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      Contribution of friction and adhesion to the reliable attachment of a gecko to smooth inclines

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          Geckos’ ability to move on steep surfaces depends on their excellent adhesive structure, timely adjustments on locomotor behaviors, and elaborates control on reaction forces. However, it is still unclear how they can generate a sufficient driving force that is necessary for locomotion, while ensuring reliable adhesion on steep inclines. We measured the forces acting on each foot and recorded the contact states between feet and substrates when geckos encountered smooth inclination challenges ranging from 0° to 180°. The critical angles of the resultant force vectors of the front and hind-feet increased with respect to the incline angles. When the incline angle became greater than 120°, the critical angles of the front- and hind-feet were similar, and the averages of the critical angles of the front- and hind-feet were both smaller than 120°, indicating that the complicated and accurate synergy among toes endows gecko’s foot an obvious characteristic of “frictional adhesion” during locomotion. Additionally, we established a contact mechanical model for gecko’s foot in order to quantify the contribution of the frictional forces generated by the heel, and the adhesion forces generated by the toes on various inclines. The synergy between multiple contact mechanisms (friction or adhesion) is critical for the reliable attachment on an inclined surface, which is impossible to achieve by using a single-contact mechanism, thereby increasing the animal’s ability to adapt to its environment.

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          Frictional adhesion: A new angle on gecko attachment.

          Directional arrays of branched microscopic setae constitute a dry adhesive on the toes of pad-bearing geckos, nature's supreme climbers. Geckos are easily and rapidly able to detach their toes as they climb. There are two known mechanisms of detachment: (1) on the microscale, the seta detaches when the shaft reaches a critical angle with the substrate, and (2) on the macroscale, geckos hyperextend their toes, apparently peeling like tape. This raises the question of how geckos prevent detachment while inverted on the ceiling, where body weight should cause toes to peel and setal angles to increase. Geckos use opposing feet and toes while inverted, possibly to maintain shear forces that prevent detachment of setae or peeling of toes. If detachment occurs by macroscale peeling of toes, the peel angle should monotonically decrease with applied force. In contrast, if adhesive force is limited by microscale detachment of setae at a critical angle, the toe detachment angle should be independent of applied force. We tested the hypothesis that adhesion is increased by shear force in isolated setal arrays and live gecko toes. We also tested the corollary hypotheses that (1) adhesion in toes and arrays is limited as on the microscale by a critical angle, or (2) on the macroscale by adhesive strength as predicted for adhesive tapes. We found that adhesion depended directly on shear force, and was independent of detachment angle. Therefore we reject the hypothesis that gecko toes peel like tape. The linear relation between adhesion and shear force is consistent with a critical angle of release in live gecko toes and isolated setal arrays, and also with our prior observations of single setae. We introduced a new model, frictional adhesion, for gecko pad attachment and compared it to existing models of adhesive contacts. In an analysis of clinging stability of a gecko on an inclined plane each adhesive model predicted a different force control strategy. The frictional adhesion model provides an explanation for the very low detachment forces observed in climbing geckos that does not depend on toe peeling.
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            Adhesion and friction in gecko toe attachment and detachment.

            Geckos can run rapidly on walls and ceilings, requiring high friction forces (on walls) and adhesion forces (on ceilings), with typical step intervals of approximately 20 ms. The rapid switching between gecko foot attachment and detachment is analyzed theoretically based on a tape model that incorporates the adhesion and friction forces originating 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 a spatula along its shaft with an angle between theta 0 and 90 degrees to the substrate, has a "normal adhesion force" contribution, produced at the spatula-substrate bifurcation zone, and a "lateral friction force" contribution from the part of spatula still in contact with the substrate. High net friction and adhesion forces on the whole gecko are obtained by rolling down and gripping the toes inward to realize small pulling angles between the large number of spatulae in contact with the substrate. To detach, the high adhesion/friction is rapidly reduced to a very low value by rolling the toes upward and backward, which, mediated by the lever function of the setal shaft, peels the spatulae off perpendicularly from the substrates. By these mechanisms, both the adhesion and friction forces of geckos can be changed over three orders of magnitude, allowing for the swift attachment and detachment during gecko motion. The results have obvious implications for the fabrication of dry adhesives and robotic systems inspired by the gecko's locomotion mechanism.
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              How animals move: an integrative view.

              Recent advances in integrative studies of locomotion have revealed several general principles. Energy storage and exchange mechanisms discovered in walking and running bipeds apply to multilegged locomotion and even to flying and swimming. Nonpropulsive lateral forces can be sizable, but they may benefit stability, maneuverability, or other criteria that become apparent in natural environments. Locomotor control systems combine rapid mechanical preflexes with multimodal sensory feedback and feedforward commands. Muscles have a surprising variety of functions in locomotion, serving as motors, brakes, springs, and struts. Integrative approaches reveal not only how each component within a locomotor system operates but how they function as a collective whole.

                Author and article information

                Tsinghua Science and Technology
                Tsinghua University Press (Xueyuan Building, Tsinghua University, Beijing 100084, China )
                05 December 2018
                : 06
                : 04
                : 407-419 (pp. )
                [ 1 ] Institute of Bio-inspired Structure and Surface Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China
                [ 2 ] School of Mechanical Engineering, Nantong University, Nantong 226019, China
                Author notes
                * Corresponding authors: Zhouyi WANG, E-mail: wzyxml@
                Zhendong DAI, E-mail: zddai@

                Zhouyi WANG. He obtained his doctor degree in 2015 from College of Mechanical and Electrical Engineering, Nanjing University of Aeronautics and Astronautics (NUAA). He currently works at the College of Astronautics at the NUAA. He was awarded the Shangyin Excellent Mechanical Doctoral Dissertation Award (Bronze) in 2016. His interested research areas include tribology, bionics, and animal kinematics and dynamics.

                Zhendong DAI. Male, professor, and tutor of PhD Students, he obtained his doctor degree in 1999 from College of Mechanical and Electrical Engineering, NUAA. He is one of the Chinese delegates of International Institute of Bionic Engineering, an executive member of the Council of Chinese Mechanical Engineering in Tribology, and a member of the Academic Committee, State Key Laboratory of Solid Lubrication. He also is member of editorial board of many magazines such as Journal of Bionic Engineering, International Journal of Vehicle Autonomous System, Tribology, and so on. His research areas include bionics, light material, control of bionics, bio-robots, and biological robots. He has successively presided and participated in many research projects and has published more than 200 papers and gotten more than 20 patents.


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                Page count
                Figures: 8, Tables: 2, References: 37, Pages: 13
                Research Article


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