During the summer of 2003, scientists at the University of Manchester, UK, produced a revolutionary new kind of adhesive tape. This tape will stick to anything, peel off anything without leaving a residue, and it is potentially infinitely reusable1. The mechanism by which it worked was completely new to technology but, like many inventions, was not new to nature. The scientists had been inspired by the humble gecko, a group of over 300 kinds of small, insectivorous lizards renowned for their ability to climb smooth, vertical walls and even across ceilings. This ability was first noted in the 4th Century BC by Aristotle, who marvelled at the gecko's ability to run up and down tree-trunks, even head-downwards. This ability has been the subject of rather more intensive scientific study for the last 100 years.
Mechanism of Gecko Adhesion
The gecko can cling to a wall with remarkable tenacity. You wouldn't be able to pull it away from the wall, and it has been said that you could hang a fully-laden rucksack from a gecko on the ceiling, and it still wouldn't fall off.
The mechanism by which the gecko is able to cling to a wall or ceiling is unique in nature. For example, it does not have little suction pads on its feet, like the tree frog, because, to make suction pads stick, you would have to squeeze them hard against the wall in order to expel the air, and this would require more power than the gecko has in the whole of its body. Some insects, such as the housefly, have sticky pads on their feet, and as the insect moves across a surface the glue leaves a sticky trail across it. Geckos leave no trace at all on a wall or ceiling, except possibly footprints on a dusty surface. Another mechanism that might potentially be used would be static electricity, as is employed by cockroaches. However, we know that geckos do not use this as they can adhere to wet walls as well as to dry ones, and water dissipates static electricity. Microscopic examination reveals that the gecko doesn't even have Velcro-like hooks on its feet.
The mechanism by which geckos are able to cling to surfaces remained a mystery until Dr Kellar Autumn, working at the University of California, discovered in 2002 that the gecko foot appears to be covered in hundreds of overlapping scales or plates. Under the scanning electron microscope, it can be seen that each plate is made up of many millions of tiny hairs. There are about half a million of these hairs, called setae, on each of the gecko's four feet. The tip of each seta has 100 to 1,000 tiny pads, called spatulae, making a total of 108 - 109 per gecko; and these are the key to the gecko's strong adhesive force. Each spatula is about four millionths of a centimetre across.
Now, if we examined a section of wall under a microscope, we would observe that it is far from being perfectly smooth; it is a landscape of bumps, hollows and crevices. The spatulae on the gecko's toe-hairs are so minute and in such intimate contact with the wall, that molecules within the spatulae are able to interact directly with the molecules that comprise the wall.
Scientists believe that as the billions of spatulae come into close contact with a surface, the weak interactions between molecules in the spatula and molecules in the surface create bonds that collectively are 1,000 times greater than the gecko actually needs to adhere to the surface. Indeed, Kellar has measured the force of attraction of a single seta, using an atomic-force microscope, and found that it can lift the weight of an ant (20 milligrams). A million setae would be able to lift the weight of a 20 kilogram child. These short-range forces that operate between atoms and molecules are known as van der Waals forces, after the Dutch scientist, Johannes van der Waals (1837 - 1923).
As a gecko climbs a wall, it uncurls its toes like the type of party whistle that uncurls when you blow into it ('blowouts'), and rolls its setae onto the surface by some 'unusually complex behaviour'. However, it's detaching itself from the surface that creates the problems. When a gecko runs, it needs to attach and detach its feet about 15 times a second, and it appears that it does this by increasing the angle of the setae with the surface, much like unrolling a reel of sellotape.
Van der Waals Forces
Van der Waals attractive forces are intermolecular forces that exist between non-polar molecules such as alkanes. They are extremely weak forces, typically less than 1% of the average covalent bond strength2.
Van der Waals interactions3 are set up when the electronic distribution in a molecule is momentarily unsymmetrical, so that uneven charges are set up in the molecule. Hence the molecule becomes transiently polar, and is thus called a 'temporary dipole'. This can then disturb the electronic distribution in an adjacent molecule by inducing an opposite, or complementary, dipole. An analogy would be the way the north pole of a bar magnet induces a weak south pole in an unmagnetised iron nail brought near it, and thus the two are attracted to each other. The two temporary dipoles also attract each other and the two molecules are pulled close together. These forces operate only transiently because the electron densities in both the wall molecules and those of the gecko's setae are constantly changing, and so the forces are continuously being switched on and off.
Prior to Dr Autumn's discovery, scientists had thought that Van der Waals forces operated only at the sub-molecular level; it was never considered that they could operate at a scale that could be observed with the naked eye. The discovery that the gecko is able to exploit van der Waals forces has opened up the possibility that Man might also be able to exploit the phenomenon for his own purposes.
Thus, in 2002, Professor Andre Geim's team of nano-technologists4 at the University of Manchester succeeded in imitating the ultra-hairy structure of a gecko's foot on a piece of plastic one centimetre square. This was achieved by using an atomic force microscope to etch a plastic mould, thus making little hair-like 'pillars'. This would be of a sufficient size to hold up a toy spiderman. If you could manufacture a palm's width of the material, then this would allow an adult human to cling to the ceiling. The product was viewed using a scanning electron microscope.
Since then scientists in the USA have improved on the idea by adding multi-walled carbon nanotubes to the surface of a polymer. MWNTs have rough surfaces and remarkable flexibility, and can thus emulate the setae of gecko feet. Indeed, their adhesive strength is 200 times greater than gecko setae!
This, however, creates its own problems. Geckos have evolved over centuries to be able to attach and detach their feet from surfaces at will, but chemists, as yet, have been unable to achieve this synthetically. Dr Ali Dhinojwala of the University of Akron, Ohio, says:
It will be a challenge to figure out how to design a strong adhesive that can provide a strong attachment to support a large force, but at the same time have the capability of detaching itself from the surface with ease.
Is There Anything to Which a Gecko Can't Adhere?
Yes there is; Teflon, which is one of the trade names of polytetrafluoroethene (PTFE). This material was a chance discovery by Roy Plunkett of Du Pont Research Laboratories at Deepwater, New Jersey in 1938. Among a number of other novel properties, it was noted that this material had a slippery feel to it, and this was to be the secret of its later commercial success. Thus, it found application as a coating for non-stick frying pans in the 1950s when a method was found to bond it to aluminium.
The reason for its 'slippery feel' is that the van der Waals forces in PTFE are very, very weak indeed; in fact, PTFE has the lowest coefficient of friction of any known material. Hence PTFE molecules do not respond to the van der Waals forces in other materials.
Current Constraints to Commercial Production
At the present time the fabrication method used is not amenable for scaling up to mass production. Furthermore, a major technological challenge is to make the artificial setae sufficiently durable for multiple re-use.
These include 'gecko boots', boots with tiny hairs on their soles to, say, enable astronauts involved with extra-vehicular activity (EVA) to adhere to the sides of their space vehicle whilst making repairs; or for robotic rovers on, say, Mars, to climb vertical rock faces with gecko-style wheels or tracks.
It is considered that this material may also have applications as dry adhesives in space or in microelectronics.