The Gecko’s Foot: How Scientists are Taking a Leaf from Nature's Book. Peter Forbes

and ‘superhydrophilicity’ (super-wettability) are buzzwords in many research departments.

      Although the Lotus-Effect could work with any number of materials, in practice the early versions all used silicones, contemporary technology’s favourite water-repellents. These are very effective but tend to be expensive. In 2003, a Turkish team of researchers found a way to make Lotus-Effect coatings from poly-propylene (the stuff kitchen bowls are made of). An advantage of this simple technique is that these lotus-style polypropylene coatings can be applied to almost any material: glass, aluminium, steel, Teflon^® and polypropylene itself. The only limitation is that the material the coating is applied to must not be attacked by the solvent used. Commercial exploitation of this technique is under way.

      For some people, the exterior walls of their house are only slightly less remote than Alpha Centauri: self-cleaning walls are fine but if this idea is so good, can’t it be used to make self-cleaning clothes? Is there any hope that in the future, accidents with red wine and coffee could be less ruinous? Yes, there is.

      A self-cleaning fabric known as Nano-Care® has been developed by the American serial chemical inventor and entrepreneur David Soane (he has about 100 patents to his name and so far has started seven companies) and marketed by his firm Nanotex. Stain-resistant jeans and khakis using Nano-Care have been available in the USA from firms like Gap, Eddie Bauer and Lee Jeans since 2001 and shirts arrived a short while later. The fabrics first appeared in the UK in September 2004 with the Rocola Shirt Tec range from Morrison McConnell, a Derby-based firm and part of the Van Heusen group.

      There have been many claims for stain-free clothes over the years and scepticism is understandable. The London Evening Standard tested them on the eve of launch by throwing lager, coffee and a particularly deep ruby red wine at the shirts. They passed: not quite every drop of the coffee was repelled but in all but the most extreme cases the shirt did what it said on the label.

      The lotus leaf of Nano-Care is the peach. Peaches have a soft fuzz of hairs on the surface that function like the bobbles on a lotus leaf. They trap air and make water sit on top of the hairs. But this is very much an analogy only. If you put a peach under the tap you will see that water does run off at first, but the downy hairs are soon swamped and the surface wetted. Nano-Care whiskers are made of stronger stuff.

      Nano-Care uses the lotus principle but the hairs are very tiny, less than a thousandth of the height of the lotus bumps. Compared to them, the cotton thread they stick to is an enormous tree trunk. The hairs are chemically bonded to the fibre and do not come off in the wash. And because they are so tiny, they do not change the feel of the cotton fabric appreciably.

      Nanotex is a 21st-century textile company. It licenses the technology to chemical companies and buys back the nanofibre polymers to sell to textile companies which must then use the Nano-Care^® trademark on the product. Nano-Care is an environmentally friendly technology in more ways than one. It makes traditional, organic cotton into a hi-tech fabric with better properties than synthetics; the process in which the nanowhiskers are attached is a normal textile process using watery solutions, and in everyday use these fabrics require fewer cleaning materials.

      Whatever the technique, there will always be a need to make self-cleaning effects last longer. As Pilkington’s Kevin Sanderson says:

      I think that’s something that the hydrophobics have got to solve: if someone comes along and puts their fingerprint on it, it’s not going to be superhydrophobic again until someone removes that smudge. The lotus leaf repairs itself because it has tiny wax crystals that grow back; if you have a surface that mimics the effect it can’t do that. The Lotus-Effect is a very nice idea and it clearly works but these kinds of questions need to be answered.

      The great thing about titanium dioxide is that it is self-renewing. Sunlight, air and water are all it needs. Lotus-Effect paint has no such renewing power. Like all normal material surfaces, it gradually loses its powers.

      Could titanium dioxide be used with Lotus-Effect coatings to produce a self-renewing capability? On the face of it this is unlikely because a waxy Lotus-Effect coating and titanium dioxide at first seem to be chalk and cheese (or oil and water). They work in opposite directions: Lotus-Effect coatings being super-water-repellent and titanium dioxide super-water-attracting. But it turns out that very small quantities of titanium dioxide can have a significant effect in breaking down organic deposits on a Lotus-Effect coating without significantly weakening its water repellency. It could so easily have been the other way round: there is an element of pure luck in technology.

      Not surprisingly, nature has already combined the Lotus-Effect and Activ technology – in the shape of a beetle that lives in the Namib Desert in southern Africa. The purpose here is not self-cleaning but water collection, for this is a harsh, arid, almost rainless environment where the only moisture comes in the form of wind-driven morning fogs. Remembering that Activ glass captures the dew, gives us the clue that creatures in this environment might want to use water-attracting surfaces to harvest what water there is in fog.

      This is just what the Namibian Darkling beetle does. The beetle is 2 cm long and its wing covers are warty, with bumps about half a millimetre in diameter. Under the microscope, the area between the bumps is also seen to be bumpy but at a nanoscale; the peaks of the big bumps are water-attracting whilst the rest of the surface is waxy and water-repelling.

      The tips of the bumps attract and collect very fine droplets from the mist; they coalesce and grow and then the waxy portions come into play. When the droplets reach a certain size (about 5 mm), they swamp the tip and begin to roll. The other bumps help the drops roll towards the mouth of the beetle. The beetle has a rather comical ‘water-collecting posture’ in which it stands into the wind, face down, to present a sloping back for the water to run down.

      The beetle’s trick with the foggy foggy dew came to light, as so often, when researchers were looking for something else. In 2001, Andrew Parker, a young zoologist at Oxford, came across a photograph of beetles eating a locust in the Namib Desert. The desert is probably the hottest on Earth and the locust, which had been blown there by the strong winds typical of the region, would have perished the instant it hit the sand. But the beetles were obviously comfortable.

      Parker investigated the beetles, expecting to find sophisticated heat-reflection surfaces. They do indeed have such a capacity but Parker also immediately noticed the bumps on their backs. Parker is a modern researcher with an eye for bio-inspiration; the fog-harvesting ability of these beetles had been noticed back in 1976 but at the time no one looked at the mechanism. Parker immediately suspected that some adaptation of the Lotus-Effect was at work in the water-collection process.

      As with the Lotus-Effect proper, you don’t need a beetle, or any kind of living thing to get the effect. Water collection from fog in arid regions is an established technology: it is usually done with large nylon nets. But experiments on coated glass slides with artificial surfaces mimicking those of the beetle and control slides with entirely waxy or water-attracting surfaces quickly showed that the beetle’s structure is the best for the job. Here was an efficient new way of collecting water. Parker is developing the idea with QinetiQ, the hi-tech research company spun off from the Ministry of Defence research department at Farnborough. In 2004, the process was patented and commercial applications are forthcoming.

      Stripped of the needs of the beetle, the system boils down to alternating regions of water-attracting and water-repelling surfaces with the latter being the background, as it is with the beetle. The width of the water-attracting regions governs the droplet size. The technical device mimics the beetle’s head-down posture by setting the collecting plates at an angle so that the water collected simply runs off into a trough. Although there is a tendency for the wind to roll the droplets back, if the size of the droplets is tuned to be large enough, they will roll against the wind into the collecting trough.

      The desert-beetle water-collection mechanism is so simple and founded on such basic properties of