Toe-tally spies: to achieve the fly-on-the-wall crawling abilities of geckos, robots will need “flexible, hairy toes”.

New research by biologists of the Nanjing University of Aeronautics and Astronautics, and the University of California, explains some geckos’ gravity-defying ability to run sideways across walls. Tokay geckos can run sideways just as quickly as they can climb upward, Yi Song and his team discover, because their toes allow them to quickly adapt to changes in direction and surface texture. Such research may inform designs for new mechanical feet, which would allow robots to exhibit the same spectacular skill set.

You’re sat outside on a scorching Summer morning. Sun pours across the ground, and the walls around you are hot with residual heat. Out of nowhere, a gecko dashes across the bricks in front of you. Small, speedy, and shockingly nimble, it moves like a flash, up and across the horizontal wall as if the surface were as flat as the ground your chair sits on. You watch it as it hurries up and across the balcony roof above you, clinging to the ceiling as if gravity did not apply.

Now imagine that it’s not a gecko you spot, but a robot, moving just as dextrously.

As the field of biomimetics expands, this scene may not be that far from plausible.  Engineering frequently takes inspiration from the natural world, and a design like this would certainly not be the first time that gecko anatomy had inspired a new development. In fact, the structures that allow geckos to travel in this way have already been replicated in materials from silicones to plastics to nanotubes. Now, moving onto designing robots with these abilities may not be such a distant development, this week’s newly published research by Yi Song et al. suggests.

But before we can speculate about robots, we must first understand geckos. Geckos possess many unique skills, most notably their ability to scale walls and cling to ceilings. The secret to their success? A structure that, in itself, is not unusual at all. In fact, you have millions of a similar kind yourself – but none that are quite this specialised. It’s hair!

Tiny rows of hairs, known as setae, run across the underside of gecko feet, allowing them to do far more than just ‘stick’. ‘Sticking’ to surfaces wouldn’t be particularly useful to geckos; they need to move, and fast. That’s where setae come in. Invisible to the human eye, these miniscule structures and their even tinier, branch-like ends, allow geckos to cross surfaces at speeds of 20 body lengths per second.

Under the pressure of a gecko’s body weight, setae are pressed closely into the microscopic contours of surfaces like walls and ceilings. So closely, that the electrons of hair molecules can interact with electrons of the wall molecules, to form weak attractions known as ‘Van der Waals’ forces. These forces create a secure temporary foothold, which is also easily broken when setae are pulled away. Think Velcro™, but with forces. The result: a gecko can traverse surfaces at any pace it chooses, without falling off or becoming stuck.

So, gecko feet aren’t ‘sticky’ at all. Instead, flexible setae on the bottom of their agile toes interact with textured surfaces, to generate temporary electrostatic forces. However, setae alone cannot be given all the credit for the lizard’s acrobatics. Another, larger, structure takes responsibility for the animal’s agility and adaptability, as Song and his team discovered.

Using a technique called ‘frustrated total internal reflection’, which causes surfaces to light up on contact with a Tokay gecko’s feet, Song was able to observe gecko toes swiftly realigning against gravity as they crossed the surface. Critically, he noticed that during sideways wall crossing, the toes of the hind and forefeet which were on the surface of the lizard nearest to the ceiling acted in the same way that both forefeet do while climbing. This allowed the gecko to maintain their attraction to the surface, even while travelling sideways at speed.

What’s more, by adding slippery patches and irregular surfaces to his experiments, Song demonstrated that in challenging terrain, geckos take advantage of their multiple soft toes to maintain contact with the surface. On these unpredictable surfaces, there is a higher likelihood of toes slipping or setae being unable to make close enough contact for attraction. To combat this, geckos can use their many toes individually, thus increasing the likelihood that at least one attaches successfully. At the same time, the toe’s soft texture makes them flexible and able to conform to rough surfaces – yet another way to ensure stability.

These discoveries not only shed light on an interesting biomechanical phenomenon, but may inform new developments in robotics. Designs for robot feet with similarly adept mechanical toes, for example, could allow robots to move more quickly, and across a wider range of surfaces. Perhaps soon, the hairy, agile toes of Tokay geckos could be seen not only on their own clever feet, but on those of new, better robots.

Article Reference: Song, Y., Dai, Z., Wang, Z., Full, R. (2020). Role of multiple, adjustable toes in distributed control shown by sideways wall-running in geckos. Proc. R. Soc. B. 287: 20200123. doi: 10.1098/rspb.2020.0123

See also: https://www.eurekalert.org/pub_releases/2020-05/uoc--tcl050820.php