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

The success of the Lotus-Effect and Activ glass has stimulated much research and the story is far from over. The range of possible applications of the Lotus-Effect is in inverse proportion to its elemental simplicity. If the Lotus-Effect were a plant it would be seen as a rampant ecological invader. ‘Superhydrophobicity’ (super-non-wettability) 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 (#litres_trial_promo) (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® (#litres_trial_promo) 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? (#litres_trial_promo) 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 (#litres_trial_promo) – 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 (#litres_trial_promo). 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 matter it seems astonishing that it should have waited till our technology had reached such a peak of sophistication before being discovered. Water is so ubiquitous that we take it for granted. But nature exploits every possible property of a substance. Having mastered all kinds of complexity, we are now catching up on some tricks that are simplicity themselves, like this new source of water we might call Beetle Juice.

Some of the ongoing Lotus-Effect research has a playful quality in keeping with the purity of this blindingly simple idea. In 2001, two French researchers came up with a Zen-like party trick by coating drops of water so that they can roll on glass without breaking up, or even float on water itself (fig. 2.8). These ‘liquid marbles’ are made with lycopodium

(#litres_trial_promo) grains coated with a silicone. This creates a lotuslike surface with almost perfect water repellency: hence their spherical shape and ability to float on water. This ‘non-stick water’ (#litres_trial_promo) may eventually find applications in the packaging and delivery of fluids, but for now it induces a Buddha-like smile at the quirkiness and eternally surprising nature of the physical world.

As usual, when we think we’ve invented something really far-out, nature seems to have got there first. There are aphids that, in an example of the crazily degraded lifestyles that are so common in the natural world, live all their lives inside plant galls. In fact, the galls – those warty lumps found on the undersides of tree leaves, especially on oaks – are created by the aphids, which interfere with the host plant’s metabolism, thus creating the galls. In choosing, in evolutionary terms, to live like this, these aphids have created a problem for themselves. Aphids feed on the sap of plants and they produce large quantities of a whitish, sugary excrement known as honeydew. Aphids that live on the surface of plants have developed a symbiotic relationship with ants, who feed on the honeydew and protect the aphids. But gall-living aphids have no such means of disposal: they risk drowning in their own excrement unless they can easily evacuate it from the gall. The honeydew is very sticky and once an aphid gets trapped in a ball of honeydew it can’t escape.

To the rescue comes super-non-wettability of an ingenious kind. The aphids produce needles that break off and line the inside of the gall with a rough waxy coating. The drops of honeydew are coated with the wax and become non-wetting honeydew parcels just like the water marbles. There is even a caste of soldier aphids whose job it is to elbow the parcelled-up honeydew balls out of the gall!

The aphid’s secret (#litres_trial_promo) was revealed in a paper, wittily entitled ‘How aphids lose their marbles’, by the young Indian physicist L Mahadevan and his team. Mahadevan, at Cambridge University when he did this work and now at Harvard, is one of the most dazzling figures in bio-inspiration. He is a mathematical physicist who works with biologists to unravel bio-inspired problems right across the spectrum. His papers have artistic references wherever possible, rigorous mathematics and, above all, they impart a sense of the remarkable creativity, chutzpah even, of nature in devising these solutions.

When I visited Mahadevan at Harvard, his computer desktop was a treasure trove of biological curiosities, involving origami, the draping patterns of clothes, biological springs and ratchets, and those aphids that lose their marbles. Mahadevan admits to having a short attention span, which means that he attacks these problems in a brilliant mercurial way and then passes on to the next. He is a delighted roamer in this new terrain of bio-inspiration, throwing out brilliant suggestions that others can follow up.

So we see that the Lotus-Effect is not just a matter of building maintenance. It sheds light on many strange corners of the natural world as well as adding some radiance to the built environment. Just as the self-cleaning properties of the sacred lotus were of philosophical, spiritual and artistic importance to eastern civilizations, the idea of self-cleaning can be a secular boon to the northern latitudes. In The Poetics of Space (#litres_trial_promo), the French philosopher Gaston Bachelard has suggested that cleaning might itself have spiritual/aesthetic value:

And so, when a poet rubs a piece of furniture – even vicariously – when he puts a little fragrant wax on his table with the woollen cloth that lends warmth to everything that it touches, he creates a new object; he increases the object’s human dignity; he registers the object officially as a member of the human household.

Water is one of our prime elements and in our whoring after complex chemistry we have forgotten how many subtle effects nature produces simply by manipulating water in some way. Repelling water is both the mechanism and the purpose of the Lotus-Effect, but at the nanoscale the subtle control of the water-attracting and water-repelling qualities of proteins can produce properties that have nothing to do with cleaning.

Spider silk is composed of such a protein and its strength comes from the way the fibre is spun from a watery solution, using water-attracting and water-repelling regions to create a composite structure that materials scientists would dearly love to mimic. Indeed, spider silk is regarded by many as the holy grail of materials science. The Lotus-Effect still has much scope for development but it has reached a degree of fruition: the spider guards many secrets still.

CHAPTER THREE Nature’s Nylon (#ulink_fd774191-c5e2-5b02-a99b-0be97072150f)

What Skill is in the frame of Insects shown?

How fine the Threds, in their small Textures spun?

RICHARD LEIGH, ‘Greatness in Little ’

The astounding properties of spider silk (#litres_trial_promo) have been recognized for decades. In the force needed to break it when pulled, spider silk is about half as strong as mild steel, so the oft-quoted ‘spider silk is stronger than steel’ is not strictly true. Steel, however, is nearly eight times denser than spider silk so weight-for-weight spider silk is about six times as strong as steel. Spider silk is much more stretchy than steel, extending by 30–40% before it breaks; it is about twice as stretchy as nylon and eight times more stretchy than Kevlar

. What is special about spider silk is that it is both stretchy and tough: a rubber band will stretch more than spider silk but its breaking strength is very low. Spider silk is the only material with exceptional stretchiness and good breaking strength.

