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

Proteins have active centres, nooks and crannies precisely fashioned so that only one specific chemical can fit into them. When, in the whirling fluids of the cell, the one and only right chemical happens to come along, it becomes tightly bound to the protein. In living cells, proteins bind some chemicals, let others pass through pores, and, in general, regulate the traffic within the cell and facilitate chemical reactions. The full implications of this are spelt out in Chapter 6 but for now the point is that we have come so far from vitalism that the old division between living and non-living substances is breaking down – we can engineer hybrids between the two.

That there are no new frontiers is a weary cliché of our time: the ancient thrill of unspoiled places on Earth has given way to the fact of life that people can and do fly anywhere anytime. The dream of new worlds in space has retreated in the face of the barrenness of the Moon and Mars; the glorious new dawn of modernism in the Arts in the early 20th century led only to the stylistic emporium of postmodernism in which any retro style could be taken up again for a few years, given a whirl, then dropped. The decadence and satiation of our world is only too apparent. Scientifically, we have gone very deep – into the nucleus of the atom and the genetic code of all life – so what can be left to discover?

Bio-inspiration is a genuine new frontier. It is a growing body of techniques for making materials with novel and startling properties: surfaces such as paint and glass that clean themselves, fabrics that exhibit shimmering colour despite having no coloured pigments, fibres tougher (weight for weight) than nylon or steel based on spider silk, dry adhesives based on the microstructure of the gecko’s foot.

It is not just a new frontier because these properties are startling but because they have something in common. The mechanisms of most of these effects are caused by physical structures of a certain size: from one billionth of a metre up to one millionth of a metre (fig. 1.1). This is the nanoregion and the structures nature builds at this level we can call nature’s nanostructures. Until recently, the nanorealm remained relatively inaccessible to science and this may seem strange since scientists are able to manipulate subatomic particles millions of times smaller. And chemistry, a precise science with a growing inventory of more than 24 million discrete substances, operates at the size range just below the nanoscale.

The key to this paradox is that there is a huge gap between what we can infer about the size of atoms and molecules (and their even smaller constituents – protons, electrons and the like) by elegantly indirect experiments in chemistry and physics, and what we can see with the aid of a microscope (#litres_trial_promo). The ability of microscopes to magnify the smallest features has improved immensely since their invention in the late 17th century but there is a limit that is set by the properties of light itself.

When light hits objects patterned at just below one thousandth of a millimetre (#litres_trial_promo) (1 micrometre or 1,000 nanometres) strange things begin to happen to it. This is because light itself is patterned on the same dimension. Light is a wave motion, with the peaks of the waves repeating at just below the 1 micrometre mark. When the waves meet patterns of a similar size, they bounce off in ways that blur the picture. This is known as interference and in itself it plays an important role in bio-inspiration (see Chapter 5).

As far as microscopy goes, though, this is simply a nuisance. With the light microscope we can see living cells and some of their contents – bacteria, spermatozoa, etc – but not the complicated large molecules that make up these structures.

Microscopy and chemistry began at more or less the same time in the late 17th century and closing the gap between them has been a long and tortuous business. At first, chemistry had nothing to do with size. The initial job was to identify which substances could not be broken down into anything simpler – these are the elements such as hydrogen, carbon, oxygen, nitrogen, sulphur. It was a matter of speculation as to what was the smallest possible part of an element. The best theory going at the time was the Atomic Theory that suggested that elements were composed of millions of identical tiny billiard-ball-like particles. For centuries, this was purely a theory. No one knew how large atoms were or if they really existed at all.

But, in the late 19th century, thanks to work on the pressure of gases,

(#litres_trial_promo) it became possible to estimate the size of these ‘atoms’ (by now most scientists accepted that they existed). The first accurate figure for the size of individual atoms was made in 1908. Atoms are very small – in fact they are just off the nanoscale. A typical small atom such as carbon is about 0.3 nm (nanometre) in diameter.

So, if atoms were less than 1 nm in size and the smallest object you could see with a microscope was 1,000 nm, what existed in this Blind Zone? To try to understand how much we were missing, imagine being able to see objects, say, up to 1 cm but nothing more until you get to 10 m. Most of what we make and live with lies within this range (micro-electronics excepted). The equivalent for nature is the region ten million times smaller – and this zone was inaccessible to us.

