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

But, at the time, chemistry had no answers to these questions. Whenever such structures and substances were mentioned in textbooks, the explanations petered out in sentences such as: ‘The hardness of the insect skeleton is due to the chitin being impregnated with another substance, called sclerotin or cuticulin; but not much is known about it chemically.’ There were some successes in getting close to nature. Nylon, for instance, invented in 1937 (#litres_trial_promo), imitated the chemical bond of natural protein fibres, but natural proteins such as wool, silk and spider silk were known to be much more complex than nylon. While the nylon molecule has the same chemical unit, linked nose to tail thousands of times, natural silks have different amino acid units, linked nose to tail (#litres_trial_promo) in a complex non-random pattern. Despite a concerted effort over the last 20 years to determine the structure of, and replicate, spider silk, it is still not fully understood.

Although science has been successful in uncovering things not directly known to our senses, the mindset required to solve the problems of nuclear physics and genetic inheritance tends to be impatient of such questions as: What lies between the molecular realm and the objects we can see? The great early 20th-century nuclear physicist Sir Ernest Rutherford notoriously used to say that ‘all science is either physics or stamp collecting’ (#litres_trial_promo). But our new science has arisen largely from the very stamp collecting Rutherford despised – descriptive biology, investigations of the habits of strange creatures, comparative studies of the microstructures of leaves.

For a Rutherford, these meanders off the central pathway were expected to be explained fully by the fundamental laws of physics. And when his kind of particle physics was at the forefront in the mid-20th century, there were no techniques available to investigate larger-scale phenomena.

The atoms of physics and chemistry are very small (about one ten billionth of a metre in diameter) and until 1971 this was far too small for any kind of microscope to see. Their size and properties were inferred from experiments on much larger quantities than single atoms: 1 gram of carbon contains about fifty thousand million million million atoms (#litres_trial_promo) (usually written 5x10

to avoid the cum-bersomeness of the expression). It was the triumph of chemistry that it was not necessary to see these tiny atoms in order to synthesize millions of new compounds whose precise structure is known.

This chemistry of inference, working in the dark, so to speak, was the chemistry I was taught at school in the 1960s – experiments were carried out with simple substances, stuff you could grasp and whose properties were clear. I might bubble carbon dioxide through limewater, say; the result was a white precipitate of solid matter. I could filter and dry this and the result would be calcium carbonate, the chemical that chalk and limestone are made of. Running parallel to this palpable experience, the books would give you an equation for their reaction, in this case:

Ca(OH)

+ CO

= CaCO

+ H

O

Like mathematical equations, these equations always balance because they represent the reactions between individual atoms and molecules, and nothing is ever lost in a chemical reaction. There is one calcium atom, four oxygens, two hydrogens, and one carbon on both sides of the equation. What happened in my test tube was this reaction, between individual calcium hydroxide and carbon dioxide molecules. And it was happening billions of times over to make enough of this substance for it to be visible to my eyes.

I chose this reaction as an example because the simple minerals of school chemistry, such as calcium carbonate and silicon dioxide, turn out to be capable of forming structures of architectural complexity in living systems, many of which are to be found in the deep oceans. The extreme conditions to be found there – intense pressure and little light, the dispersed nature of prey, the single medium of water – have inspired some ingenious devices. The romance of the oceans is epitomized by the Venus flower basket, a sea sponge and a baroque extravaganza of mineral basketwork so ornate that Joanna Aizenberg, the biomineralization expert at Bell Labs, who is studying it for its fibre-optical properties, cannot yet see how such a structure can grow from an egg. A new frontier indeed! To its beauty and mystery have now been added the fact that it possesses in the long hairs that surround the base of its latticework some brilliantly effective fibre-optic filaments. These, in human engineering terms, are the conduits used for high-capacity telephone and internet lines. The Venus flower basket has evolved these structures to manipulate what little light there is on the sea floor (at least we think it has – as with much else about the creature, biologists are not entirely sure).

