Many animals hold sticky secrets.
They’re able to attach themselves to nearly any surface — dry, wet, rough or smooth — and then unstick themselves when they need to move on.
“The way that animals and plants produce stickiness is the inspiration for so many of the ways that you stick things to other things in your everyday life,” says Ann Jones, presenter of RN podcast What the Duck?!
She says the first natural example that comes to mind is the gecko.
Geckos use hairs and weak forces
They walk up walls and across ceilings, seemingly defying the rules of gravity. What is it about geckos that makes them so sticky?
Gecko toe pads are shaped like spatulas and covered in hairs, says Rishab Pillai, a doctorate student at James Cook University.
He recently returned from a gecko round-up in Queensland, where he found 27 different species.
“When we look at these toe pads under a microscope, each pad has thousands of hairs,” he told What the Duck?!
The hairs are tiny — at just 1 to 5 microns, compared to 9 microns for humans.
Weirdly, these tiny hairs are also covered with a layer of hairs.
“Each of these tiny tips then contacts the little bumps and valleys that you see on the surface,” Mr Pillai says.
“On a micro-scale, they use friction and van der Waals force — that is where the true sort of stickiness comes from.”
Van der Waals force is the attraction or repulsion of molecules.
This force is what brings raindrops together, so it’s stronger when they’re closer and weaker when they’re further apart.
The way geckos stick relies on tiny hairs and forces between molecules.
“Geckos have a dry mechanism — the way I think of it loosely is kind of like velcro,” Mr Pillai says.
But that’s not the case with ants.
Ants use pads at the end of their toes
Ants can walk upside down on a glass surface carrying a hundred times their body weight, says Christopher Clemente, an animal ecophysiologist at the University of the Sunshine Coast.
A soft balloon-like pad on the end of their toes is what helps ants — and other insects — stick to surfaces.
“We looked at the shape of the pads [at the end of their toes] and tried to figure out what it is about these pads that allow them to stick and also unstick from the surfaces,” Professor Clemente says.
“Having a very soft compliant pad will allow you to get really close to the surface. And that is going to generate some adhesive force.”
His lab is currently building little robots that use some of the ideas behind “stickiness” in animals to allow the robots to climb up and down different surfaces.
Mr Clemente’s interest in the stickiness of animals started when he was studying the Asian weaver ant.
“In the case of these ants, they have this wonderful folding pad that allows them to spread their pad out and grip onto the surface,” he says.
“They can retract [the pad] and fold up in a certain way that makes the pad less sticky, so they’re able to lift their foot off the surface.”
Professor Clement says if you view an ant’s foot under a microscope, it will leave a microscopic drop of fluid behind with every step.
“They’re producing a type of fluid and their feet are constantly wet.
“The fluid is going to allow this soft pad to stick to the surface by a mechanism called capillary adhesion.”
Barnacles have a ‘cement gland’
Another animal known for its “stickiness” is the barnacle. Somehow these hardy creatures manage to stick to rocks or boat hulls in the most challenging situations, like being pounded by waves.
The antennae of an adult barnacle has a cement gland that allows them to stick to all sorts of surfaces, says ecologist Eleanor Velasquez, who’s also the education and training manager at TERN Australia.
“It’s amazing that barnacles are able to withstand a pulling strength of about 2,000 kilograms per square inch,” Dr Valesquez says.
“This is a sticking strength of 10 to 30 kilograms per square inch, which is incredible when you think about it.”
The technique barnacles use to attach themselves to objects is poorly understood, Dr Valesquez says.
“Scientists know that the cement gland actually secretes a multi-protein complex which creates the cement and it’s incredibly good at attaching to things in the marine environment.
“But we still don’t understand how they produce it and we can’t replicate it yet.”
Researchers are looking at potential applications for wet environments — such as underwater or in dentistry.
“It makes a lot of sense because our teeth are always breaking and having a very, strong natural glue to fix teeth would revolutionise the industry,” Dr Valesquez says.
Slugs can be sticky too
The dusky arion is unique among slugs, says Andrew Smith, a biology professor at Ithaca College in New York.
Found around New York State in the US, this bright orange slug produces a gel-like glue.
“If you touch the slug, it just starts oozing orange goop all over and it sets within seconds … that’s sticky!”
He says all slugs and snails have a layer of cells that secrete mucus and release slime, but the dusky arion slug has a unique defensive secretion.
“It produces tons of the mucus … several per cent of its body mass all at once. It flows off the animal and sometimes forms little puddles around it.”
Professor Smith says the orange gel can be stretched to about five times its original length before it breaks, and can also stick to wet surfaces.
So, this slug-produced glue is next level – sticky as anything, but also pliable and stretchy at the same time, which is a rare combination.
“It’s got very similar mechanical properties to the skin,” Professor Smith says.
He says the way the glue can bend and stretch without breaking and stick to wet surfaces means it would make a great replacement for sutures.
“Current sutures aren’t great as you have to cut holes in the skin and it’s a place where infection can occur.
“So if you had something that you could just squeeze into a wound, slap it shut and have it set in a perfect seal, then that would be incredible. That’s what we’re after.”
Frog glue that holds beer cans together
The late Professor Mike Tyler was a renowned herpetologist known for his research on frogs and toads.
He also discovered that the Australian holy cross frog (Notaden melanoscaphus) could secrete glue as a defence mechanism.
The glue would be secreted when ants climbed on to the frog’s back. The sticky substance would trap the ants, providing a handy meal for the frog.
His wife, Ella Tyler, says frog glue was a sudden and unexpected discovery.
“He found that whenever he picked up this frog, it stuck to him.
“He found it to be a bit like superglue, so he looked into it and found it had very special properties. “
Professor Tyler discovered that the glue was so strong it could stick two full beer cans together.
That’s pretty sticky!
