Geckos can turn stickiness of their feet on and off

Geckos can turn the "stickiness" of toe hairs on their feet on and off, which enables them to run at great speeds or cling to ceilings without expending much energy.
Oregon State University (OSU) researchers have explored the subtleties of geckos' adhesion system mechanism.
"Since the time of the ancient Greeks, people have wondered how geckos are able to stick to walls - even Archimedes is known to have pondered this problem," said Alex Greaney, co-author and an assistant professor of engineering at OSU.
"It was only very recently, in 2000, that Kellar Autumn and colleagues proved unequivocally that geckos stick using van der Waals forces," Greaney said.

Van der Waals forces are weak atomistic level forces, "but geckos are able to take advantage of them because of a remarkable system of branched hairs called 'seta' on their toes," Greaney explained.
"These seta and their hierarchy can deform to make intimate contact with even very rough surfaces - resulting in millions of contact points that each are able to carry a small load," Greaney said.

Geckos - as well as spiders and insects - have independently evolved the same adhesion system mechanism and have been using it for millions of years.
"By using mathematical modelling, we've found a simple, but ingenious, mechanism allows the gecko to switch back and forth between being sticky or not," Greaney said.

"Geckos' feet are by default nonsticky, and this stickiness is activated through application of a small shear force. Gecko adhesion can be thought of as the opposite of friction," he said.
Greaney and colleagues also found that the entire process is quite subtle, so a synergistic combination of angle, flexibility, and extensibility of the hairs exists that results in incredibly robust and tough adhesion - but still allow geckos to unstick without expending energy.
The findings may help improve 'synthetic dry-adhesives' that aim to replicate the gecko. These types of adhesives are already being used in climbing robots that can search through earthquake rubble in search of survivors.

The study is published in the Journal of Applied Physics, from American Institute of Physics (AIP) Publishing.