̽��ֱ�� of Cambridge - adhesion

Why Spider-Man can’t exist: Geckos are ‘size limit’ for sticking to walls

jeh98 — Mon, 18 Jan 2016 20:05:00 +0000

A new study, published today in PNAS, shows that in climbing animals ranging in size from mites to geckos, the percentage of body surface covered by adhesive footpads increases as body size increases, setting a limit to the size of animal using this strategy because larger animals would require impossibly big feet.

Dr David Labonte and his colleagues in the ̽��ֱ�� of Cambridge’s Department of Zoology found that tiny mites use approximately 200 times less of their body surface area for adhesive pads than geckos, nature's largest adhesion-based climbers. And humans? We’d need as much as 40% of our total body surface, or roughly 80% of our front, to be covered in sticky footpads if we wanted to do a convincing Spider-Man impression.

Once an animal is so big that a substantial fraction of its body surface would need to be sticky footpads, the necessary morphological changes would make the evolution of this trait impractical, suggests Labonte.

“If a human, for example, wanted to climb up a wall the way a gecko does, we’d need impractically large sticky feet – and shoes in European size 145 or US size 114,”says Walter Federle, senior author also from Cambridge’s Department of Zoology.

“As animals increase in size, the amount of body surface area per volume decreases – an ant has a lot of surface area and very little volume, and an elephant is mostly volume with not much surface area” explains Labonte.

“This poses a problem for larger climbing animals because, when they are bigger and heavier, they need more sticking power, but they have comparatively less body surface available for sticky footpads. This implies that there is a maximum size for animals climbing with sticky footpads – and that turns out to be about the size of a gecko.”

̽��ֱ��researchers compared the weight and footpad size of 225 climbing animal species including insects, frogs, spiders, lizards and even a mammal.

“We covered a range of more than seven orders of magnitude in body weight, which is roughly the same weight difference as between a cockroach and Big Ben” says Labonte.

“Although we were looking at vastly different animals – a spider and a gecko are about as different as a human is to an ant – their sticky feet are remarkably similar,” says Labonte.

“Adhesive pads of climbing animals are a prime example of convergent evolution – where multiple species have independently, through very different evolutionary histories, arrived at the same solution to a problem. When this happens, it’s a clear sign that it must be a very good solution.”

There is one other possible solution to the problem of how to stick when you’re a large animal, and that’s to make your sticky footpads even stickier.

“We noticed that within some groups of closely related species pad size was not increasing fast enough to match body size yet these animals could still stick to walls,” says Christofer Clemente, a co-author from the ̽��ֱ�� of the Sunshine Coast.

“We found that tree frogs have switched to this second option of making pads stickier rather than bigger. It’s remarkable that we see two different evolutionary solutions to the problem of getting big and sticking to walls,” says Clemente.

“Across all species the problem is solved by evolving relatively bigger pads, but this does not seem possible within closely related species, probably since the required morphological changes would be too large. Instead within these closely related groups, the pads get stickier in larger animals, but the underlying mechanisms are still unclear. This is a great example of evolutionary constraint and innovation.”

̽��ֱ��researchers say that these insights into the size limits of sticky footpads could have profound implications for developing large-scale bio-inspired adhesives, which are currently only effective on very small areas.

“Our study emphasises the importance of scaling for animal adhesion, and scaling is also essential for improving the performance of adhesives over much larger areas. There is a lot of interesting work still to be done looking into the strategies that animals use to make their footpads stickier - these would likely have very useful applications in the development of large-scale, powerful yet controllable adhesives,” says Labonte.

This study was supported by research grants from the UK Biotechnology and Biological Sciences Research Council (BB/I008667/1), the Human Frontier Science Programme (RGP0034/2012), the Denman Baynes Senior Research Fellowship, and a Discovery Early Career Research Fellowship (DE120101503).

Reference:

Labonte, D et al "Extreme positive allometry of animal adhesive pads and the size limits of adhesion-based climbing." PNAS 18 January 2016. DOI: 10.1073/pnas.1519459113

Inset images: Vallgatan 21D, Gothenburg, Sweden (photo by Gudbjörn Valgeirsson, footprints added by Cedric Bousquet, ̽��ֱ�� of Cambridge); How sticky footpad area changes with size (David Labonte); Diversity of sticky footpads (David Labonte).

Latest research reveals why geckos are the largest animals able to scale smooth vertical walls – even larger climbers would require unmanageably large sticky footpads. Scientists estimate that a human would need adhesive pads covering 40% of their body surface in order to walk up a wall like Spider-Man, and believe their insights have implications for the feasibility of large-scale, gecko-like adhesives.

