Secrets of sticky feet revealed

The sure-footed tokay gecko Dry toes stop geckos from becoming unstuck

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Sure-footed lizards and beetles keep their feet dry to secure their grip, according to scientists.

Previous studies have attributed both species' remarkable 'sticking power' to microscopic hairs on their feet.

These hairs are attracted to surfaces by the forces between molecules, creating a strong footing.

Two studies now suggest that the animals 'walk on air' to overcome wet surfaces, trapping bubbles between these tiny hairs.

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Researchers from the University of Akron, Ohio, US, put tokay geckos through their paces in the lab.

Their results are published in the Journal of Experimental Biology.

To test the tropical geckos' powers of adhesion when dry, the team lowered them onto a sheet of glass. Once a strong bond had been made scientists gently pulled the harnessed lizard away from the glass using a small motor.

The steadfast reptiles only became unstuck at forces around 20 times their own body weight.

But when water was introduced to the environment, their sticking power was really put to the test.

Slippery when wet

"Tokay geckos are of interest to us because they are found in the tropics where surfaces may become wet with water from rain or slick from high humidity," said Alyssa Stark who led the study.

Tokay gecko on misted glass Geckos find wet surfaces more difficult to deal with

With dry toes on a glass sheet covered in a fine mist of water to mimic wet tropical leaves, the geckos were still able to hold their own body weight.

But if their feet were soaked, or the surface water was more than 0.5cm deep, the lizards lost their grip.

"Our results show that geckos need to have dry feet to stick to wet or even dry surfaces," said Ms Stark.

Under a microscope, the researchers saw that dry toes repelled water due to air bubbles trapped between the tiny hairs.

"[Dry] gecko toe pads are superhydrophobic, meaning water beads up on them and also rolls off the toes easily," said Ms Stark.

But when the lizards' feet were soaked the toes were unable to displace water in the same way.

The researchers hope to apply their results to develop artificial "gecko tape" that is reusable and maintains its grip when wet.

Beetle feet

In a different study, Japanese researchers have been testing the grip of beetles, another group known to use tiny hairs, known as "setae", for grip.

Rather than the molecular attraction known as Van der Waals forces that give the geckos their grip, green dock beetles rely on the attraction of liquids toward solids, known as capillary action.

Gastrophysa viridula, green dock beetle Green dock beetles have oily hairy feet

The tiny hairs on the feet of these beetles are covered in an oily liquid that is attracted to solid surfaces by the forces between molecules, giving the beetles impressive grip.

"These leaf beetles, bearing specialized hairy attachment devices, are able to adhere to and walk along various vertical surfaces and even ceilings," explained Dr Naoe Hosoda from the National Institute for Material Science, Japan.

In their study published in the Royal Society journal Proceedings B, Dr Hosoda and colleagues wanted to investigate if the presence of water disrupted the existing fluid mechanism.

When beetles were introduced to water baths, microscope analysis revealed trapped air between the hairs on their feet, working in a similar way to geckos' toes.

Beetle foot under a microscope Air (white patches) is trapped between the hairs (black dots) while the foot is surrounded in water (black area)

The Japanese team found that this air created a bubble around the foot that dried the surfaces the beetles made contact with, even totally submerged surfaces.

The oily liquid used for sticking was therefore protected from the water and the beetles were able to walk unhindered.

Inspired by this, the team created an artificial structure from silicone to mimic the adhesion and were able to successfully stick a plastic toy bulldozer to the bottom of a fish tank.

Dr Hosoda told BBC Nature that understanding this technique in the beetles could have implications for the development of clean underwater adhesives in human technologies, such as robotics for environmental monitoring.

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