Peering into this realm in the early 1960s, we were as blind as the moles in a fable by the Czech immunologist and poet Miroslav Holub: his poem ‘Brief reflection on cats growing in trees (#litres_trial_promo)’ imagines the moles trying to make sense of the world. Lookouts emerged at different times of day to report on the way things were above ground. The first scout saw a bird on a tree: ‘birds grow on trees’, he reported; the second found mewing cats in the branches: ‘cats grow on trees, not birds’. The conflict worried one of the elders, so up he went:

By then it was night and all was pitch-black.

Both schools are mistaken, the venerable mole declared.

Birds and cats are optical illusions produced

by the refraction of light. In fact, things above

Were the same as below, only the clay was less dense and

the upper roots of the trees were whispering something,

but only a little.

‘Things above were the same as things below’, or vice versa in our case. We had only our knowledge of chemistry at the bottom and the world of visible objects at the top to guide us. When we look around we can see only such objects as can be seen with eyes like ours. We make use of materials that we can grasp and manipulate to make objects on a scale that suits creatures around 1.5–1.8 m tall. We may not like to think of ourselves as being as cramped in our perception as the moles, but on the scale of the universe, from quarks to galaxies, we are. In the scale of things, we are trillions of times larger than the smallest things known, evanescent subatomic particles, and trillions of times smaller than the largest cosmological objects known.

What exists in the Blind Zone are large molecules of complex non-random chemical composition that are assembled to make the working structures of the cell: pumps and engines and factories for making everything the cells need, including copies of themselves. The contents of the Blind Zone comprise nature’s nanotechnology. And these are the nanomachines and structures we wish to harness for our own purposes.

But how could the gap be closed? How could we see nature’s nanomachines at work? The answer was to nibble at the problem from both ends. As chemists gained in confidence throughout the 19th century, the chemical structures of some of the molecules used by living things began to be deduced: sugars, for instance, and the amino acids that are the ingredients of the fabulously complicated proteins. And as the 20th century progressed, the structures of larger and larger natural molecules were worked out.

Although the limitations of light microscopy were unbridgeable, even in theory, new techniques of investigation became available. By far the most important new investigative technique in the mid-20th century was the use of X-rays; with a wavelength thousands of times smaller than that of light (see fig. 5.2, page 105), these allow us to penetrate deep into molecules such as proteins. When X-rays hit molecules they produce complex reflection patterns that mirror the actual structure of the molecules themselves. Strangely, this reflection of X-rays is exactly the same property that sets a limit to light microscopy. The result of an X-ray analysis is not a photograph in the conventional sense. When X-rays hit a crystalline substance they are scattered in a regular geometric fashion and the patterns produced give information about the position of the atoms in the crystal. So this is not a picture so much as the result of complex mathematical analysis of data.

And it was a combination of chemistry and X-ray analysis that led to the greatest biological breakthrough of the 20th century, the elucidation of the double helix of DNA. The chemistry of DNA had already shown that it was composed of certain known substances: sugars and four different bases, with these bases, intriguingly, seeming to be paired. In any DNA sample, from whatever source, there was always as much adenine as thymine and as much cytosine as guanine. With this knowledge, it was possible for Watson and Crick to interpret the X-ray picture and to deduce the double helical structure.

From the 1950s onwards, this technique – the combination of chemistry and X-ray analysis – allowed scientists to work out the structure of many significant biological molecules, especially proteins. However, X-ray techniques are limited by the fact that the specimen has to be a crystal, and many biological molecules cannot be crystallized. And also, we want to see the larger structures that the molecules make up.

In a sense, the beginning of a sustained interest in the nanorealm can be dated precisely, for it was on 29 December 1959 that Richard Feynman gave that talk. Feynman’s was a rallying call and it was heeded first in solid-state physics, as the relentless development of ever smaller and more integrated electronic circuits began. Finally, the better microscope requested by Feynman did arrive and biologists were allowed a glimpse into the nanoworld. This was the scanning electron microscope (SEM), invented in 1965 by Cambridge Instruments after decades of pioneering work at Cambridge University. Since then, many more advanced electronic instruments, such as the atomic force microscope, have followed, and a battery of different techniques can be brought to bear on natural structures. Ron Fearing, fabricator of gecko tape and micro air vehicles at Berkeley, University of California, talks of the ‘psychological barrier that was broken in the sixties with micro-machining, the atomic force microscope coming along. Before, people would have looked at these structures and said, “Oh, that’s too small to know what’s going on”.’

