Gecko Has Self-Cleaning Feet
Imagine self-cleaning, reusable tape. No matter where you stick it, you can remove it and stick it on another surface, no matter how dirty, and it always acts like new. The tokay gecko has achieved such a feat on its feet, according to a physicist and a biologist from Lewis and Clark College, Oregon, publishing in PNAS.1 The abstract sets up the problem:
A tokay gecko can cling to virtually any surface and support its body mass with a single toe by using the millions of keratinous setae on its toe pads. Each seta branches into hundreds of 200-nm spatulae that make intimate contact with a variety of surface profiles. We showed previously that the combined surface area of billions of spatulae maximizes van der Waals interactions to generate large adhesive and shear forces. [see 09/05/2003 headline]. Geckos are not known to groom their feet yet retain their stickiness for months between molts. How geckos manage to keep their feet clean while walking about with sticky toes has remained a puzzle until now. Although self-cleaning by water droplets occurs in plant and animal surfaces, no adhesive has been shown to self-clean. In the present study, we demonstrate that gecko setae are a self-cleaning adhesive. (Emphasis added in all quotes.)
How do they do it? The authors did a force analysis on the gecko spatulae, the surface, and contaminant particles. They found that with the small size of the spatulae, contaminants are more likely to stick to the surface than to the spatulae. (The spatulae look like split ends on a broom straw, with the setae represented by the straws). As a result, the gecko feet possess a passive self-cleaning mechanism that is intrinsic to the structure of the setae and spatulae. Furthermore, the spatulae have an “anti-self” property that keeps them from sticking to each other.
The fact that this operates by a mechanical, passive property rather than an active cleaning process gave the authors hope that man can invent a similar self-cleaning adhesive. “Thus, the self-cleaning and anti-self conditions may represent a sweet spot in the evolutionary and engineering design spaces for adhesive nanostructures.”
1W. R. Hansen and K. Autumn, “Evidence for self-cleaning in gecko setae,” Proceedings of the National Academy of Sciences USA, 10.1073/pnas.0408304102, published online before print January 3, 2005.
Question: why is this paper classified under Evolution? Anyone see any evolution here? Evolution is not an engineer. Without all the structural properties present simultaneously, the gecko would quickly die of clogged feet that could not cling to anything. These guys said nothing about how this intricate and effective structure could have evolved by a chance process. Let’s keep Charlie out of it.