Sticky Gecko Feet Nothing to do With Glue

Close up of a gecko's foot on glass. Source: Wikimedia Commons

A few years ago, I was persuaded to watch Spider-man. There was one scene that has remained in my mind after Peter Parker was bitten by a genetically modified spider. It is the scene where Peter looks down at his hands finding black hairs sprouting from them allowing him to stick to walls and ceilings as if they were velcro. This makes Spiderman share a similarity with geckoes. In 2002, scientists found that geckoes cling onto surfaces using small hairlike projections from their feet.

Geckoes have an uncanny ability to cling to a number of surfaces, including smooth surfaces like polished glass. The hairlike projections called setae which themselves are made up of smaller hairlike structures called spatulae. Using a scanning electron microscope, (SEM), the setae look like hairs with many split ends.

The intermolecular forces which David Attenborough mentions in the video are ones that occur between neighbouring molecules. These are different to to the forces that hold an individual molecule together like ionic bonds or covalent bonds.

All molecules experience intermolecular attractions. In a gas like hydrogen or helium, the intermolecular attractions are quite weak, but in liquids and solids, the attraction is stronger and molecules “stick” to one another.

The spatula tipped setae on the footpads of geckoes exploit intermolecular attractions to help them to adhere to different surfaces. Research has shown that the specific intermolecular force being utilised by geckoes are the van der Waals forces. Johannes Diderik van der Waals first described this force as the sum of attractive and repulsive forces between molecules of a substance. They occur due to the movement of electrons around a molecule. In one moment they could be at one end of the molecule making this end to have slightly negative charge and the other end slightly positive. Now, opposites attract so another molecule nearby in this moment of time could line up near the first molecule with its positive end attracted to the negative end of the first molecule. This occurs throughout the entire substance and happens extremely quickly as the electrons move around at high speeds.

The van der Waals forces are small so to use them, a very large surface area is needed to maximise contact at the molecular level. One square millimeter of the footpad of a gecko has 14,000 setae. Each individual seta has a diameter of 5 micrometers, narrower than human hair which has a diameter between 18 and 180 micrometers. There are between 100 and 1000 spatulae which make up each setae and these are 0.2 micrometers in diameter. The setae are even self cleaning.

Scientists are using the inspiration from the feet of geckoes to develop new types of adhesives that do not use a sticky paste.

There is a 2005 study suggesting that the very thin layer, (about one atom in thickness), of water molecules present on natural surfaces also plays a role in how geckoes attach themselves to surfaces.

[UPDATE 7th March 2011]: An email from Professor Kellar Autumn who was part of the initial research team that uncovered the link between van der Waals forces and geckoes feet has alerted me to a 2010 study that he was also part of that provided evidence that the thin layer of water molecules on surfaces do not play a role in geckoes gripping to surfaces.

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• Sticky feet send insect-bot climbing up the walls (newscientist.com)

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