Geckos, spiders and the comic-book hero Spiderman seem to defy gravity by scurrying along smooth walls and ceilings. Now, a physicist in Italy claims that humans could soon do the same by donning a sticky "Spiderman suit" woven from carbon nanotubes. Nicola Pugno of the Politecnico di Torino in Italy has calculated that -- assuming the material could actually be made -- a person wearing the suit would be able to cling safely to smooth surfaces such as the side of a skyscraper (J. Phys.: Condens. Matter 19 395001).
The stickiness of geckos and spiders comes from thousands of tiny fibres on their feet that grab hold using a combination of three effects – capillary forces arising from a thin layer of liquid water between the fibres and the surface; van der Waals attraction between the fibres’ molecules and those on the surface; and Velcro-like interlocking of the fibres with tiny structures on the surface. Unlike glue, these effects still allow the feet to easily detach from the surface and thus allow the creatures to walk, and they also seem to prevent the feet from accumulating dirt.
Pugno claims that gloves and boots for humans employing the same effects could be made by weaving millions of carbon nanotubes – which are each only about 10 nm thick – into threads about 1 cm thick. The thickness of the individual nanotubes and their spacing could be chosen to make the thread transparent to visible light, which Pugno claims would make them invisible.
The nanotubes at the end of a thread would be splayed in a fan-like structure, which would ensure that there were millions of contact points between the thread and a surface in order to maximize its stickiness. Pugno claims that a combination of capillary, van der Waals and mechanical forces would allow one such thread to support the weight of a man (70 kg) and that a pair of gloves covered in them could support over 1000 kg.
Pugno says that the material would be self-cleaning because carbon nanotubes are hydrophobic and therefore shed water, which takes dirt with it. Because adhesion involves million of tiny sticking points – each of which is relatively weak on its own – Pugno believes that the material could be peeled off the surface with a minimum of effort, provided the user was specially trained in the required hand motions.
Pugno even goes so far as to suggest that the material could be made into a sticky and invisible web that could be used to capture villains. Strains on the material caused by a struggling victim would change the material’s optical properties, rendering it visible to an aspiring Spiderman.
Although the idea of a Spiderman suit may seem far fetched, several research groups have already made sticky materials inspired by geckos – using polymer fibres rather than nanotubes. Also, researchers in the US have made fan-like structures from carbon nanotubes, which Pugno believes could be used in his suit.