Adhesion and friction in gecko toe attachment and detachmentKellar Autumn, and Jacob Israelachvili Yu Tian, Noshir Pesika, Hongbo Zeng, Kenny Rosenberg, Boxin Zhao, Patricia McGuiggan,doi:10.1073/pnas.0608841103 2006;103;19320-19325; originally published online Dec 5, 2006; PNAS This information is current as of March 2007. & ServicesOnline Information www.pnas.org/cgi/content/full/103/51/19320etc., can be found at: High-resolution figures, a citation map, links to PubMed and Google Scholar, Supplementary Material www.pnas.org/cgi/content/full/0608841103/DC1Supplementary material can be found at: References www.pnas.org/cgi/content/full/103/51/19320#BIBLThis article cites 14 articles, 8 of which you can access for free at: www.pnas.org/cgi/content/full/103/51/19320#otherarticlesThis article has been cited by other articles: E-mail Alerts. click hereat the top right corner of the article orReceive free email alerts when new articles cite this article - sign up in the box Rights & Permissions www.pnas.org/misc/rightperm.shtmlTo reproduce this article in part (figures, tables) or in entirety, see: Reprints www.pnas.org/misc/reprints.shtmlTo order reprints, see: Notes:Adhesion and friction in gecko toe attachmentand detachmentYu Tian*†, Noshir Pesika*, Hongbo Zeng*, Kenny Rosenberg*, Boxin Zhao*, Patricia McGuiggan*, Kellar Autumn‡,and Jacob Israelachvili*§*Department of Chemical Engineering and California NanoSystems Institute (CNSI), University of California, Santa Barbara, CA 93106;†State Key Labof Tribology, Department of Precision Instruments, Tsinghua University, Beijing 100084, People’s Republic of China; and‡Department of Biology,Lewis and Clark College, Portland, OR 97219Contributed by Jacob Israelachvili, October 5, 2006 (sent for review August 4, 2006)Geckos can run rapidly on walls and ceilings, requiring high frictionforces (on walls) and adhesion forces (on ceilings), with typical stepintervals of ⬇20 ms. The rapid switching between gecko footattachment and detachment is analyzed theoretically based on atape model that incorporates the adhesion and friction forcesoriginating from the van der Waals forces between the submicron-sized spatulae and the substrate, which are controlled by the(macroscopic) actions of the gecko toes. The pulling force of aspatula along its shaft with an angle␪between 0 and 90° to thesubstrate, has a ‘‘normal adhesion force’’ contribution, produced atthe spatula-substrate bifurcation zone, and a ‘‘lateral frictionforce’’ contribution from the part of spatula still in contact with thesubstrate. High net friction and adhesion forces on the wholegecko are obtained by rolling down and gripping the toes inwardto realize small pulling angles␪between the large number ofspatulae in contact with the substrate. To detach, the high adhe-sion兾friction is rapidly reduced to a very low value by rolling thetoes upward and backward, which, mediated by the lever functionof the setal shaft, peels the spatulae off perpendicularly from thesubstrates. By these mechanisms, both the adhesion and frictionforces of geckos can be changed over three orders of magnitude,allowing for the swift attachment and detachment during geckomotion. The results have obvious implications for the fabrication ofdry adhesives and robotic systems inspired by the gecko’s loco-motion mechanism.tape model 兩 pulling angle 兩 lever function 兩 spatula 兩 setaThe extraordinary climbing ability of geckos is considered aremark able design of nature that is attributed to the finestr ucture of its toes, which c ontain set al arrays consisting ofhundreds of spatulae on each set a. These fine str uctures allow forintimate c ontact between the spatulae and surfaces to obtainhigh adhesion and friction forces on almost any surface, whetherhydrophilic or hydrophobic, rough or smooth, through weak vander Waals forces (1–4). The focus of studies (4–26) has been tounderst and the friction and adhesion of geckos with a goal topotentially fabricate dry ‘‘responsive’’ adhesives.The macro-, meso-, micro-, and nanoscale structures thatmake up the hierarchical str ucture of the gecko toe pads havebeen imaged by different microscopy techniques (5). The wholehierarchical structure of a Tokay gecko is shown in Fig. 1 a–f: onebody with four feet, each foot with five toes, each toe with ⬇20rows of sticky lamellae, each lamella with many setal arraysc onsisting of thousands of setae, which amounts to ⬇200,000set ae per toe, and each seta consisting of hundreds to 1,000spatulae at its end.The corresponding forces achieved by the different hierarchi-cal structures are shown in the right of Fig. 1 g–i. The lateralf riction force FLand normal adhesion force Fnresulting fromeach spatula (Fig. 1i) must be vectorially summed over all thespatulae to obtain the net (normal and lateral) force Ftotactingon the entire body of the gecko (Fig. 1g). Because the toes oneach foot, and the feet themselves, all point in different direc-tions, the summation of F(␪) ⫽ [Fnsin␪⫹ FLcos␪] over all angles␪is not a trivial matter, and neither is the calculation (andmeasurement) of the normal and lateral forces Fnand FL, whichare not constant but also␪-dependent. The above equation doesshow, however, that both the (local) adhesion and friction forcestogether determine the net or tot al force Ftot.The hierarchical str uctures with large dimensions, i.e., thefeet, toes, lamella, and setal arrays, are relatively easy toapproach. The kinetics of gecko motion using the friction forcesof their feet have been reported recently (27, 28). The testedf riction force of two front feet is ⬇20.1 N (6). The ‘‘frictionaladhesion’’ of a setal array on a glass surface has also been testedrecently (7). In contrast, it is hard to study the mechanics of asingle seta or a single spatula due to their small dimensions. Thelimited data on set ae or spatulae are as follows. First, themaximum friction force of a single seta (with 100–1,000 spatu-lae) is ⬇200␮N, and the adhesive force is 20–40␮N (2, 8). Amodel of the seta as a cantilever beam has been proposed thatagrees with the measurements on set al arrays (9). Second, Huberand colleagues (10, 20) glued a set a perpendicular to the end ofan atomic force microscope cantilever beam. With only a fewspatulae on the end of the setal shaft, they measured theadhesion force of a single spatula to be ⬇10 nN. Sun et al.