Then there are the brittlestars, with primitive eyes that focus light through exquisitely engineered lenses made from single crystals of calcium carbonate (see Chapter 5). In these creatures, the crystallization of calcium carbonate is directed by proteins and this is one of the prime routes being explored in bio-inspiration: to direct the formation of engineered structures of minerals such as calcium carbonate and silica, using proteins, as nature does.

But simple chemistry was inadequate to explain how proteins organize minerals to produce these complex forms. Proteins are, unlike calcium carbonate, very large molecules. The molecular weight of CaCo

is 100 D (D stands for ‘Dalton’ and is a measure of the relative mass of atoms and molecules, hydrogen being 1 Dalton) but a protein can contain thousands of different amino acid building blocks in one molecule, and the molecular weight might be 300,000 D.

Although attempts to derive engineering solutions from natural mechanisms have only begun to be made in the last 15 years, earlier biologists came close to guessing their potential. Sir Alistair Hardy, in The Open Sea (1956), repeatedly marvelled at natural mechanisms as feats of engineering. This is Hardy on the stinging hairs found on many jellyfish:

It is not a living thing (#litres_trial_promo); it is a dead structure, an elaborate tool made ready for work – and made to perfection – by the semi-fluid living substance of the cell. Here is something to wonder at, for it looks as if it were designed.

Behind this you feel the lurking suspicion that we ought to be able to design such a structure. In this case, we haven’t yet done so but the action of biological springs like the jellyfish’s sting is definitely on the bio-inspired agenda. Hardy has the true spirit of bio-inspiration before its time. My second-hand copy of The Open Sea: The World of Plankton (The Open Sea is in two volumes, one on the world of plankton, the other on fishes) has an interesting history. It is stamped inside: ‘MoD Library Services: withdrawn from stock.’ These days, the Ministry of Defence is a principal funder of work in bio-inspiration. I hope they have bought a new copy.

Bio-inspiration has an appeal denied to other cutting-edge sciences. Firstly, it involves some attractive creatures, adding an extra dimension to the allure of butterflies, geckos, lotus plants and the like. Then there is the utility of the products – this is a technology, not science for science’s sake. Bio-inspired solutions are often comprehensible in a way that much science is not: they involve structures whose functions are clear, even if they need a microscope to see them. Finally, some subjects of bio-inspiration are amenable to kitchen-table experimentation, as this book will demonstrate.

Inevitably with a new subject, there is some uncertainty about the boundaries of bio-inspiration. Scientists working in bio-inspiration generally fall into one of two camps: biomechanics or materials science. Biomechanics is concerned with large-scale mechanisms, such as how insects fly, materials science with fine-scale structure and chemical composition. It is worth remembering that, historically, these two disciplines come from very different traditions. The materials scientists prefer the term ‘biomimetics’ for this new subject but the biomechanics don’t like this because it suggests to them a slavish copying of nature (mimesis = ‘copying’). When he lectures, Professor Bob Full, the ebullient master of animal locomotion at Berkeley, University of California, even has a slide with a big red slash through the word: ‘Biomimetics? No,’ he says, ‘Bio-inspiration is the way to do it.’ In an important sense Full is right. Scientists try to unravel nature’s mechanisms, but technologists use whatever will work. Bio-inspired technical products will almost certainly not mimic the actual materials used by nature. The self-cleaning Lotus-Effect

(see Chapter 2) is the most advanced of these techniques in terms of coming to market, with several products available, but it does not use the actual substances found in lotus leaves.

It is worth thinking about how nature and the human engineer went about producing their structures before we reached this point of rapprochement at which engineers are eager to learn from nature. Design in nature and in engineering are achieved by totally opposite methods. The human engineer can start from scratch, designing on paper something never seen before and then assembling the parts until it is all connected up and ready to go. For example, for birds to reach their present sublime level of design, it has taken millions of years of evolution. In the 1940s, aeroplanes made the abrupt jump of moving to jet engines from piston engines that drive propellers. The jet engine was perfected by Frank Whittle between its invention in 1928 and the first flight in 1941. If nature had wanted to evolve towards something similar there would have been an intermediate creature that could still fly by the old method while the new one was developing.

