Silkworms’ Colorful Diet

In a colorful twist on silk production, scientists in Singapore are having silkworms gobble up dyes that colorize the worms and their silk cocoons (Adv. Mater., DOI: 10.1002/adma.201003860). The approach is a potential low-cost, environmentally friendly method for making colored silks that avoids traditional chemical dyeing. A silkworm’s diet is known to affect its color and the color of the silk it produces. But the color change of the silk is largely limited to the outer sheath of the fiber, a gummy protein called sericin. This layer must be removed enzymatically to get at silk’s usable fiber core, a protein known as fibroin, which doesn’t normally take up pigments. Researchers led by Ming-Yong Han of Singapore’s Agency for Science, Technology & Research fed silkworms fluorescent rhodamine dyes mixed in with mulberry leaves, the silkworms’ favorite food. Various concentrations of single rhodamines and mixtures of rhodamines caused the worms to change color and produce a rainbow of silk cocoons in which the color is incorporated into fibroin. As an added bonus, under UV light the rhodamine-infused silk luminesces in different colors. The researchers suggest hydrophilic-hydrophobic interactions between the dye molecules and fibroin, which were missing in previous dietary silk-coloring attempts, make the colorful silks possible. It’s not clear yet whether the new silks are colorfast.