Spider silk has been brought to a pitch of perfection by millions of years of evolution. And this optimization means that there isn’t just one generic spider silk: a single spider can make up to seven different kinds of silk, each tailored towards a specific task: the dragline from which the web is hung is the strongest, the capture threads have the greatest extensibility, and so on. Spider silk’s great resilience has long suggested human applications. The web has to catch a heavy insect at speed, and bring it to a standstill without snapping and without flinging it back out again in recoil, a process reminiscent of the arrester wires used to bring jets landing on aircraft carriers to a halt.

Spiders have been working their magic for over 400 million years – that’s pre-dinosaur time. The oldest existing strand of spider silk (#litres_trial_promo) was reported in 2003, preserved in Lebanese amber. It dates from the Early Cretaceous Period, more than 120 million years ago and what is fascinating about this specimen is that the small globules of ‘glue’ that are strung along the capture threads are still clearly visible, as they are on spider webs today.

We think of spider webs as delicate filigree structures, best seen with dew or frost accentuating their patterns. The garden spider (Araneus diadematus) is one of the best web spinners (fig. 3.1). But tropical spider webs can be very large (#litres_trial_promo): the queen of spinners is the golden orb-weaving spider (Nephila claviceps), which can be 5–8 cm long and 20 cm in total span: her webs are up to 2 m in diameter – big enough in fact to be useful economically. In Papua New Guinea, they have been draped across bamboo poles and looped at the end to make fishing nets. Early Western explorers also encountered such webs: in 1725, Sir Hans Sloane reported the nets were “so strong as to give a man inveigled in them trouble for some time (#litres_trial_promo) with their viscid, sticking quality”.

For those who fear spiders, a web large enough to enmesh a person is the stuff of nightmares. The fear of spiders is a widespread cultural phenomenon, and my interest in the silk made me question my own attitude to spiders. Primo Levi, who is always a good guide to our reactions to the natural world, summed up the symbolism of spiders like this:

The old cobwebs in cellars and attics (#litres_trial_promo) are heavy with symbolic significance: they are the banners of desertion, absence, decay and oblivion. They veil human works, envelop them as though in a shroud, dead as the hands which through years and centuries built them.

Other People’s Trades

There is a constellation of factors that creates a general sense of unease. There are very few large, hairy poisonous spiders (the tarantula, despite the legend, is not much more poisonous to a human being than a wasp), but all spiders, by association, share in a little of the horror that these monsters can conjure up. The spider’s snaring and ambushing techniques worry some people. Something as deep-seated as spider phobia most likely has sexual connotations: the fact that the female sometimes consumes the male after mating suggests that men can associate them with women who symbolically castrate, if not devour. But women in particular are sufferers from arachnophobia. One attribute of spiders that would cause unease if it was generally known would be the fact that they have eight eyes. But most people have never seen them because they are only visible through a microscope.

Before I became seriously interested in spider silk, I had realized that not only is the web of the garden spider very beautiful but the creatures themselves, with their light speckled colouring, are much the most comely spiders you are likely to come across unless you become a dedicated arachnologist. Once I learned about the silk, my conversion to spider-worship was complete and I became ashamed of my earlier hostility.

The first documented attempt to exploit spider silk was by a Frenchman (#litres_trial_promo): in 1709, Xavier Saint-Hilaire Bon made gloves and stockings from the silk and presented them to Louis XIV. He wrote ‘A Dissertation on the usefulness of spider silk’. The scientist René Réaumur investigated these claims (#litres_trial_promo) in 1710 and concluded that only egg cocoon silk is good enough for spinning but it lacks lustre (a surprising finding given that all modern research focuses on the dragline silk from which the web is hung). Réaumur estimated that it would need 27,468 female garden spiders to make 1 lb of silk. Despite this discouraging report, the Chinese Emperor requested a copy of his paper and Chinese silk experts attempted to exploit spider silk. In 1876, the Chinese Emperor gave Queen Victoria a spider-silk gown. Whether this had been a century and a half in the making, since their initial interest, we do not know. Despite the impression we have of the evanescence of spider webs, the silk is durable. In Austria in the late 18th century, there was a tradition of painting on spider webs and some of these pictures exist to this day; one of them hangs in Chester Cathedral.

There is no great problem in spinning silk from a single spider. You can just about do it yourself with some improvised kit and patience. First, wait for the webs made by the garden spider, which appear in late July, then catch a spider. The spider has to be restrained, obviously, and a Styrofoam block makes a handy mount. The spider must gently be turned onto its back and a couple of very light rubber bands used to pin four legs on each side close to the body. Once the spider is stable you can start a thread by lightly touching the spinneret under the belly with a glass rod and pulling gently.

If you want to create a reel of silk, the Styrofoam block can be mounted on a piece of wood, with an improvised reel at the other end. This could be a cotton reel on a spindle with a handle. If you happen to know anything about hand-spinning, you could collect a few bobbins of spider silk and try to braid them into a thicker thread that would bring it closer to the dimensions of usable textiles. Whatever you do with the silk, the last stage is to let the spider go, when it will instantly return to work, repairing its web.

Although you cannot see any of this without a microscope, it is worth knowing that a garden spider has three pairs of spinnerets (#litres_trial_promo), each with multiple spinning tubes – more than 600 in all (fig. 3.2). Without an explanation, a picture of this apparatus might appear to be some kind of technical glue nozzle system.