1932

Abstract

Geckos possess a superlative climbing adaptation in the form of hierarchical arrays of adhesive nanostructures on the underside of their toes. These structures permit rapid, robust, and reliable adhesion to nearly any substrate during full-speed locomotion. We review the fundamental principles and properties of this system, describe its ecological and evolutionary aspects, and offer our assessment of the field alongside suggestions for future research in this direction.

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2014-11-23
2024-07-23
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    Zoom into the tokay gecko's adhesive system from the macro- to nanoscales. For information on the helium ion scanning electron microscopy technique used in this video, see Yang et al. (2011).

    Video and animation illustrating the mechanical requirements for attachment and detachment of a single isolated gecko seta (Autumn et al. 2000). The video shows a single seta glued to a minutien pin. The vertical bar at the left side is a 25-μm aluminum wire force gauge. Initial attempts to adhere a single isolated seta to a surface failed because we simply touched the tip of the seta into the surface and pulled away vertically. Instead, a slight preload force, followed by a micrometer-scale drag along the direction of curvature of the seta (i.e., toward the rear of the animal) switches the spatulae from their default unloaded state to the adhered state. The seta can now sustain a perpendicular pull because the adhesive van der Waals forces at the spatula tips resist detachment. Detachment occurs when the angle between the setal shaft and the surface exceeds 30°. This experiment illustrates the mechanical program for attachment and detachment required for controllable adhesion in gecko setae.

    Toe peeling (digital hyperextension) during climbing by a tokay gecko. The motion of gecko toes is superficially similar to that of peeling tape. However, because adhesion of gecko toes is governed by the micro-mechanics of their setae, a tape peeling model can be rejected (Autumn et al. 2006a). In contrast to the peeling of tape, gecko toes function by “frictional adhesion”: Pull-off forces increase linearly with shear load and detach when the angle of the resultant force exceeds 30° relative to the surface.

    Sample data and videomicroscopy demonstrating anisotropic frictional adhesion in isolated tokay gecko setal arrays (Autumn et al. 2006a). In each video, the upper section shows in side view an array of ∼10,000 setae taken from one scansor of a toe. A multiaxis sensor measures the forces acting on the setal array. Nanopositioners move a glass substrate through load, drag, and pull steps. The lower left sections of the videos show the time course of shear () and normal () forces. Positive normal forces represent compression, whereas negative normal forces represent adhesion. The sign of shear force is arbitrary and represents sliding to the left or right. The lower right sections show force space, a plot of shear force on the horizontal axis versus normal force on the vertical axis. shows how gecko setae are slippery, not sticky, when pushed away from the animal, against the direction of curvature of the setae (this is the opposite direction geckos use when they climb). In the lower left, a compression force () develops during the load step. Friction (shear force) is approximately 0.25 of the compression force, as expected for conventional friction of hard dry materials in contact. There is no measurable adhesion when setae are pushed against their curvature: This is the anti-adhesive direction. shows how gecko setae adhere when preloaded and dragged along the direction of curvature of the setae (this is in the same direction geckos use when they climb). In the lower left, the normal force () is compressive initially during the load step, but immediately following the drag step, strong adhesion occurs (negative normal force) and the setae are drawn into tension. Notably, adhesion is sustained even as the setae slide across the substrate. The lower right plot illustrates frictional adhesion: Adhesion is coupled to friction, and the resultant force angle is approximately 30°. Adhesion is controlled by the shear force. During the pull step, shear force decreases, and adhesion returns to zero without the tacky behavior common to conventional adhesive tapes.

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