London, Jan 19: Humans would need adhesive pads covering 40 per cent of their body surface in order to walk up a wall like Spiderman, according to a new Cambridge study that may lead to development of large-scale, gecko-like adhesives.
The study shows why geckos are the largest animals able to scale smooth vertical walls - even larger climbers would require unmanageably large sticky footpads.
In climbing animals from mites and spiders up to tree frogs and geckos, the percentage of body surface covered by adhesive footpads increases as body size increases, setting a limit to the size of animal that can use this strategy because larger animals would require impossibly big feet.
Dr David Labonte and his colleagues in the University of Cambridge's Department of Zoology found that tiny mites use approximately 200 times less of their total body area for adhesive pads than geckos, nature's largest adhesion-based climbers.
Humans would need about 40 per cent of total body surface, or roughly 80 per cent of front, to be covered in sticky footpads if we wanted to do a convincing Spiderman impression. Once an animal is big enough to need a substantial fraction of its body surface to be covered in sticky footpads, the necessary morphological changes would make the evolution of this trait impractical, said Labonte.
"If a human, for example, wanted to walk up a wall the way a gecko does, we'd need impractically large sticky feet - our shoes would need to be a European size 145 or a US size 114," said Walter Federle, senior author also from Cambridge's Department of Zoology.
The researchers said 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.
"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 a blue whale is mostly volume with not much surface area," said Labonte.
"This poses a problem for larger climbing species because, when they are bigger and heavier, they need more sticking power to be able to adhere to vertical or inverted surfaces, but they have comparatively less body surface available to cover with sticky footpads," he said.
"This implies that there is a size limit to sticky footpads as an evolutionary solution to climbing - and that turns out to be about the size of a gecko," said Labonte.
Larger animals have evolved alternative strategies to help them climb, such as claws and toes to grip with, researchers said.
They compared the weight and footpad size of 225 climbing animal species including insects, frogs, spiders, lizards and even a mammal. The study was published in the journal PNAS.