'Spider-man' suit secret revealed
A "Spider-man" suit that enables its wearer to scale vertical walls like the comic and movie superhero could one day be a reality, according to a study.
Natural technology used by spiders and geckos could help a human climb the side of a building or hang upside down from a roof, the analysis suggests.
The findings are published in the Journal of Physics: Condensed Matter.
Both spiders and geckos possess tiny "hairs" that allow them to stick to surfaces.
Some studies suggest that geckos can hold hundreds of times their own body weight.
In 2002, US research suggested this adhesion in geckos was due to very weak intermolecular forces.
These are produced by billions of hair-like structures of different sizes that are arranged in a hierarchical structure on each gecko foot.
The intermolecular "van der Waals" forces arise when unbalanced electrical charges around molecules attract one another.
The cumulative attractive force of billions of gecko hairs allows the reptiles to scurry up walls and even hang upside down on polished glass.
Professor Nicola Pugno, from the Polytechnic of Turin, Italy, has calculated how sufficient stickiness could be generated in the same way to support an adult human's body weight.
But the bigger the surface that needs to stick, the lower its adhesion strength. So a glove able to fit a man's hand, and covered with artificial gecko hairs, should not be as sticky as a gecko's foot.
Luckily, the gecko only uses a fraction of the theoretical stickiness available through van der Waals forces.
"Some researchers were able to measure a [theoretical] adhesion strength 200 times higher than the adhesion strength in the gecko. But between theory and practical applications there is a large gap," said Professor Pugno.
"If we are able to make a surface a little bit stronger, so that the size effect vanishes, we might be able to make a suit with the same adhesion as a gecko."
The Turin-based researcher proposes that carbon nanotubes could be used as an artificial alternative to the gecko's hairs.
Carbon nanotubes are tiny cylinders of carbon that measure just a few billionths of a metre across. They are ultra-strong and can be organised into larger fibres.
Professor Pugno also outlined three properties which a real Spider-man suit must demonstrate.
Firstly, and most obviously, it must be able to demonstrate strong adhesive properties. Secondly, the suit must be able to detach easily from a surface after it has stuck. Thirdly, the suit must, to some degree, be able to clean itself.
The latter requirement is considered important because dirt particles could get in the way, interfering with the adhesive properties of the suit.
One way to do it is to make the suit "superhydrophobic", so that it strongly repels water. As water droplets are forced away from the contact areas of the outfit, they should wash away particles of dirt.
This property could be achieved simply by altering the geometrical properties, or topology, of the surface.
"To have all these mechanisms working together is difficult, because they are in competition with one another," Professor Pugno told the BBC News website.
"But geckos and spiders provide a natural demonstration that this can be done."
He added that there were many interesting applications for adhesive suits, in areas ranging from space exploration to defence. The work could also aid the design of gloves and shoes for window cleaners working on tall skyscrapers.
But human muscles are very different to those of geckos, so people would probably suffer from muscle fatigue if they tried to stick to a wall for many hours.
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