Bob Full makes the point like this: ‘If I told you to take my ‘84 Toyota and make it the fastest car possible using any material that you have, you could make a pretty fast car if you could replace 20 things. But you can’t throw away the whole genome and start from scratch. That’s a pretty heavy compromise.’

Put like this, it would seem that the human engineer holds all the aces. If, as a designer, nature is hobbled in this way, surely the human engineer ought to win hands down? But, despite her apparent constraints, nature has still produced devices for which engineers would give their eyeteeth. With regard to flight, for example, human aviation is impressive but in terms of manoeuvrability, the fly leaves a modern jet fighter standing, being able to turn a right angle at speed in only one twentieth of a second.

We are fortunate that we can have it both ways, using nature when it has developed structures we can adapt, while at the same time retaining the engineer’s radical risk-taking advantage over evolution’s necessarily conservative processes.

There are times when it seems that bio-inspiration should be called ‘technomimetics’: only too often physicists, engineers or chemists invent something; biologists then discover that nature has already invented it (often hundreds of millions of years before), but the phenomenon itself was not known until discovered by the technologists! Obvious examples are echo-location in bats and sonar in whales and dolphins: before ultrasound was invented scientists could have dissected bats for eternity and still not understood their echo-location mechanism.

The most dramatic recent example has been the photonic crystal (#litres_trial_promo): a nanostructured crystal that will enable light to be guided at fantastic speeds through the crystal to create pathways in which information can be stored and manipulated. The photonic crystal was predicted as a theoretical possibility by physicists in 1987, first created technically in 1991, and discovered in butterflies and marine creatures in the late 1990s. In other areas, biological discovery has led to technical invention in the true spirit of bio-inspiration. In fact, in the case of the Lotus-Effect, once the biological effect was established – that some plant leaves have a micro-structure that produces highly developed water repellency and self-cleaning – it was realized that physicists had produced a general theory to account for this 50 years earlier, but its importance had not been recognized. Now, super-water repellency is a respectable subject in many physics and materials science laboratories where you won’t find a leaf of any kind.

Bio-inspiration is not a narrow discipline. Origami was once thought merely to be an amusing game, nothing to do with science. Then mathematicians realized that it could be interesting to them, as a branch of topology: the maths of shapes. Origami is used by nature because some structures such as leaves and wings need to be folded. Now whenever human engineers want to deploy structures (erect something that is usually kept folded), they look at the ways nature uses origami.

Although Primo Levi, the great Italian writer and chemist who died in 1987, did not live long enough to see the birth of bio-inspiration, he did have an abiding interest in the natural world. He was especially fascinated by insects and in his essays (Other People’s Trades) he said of beetles:

These small flying fortresses (#litres_trial_promo), these portentous little machines, whose instincts were programmed one hundred million years ago, have nothing at all to do with us, they represent a totally different solution to the survival problem.

But beetles, like every other major group in the natural world, do have something to offer us. The flashing light of the firefly (#litres_trial_promo) (a beetle despite the name) is caused by a chemical reaction that produces almost no heat and this has been mimicked to produce biomedical diagnostic tests. The Oxford zoologist Andrew Parker has discovered a desert beetle that has a novel way of capturing the sparse water that comes its way and this too will have technical applications.

And what of the bombardier beetle (#litres_trial_promo), a creature that seems to have anticipated many of the principles of human rocketry? It has a powerful defence mechanism that involves directing a hot irritant spray in the direction of an attacker. The chemical propellant for the spray turns out to be hydrogen peroxide, a well-known human rocket fuel. The peroxide is mixed with hydroquinones in a ‘reaction chamber’; the reaction is hot (80°C) and the gases produced result in an explosive exhaust. The reaction chamber can be swivelled like a rocket motor to point towards the attacker. The whole business sounds far more like human technology than a natural creature. The more we know about beetles the more they seem to be little compendia of bio-inspirational properties.

