Hanging by a silk

It’s flexible yet tougher than a bullet-proof jacket. Are we anywhere near deciphering the fibre?

By Dinsa Sachan
Published: Thursday 30 September 2010

imageHOW strong can silk get? Ask a biomedical engineer and the answer would be: as tough as a bullet-proof jacket made from the advanced fibre system, Kevlar. Sixteen layers of certain silks can stop a nine mm speeding bullet.

Spun by silkworms and spiders, the protein fibre has been in use through history: for weaving shimmering garments to making non-absorbable surgical sutures. With advancement of technology, silk has been used to create several hi-tech products such as ligaments and brain implants. Yet the engineers are nowhere near copying nature’s silk spinners and replicating the fibre synthetically.

In the July 30 issue of Science, David Kaplan and Fiorenzo G Omenetto, biomedical engineers at the Tufts University in USA, reviewed the state of silk research and remaining challenges.

Scientists have understood the structure and chemistry of silk fibre, notes the review. It is made of simple proteins. It has large repeats of amino acids flanked by shorter fragments with either nitrogen or carbon at the end. But they have failed to decipher how the insects convert their proteins into silk fibre. The protein remains stable in the glands of the insect at high concentration. But when synthesised in the laboratory, the molecules aggregate and precipitate. It could be that nitrogen and carbon at the end of the chain which helps stabilise silk during processing in the gland.

Though silk from silkworm is easily available, researchers have found that silk from spiders has more uses. “The two types of silk proteins are very different to start with. Spider silk is stronger and more elastic,” said Randy Lewis, who studies spider silk at University of Wyoming in USA. Lewis is studying both natural and novel spider silk proteins his team generates to develop ligaments and repair tendons.

Todd A Blackledge, biology professor at University of Akron in USA, has found out how spider silk attains its incredible mechanical property. Parts of the protein fold into nanocrystals, which are extremely strong. The nanocrystals are loosely interconnected by amorphous regions that allow the fibre to be stretched. “This extreme ability of certain spider silks to resist breaking when pulled is why researchers are so interested in mimicking its properties in synthetic fibres. The property can help prepare artificial tendons and tissue scaffolds,” said Blackledge who is engineering biomimetic muscle that has implications in controlled drug release and robotics.

Spider silk can be processed without the use of chemicals and is fully degradable, said Kaplan. The latest research involves producing silk by transferring engineered silk protein genes into bacteria, plants and goats. The researchers hope someday plants could be modified to produce silk as a crop, like cotton is harvested today.

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