Tag Archives: Tokay Geckos

Gecko-like toes needed for climbing robots

Caption: The spotted belly of a Tokay gecko used by UC Berkeley biologists to understand how the animal’s five sticky toes help it climb on many types of surface. Credit: Yi Song

Those are fabulous toes. Geckos and the fine hairs on their toes have been of great interest to researchers looking to increase qualities of adhesion for all kinds of purposes including for robots that climb. The latest foray into the research suggests that it’s not just the fine hairs found on gecko toes that are important.

A May 8, 2020 news item on ScienceDaily makes the proclamation,

Robots with toes? Experiments suggest that climbing robots could benefit from having flexible, hairy toes, like those of geckos, that can adjust quickly to accommodate shifting weight and slippery surfaces.

Biologists from the University of California, Berkeley, and Nanjing University of Aeronautics and Astronautics observed geckos running horizontally along walls to learn how they use their five toes to compensate for different types of surfaces without slowing down.

Close-up look at the toe pads of a Tokay gecko. They have about 15,000 hairs per foot, each of which has split ends that maximize contact with the surface and support the animal’s weight by interacting with surface molecules via van der Waals forces. (Photo by Yi Song)

You can find that image and more embedded in the May 8, 2020 University of California at Berkeley news release (also on EurekAlert) by Robert Sanders. The news release delves further into the work

“The research helped answer a fundamental question: Why have many toes?” said Robert Full, UC Berkeley professor of integrative biology.

As his previous research showed, geckos’ toes can stick to the smoothest surfaces through the use of intermolecular forces, and uncurl and peel in milliseconds. Their toes have up to 15,000 hairs per foot, and each hair has “an awful case of split ends, with as many as a thousand nano-sized tips that allow close surface contact,” he said.

These discoveries have spawned research on new types of adhesives that use intermolecular forces, or van der Waals forces, to stick almost anywhere, even underwater.

One puzzle, he said, is that gecko toes only stick in one direction. They grab when pulled in one direction, but release when peeled in the opposite direction. Yet, geckos move agilely in any orientation.

To determine how geckos have learned to deal with shifting forces as they move on different surfaces, Yi Song, a UC Berkeley visiting student from Nanjing, China, 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, Song, also measured the area of contact of each toe. The technique made the toes light up when they touched a surface.

To the researcher’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, researchers 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 said. “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 team, which also includes Zhendong Dai and Zhouyi Wang of the College of Mechanical and Electrical Engineering at Nanjing University of Aeronautics and Astronautics, published its findings this week in the journal Proceedings of the Royal Society B.

Here’s a link to and a citation for the paper,

Role of multiple, adjustable toes in distributed control shown by sideways wall-running in geckos by Yi Song, Zhendong Dai, Zhouyi Wang, and Robert J. Full. Proceedings of the Royal Society B; Biological Sciences 29 April 2020 Volume 287Issue 1926 DOI: https://doi.org/10.1098/rspb.2020.0123 Published [online]:06 May 2020

This paper is open access.

How do you make a harness for a gecko?

It’s the first question (how do you make a harness for a gecko?) I had on reading the latest research about geckos and their ability to adhere to various surfaces, dry and wet. From the Aug. 9,2012 news item on Nanowerk,

But first they had to find out how well their geckos clung onto glass with dry feet. Fitting a tiny harness around the lizard’s pelvis and gently lowering the animal onto a plate of smooth glass, Stark [Alyssa Stark] and Sullivan [Timothy Sullivan] allowed the animal to become well attached before connecting the harness to a tiny motor and gently pull the lizard until it came unstuck. [emphasis mine] The geckos hung on tenaciously, and only came unstuck at forces of around 20N, which is about 20 times their own body weight. ‘The gecko attachment system is over-designed’, says Stark.

Here’s more about the research and the geckos (from the news item),

Geckos are remarkable little creatures, clinging to almost any dry surface, and Alyssa Stark, from the University of Akron, USA, explains that they appear to be equally happy scampering through tropical rainforest canopies as they are in urban settings. ‘A lot of work is done on geckos that looks at the very small adhesive structures on their toes to really understand how the system works at the most basic level’, says Stark. She adds that the animals grip surfaces with microscopic hairs on the soles of their feet that make close enough contact to be attracted to the surface by the minute van der Waals forces between atoms. However, she and her colleagues Timothy Sullivan and Peter Niewiarowski were curious about how the lizards cope on surfaces in their natural habitat.

