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Sticky Situation

Researchers reveal intricacies of a gecko's grip
aging



J. W. Stewart
gecko portrait
Image:Anthony Herrel

TOKAY GECKO yielded some of its sticky secrets.
They don¿t use glue, or suction, or even nature¿s version of Velcro. So just how do geckos scamper up walls and across ceilings so effortlessly? Researchers have pondered this question for decades but were unable to identify the adhesive mechanism. Until now. According to a report in the June 8 Nature, investigators examining the feet of Tokay geckos have determined that these lizards most likely utilize intermolecular forces called van der Waals forces to get a toehold on even the slipperiest surfaces.

The bottom of a gecko's foot bears about half a million microscopic hairs called setae, each one tipped with hundreds of teeny pads, or "spatulae." These spatulae, say Kellar Autumn of Lewis and Clark College, Robert J. Full of the University of California at Berkeley and their colleagues, hug the surface so closely that they interact with its molecular structure.

But simply placing its foot on or removing it from the substrate perpendicularly is not enough to engage or disengage the gecko's spatulae. Rather, these creatures have a unique behavior of toe uncurling and peeling that enables them to rapidly attach and detach their feet--15 times a second when running-while achieving 600 times more adhesive force than friction alone could account for.

gecko on glass
Image: Kellar Autumn

GECKO ON GLASS, upside-down, clings easily.
By determining the sticking power of a single seta, the team ruled out both friction and suction as mechanisms of gecko grip. (Earlier studies had also indicated that these explanations were insufficient, as gecko feet appear to work quite well on glass and in vacuums.) Neither electrostatic attraction nor glue seem plausible, because the setae still function in ionized air, and gecko feet do not have glue glands. The researchers have not, however, eliminated the potential role of water adsorption, or some other watery interaction, in the adhesion of gecko feet to seemingly any surface.

Another fascinating feature of the gecko foot hairs is that they appear to be self-cleaning. Microspheres embedded by researchers into the hairs disappeared as if by magic after the gecko had taken only a few steps. Considering the extraordinary efficacy of this lizard's setae and spatulae system, it should come as no surprise that members of the team are investigating commercial applications: a dry adhesive modeled after gecko feet may not be far off.

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