Spider silk is already tougher and lighter than steel, and now scientists have made it three times stronger by adding small amounts of metal.

The technique may be useful for manufacturing super-tough textiles and high-tech medical materials, including artificial bones and tendons.

"It could make very strong thread for surgical operations," researcher Seung-Mo Lee of the Max Planck Institute of Microstructure Physics in Halle, Germany, said in a telephone interview.

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Lee and colleagues, who published their findings in the journal Science, found that adding zinc, titanium or aluminum to a length of spider silk made it more resistant to breaking or deforming.

They used a process called atomic layer deposition, which not only coated spider dragline silks with metal but also caused some metal ions to penetrate the fibers and react with their protein structure.

Lee said he next wanted to try adding other materials, including artificial polymers like Teflon.

The idea was inspired by studies showing traces of metals in the toughest parts of some insect body parts.

The jaws of leaf-cutter ants and locusts, for example, both contain high levels of zinc, making them particularly stiff and hard.

Spider silk has long fascinated scientists but producing it in commercial quantities is difficult because spiders kept in captivity tend to eat each other.

As a result, researchers have looked at alternative ways of producing silk without spiders, by duplicating their spinning technique.

Approaches being tried include deriving fiber from the milk of transgenic goats with an extra spider-silk gene and adapting silk produced by other insects, such as silkworms.