Bio-inspiration can work across the whole size spectrum but there is no doubt that most of the work presently being carried out is in the former Blind Zone, the nanoregion. The idea that the properties of things as experienced by us derive from tiny structures goes back a very long way: back to the 5th-century-BC Greek philosophers Democritus and Leucippus who proposed the atomic theory of matter. Their ideas are known to us through the exuberant epic poem De Rerum Natura (#litres_trial_promo) by the Roman poet Lucretius (c. 100–c. 55 BC). Lucretius would have loved bio-inspiration. He tried to answer fundamental questions: What is the world made of? Can matter be created or destroyed? Are conscious beings made of conscious stuff? How does life renew itself? And despite three centuries of modern science that would have astonished Lucretius, many everyday things remained unexplained until recently. As he makes clear in De Rerum Natura, he was aware of the mystery of nature’s tiny functioning organs:

How small can anything be? We know of creatures

So tiny they would seem to disappear

If they were less than half their present size.

How big do you suppose their livers are?

Their hearts? The pupils of their eyes? Their toes?

Pretty minute you must admit.

Lucretius believed that the underlying particles of the material world could not have the same properties that appear to our eyes. They had to be colourless, odourless and tasteless (and lacking consciousness). It was a subtle idea; you might think that if you kept chopping something up until it was very small it would be the same all the way through – just smaller – but it was Lucretius’s intuition (I refer to this as the Lucretian Leap) that, at the smallest scale, things just had to be different. In this sense, Lucretius and his forebears were the first nanotechnologists, although the subject had only a notional existence in their imaginations.

De Rerum Natura gives us, in a language all can understand, a passionate explanation of the way things are. Indeed, it is unfortunate that modern science has not proved amenable to the Lucretian treatment. But bio-inspiration is remarkably Lucretian in spirit. It answers simple bold questions about aspects of nature: Why is the lotus leaf always clean? How does the gecko walk upside down? How can a spider’s web be stronger than steel? How can a fly of little brain be more manoeuvrable than a Eurofighter?

The answers to these questions are also Lucretian. Lucretius constantly argues that the causes of the effects that we see are different in kind to the effects. This could almost be the first law of bio-inspiration: the tidiest surfaces are the roughest at the nanolevel; the structures that cause the colour of the peacock’s tail are not coloured; the hairs on the feet of the gecko are not sticky.

Take silk, a byword for slinkiness. But what is the gorgeous crackle it makes when you rub it against itself (known as ‘scrooping’) and what causes the colour changes when dyed silks are viewed from different angles? Early synthetic silks did not have these properties because the fibres were smooth and of rounded section. But under the SEM a natural silk fibre will be observed to have micro-structured rough edges (#litres_trial_promo) – not at all what might be expected from the feel of it. When, in the 1980s, Japanese textile manufacturers realized this, at last they were able to make close synthetic copies of natural silks: they called them Shin-Gosen (‘New Feel’).

Lucretius was not the only poet whose imagination was caught by these natural phenomena. In his poem ‘Greatness in Little (#litres_trial_promo)’, the 17th-century English poet Richard Leigh was intrigued by tiny things, long before such a fascination could be satisfied. At one point he bursts into praise of minuteness itself:

Ah, happy littleness! That art thus blest,

That greatest glories aspire to seem least.

Even those installed in a higher sphere,

The higher they are raised, the less appear…

Bio-inspiration usually works at the nanolevel, but that does not make it synonymous with nanotechnology. Most nanotechnology is not bio-inspired; it is the province of materials technology, comprising things like smaller electronic components and nanoparticles in cosmetics and systems for delivering targeted doses of drugs. Another name for bio-inspiration makes clear the distinction: bio-inspiration is ‘nature’s nanotechnology’.