Abstract
Recent years have witnessed a multitude of studies focusing on gekkotan adhesion. Intense interest in this phenomenon was triggered by the discovery of the manner and magnitude of the forces generated by the hair-like filaments (setae) on the toe pads and inspired the development of the next generation of smart, reversible synthetic adhesives. Most studies pursuing these goals have concentrated on the generalized form and properties of gekkotan setae outlined in those key early studies, resulting in the fabrication of synthetic filaments of uniform dimensions. Although there are over 1,800 species of extant geckos, and hundreds of species of anoles (a separate lizard lineage that has convergently evolved adhesive toe pads), most investigations have used relatively few species as the source of basic information, the Tokay gecko (Gekko gecko) being the most prominent among these. Such exemplar taxa generally exhibit structurally intricate setae and morphologically complex configurations of the adhesive apparatus. Setal structure taken to be characteristic of these taxa is generally reported by singular statements of maximal length, diameter, density and branching pattern. Contemporaneous work focusing on the configuration of setae at locations across the toe pads and upon the evolutionary origin of adhesively competent digits in anoles and specific lineages of geckos, however, has revealed extensive variation of setal structure within individuals, information about how setae may have arisen from non-adhesive filamentous precursors, and how newly adhesively competent digits have been integrated into pre-existing patterns of locomotor mechanics and kinematics. Such observations provide insights into what is minimally necessary for adhesively competent digits to function and reveal the simplest configuration of components that make this possible. We contend that information gleaned from such studies will assist those seeking to employ the principles of fibrillar-based adhesion, as exemplified by lizards, for bio-inspired applications.
Highlights
Recent years have witnessed a deluge of studies focusing on gekkotan adhesion, a remarkable phenomenon whereby geckos can attach to, and move on, smooth, low friction surfaces using series of expanded scales that possess arrays of hair-like fibers carrying multiple flattened, triangular-shaped tips (Ruibal and Ernst, 1965; Autumn et al, 2000; Autumn et al, 2002)
The last two decades have witnessed a flood of investigations examining the form, function, and properties of the gecko adhesive system, many of them aiming, directly or indirectly, to inform the design and fabrication of the generation of smart, reversible, fibrillar synthetic adhesives
We contend that studies examining setal transects of adhesive pad-bearing lizards, evolutionary transitions from non-adhesive to adhesively competent digits, and the convergent evolution of the adhesive apparatuses of geckos and anoles provide a wealth of information from which researchers may gain engineering inspiration
Summary
Recent years have witnessed a deluge of studies focusing on gekkotan adhesion, a remarkable phenomenon whereby geckos can attach to, and move on, smooth, low friction surfaces using series of expanded scales (scansors or lamellae) that possess arrays of hair-like fibers (setae) carrying multiple flattened, triangular-shaped tips (spatulae) (Ruibal and Ernst, 1965; Autumn et al, 2000; Autumn et al, 2002). A recent investigation of the morphometrics of the setal arrays of the Cuban knight anole (Anolis equestris), a species similar in size to Gekko gecko, reveals that its setae increase in length and decrease in basal diameter proximodistally along regions (proximal, intermediate, distal) of the subdigital pad (Figure 8B) (Garner et al, 2020). Our examination of transects of setal fields constituting the toe pads of geckos, the evolutionary transition to functionally adhesive digits in the naked-toed gecko genus Gonatodes, and the comparison of the structure of gecko and anole adhesive systems has allowed us to deduce what we consider to represent the most basic level of organization and mechanical operation of such a system Both setal structure, at its most elaborate, and the configuration and operation of the entire adhesive system of the Tokay gecko (Gekko gecko) are complex (Russell, 2002). The free margin is overlain by the compliance mechanism
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