Gecko’s Adhesive System: The Secret of it
- Fig. 1: Hierarchical fibrillar structure of a day gecko: from (a) toe pads to (b) lamellae to (c) top view and (d) side view of setal arrays to (e) spatulae. Credits: (a) Edward Ramirez; (b) Tim Sullivan and Joel Thorson; (c) Tim Astrop; (d) Tim Sullivan; (e) Tim Sullivan
- Fig. 2: (a) Picture of gecko toe peeling via digital hypertension (DH). Schematics of (b) a single digit under the action of DH and (c) the setal jump-off process at the separation front. Credit: (a) Edward Ramirez
- Zhenhai Xia, Associate Professor, Department of Materials Science and Engineering, University of North Texas, USA
- Peter H. Niewiarowski, Professor, Department of Biology and Integrated Bioscience Program, The University of Akron, Ohio, USA
- Shihao Hu Postdoct, Department of Macromolecular Science and Engineering, Case Western Reserve University, Ohio, USA
- © Edward Ramirez
Geckos have fascinated generations of researchers with their extraordinary ability to move on vertical surfaces and ceilings. Given the strong adhesion generated by the animal, the mechanism that keeps this adhesion over thousands of cycles of contact and release is even more remarkable. Here, we discuss three critical aspects of the gecko adhesive system - strong adhesion, easy detachment and dynamic self-cleaning, and how these functions combine with each other to keep gecko's feet sticky yet clean.
Biological systems are sources of remarkable designs that have and will continue to inspire human invention and innovation. Geckos, for example, are renowned for their wall climbing talents, with an ability to scale almost any surface at any angle with seemingly little effort . Strong adhesion, easy detachment and self-cleaning are possibly the three major functionalities that make the adhesive system successful in geckos' daily experience. Seemingly paradoxical, these three aspects of performance are intrinsically connected and mutually enhancing. Indeed, these attributes interact during a gecko's locomotion given the anisotropic hierarchical fibrillar structure of the keratinous foot hairs as shown in figure 1. Here, the focus lies on the mechanics that lead to the superior self-cleaning property; most importantly, its relations to and implications strong adhesion and easy detachment as a whole.
Intimate contact between the toe pads and substrate are achieved when geckos flatten their feet and place them onto a surface. This motion induces a bend and shear on the distally oriented setal arrays, where the setae and spatulae deform according to the surface topography at corresponding length scales (i.e. micro- and nano-scales), maximizing the effective contact points . By virtue of the anisotropic and hierarchical fibrillar design, great conformability and robustness are achieved at the same time by employing relatively stiff fibrils, composed of β-keratin and other materials . The ubiquitous and unselective van der Waals force becomes significant when hundreds of thousands to millions of ‘adhesive points' fall in intermolecular working rage.
This collective effect results in a phenomenal adhesive strength that enables geckos to literally defy gravity. Specifically, each gecko foot can generate ~10 N adhesive force in whole animal measurements against smooth acrylic or glass surfaces . If estimated from single seta measurements (e.g. ~200 µN in shear) , a single gecko foot could sustain ~100 N force under maximum conditions derived from setal density and toe pad area.
To be useful as a means of traction and adhesion during locomotion, geckos must be able to rapidly and easily detach their toes when walking or running across a surface. This is where the easy release has to come into play. During foot removal, before taking each step, geckos hyperextend their digits and scroll up each toe pads to detach by peeling in a distal to proximal direction (fig. 2a, b). This peeling motion leads to a drastic decrease of the releasing force preparing the detached foot for the next step. At any instant during toe peeling, there is only a small fraction of setal patch (e.g. ~0.3 mm wide) being pulled away from the substrate with a force distribution that extends from the adhesive strength of individual setae to zero. A conservative calculation suggests a 3-order-difference in magnitude between strong attachment and easy removal at the whole animal scale .
Although gecko toe pads show strong adhesion on various surfaces, they remain remarkably clean around everyday contaminants. A recent experiment demonstrates that geckos keep their toes clean through a unique dynamic self-cleaning mechanism activated by digital hyperextension (DH; fig 2b). When walking naturally on a surface, geckos shed dirt from their toes twice as fast as they would if walking without DH, returning their feet to nearly 80 % of their original stickiness in only 4 steps .
Theoretical analysis shows that as geckos scroll up the toe pads via DH, they generate a peeling zone that propagates from distal to proximal ends (fig. 2b). In this region each compressively bent and laterally dragged seta (fig. 2c; black) sequentially returns towards its original position in the non-contact state laterally, while it returns and is pulled upward into an opposite bending state, building up a large tension-induced elastic energy (fig. 2c; red). Based on our calculations, the peeling motion exerts a nearly vertical pull on the individual setae within the peeling zone before they finally jump off the surface (fig. 2c; blue) and release at the separation front (fig. 2b). This sudden release of the restored elastic energy generates a very large inertial force at the tip, which is large enough to dislodge dirt particles strongly attached to the setae.
Integration of Dynamic Self-cleaning with Attachment/Detachment
The unique hierarchical fibrillar structure and the ability to hyperextend their toes are two key factors to the success of the gecko's locomotor adhesive system . First, strong adhesion is generated when geckos place their feet, and initialize intimate contact with target surface. When moving, a gecko releases this strong adhesion by peeling their toe pads via digital hyperextension. During release, due to the relatively strong adhesion for individual setae, each seta sequentially jumps off the substrate, generating the dynamic effect to dislodge attached dirt particles. The inticrate design of gecko toe pad structure perfectly combines dynamic self-cleaning with attachment/detachment, making gecko feet sticky yet clean.
Self-cleaning due to superhydrophobicity of hierarchically structured surfaces is a well-known phenomenon in many plants  and insects , which has been coined the ‘lotus effect'. Titanium dioxide coatings, on the other hand, offer another self-cleaning mechanism on the basis of UV-light (or sunlight) induced superhydrophilicity and chemical decomposition . These mechanisms, however, are distinct from gecko contact self-cleaning, where water is not involved as a ‘cleaning agent'. In other words, gecko toe pads are self-cleaned even under dry conditions during locomotion. Currently, there already exits a whole range of synthetic mimics made mostly out of polymers and carbon nanotubes, successfully replicating the features of strong adhesion and easy detachment of the gecko's adhesive system . However, progress still lags in the self-cleaning properties, especially for the dynamic effects. Hence, incorporating gecko-like self-cleaning mechanisms into the artificial mimics will open up a new route for applications such as smart climbing robots and others.
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