Gecko Adhesion

1) How do geckos adhere to nearly any surface?




Myriad gecko feet (Paul Stewart) [PNAS Cover image]

 
(Autumn, et al. 2002, PNAS [Cover]).

It turns out that gecko feet are covered in millions of tiny hairs called setae. Each of these hairs branches into 100-1000 spatulae each of which is narrower than the wavelength of visible light (< 300 nm)! When I joined this project, prior research had shown that intermolecular forces binding the gecko’s setae to the surface are responsible for adhesion, but it was not yet known whether polar or non-polar, van der Waals interactions, are ultimately responsible. Together with our collaborators, we were able to formulate a simple experiment using semiconductors of different surface properties to separate polarity from polarizability (i.e. van der Waals interactions). The results of this experiment conclusively demonstrated that van der Waals forces are the predominant mechanism in gecko adhesion. Van der Waals forces are actually based on quantum mechanical properties of the materials, specifically the degree of flexibility, or polarizability, of the electron cloud surrounding the individual molecules. Thus in order to fully understand the principle of how geckos adhere to walls, we had to consider the quantum mechanics of their feet. Application of this research by engineers at Berkeley and Stanford (as well as many other groups) has led to the development of biologically-inspired synthetic adhesives that adhere like gecko pads. These have even been put on robotic climbers. Check out Stickybot and Stickybot III, from Professor Mark Cutkosky’s lab at Stanford.

 

Simon Sponberg

24 Kincaid Hall

Box 351800

Department of Biology

University of Washington

Seattle, WA 98195-1800

bergs [at] uw [dot] edu

2) What happens to gecko adhesion (and friction) when the animal begins to slide?




Frictional (a) and adhesive forces (b) of a gecko’s adhesive foot pads increase with increasing sliding velocity. Negative force in (b) indicates sticking to the surface. Figure from Gravish, et al. 2010, Interface.

 
(Gravish, et al., 2010, Proc. Royal Soc. Interface).

Even geckos encounter perturbations and environments that can challenge their ability to stick. The hairs of a gecko’s foot are composed of dry, hard keratin. Classic physics predicts dry, hard solids to have frictional forces that decrease at the onset of sliding (kinetic friction is less than static friction). We discovered that gecko adhesives actually become stickier, increasing both their friction (shear) and adhesion (normal) forces as sliding velocity increases. There is no drop in frictional forces upon the onset of sliding and the adhesive’s performance can be maintained for over 30,000 repeated cycles of contact, sliding, and detachment. We have demonstrated that these properties can be captured in gecko synthetic adhesive (GSA) arrays composed of a hard silicone polymer. Finally we have shown that this behavior of both the gecko and the synthetic arrays occurs from the random stick-slip events of the many individual hairs of adhesive fibrils. This fibrillar structure enables a greater energy dissipation as the number of stick slip events increases to a critical level. These results suggest that geckos do not face imminent failure of adhesion if sliding does begin to occur. Rather they can simply maintain foot contact and adhesion and friction will passively increase.

 

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One of the animals that Aristotle marveled at was the gecko with its ability to "run up and down a tree in any way, even with the head downwards" (Aristotle. The History of Animals.). Since Aristotle, geckos have continued to amaze scientists. During my earlier research and collaboration with Professor Kellar Autumn at Lewis & Clack College, I investigated geckos’ abilities to hang from a single toe, adhere and detach a foot from a surface in less than 5 milliseconds, and maximally stick with a force up to 240 times their own weight.

Systems Neuromechanics Projects

Locomotor control strategies                             The neuromechanical transform

Behaviorally-relevant sensing                                            Mechanisms of muscle

Complementary Research Activities

Gecko adhesion                                                         Evolution of whale body size