Scientists have created genetically-modified silkworms that can spin the much stronger silken threads of spiders in a technological breakthrough that promises to revolutionise the production and use of new materials made with spider silk.

For more than a quarter of a century scientists have been trying to find ways of producing industrial-scale quantities of spider silk because, weight for weight, it is stronger than steel and almost as tough as bulletproof Kevlar. A net weaved from pencil-thick rope spun from spider silk, for instance, could in theory catch a fighter jet in flight without breaking.

However, unlike the caterpillars of the silk moth Bombyx mori, spiders are territorial, aggressive and prone to cannibalism, making it impossible to rear them in the population densities required for commercial silk production.

Researchers have attempted to overcome this difficulty by transferring into silkworms the key spider genes responsible for making the silk threads used in the draglines of the golden orb-web spider. The result was a genetically-modified "transgenic" silkworm that produced a mixture of its own silk combined with the far tougher and stronger threads of spider silk within the mile-long threads of its cocoon.

The researchers, led by Professor Don Jarvis of the University of Wyoming, in Laramie, yesterday published their study in the journal Proceedings of the National Academy of Sciences, showing how they created transgenic silkworms capable of making composite fibres with silk threads from both spiders and commercial silkworms.

"On average, the composite fibres produced by our transgenic silkworm lines were significantly tougher than those produced by the parental animals and as tough as native dragline spider silk fibre. In best-case measurements, the composite fibre produced by one of our transgenic silkworms was even tougher than the native dragline spider silk fibre," the scientists said.

Some possible uses for spider silk have already been identified in medicine, such as new kinds of biomaterials for wound dressings, artificial ligaments, tendons, tissue scaffolds and microcapsules for drug delivery, they say. Other uses could include materials used in bulletproof jackets and engineering.

Ever since scientists first identified the spider genes involved with silk production, biotechnologists have tried to created genetically-modified alternatives to spiders. Synthetic spider silk genes have been transferred into bacteria, tobacco plants and even goats, which produced limited quantities of silk proteins in their milk.

However, none of the transgenic microbes, plants or animals carrying spider silk genes have been able to produce sufficient quantities of the pure proteins needed for commercial-scale production.