Tag Archives: Alyssa Stark

Staying stuck when it’s wet; learning from the geckos

Researchers from the University of Akron have published another study on geckos and their ‘stickability’ in watery environments. Last mentioned here in my Aug. 10, 2012 posting, doctoral candidate Alyssa Stark  and her colleagues were then testing the geckos by placing them on wetted glass plate surfaces and also by immersing them on water-filled tubs with glass bottom,

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.

In this latest research, from the Ap. 1, 2013 news release issued by the University of Akron on EurekAlert, Stark and her colleagues announce they’ve discovered the conditions under which geckos can adhere to wet surfaces,

Principal investigator Stark and her fellow UA researchers Ila Badge, Nicholas Wucinich, Timothy Sullivan, Peter Niewiarowski and Ali Dhinojwala study the adhesive qualities of gecko pads, which have tiny, clingy hairs that stick like Velcro to dry surfaces. In a 2012 study, the team discovered that geckos lose their grip on wet glass. This finding led the scientists to explore how the lizards function in their natural environments.

The scientists studied the clinging power of six geckos, which they outfitted with harnesses and tugged upon gently as the lizards clung to surfaces in wet and dry conditions. The researchers found that the effect of water on adhesive strength correlates with wettability, or the ability of a liquid to maintain contact with a solid surface. On glass, which has high wettability, a film of water forms between the surface and the gecko’s foot, decreasing adhesion. Conversely, on surfaces with low wettability, such as waxy leaves on tropical plants, the areas in contact with the gecko’s toes remain dry and adhesion, firm. [emphasis mine]

“The geckos stuck just as well under water as they did on a dry surface, as long as the surface was hydrophobic,” Stark explains. “We believe this is how geckos stick to wet leaves and tree trunks in their natural environment.”

For interested parties, this is where the paper can be found,

The discovery, “Surface Wettability Plays a Significant Role in Gecko Adhesion Underwater,” was published April 1, 2013 by the Proceedings of the National Academy of Sciences. The study has implications for the design of a synthetic gecko-inspired adhesive.

Here’s an image of a gecko (from the University of Akron’s webpage with their Ap. 1, 2013 news release),

Courtesy University of Akron [downloaded from http://www.uakron.edu/im/online-newsroom/news_details.dot?newsId=ec9fd559-e4af-487f-a9cc-2ea5f5c9612d&pageTitle=Top%20Story%20Headline&crumbTitle=Geckos%20keep%20firm%20grip%20in%20wet%20natural%20habitat]

Courtesy University of Akron [downloaded from http://www.uakron.edu/im/online-newsroom/news_details.dot?newsId=ec9fd559-e4af-487f-a9cc-2ea5f5c9612d&pageTitle=Top%20Story%20Headline&crumbTitle=Geckos%20keep%20firm%20grip%20in%20wet%20natural%20habitat]


Not mentioned in this news release, one of the relevant applications for this work would be getting bandages and dressings  to adhere to wet surfaces.

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,

Description
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.

Personality
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.