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

The industry of garden spiders is prodigious. When I brought one in to be ‘silked’, its web was damaged in the process, but a few hours after releasing the spider the web had been rebuilt. Spiders keep their webs in good repair. After a few days, a web will become tatty from insect collisions, wind, dust and the spider’s own movements across her domain. Every two or three days, the spider will consume the old web and build a new one, usually in exactly the same place, since they are highly territorial. Around 80–90% of a new web is protein recycled from the old one. This means that a spider catches food mainly to get the energy to build the web; it doesn’t need food to supply much of the material – an example of the amazing efficiency of living processes.

Attempts have been made to silk spiders on an industrial scale. Properly set up, a single golden orb-weaver can produce 300 metres of silk in one session. The problem is that spiders cannot be farmed intensively. They are aggressive, solitary creatures who, if confined in one space, eat each other.

This naturally turns the mind towards the idea of making a synthetic silk. That this might be possible was suggested as far back as 1665 by Robert Hooke:

Probably there might be a way found out, to make an artificial glutinous composition (#litres_trial_promo), much resembling, if not full as good, nay better, than that excrement, or whatever other substance it be, out of which the silkworm wire-draws his clew. If such a composition were found, it were certainly an easy matter to find very quick ways of drawing it out into small wires for use. I need not mention the use of such an invention.

For centuries the only way of making silk was with the silkworm (#litres_trial_promo). Archaeological evidence has shown this to be an ancient craft, going back to around 2600 BC in China. The silk moth is the only domesticated insect, having lost the power of flight, all pigmentation, and just about any desire to move or to do anything. The silk is produced by the caterpillar to cocoon the chrysalis and for this reason is not as strong as spider dragline silk. But it is a natural product that no synthetic has ever been able to match, although Japanese textile technologists have now come very close.

The basic process is as follows. The eggs are hatched and the caterpillars fed on mulberry leaves. They moult four times before they are ready to spin a cocoon in which the chrysalis will develop. The chrysalises within the cocoons are then killed by steam or fumigation. The cocoon silk consists of two filaments of the silk protein fibroin stuck together by another protein, sericin. To process the silk, the sericin is removed with hot water and the filaments drawn from water and combined to make yarn. The yarn undergoes stretching and is wound onto reels as raw silk.

Because of the finicky nature of the silkworms and the demanding cultivation regime, increasing the production of natural silk is not easy, and silk production has often been threatened by disease. In 1855, silkworms, particularly those in Europe, were afflicted by a parasitic disease called pébrine (#litres_trial_promo). This episode is the centrepiece of Alessandro Baricco’s novel Silk, which captures in delicate prose the aura we associate with the fabric:

He felt the lightness of a silken veil dropping onto him. And the hands of a woman – of a woman – drying him all over, caressing his skin; those hands and that material spun out of nothing. He never stirred, not even when he felt the hands move from his shoulders to his neck and the fingers –the silk and the fingers – climb to his lips and brush them once, slowly, then vanish.

Pasteur was called in to solve the pébrine crisis but progress was slow and this seriously focused minds on the possibility of imitating the natural process. At the time, knowledge of the chemistry of silk and all such natural substances was non-existent. Because the caterpillars grew on a diet of the leaves of the white mulberry, Count Hilaire de Chardonnet, who had worked with Pasteur on pébrine, tried ways of by-passing the silkworm by digesting mulberry leaves and creating a solution that could be squeezed through a nozzle similar to the silkworm’s spinnerets. In fact, the main component of leaves is cellulose, a material very different to silk proteins but also a long-chain molecule. Amazingly, it did prove possible to create silk-like substances from cellulose by several processes, the best-known being rayon (1891).

