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Diffraction gratings made of silk. Credit: Fiorenzo Omenetto
In an optics lab at Tufts University near Boston, a $20,000 vibration-controlled tabletop is going to waste. Where an ultrafast laser should be, there is instead a disheveled array of Petri dishes. In place of a spectrum analyzer sits a cardboard box full of leafy twigs.
“Here’s our farm,” joked Dr. Fiorenzo Omenetto, a professor of biomedical engineering and physics at Tufts who was recently named one of “the smartest people in tech” by Fortune magazine. Through a pair of hip plastic-framed glasses he peered down into the Petri dishes, giving one or two a friendly flick. In each dish, a single, plump silkworm was wallowing on a bed of mulberry leaves.
Three years ago Omenetto wouldn’t have been able to imagine a less likely scenario. At that time, he ran an ultrafast laser lab with a focus on photonics and didn’t know a silkworm from a white grub. But a lot has happened since then. A hallway conversation with Dr. David Kaplan, the chair of the biomedical engineering department, has evolved into the collaboration of a lifetime, generating dozens of publications, hundreds of talks and interviews, and a Renaissance of ideas about silk.
“Ah!” Omenetto’s face lit up. “Look—this one is spinning.” He pointed to a worm looking more purposeful than the others. As if in a trance, it moved its raised head in a figure-eight pattern. With spinnerets on either side of its mouth it was reeling out a fine strand of silk through a hole in its head, affixing the strand at different points on the walls of its plastic container. The silkworm was erecting a scaffold around itself; a kilometer of silk to go and its cocoon would be so thick and rigid you couldn’t squeeze a dent in it.
But the silkworm’s hopes of metamorphosing inside, I was told, were ill-fated. It was to share the destiny of the vast majority of silkworms reared by the hundred billion every year in Southeast Asia: As soon as construction is done, they are done in, and their cocoons are harvested, soaked, and unraveled. Only a handful of worms per farm are allowed to mature and burst forth from their shrouds as moths. These lucky few are entrusted with the task of passing on the secret of their trade to the next generation: For thousands of years, the secret of how to actually make silk has been one the worms have kept to themselves.
Silkworm cultivation, or sericulture, began sometime around the year 2700 BCE in China, and was kept under wraps for the next three millennia. By exporting silk cloth cloaked in total mystery, the Chinese maintained the longest industrial monopoly in history. That mystery deepened desire. The rest of the world was so completely enamored with silk—and in the dark about the material’s origins—that the Roman poet Virgil praised its beauty in his work, and suggested it might come from fluff combed out of unknown Chinese leaves. Soaring demand for the lustrous fabric drove the establishment of trade routes—collectively, the “Silk Road” —forever linking the East and West.
Eventually greed and gossip joined forces on the matter of silkworm cultivation, and in the third century CE the secret finally spilled over the Chinese border. Soon sericulture was thriving all over Asia, and then all over Europe. Silk, which was once worth its weight in gold, plummeted in price. Prices have continued to drop ever since, as farming and processing technologies incrementally improve.
But farmers can only do so much. Silkworms themselves are the bottleneck in production. They must fatten up for a month on vast quantities of mulberry leaves before spinning, and they don’t spin at all unless conditions are just right. Like artistic geniuses, their foibles must be endured for the sake of their talents. And like the creation of art, that of silk—one of the strongest and longest protein chains found in nature—is wholly, beautifully inessential. Silk is much stronger on the molecular level than it needs to be to make an effective cocoon, and far more spectacular.
Silk protein fibers “embody strength and beauty,” in the words of Omenetto and Kaplan in a review they co-authored in the July 30th issue of Science. Kaplan has been studying silk for more than 20 years with ever-waxing enthusiasm. At this point he may be the Western world’s leading silk expert, so I asked him what it is, exactly. Silk, he explained, is an incredibly long chain of repeating sequences of a few types of amino acids. Firm bonds that form between adjacent amino acids make the fiber as much as five times stronger than steel and three times tougher than Kevlar (a synthetic fabric used in bulletproof vests and other body armor). And yet, Kaplan noted with awe, even though the silk molecule is 400,000 amino acids long and unbreakably strong, “it has a relatively simple form, so it appears over and over again in nature.”
Silkworms and spiders, for example, each evolved the ability to synthesize silk independently. Spiders weave the slightly stronger kind, “drag-line” silk, but due to their aggression and the tendency of their webs to tangle, silk from silkworms is the textile industry standard. In fact, that industry has created its own species: Bombyx mori, which was domesticated from a wild species of silkworm by the Chinese. Having lost the ability to fly or reproduce without assistance, B. mori depends largely on humans for survival. And yet, because it alone knows how to assemble amino acids in the right way to make silk, throughout the triumphs and treacheries of history it alone has held a true monopoly on the fabric’s manufacture.
That is, until now.
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