By feeding a modified diet, researchers have coaxed silkworms to produce coloured silk, an approach that may help get rid of environment-unfriendly dyeing of the fibre1.

Commercial silk fibre produced by mulberry silkworm is generally white. The modern textile industry demands that silk fibre be dyed into a variety of colours that can be weaved into attractive textiles. But dyeing is one of the most polluting industries. It requires huge quantities of water for bleaching, washing and rinsing, resulting in untreated wastewater laced with harmful toxins that get into canals and rivers.

Coloured cocoons (left) and threads (right) produced by silkworm fed on red-dyed leaves. Credit: Nisal et al

The "green" method developed by researchers at the National Chemical Laboratory (NCL) in Pune and Central Sericultural Research and Training Institute in Mysore enables the silkworms to directly produce coloured silk. The technique involves feeding the larvae their favourite food — mulberry leaves — dipped in dye. The dye is transported along the biochemical pathways of the silkworm to produce a coloured cocoon and coloured silk fibre.

This idea is not new and has been studied before by other groups, the researchers admit. However, all the reported studies on colour silk have so far focused on the fluorescent dye Rhodamine which is too expensive for large-scale production.

So the Indian team turned to "azo" dyes — the name comes from azote, French for nitrogen — which are inexpensive and account for more than half of the textile dyes used today. The researchers say their work was prompted by the fact that for large-scale commercial synthesis of coloured silk, the usage of common cheap textile dyes is necessary.

The scientists evaluated seven different azo dyes by feeding dye-dipped or dye-sprayed mulberry leaves to Bombyx mori silkworm larvae to see which dyes were transferred to the silk. They found three dyes get incorporated into the caterpillars' silk and none seemed to harm the silkworm or affect its growth.

Using ultraviolet spectroscopy the scientists noticed that certain properties of the dyes, such as the ability to dissolve in water, were crucial for diffusion of the dye from the alimentary canal of the silkworm larva and eventually into the silk glands.

"These insights are extremely important in development of novel dye molecules that can be successfully fed to Bombyx mori silkworm larvae for producing intrinsically coloured silk of various colours and shades," the researchers said in their paper.

Sayam Sengupta, one of the authors from NCL, said they investigated a series of dyes that could produce colored silk in several shades. "Of course, many things need to work out to make it a successful technology — the dyes need to be non-toxic, they should preferentially color the fibroin and the color fastness properties need to match the color silk produced by traditional dyeing," he said.

Also, the economics needs to work out in favour of the process. "If all of these are successful, this could prove to be a process that can eliminate dyeing of silk, which is known to be a polluting technology," Sengupta said.