The potential of silks in one of the toughest applications imaginable (#litres_trial_promo) was realized in the late 19th century by a physician in Tombstone, Arizona: ‘In the spring of 1881 I was a few feet distant from a couple of individuals who were quarrelling,’ George Emery Goodfellow wrote in his diary. ‘They began shooting.’ Two bullets pierced the breast of one gunman, who expired from his wounds. But, on examining the body, Goodfellow found that, ‘not a drop of blood had come from either of the two wounds’. He noted that ‘from the wound in the breast a silk handkerchief protruded’. When he tugged on the handkerchief, it came out with a bullet wrapped inside. Evidently, the bullet had torn through the man’s clothes, flesh and bones but had failed to pierce his silk handkerchief. Intrigued by this discovery, Goodfellow began to document other cases of silk garments halting projectiles – including one incident in which a silk bandanna tied around a man’s neck kept a bullet from severing his carotid artery.

If silk was ever going to be used seriously for such applications it needed to be made in quantity. The mimicking of natural silks on a commercial scale began with the invention of nylon (#litres_trial_promo) in 1937. Nylon is derived not from plant products but from very small chemical units, linked together to form long-chain molecules. Such compounds, now ubiquitous in modern civilization, are called polymers. In nylon, the link – the amide group – was the same as that in natural silks although the rest of the molecule was very different. Nylon has a much more regular structure than natural silks.

The first serious flak-jacket silk was kevlar, a tougher variant of nylon, invented in 1963 (#litres_trial_promo). Even with nylon, kevlar and other fibres established as industrial staples, the superior properties of spider silk were alluring, but no bulk industrial or military use was proposed until very recently. The first serious modern application was very small scale. In the Second World War, single fibres of spider silk were used as cross-hairs for accurate range-finders – it came from black widows in the USA, garden spiders in the UK. Pioneer spider-silk researcher David Knight tells the story of the major US chemical company Du Pont, inventors of nylon and kevlar, who supplied a spider-silk sample to the US Army during the war, hoping for an order. Three years later, they politely enquired about the silk and asked whether the Army would be making an order. ‘Oh, we don’t need any more,’ they were told, ‘what you sent was fine.’

The picture changed dramatically, at least in prospect, with the arrival of genetic modification (GM) technologies in the late 1970s. In GM, a gene can be inserted into a foreign organism; the organism will function normally and produce the proteins programmed by that gene. So, in theory, if you took the gene for spider silk, and inserted it into an animal, you could make industrial quantities of silk.

Work began on this project in the 1980s and was bedevilled by nature’s cussedness. Spider-silk genes turned out to be harder to handle than the insulin gene, GM’s first great success. But, in June 2002, Nexia Biotechnologies in Quebec, Canada, claimed that they were able to produce industrial quantities of spider silk from the milk of genetically engineered (#litres_trial_promo) goats. The story had a strange blend of hard military exploitation and New Age greenery. On the one hand, the US Army had been working on spider silk for many years; Nexia’s silk, named BioSteel

, was developed under an Army contract for flak jackets and one of the two herds of modified goats was kept on a former United States Air Force B52 bomber base at Plattburgh, New York State. On the other hand, Nexia’s President and CEO, Jeffrey Turner, waxed lyrical about this new fibre produced from meadows, goats, sun and water and spun at room temperature from a watery solution. Nylon and kevlar, the closest things we have to spider silk, are made using toxic chemicals and high temperatures and they generate toxic wastes. Turner said: ‘We use water and hay (#litres_trial_promo); to make nylon – which has a half-life of 5,000 years [which means it’s not biodegradable] – you have to sink a hole in the ground. That’s not the kind of world I want to leave my kids.’

If we could manufacture large quantities of spider silk and spin it the way the spider does we would have a very special material. But 16 months on from the excited press reports of June 2002 the spider-silk story looked very different. The US Army withdrew from its collaboration with Nexia because BioSteel, as it then was, could not meet their requirements for quality or quantity.

By mid-2004, Biosteel had been downgraded even further. Development of spinning for general yarn and fabric was suspended due to the ‘ongoing technical challenges of producing bulk, cost-competitive spider-silk fabrics with superior mechanical properties’. On 8 March 2005 this particular strand of the spider-silk story was fractured. Nexia’s principal asset Protexia