To climb, maybe robots need toes

At right is a close-up of the toepads of a Tokay gecko (Gekko gecko), and it has links to a few branches of science. 

Biology, of course, but also chemistry and physics, because each of the 15,000 hairs on each gecko foot has split ends – each with maybe thousands of nano-size tips – which maximise contact with a surface and support the animal’s weight via van der Waals forces.

And that intrigues robotics engineers, who want to know whether climbing robots could benefit from having similarly flexible, hairy toes to help them adjust quickly on steep and slippery surfaces.

To find out more, researchers from the University of California Berkeley (UCB), and China’s Nanjing University of Aeronautics and Astronautics watched geckos running horizontally along walls to learn how they use their five toes – and, indeed, why they have five.

In previous work, UCB biologist Robert Full has shown that gecko toes can stick to the smoothest of surfaces, inspiring research into new types of adhesives that similarly use intermolecular forces.

It’s still a puzzle, however, that gecko toes only stick in one direction. They grab when pulled in one direction, and release when peeled in the opposite direction, but still move agilely in any orientation.

Nanjng’s Yi Song ran geckos sideways along a vertical wall while making high-speed video recordings to show the orientation of their toes. The sideways movement allowed him to distinguish downward gravity from forward running forces to best test the idea of toe compensation.

Using a technique called frustrated total internal reflection, he also measured the area of contact of each toe. The technique made the toes light up when they touched a surface.

To his and Full’s surprise, geckos ran sideways just as fast as they climbed upward, easily and quickly realigning their toes against gravity. The toes of the front and hind top feet during sideways wall-running shifted upward and acted just like toes of the front feet during climbing.

To further explore the value of adjustable toes, Song added slippery patches and strips, as well as irregular surfaces. To deal with these hazards, geckos took advantage of having multiple, soft toes. The redundancy allowed toes that still had contact with the surface to reorient and distribute the load, while the softness let them conform to rough surfaces.

“Toes allowed agile locomotion by distributing control among multiple, compliant, redundant structures that mitigate the risks of moving on challenging terrain,” Full says. 

“Distributed control shows how biological adhesion can be deployed more effectively and offers design ideas for new robot feet, novel grippers and unique manipulators.”

The findings are published in the Proceedings of the Royal Society B.

The Royal Institution of Australia has an education resource based on this article. You can access it here.