Designers Godley & Peers just revealed a full length silk cape woven from the silk of the golden orb spider. But how exactly does one go about getting silk from a spider? Greene’s tutor, Catherine Offord, explains the miracle of spider silk. Catherine is a Biologist specialising in animal behaviour; she gained her first degree (B.Sc.) in Biological Sciences at the University of Oxford (St. Hilda’s College). Having graduated, Catherine worked as a laboratory assistant and as the Greene’s Science Co-ordinator. She is now studying for a postgraduate degree at the University of Princeton, U.S.A.
Silks are protein fibres made by a number of animal species. As well as spiders, many insects produce some form of silk: Lepidopterans (butterflies and moths) produce silk cocoons, and commercial silk production uses ‘silkworms’ (actually moth larvae) to produce fibres on an industrial scale.
All spiders make several different types of silk for various purposes, although not all spiders spin webs – more ancient species of spiders, such as tarantulas, hunt their prey on the ground without the aid of dinner-catching nets. Inside the spider, the silk is produced in complex glands and stored as a liquid gel called ‘dope’, but as soon as it leaves the spider it takes on a ‘liquid crystalline’ structure. This transition from internal gel to external fibre makes silk a very unusual substance.
Historically, spider silk has had a number of interesting uses. It was applied to wounds by the ancient Greeks to accelerate healing (a capacity now attributed to the antiseptic coating on the outside of the silk fibre) and used during World War II to make the cross hairs of viewfinders on telescopes and sniper rifles. However, today’s interest in spider silk primarily lies in its mechanical properties, such as its incredible strength and toughness. Dragline silk, the line that web-weaving spiders dangle from, is the most impressive silk mechanically, and it is this fibre that scientists are now working hard to reproduce.
Barely visible to the naked eye, a strand of silk just four thousandths of a millimetre in diameter can support a spider weighing over three grams. On a more familiar scale, a spider-silk rope with a diameter of under a millimetre can support an 83kg person. The silk of the golden orb weaver, weight for weight, has a tensile strength nearly equivalent to that of high-grade steel, but its elasticity means that it has a far greater toughness (a measure of the energy needed to break the fibre). A single thread in a web, thirty times thinner than a human hair, can stop an unfortunate fly mid-flight. Scaling up, it has been suggested that a silk thread the diameter of a pencil could therefore halt a Boeing 747. Ignoring the practicalities of verifying such a claim, it is not difficult to see, then, why spider silk has attracted the attention of biologists, engineers, physicists and material scientists, as well as numerous biotech companies, interested in making super-materials for protective – even bulletproof – light-weight clothing, artificial ligaments, or strong surgical thread.
So why not harvest spider silk on a mass scale, just like we do with silkworm silk? Answer: spiders in general are extremely antisocial. Force a group of spiders to occupy a confined space and most will not survive the experience.
Ideally, then, we would like to produce our own spider silk, with all the same impressive properties, but without relying on the cooperation of the spider. A number of attempts have been made to modify genetically other organisms to produce silk proteins for us by inserting the genes coding for spider silk proteins (or ‘spidroins’) into a host. In 2002, Nexia Biotechnologies Ltd., a company specialising in the development of materials from biological origins, genetically engineered a herd of goats to produce silk proteins which could then be harvested from their milk. More recently, scientists at the Korea Advanced Institute of Science and Technology (KAIST) have modified E. coli bacteria to produce spider silk proteins which can be spun into a fibre in much the same way we make nylon.
However, although we are now able to make the appropriate proteins, artificially spun fibres have hugely inferior properties to natural fibres. Evidently, there is something taking place inside the spider which is vital to the strength and toughness of the fibre that we are not able to reproduce using our own spinning methods. Perhaps the shape of the glands and the passages that the silk dope passes through within the spider is crucial in silk production. Maybe the chemical environment inside the abdomen is an essential factor in determining the properties of the fibre. The international race is now on to learn how to copy nature’s design and produce this remarkable substance on an economically sustainable scale.
Spiders have been producing silk for 300 million years. It is hardly surprising that the process is a complex one, balancing the internal conditions in which silk is produced with the harsh external world where silk must perform, and we are a long way from understanding exactly how this process works. But the next time you are about to brush a spider out of the house, just bear in mind that it produces one of the most incredible fibres in the natural world—something that decades of human endeavour is as yet unable to reproduce.
German documentary showing a man lifted by a thin rope of silk fibres reeled by Oxford Silk Group: http://www.wdr.de/tv/kopfball/sendungsbeitraege/2010/1212/
BBC documentary Bang Goes the Theory explaining how Oxford Silk Group reels silk from Nephila spiders: http://www.bbc.co.uk/programmes/p004kkn5