The SEM was a big breakthrough and it has had huge consequences for bio-inspiration. The pictures revealed by the SEM (#litres_trial_promo) look like engineering of an exquisite kind. The organs of minute insects and the parts of plants are revealed as wonderfully tooled artefacts. Bio-inspirationists constantly have to track back and forth between the nanorealm and the everyday scale of things. According to the Russian novelist and serious amateur lepidopterist Vladimir Nabokov in Speak, Memory, this is an intrinsically artistic activity:

There is, it would seem, in the dimensional scale of the world a kind of delicate meeting place between imagination and knowledge (#litres_trial_promo), a point arrived at by diminishing large things and enlarging small ones, that is intrinsically artistic.

When the first pictures were seen, the question of how nature achieved these wonders of micro-engineering was completely off the agenda – scientists could only goggle at the structures. But now we know a lot more about how nature creates such shapes. The Gecko’s Foot is the story of how we are closing in on this last frontier of natural exploration.

The nanoworld is like a complex jigsaw puzzle in three dimensions. We try to piece it together by viewing it with different magnifications and techniques. Behind the picture we can see with the unaided eye, there is another picture we have to zoom in on with the light microscope; behind that is a more detailed picture that we need the electron microscope to see; beyond that is the picture revealed by X-rays; and there are new types of microscope, such as the atomic tunnelling microscope, that all add information to the puzzle. To add to this, our knowledge of chemistry also sheds light on the three-dimensional structure. By combining all the information, we come to a picture that begins to approach completeness.

In retrospect, it seems curious that we have been ignorant for so long about how nature makes stuff. While we are pretty good at making intricate structures ourselves, when it comes to the miracles of the human body our role in the construction process is crude andlumbering. Anne Stevenson’s poem ‘The Spirit is too Blunt an Instrument (#litres_trial_promo)’ makes this point:

The spirit is too blunt an instrument

to have made this baby.

Nothing so unskilful as human passions

could have managed the intricate

exacting particulars…

Observe the distinct eyelashes and sharp crescent

fingernails, the shell-like complexity

of the ear, with its firm involutions

concentric in miniature to minute

ossicles. Imagine the

infinitesimal capillaries, the flawless connections

of the lungs, the invisible neural filaments…

So, if not the spirit, what is nature’s organizing principle? How does nature create intricate structures? There is still much to learn and our own attempts at mimicking these processes are fumbling, but we are now on the trail.

To understand why the realm of bio-inspiration is such a terra incognita, something really new under the sun, we need to look at the two great currents of 20th-century science. So powerful were these two prongs of attack that many people were dazzled into thinking that they revealed all we needed to know about the material world. These sciences were nuclear physics and molecular biology. Both ignored the multiplicity of the natural world – the several million species of living creatures (some estimates go as high as 30 million or more (#litres_trial_promo)), all with different shapes, sizes, habits and curious adaptations; the more than 24 million known chemical combinations (#litres_trial_promo) of the 92 natural elements; the architecture of matter in the honeycombs of the beehive, the fantastic filigree forms of the radiolarians of the ocean, and the interlocking spirals of a sunflower head. These were cast aside in the search for the ultimate, universal components and principles of matter (physics) and the chemical unit and mechanism of genetic inheritance in biology.

The idea behind these quests was that if successful, they would somehow explain everything else. And, of course, they were successful. Nuclear physics uncovered the unexpected power of nuclear forces and molecular biology determined the mechanism of inheritance: a precise sequence that has a chemical form (the DNA molecule) but which functions as a code for the synthesis of proteins, nature’s prime functional substances.

But, dramatically brilliant as these sciences were, they left enormous gaps. They did not begin to explain complex forms of nature, nor did they determine the composition of these forms. What the physics and biology obscured was the fact that to create functioning organs, the fundamental building blocks of atoms and molecules have to be synthesized into large structures whose properties cannot really be explained by a knowledge of which molecules compose them. The biologist Helen Ghiradella wrote in 1991, just before the bio-inspired explosion:

Many of us working in biological fields have perhaps unconsciously assumed that small things must be simple (#litres_trial_promo), at least more accessible to human understanding than those on a human scale. This may not be the case, and indeed, the further we investigate the more complexity we seem to find.

When, as a schoolboy in the early 1960s, I became fascinated by chemistry, what I wanted to know was: What are familiar objects made of? How is a tiny insect engineered from biological materials? What is the chemical structure of wood? What, in chemical terms, is a spider’s web? In The Periodic Table, Primo Levi beautifully expressed this chemist’s lust to know the fabric of the world:

Everything around us was a mystery (#litres_trial_promo) pressing to be revealed: the old wood of the benches, the sun’s sphere beyond the window panes and the roofs, the vain flight of the pappus down in the June air. Would all the philosophers and all the armies of the world be able to construct this little fly?