Explaining that previous studies had focused on the reptiles clinging to artificial dry surfaces, Stark says ‘We know they are in tropical environments that probably have a lot of rain and it’s not like the geckos fall out of the trees when it’s wet’. Yet, the animals do seem to have trouble getting a grip on smooth wet surfaces, sliding down wet vertical glass after a several steps even though minute patches of the animal’s adhesive structures do not slip under humid conditions on moist glass. The team decided to find out how Tokay geckos with wet feet cope on wet and dry surfaces, and publish their discovery that geckos struggle to remain attached as their feet get wetter in The Journal of Experimental Biology (“The effect of surface water and wetting on gecko adhesion” [behind a paywall]).

According to the news item, Tokay geclos were used for this study. These are neither small, nor amiable geckos according to the webpage devoted to Tokay Geckos on the anapsid.org website,

Native to SE Asia, these relatively large (12″) geckos are pale gray with bluish spots when they have been in the dark, darkening to dark gray with reddish spots in the light. Like most geckos, tokays are oviparous insectivores.

Young are 2-3″ at hatching. Eggs are laid in rocky crevices or under the eaves of houses. The 2-3 eggs, laid several times a year, are sticky and adhere to surfaces. In captivity, they may be laid on the glass sides of their terraria. Incubation time for the eggs ranges from 2-6 months for the oviparous Gekko species.

Tokays have the specialized lamellae on the pads of their toes which enable them to walk on vertical surfaces, including ceilings. Contrary to popular misconception, these pads are not “sticky” but rather are composed of tiny, microscopic filaments which find equally tiny imperfections in surface – including glass.

Like many lizards, tokays can darken or lighten their ground and spot colors to better blend in with their background.

Despite the fact that they follow human habitation, finding human dwellings to be great places to find prey, Tokays are the least lovable of the geckos. They are known for their nasty temperament, cheerfully biting the hand that feeds, cleans or otherwise comes into anything resembling close proximity to them. Their bites are powerful–one might say they are the pit bulls of the gecko world…they hang on and let go only when it suits them. Equipped as they are with numerous sharp teeth, the bites can bleed profusely and, even barring subsequent infection, are annoying for days. Note that while I am a strong believer that almost any animal can be habituated to human contact, such contact can be stressful for many species, and geckos as a whole are known for their marked preference to be left alone.

That harness question gets a lot more interesting after you’ve read about the Tokay Geckos, yes? I found the parts about being “the least lovable of the geckos’ and being known for their nasty bites particularly interesting.

Kathryn Knight’s article about the study for the Journal of Experimental Biology (which originated the news item) offers details about the testing on wet surfaces  (but no more about the harnesses),

Next, the trio sprayed the glass plate with a mist of water and retested the lizards, but this time the animals had problems holding tight: the attachment force varied each time they took a step. The droplets were interfering with the lizards’ attachment mechanism, but it wasn’t clear how. And when the team immersed the geckos in a bath of room temperature water with a smooth glass bottom, the animals were completely unable to anchor themselves to the smooth surface. ‘The toes are superhydrophobic [water repellent]’, explains Stark, who could see a silvery bubble of air around their toes, but they were unable to displace the water surrounding their feet to make the tight van der Waals contacts that usually keep the geckos in place.

Then, the team tested the lizard’s adhesive forces on the dry surface when their feet had been soaking for 90 min and found that the lizards could barely hold on, detaching when they were pulled with a force roughly equalling their own weight. ‘That might be the sliding behaviour that we see when the geckos climb vertically up misted glass’, says Stark. So, geckos climbing on wet surfaces with damp feet are constantly on the verge of slipping and Stark adds that when the soggy lizards were faced with the misted and immersed horizontal surfaces, they slipped as soon as the rig started pulling.

Therefore geckos can walk on wet surfaces, so long as their feet are reasonably dry. However, as soon as their feet get wet, they are barely able to hang on and the team is keen to understand how long it takes geckos to recover from a drenching.

Given the number of studies using geckos, I wonder if there are specialists devoted to creating gecko harnesses. In any case, one certainly can appreciate that the practice of science can sometimes be a blood sport. I think the question being asked is intriguing and it’s the first time I’ve seen any study of the gecko’s adhesive qualities being tested on wet surfaces.