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The Secret of Wall Climbing

Originally published in Green Country, Issue 128 (Oct 2017)
Author: Green Power

The superhero in the movie Spider Man acquired the ability to climb walls like a spider—typically on vertical walls but also crossing the ceiling high above. In the technologically advanced world of today, is it possible that we humans can also climb walls with our bare hands?

To answer this question, we have to understand the mechanism by which animals climb. We may start from the observation made by Prof. Kellar Autumn, researcher from the University of California at Berkeley, during a holiday – as he was lying on his bed and looking up the ceiling. He was inspired by a gecko moving about and preying on a spider on the ceiling. He decided to find out how the gecko can move as it wishes on the ceiling.

Van der Waals’ force is formed between two very close surfaces, bonding them together. Prof. Autumn observed the gecko’s foot structure under an electron microscope and found that there are about 5,000 setae on every square millimeter of a foot’s surface, and each seta has hundreds of spatula-shaped structures at its end. All these greatly increase the contact area between a gecko’s foot and a climbing surface, resulting in sufficient Van der Waals’ force to allow the gecko to strongly cling to the surface. From this discovery, Prof. Autumn and his team invented and patented the gecko-like dry adhesive “gecko glue” in 2004.

Size No. 145 shoes

If a gecko can move freely on the wall with Van der Waals’ force, is a human capable of doing so? In 2016, Cambridge University researcher Dr. David Labonte and his team published a report saying that gecko is the largest animal capable of adhesion-based climbing. They studied 225 climbing species including ants, tree frogs and spiders, and recorded the pad area in touch with the climbing surface and the total body surface and weight. It was found that when body weight increases, the proportion of pad area to total surface also increases. As an illustration, the figure for spiders is 0.92% and that for geckos is 4.3%.

Extrapolating from these figures, if a human wanted to climb up a wall the way a spider does, the pad area should be 40% of the total surface. In other words, we cannot climb walls with our hands and feet alone. The research team indicated in the report that, “We’d need impractically large sticky feet – and shoes in European size 145,” in order to climb up a wall. This is equivalent to having feet more than a metre long!

Amazing abilities of different creatures cannot be readily imitated, but are sources of all kinds of inspiration!

Image
The spatula-shaped setae on a gecko’s footpad are key to its wall-clinging ability.
Photo from Pixabay
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Tree frogs are among the animals that can cling to vertical surfaces.
© Henry Lui
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The red spot represents the proportion of an animal’s pad area. As body weight increases, the proportion has to be increased in order to cling to a vertical surface.

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