Now, look for luminescent silk

The technique could eliminate the need for energy-intensive traditional dyeing, which results in large amounts of polluted wastewater.

It could also open the door to large-scale production of revolutionary biomaterials with new functions, notably in medical science, say researchers conducting the experiment at the Institute of Materials Research and Engineering.

The feeding technique could be used, for example, to produce thread with antibacterial, anticoagulant or anti-inflammatory properties.

“What we have now is an understanding of how to integrate molecules into the core of silk filament,” said Dr Natalia Tansil, the lead researcher on the project. “It doesn’t have to be dye. The same principle can be applied to other chemical molecules. They just need to have a certain molecular structure.” “We can load drugs into the silk, and control the interaction between drug molecules and the silk so that drugs will be gradually released,” Natalia said.

Diet

The researchers are now testing silk produced by silkworms fed on a diet that includes cancer-fighting drugs to monitor the effects on cancer cells.

Silkworms are the larvae of the silk moth Bombyx mori. They grow rapidly, eating mulberry leaves, shedding their skin five times, until they are about 7.6 centimeters long. They then form a cocoon, spinning protein around them. It is the cocoon that is harvested for silk.

The researchers started adding rhodamine chemical dyes to the feed on the third day of the last phase of the worms’ larval stage, shortly before the larvae started spinning their cocoons. The silkworms changed colour within several hours, and colour was transmitted to the silk of the cocoons — and in particular the core filament, called fibroin.

Pierre Couble, a French researcher who directs the Molecular and Cellular Genetics Center at CNRS-Universite Lyon I, said that the technique of giving dye-laced feeds to silkworms had been tried before, but without lasting results.

“One can rear silkworms that produce dark yellow, intense golden, pink, light green cocoons, etc. However, when one dips the cocoons in water the pigments are dissolved out, since they are bound by very loose chemical interactions,” he wrote in an e-mail.
But Natalia said previous efforts to incorporate dye-laced feeds had failed because the dye used had an inappropriate molecular structure, “so it was just passing through the body of the larvae rather than being absorbed to be expressed with the silk.”

“In the past others have produced coloured cocoons but these colour come from pigments that reside mainly in the outer layer of sericin,” she said, referring to the gelatinous protein that cements the two fibroin filaments in a silk fibre.

“Sericin is hydrophilic and located at the outer layer, therefore these colour are easily removed upon dissolving in water. Our silk fibroin are still strongly coloured even after degumming, the process of removing the sericin, which involves immersing the raw silk in hot water for at least one hour,” she added.

“The incorporation of dye molecules while the silk molecules are being synthesised in the silkworm’s silk gland ensures a strong interaction at a molecular level,” she said.

White silk is already widely used as a scaffold in engineering the growth of tissue cells. Now, the use of luminescent silk could help to improve the visualisation of the cells, allowing scientists to better understand the performance of the scaffold.

Cells that are growing on or in direct contact with the scaffold material appear yellow (due to the overlap of various fluorescences) while other cells appear green. “That, we’ve already demonstrated in our work,” she said.