Silk is pretty weird to begin with. The natural protein fiber, one of the strongest found in nature, is harvested from the larval cocoons of Bombyx mori, aka the domesticated silk moth. After some 5,000 years of selective breeding, said moth exists on Earth solely for the benefit of the silk trade—an insect carefully optimized to yield a single cacoon consisting of up to 3,000 feet of fine thread wrought from caterpillar spit.
The silk industry has been in decline over the past half-century or so thanks to the advent of synthetic textiles, particularly nylon. We don't really need catepillar spit anymore to make things like parachutes and stockings. But that's not the end of it—chemists are hard at work making a better silk, still with help from the industry's old friend, Bombyx mori. In a paper published last week in Nano Letters, a group of Chinese chemists describe the creation of a new reinforced silk fiber yielded by silkworms fed a diet of carbon nanotubes and graphene. It's pretty weird.
Hybrid supersilk itself isn't a brand-new idea. In the past, researchers have added dyes, antimicrobial agents, conductive polymers, and nanoparticles to silk, either by directly incorporating it into threads or by feeding it to the silkworms themselves.
Silkworms are pretty basic creatures. They mostly consist of stomach space, which is packed overfull with mulberry leaves in the 27 days or so that a given silkworm has to exist as a silkworm before spinning its cocoon, going into hiding, and undergoing metamorphosis into a moth. Silk is produced by pairs of salivary glands and consists of two distinct components: fibroin and sericin. Fibroin is the solid, durable component of silk. It's excreted as liquid through two holes in the silkworm's face where it solidifies into two hardened filaments as it hits the air. Meanwhile, the worm is secreting sericin, which acts as a bonding agent and glues the filaments together.
Image: Zhang et al
The Tsinghua University-based researchers behind the new study don't actually know how introducing graphene and carbon-nanotubes into the silkworm diet—a process in which mulberry leaves are sprayed with a liquid containing bits and pieces of added material—fits into all of this. Somehow the added synthetic material is conserved rather than excreted as waste and is then reincorporated into newly produced silk.
"This natural feeding strategy could be easily scaled up, paving a new path for the production of supertough silk fibers at a large scale," the authors note. "It is worth noting that there are still several interesting and important questions that cannot be answered by our current work, such as what is the safety limit of nanocarbons in the diets for the silkworms, how much of the nanocarbons taken by the silkworms are incorporated into the silk, and what is the detailed biological process."
The enhanced silk threads were about 50 percent stronger than normal silk, and were also electrically conductive. This latter property might make the material useful for sensors in future smart-materials, while the increased strength could lend itself to durable biomedical implants. But understanding how silkworms are even doing this is a prerequisite for getting to these real-world applications. That comes next.