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

experiments in chemistry and physics, and what we can see with the aid of a microscope. 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 (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,^* 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’ 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 look