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 News :. Secrets of the silkworm revealed

The mystery of how silkworms manage to spin their silky fibres has been unravelled by U.S. researchers, bringing the engineering of new artificial silks a step closer.

Professor David Kaplan and Hyoung-Joon Jin from Tufts University in Boston report their research in today's issue of the journal Nature.

Silk is a strong fabric widely used in clothing, carpets and parachutes. Traditionally it is made by the meticulous unravelling and weaving of fibres from the cocoons of the silkworm Bombyx mori. But how this creature manages to convert an aqueous solutions of silk proteins (fibroins) into threads without gumming up their silk glands has until now been unclear.

Kaplan and Jin extracted silk from a silkworm cocoon and then redissolved it in water. They found that by altering the water levels the molecular structure of the solution changed. Silk fibre, they found, was formed in phases, which depended on how much water was present in the solution at a given time.

The watery solution of silk fibroins is a mixture of hydrophilic (water loving) and hydrophobic (water hating) areas. To test how the amount of water affected the silk production, the researchers added a compound that competed with the silk fibroin for the water molecules in which the protein was dissolved.

As the water was taken up by the added compound, the silk folded together in chains through the hydrophilic sections of the solution. This in turn meant the hydrophobic sections were forced together. As the water level continued to drop, the folded chains of silk pushed together to form nano-scale globular islands called "micelles". The micelles then joined together to form larger and larger gel-like structures all the while remaining water soluble and avoiding premature crystallisation.

The researchers also found globular structures on natural silk fibres suggesting the process observed in the laboratory is indeed close to what happens in nature.

The remaining missing piece of the silk spinning puzzle is how the gel-like material forms fibres as it is stretched: "Quite how the deformed globules then form fibres is a question for the future," writes Professor Edward Atkins from the University of Bristol in an accompanying News and Views article.

It is known that as the silk solution is 'pulled', it forms a partly crystalline, insoluble fibrous thread and that in the thread, most of the polymer chains are oriented parallel to the fibre axis.

While some research has been done on the strength and toughness of different kinds of silk, Atkins argues "it is important that any remaining puzzles about the mechanism of silk spinning should be solved."

"Jin and Kaplan's findings provide a basis for researchers to investigate how far it is possible to manipulate silk spinning, and how to engineer new silk proteins that can still form fibres," he writes.

"Ultimately [polymer design and processing] should lead to materials engineering in aqueous systems with desired functional properties, such as for biomaterials and tissue engineering," write Jin and Kaplan.

Some manufactured silks are available now, formed from spider silk proteins genetically engineered to be expressed in goats' milk.

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