The use of synthetic spider silk to replace current materials in a host of applications has, until recently, been a step too far for materials science. But earlier this year man-made spider silk moved a step nearer with the news that Canadian-based Nexia Biotechnologies Inc and the US Army Soldier Biological Chemical Command have collaborated to spin the world’s first man-made spider silk. Its commercial production has been something of a holy grail for materials scientists for many years, not least because it is known to be tougher, in terms of energy required to break, and less dense, than steel or Kevlar.
Dr Jeff Turner, President of Nexia Biotechnologies, is a former professor at McGill University and a specialist in molecular genetics and goat farming. He founded the company about eight years ago with a view to producing man-made spider silk and aims to have the first products in the market place in 2004. But it’s been a tricky process getting this far. ‘Spider silk is really the elusive child of evolution - it really dwarfs man’s achievements in petroleum chemistry,’ he says.
Different Types of Spiders’ Silks
The silk fibres produced by spiders are composed of proteins that have made a transition from a soluble silk protein solution inside the spider’s silk gland into an insoluble fibre of crystalline structure when spun. Spiders produce a number of different silks with different mechanical properties - of these silks the ‘dragline’ or ‘frame’ silk has been the most desired. ‘We have intellectual property that governs all known properties of spider silk genetics,’ says Turner. ‘We use the orb weaver group of spiders - all spiders produce silk, but work carried out by our partners in the Pentagon, where they did an exhaustive study of spiders, gave us a variety of different genes. They’ve isolated those genes and licensed them to us, so you can get spider silk that breaks down quickly or not, by choosing the appropriate spider gene.’
Problems Encountered Trying to Manufacture Man-Made Spider’s Silk
Isolating the right spider gene for a suitable silk is only half the problem - scaling up for commercial production is equally difficult. Farming, the traditional route of collecting industrial quantities of animal product, is not an option - unlike silkworms, spiders are both territorial and carnivorous. Furthermore, the silk itself, composed of complex proteins, has been proved extremely difficult to replicate by industrial chemists. Even in the field of biotechnology, in which protein-producing cells can be genetically engineered to allow proteins to be made in yeast, bacterial or mammalian cell cultures, the task of producing spider silk has proved prohibitively expensive, or in the case of bacterial fermentation, the silk protein itself far to complex.
Using Transgenic Technology to Overcome These Problems
To overcome these problems researchers have turned to transgenic animal technology. This is the process of introducing a piece of genetic code from one organism into the genetic code of another and using it - in this case - to produce complex proteins, which are usually expressed in the animal’s milk. In the case of spider silk, Nexia uses in vitro microinjection to introduce a genetically engineered segment of spider DNA into the genetic material of fertilised goat egg. This is transferred to suitable female dairy animal, in this case goat, that nurtures and gives birth to the transgenic animal, which will, when mature produce the desired proteins in its milk. Only in a small percentage of embryos is this process successful, but when it is, the implanted gene will be passed down future generations through tradition breeding.
Raw Materials and Host Animals
The advantage of using goats is that, while not quite working for peanuts, they will lactate on a diet of hay. ‘Cost of goods is critical,’ says Turner. ‘One way to reduce this is to have your input materials as cheap as possible and in our case it’s hay. We don’t need horse-quality hay made of alfalfa, just mid-quality hay, which is inexpensive and, as a commodity, easily obtainable. That and water is all they require. The beauty is that, using age-old animal husbandry practices, we get a product that isn’t just worth 40 cents a litre, but $20,000 a litre.’ By using a type of Central African dwarf goat that breeds early and lactates early, Nexia can reduce the time taken to produce the spider protein-laden milk to 16 months, two months earlier than possible with normal sheep and goats, and less than half the time taken by cows.
The First Spiders’ Silk-Based Products
According to Turner, the size of herd you need to get meaningful amounts of protein depends on the application. So what applications is Nexia exploring? Its first product will be that which is easiest to produce with the least regulation - in the Turner’s words, ‘the low-hanging fruit’.
The earliest product to market will most likely be fishing line. It will be biodegradable, with a spider silk core and a Xylane coating to make it waterproof. Turner envisages it lasting for a year before breaking down, but should it be lost, the broken end would expose the spider silk to water and it would break down in a few months. ‘This is predicted for 2004, with the rest of this year spent scaling up,’ says Turner ‘2003 will be spent doing product development work - how does the line work in cold fresh water, warm fresh water, cold salt water, warm salt water and so on, and our partner, a global player in the fishing line business, will be announcing progress in the next quarter.’
Another area of early application is in the medical market. ‘Our first application will probably be micro sutures in opthalmic and neurological applications,’ says Turner. ‘We’re looking at about the same time-frame for producing fishing line – we’re currently in pre-clinical work confirming that our spider silk acts like traditional silk - i.e. it’s not harmful to the patient - and regulatory approval should take about another year, which means we’re looking at a date of about 2004.’
A potential life-saving application for spider silk is as the reinforcing fibres in lightweight body armour. However, the specifications for ballistic protection are extremely high, as is the quality control. Obviously, every article of body armour has to stop a bullet every time. ‘From that point of view our quest for perfect biomimicry has not been achieved, so the types of mechanical properties that you need to get perfect ballistic protection won’t be around until 2010,’ says Turner. ‘But we’ve already got two of the four attributes needed for perfect biomimicry toughness and energy management.’ Another difficulty would be the amount of spider silk required.
‘For medical devices we only need about 100 goats for silk production and we’ve got that many goats at the moment,’ says Turner. ‘With regard to more industrial applications, it’s a much bigger undertaking. Fishing line would need a few thousand goats, while for ballistic protection, the army would want 345,000 vests, which would need between 5,000-8,000 goats.’
Modifying Properties of Spiders’ Silk
Beyond this the applications for spider silk are legion. Spider silk has a biphasic modulus - initially it’s very stiff, like Kevlar, but just before the yield point it becomes very elastic, like Nylon. It also undergoes hysteresis, so if released from tension it comes back into shape. According to Turner, Nexia have not yet got to the full strength of natural spider silk, but the company has got more elongation before the yield point in its product compared to natural spider silk. ‘We think we can correct both attributes by spinning again,’ he says. Other ways to change the properties of the spider silk include choosing different genes, and by changing the spinning process itself, mimicking the spider, using shear and dehydration. It is possible to make thicker or thinner fibres, and doping agents can be added to control the level of crystallinity. At the same time conventional coatings can be applied - Xylane to make it waterproof, anti-UV coatings to resist UV damage and so on. Finally, the use of the silk will determine its properties necessary - if one was to exploit its strength property inside a composite element, the resin of the composite would protect the spider silk, in effect, for ever. ‘Spider silk itself, if It’s kept dry and away from bacteria, is stable indefinitely,’ says Turner. ‘There have been spider webs hanging in enclosed Egyptian tombs discovered thousands of years later and they’re perfectly intact.’
And the future? ‘We are working on other manufacturing platforms, in particular plants, but that’s still at an early stage,’ says Turner. Even so, the next few years could give rise to a new meaning for the phrase ‘web-based industry’.