Silkworms in a Japanese lab are busy spinning silks that glow in the dark. But these silkworms, unlike others that have been fed rainbow-colored dyes, don't need any dietary interventions to spin in color: They've been genetically engineered to produce fluorescent skeins in shades of red, orange, and green.

Now, scientists have tweaked the silk production process and made it possible to turn these somewhat freakish threads into useable fabrics.

The resulting silks glow under fluorescent light, and are only ever-so-slightly weaker than silks that are normally used for fabrics, scientists reported June 12 in Advanced Functional Materials. Already, the glowing silks have been incorporated into everyday garments such as suits and ties, and Japanese wedding dress designer Yumi Katsura has designed and made gowns that glow in the dark.

This isn't the first study that has genetically modified silkworms (Bombyx mori). Scientists have previously engineered silkworms to produce a variety of substances, including spider silk, human collagen proteins, and glowing proteins.

"When we produced green fluorescent protein in transgenic silkworms, we obtained very beautiful silks," said study coauthor Toshiki Tamura, a molecular biologist at the National Institute of Agrobiological Sciences. "We extended the method to produce three different colored fluorescent silks in large amounts."

Fluorescent cocoons produced by transgenic silkworms, shown under both white and fluorescent light. Unmodified silk is on top. ( Iizuka et al., Advanced Functional Materials)

Creating the glowing silks meant borrowing from organisms that already produce fluorescent molecules. Scientists inserted the DNA sequences that produce these foreign fluorescent proteins into the silkworm genome, creating what’s called a transgenic animal. One batch got a red, glowing protein normally found in Discosoma corals; another got a glowing orange protein from the Fungia concinna coral. The third strain incorporated the green fluorescent protein derived from jellyfish.

When the silkworms started spinning, the glowing sequences turned on and produced silk in three different colors – and the colors stayed vibrant and glowing for more than two years. Scientists bred and reared more than 20,000 of these transgenic silkworms in the lab, feeding them mulberry leaves, harvesting their shimmering threads and working out how to turn the raw, glowing cocoon silk into a functional material. Because the processing steps for normal silk – such as cooking cocoons at 100 degrees Celsius – destroy fluorescent proteins, the scientists needed to find a slightly different way to produce the fabrics. In the end, they found that a combination of slightly lower temperatures, an alkaline solution, and a vacuum produced softened cocoons that could be reeled.

Now, the team hopes these glowing fabrics will find a use in other areas, such as medical technologies. The silks, if commercially available, would only be a bit more expensive than normal silk, Tamura said, noting that additional costs are derived from the different processing steps.

Malcolm Fraser, a molecular biologist at the University of Notre Dame, suggests that while the fluorescent silks are interesting, they'd be even better, and more practical, if the colors were more robust. In 2011, Fraser and his colleagues engineered a silkworm that spun spider silk. "The coloration conferred by these fluorescent protein sequences is not strong enough to impart a significant coloration in visible light," he said. "Coloration using dyes is much more versatile in terms of the color palette, and certainly more practical in terms of creating fabrics."