If a human wanted to climb up a wall the way a gecko does, we’d need impractically large sticky feet – and shoes in European size 145

Walter Federle

A Hackmann & D Labonte

Gecko and ant

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. For image use please see separate credits above.

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How the stick insect sticks (and unsticks) itself

jeh98 — Wed, 07 Oct 2015 13:12:35 +0000

Geckos, tree frogs, spiders and insects all share a special skill – they can walk up vertical surfaces and even upside down using adhesive pads on their feet. But geckos have ‘dry’ feet, while insects have ‘wet’ feet.

Scientists have assumed that the two groups use different mechanisms to keep their feet firmly attached to a surface, but new research from David Labonte and Dr Walter Federle in the ̽��ֱ�� of Cambridge’s Department of Zoology provides evidence that this isn’t actually the case.

“It has generally been assumed that the fluid on their feet must be involved in helping insects like stick insects adhere to a surface by capillary and viscous forces – in the same way that a beer glass will stick to a glass table if it’s wet on the bottom,” explains Labonte, lead author of the study published in Soft Matter, “but our research shows that the fluid is likely used for something else entirely – it may even help insects unstick their feet.”

By measuring how much force was required to detach the foot of a stick insect from a glass plate at different speeds and applying the theory of fracture mechanics, Labonte and Federle found that only a ‘dry’ contact model could explain the data. They also carried out a comparison of the sticking performance of wet and dry adhesive pads, which revealed that there is a striking lack of differences between the two, contrary to previous opinion.

Insects and geckos need to walk up vertical surfaces and even upside down in order to get to the places where they feed and to escape from predators. As smooth surfaces don’t allow them to grip with their claws, they need soft adhesive pads on their feet and legs. This means they need to have excellent control over adhesion – to ensure their feet stick when they want them to, but can also unstick easily to allow them to walk around or run away from predators.

“Both wet and dry adhesive pads behave in a similar way to soft, rubbery materials in that, when they are pressed against another surface, there is a large area of contact between the two surfaces,” says Labonte. Both pad types then rely on shear forces to control their stickiness: insect and gecko feet are much stickier when they are pulled towards the body.

“ ̽��ֱ��fluid that insects have on their adhesive pads doesn’t seem to increase the pads' stickiness by means of capillary or viscous forces, and the same may hold for the fluid on the feet of spiders and tree frogs.”

So what is this fluid for?

Labonte and Federle believe it may act as a ‘release layer’ to help insects unstick their feet when they want to move. “If you think of commercial adhesives, like Scotch tape, there are often bits of tape or residue left behind when you remove it quickly. But a stick insect needs to be able to unstick its feet without expending a lot of energy or leaving bits of its foot still stuck to a leaf,” explains Federle.

“ ̽��ֱ��fluid may act as a lubricant to make detachment easier, giving insects greater control over adhesion at very short timescales.”

“When the first microscopes were invented in the 17th century, one of the first things scientists looked at was a fly’s foot. ̽��ֱ��purpose of the fluid that you find on insects’ feet has remained a fascinating question ever since,” says Labonte.

But it’s not just an age-old question that this research is helping to answer. ̽��ֱ��researchers say there may be lessons to learn for modern manmade devices.

“Understanding how insects control adhesion could have applications where adhesion is needed in a dynamic context, for instance in the production of small electronic devices, where it’s necessary to pick up and place down tiny parts with ease and accuracy,” adds Federle.

This research was enabled by funding from the Biotechnology and Biological Sciences Research Council and the Human Frontier Science Programme.

Reference:

David Labonte and Walter Federle ‘Rate-dependence of ‘wet’ biological adhesives and the function of the pad secretion in insects’ Soft Matter (2015).

Inset images: Composite figure showing the adhesive pad on the foot of a stick insect (T Endlein and David Labonte); Stick insect (T Endlein); Ant's foot showing a fluid trail (Walter Federle).

New research shows the fluid found on insects’ feet does not help them adhere to vertical and inverted surfaces, as previously thought, but may in fact help them to unstick their feet more easily to allow greater control over their sticking power.

When the first microscopes were invented in the 17th century, one of the first things scientists looked at was a fly’s foot. ̽��ֱ��purpose of the fluid that you find on insects’ feet has remained a fascinating question ever since

David Labonte

Walter Federle

Ant's foot showing a fluid trail

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. For image use please see separate credits above.

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