20 August 2004
20 August 2004
The US Defence Department is encouraging research into developing web-slinging weapons from spider silk, one of nature's strongest fibres.
Scientists have long been trying to unravel the molecular secrets of spider silk itself. The hope is that the silk could some day be woven into objects ranging from impregnable body armour to wear-resistant ropes, parachutes and uniforms.
Thinner than human hair and lighter than cotton, the strongest silk is three times tougher than Kevlar, the human-made material used in bullet-proof vests, and five times stronger than steel cable.
Unlike Kevlar, which requires intense pressure and poisonous sulphuric acid to produce, spiders can unspool silk at room temperature, under normal pressure, using little more than proteins and water.
But efforts to produce large quantities in the lab have progressed slowly, says Mr Frank Ko, a textile engineer at Drexel University in Philadelphia, who studies the material's mechanical properties. 'Spiders,' he says, 'are only ahead of us by a few million years.'
Scientists long ago ruled out the most obvious solution to harvesting silk: 'spider farms'. Unlike mulberry-munching silkworms, whose shimmering fibre is prized in the fashion world, spiders are territorial cannibals. Cage them in a pen, says Mr Ko, and they will soon consume each other. So scientists have turned to genetics.
In 1990, a team led by molecular biologist Randolph Lewis at the University of Wyoming decoded the first pair of spider silk genes.
The genes produce 'dragline' silk. The strongest of the spider's seven silks, it is used to frame webs and as a lifeline when the arachnid dangles.
The next trick is to produce the proteins made by these genes. Nexia Biotechnologies in Montreal has created a flock of genetically engineered goats designed to churn out silk proteins in their milk. After processing, the company is left with a pure powder that it markets under the name BioSteel.
But at US$1,500 (S$2,580) a gram, the silk is not cheap. And so the company has moved away from bullet-resistant textiles, says Nexia chief executive Jeffrey Turner.
'We'll leave that to Spider-Man,' he says. The company is looking at applications for its silk ranging from super-strong composites to cosmetics.
But churning out cheap silk protein is only part of the challenge; scientists must then spin it. In reality, spiders don't spin, they squeeze. Liquid silk protein is forced through fingerlike glands on a spider's abdomen called spinnerets. Spiders then stretch these filaments with their legs to align the silk protein molecules.
In 2002, researchers at Nexia and the US Army announced in the journal Science that by mimicking this squeezing process mechanically, they created a filament that was a little stretchier than the real thing and not quite as strong.
Dr Lewis, whose research is partially funded by the Defence Department, is pushing silk research towards the next frontier: customised silks.
Some engineers are side-stepping the challenges of natural silk and devising applications that are only loosely spider-inspired.
Mr Arnis Mangolds at Foster-Miller, an engineering company near Boston, has spent the last 15 years or so dreaming up various web-slinging weapons, mostly for the government. One of the first was Webshot, a flare-gun-like device that fires a 4.5m diameter nylon net to subdue fleeing suspects. Tested in the late 1990s by several US police departments, the device worked but never caught on, Mr Mangolds says.