Could studying the slow moving stick insect help Olympic sprinter Usain Bolt cover 100 meters faster? Researchers at Cambridge believe it could. It's all to do with sticky toes versus hairy toes.

Materials scientists can learn a lot from nature. Take the humble stick insect as an example. While hanging upside-down the gravity defying invertebrate uses specialized pads on its feet to cling to virtually any surface. This ability could seriously impede movement in not-so-death-defying conditions however, such as walking around on the ground like the rest of us. So how does the stick insect get around this sticky problem?

What the latest research from University of Cambridge has uncovered is that in an interesting combination of hard and soft materials the stick insect is able to overcome this contradiction in functional needs. Using a mixture of hard pads for suction and hairy pads for powerful friction (rather than actually "sticking") the insect is able to conjure gravity-defying grip, while also allowing it to move about freely when upright.

“Stick insects have developed an ingenious way of overcoming the conflict between attachment and locomotion, with a dual pad system that alternates between stick and grip depending on the situation," says David Labonte, lead researcher from the Department of Zoology. "Just by arrangement and morphology, nature teaches us that good design means we can combine the properties of hard and soft materials, making elemental forces like friction go a very long way with just a small amount of pressure.”

The research revealed that stick insects have two distinct attachment footpads at the end of each leg. The adhesive toe pad provides the stick needed to scale walls and move about on the underside of branches, while a hairy heel pad provides the friction needed to grip but not stick when upright. This level of friction is easily manipulated due to the arrangement and shape of the tiny hairs which are of varying length.

By applying more or less pressure the insect is able to gain or lose friction. This seems to be due to a combination of three factors that by Prof Labonte and his team describe as follows:

  • Both the pad itself and the tips of the hairs are rounded. This means that, when pressure is applied, more contact area is generated – like pushing down on a rubber ball;
  • Some hairs are shorter than others, so the more pressure, the more hairs come into contact with the surface;
  • When even more pressure is applied, some of the hairs bend over and make side contact – greatly increasing contact area with very little extra force.

These design elements generate large amounts of friction with comparatively small amounts of pressure from the insect. Also there is hardly any contact area without some pressure, meaning these specialized frictional hairs do not stick.

Learning from nature has led to plenty of improvements in modern technology already – the development of ultra-thin insulation from studying polar bear fur is just one recent example.

In the case of the stick insect, scientists hope new innovations in materials used for the soles of sports shoes might be one application. While they might not cover the 100 meters as fast as Usain Bolt, stick insects might just help the champion sprinter get across the line a little quicker.

Prof Labonte explains: “If you run, you don’t want your feet to stick to the ground, but you also want to make sure you don’t slip. We investigate these insects to try and understand biological systems, but lessons from nature such as this might also be useful for inspiring new approaches in man-made devices.”

The research was published in the Journal of the Royal Society Interface.

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