolenak, K Mecke,K Jacobs, S. N. Gorb and E.Arzt. 2005. Evi-dence for capillarity contributions to geckoadhesion from single spatula nanomechani-cal measurements. Proceedings of theNationalAcademy of Sciences of the U.S.A.102:16293-16296.

Irschick, D. J.,c. C. Austin, K Petren, R Fisher,J. B. Losos and O. Ellers. 1996. A compara-tive analysis of clinging ability among pad-bearing lizards. Biological Journal of the Lin-nean Society 59:21-35.

Northen, M. T., and K L.Turner. 2005. A batchof fabricated dry adhesive. Nanotechnology16:1159-1166.

Ruibal, R, and V. Ernst. 1965. The structure ofthe digital setae of lizards. Journal of Mor-phology 117:271-294.

Sun, W., P. Neuzil, T. S. Kustandi, S. Oh andV.D. Samper. 2005. The nature of the geckolizard adhesive force. Biophysical Journal 89:L14-L17.

Yurdumakan, B., N. R Raravikar, P.M. Ajayanand A. Dhinojwala. 2005. Synthetic geckofoot-hairs from multiwalled carbon nano-tubes. Chemical Communications 30:3799-3801.

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