Unravelling the secrets of silk production
Oct 11, 2010
The secrets behind the mighty strength of silk could be unravelled by neutron-scattering experiments being carried out in France. Early results have revealed that silk worms spin their silky threads in a process that seems completely counterintuitive to what is expected. On a domestic note, the findings provide some practical tips for anyone wondering how to wash their silk products.
For such an elegant material, silk is incredibly durable, possessing a tensile strength comparable with steel. These properties combine to make silk a highly desirable product. But despite being ubiquitous in luxury textiles throughout the centuries, the process of silk formation inside the body of silk worms has remained something of a mystery, with different scientists proposing varying explanations.
One of the practical limitations when studying silk is that at any given time each silk worm has only minute amounts of silk's precursor proteins inside its body. So any scientific programme to study silk requires the upkeep of large numbers of silk worms followed by the careful extraction of silk samples. Given these difficulties, previous silk studies have used "regenerated" silk proteins, obtained by breaking down silk worm cocoons with high salt concentrations then mixing samples.
In this research, a team led by Cedric Dicko of the University of Oxford has for the first time studied the production of pure silk, extracted in small quantities from silk worms. Using a series of small angle neutron-scattering experiments at the Institut Laue-Langevin (ILL), the team was able to hone in on relatively small samples of the large biological molecules that form silk.
Dicko's team discovered that proteins are abundant inside the worm, with concentrations of up to 400 mg/ml. Unusually for this concentration, the proteins showed very little interaction, instead forming a compact helical structure with a radius of gyration of 90 nm. However, the situation changed as the researchers diluted the silk solution with water, which caused the proteins to unfold to 130 nm and start to combine into the ordered filaments of silk.
"This is an extraordinarily high concentration for the proteins to remain stably dispersed throughout the solution," says Dicko. "Even stranger, as the concentration drops the proteins begin to expand and flow, until they eventually clump together – this is the reverse of what we'd expected."
Saving silk shirts
The finding that water plays such a key role in giving silk its strength has implications for those in possession of silk products. "Dry-cleaning silks can strip away the moisture and weaken the fibres in silk garments, leaving them more likely to get damaged," says Phil Callow, one of the researchers, based full-time at the ILL. Callow provides reassurance, however, that if one were to make the mistake of dry-cleaning a silk shirt, they should be able to return it to its original condition by steaming it gently.
Callow explained that neutrons were used to examine silk because they offer advantages over other diffraction experiments, such as X-rays, which can damage the samples under study. He revealed that Dicko is set to return to the ILL in December to continue this research by investigating the effects of temperature variation on silk production.
The latest findings are described in a paper in Soft Matter.
About the author
James Dacey is a reporter for physicsworld.com