Silk, The Ancient Material With High-Tech Possibilities

The following is an excerpt from Silk: A World History by Aarathi Prasad.

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Silk: A World History

Silk is probably the only material that has lived so long, remained in continuous use, and that has been invented and reinvented. But it is also an ancient material with a surprisingly technological future. This substance made by an insect to protect it from predators and disease, that, in human hands, became a material for the most luxurious of clothing, is now being tested for ingenious applications in medicine, technology, and as a sustainable material to help mitigate the planetary catastrophe we now face.

Over the centuries, many were the scientists who became obsessed with its potential, and the knowledge of the natural world that the study of silk brought. The fact that the Bombyx mori was bred for so long in homes and factories specifically for its silken cocoons had made this caterpillar so docile, prevalent, and immobile that it would also quite seamlessly become the focus of intense scientific study. In the words of the Italian physician, Marcello Malpighi, published in 1668, “The silk- worm . . . is the most well-known insect among our countrymen . . . in which so wonderful Metamorphoses happens, and the work of nature so shines forth, that it is necessary to consider the unique aspects of its life.”

It was inevitable that the entry point into the study of the metamorphosis of all animals would be the domesticated silkworm. It was abundant. It could not fly. It was kept only because of the desire for the silk that emerged from inside its convoluted glands, threads it made solely for the purpose of its transformation. It was there to be observed by those who cared to study it. But, more than this, from the sixteenth century, Bombyx mori would become a fortuitous model for an unanticipated amount of scientific discovery—microbiology, immunology, physiology, zoology, evolution, and genetics—at a time when enormous paradigm changes were also taking place providing new understandings of biology—and the entirely new field of ecology championed by Malpighi’s contemporary, Maria Sybilla Merian, who showed the men of science, by example, that it was vital to move beyond studying their dead insects pinned to cards, to understanding the life cycles of animals, in the ecosystems that sustained them.

Our quest for silk has led to the unravelling of the mysteries of insect metamorphosis, and to the characterization of pheromones (hormonal substances emitted for attraction or warning). Silkworm studies gave Louis Pasteur a robust body of evidence for contagion, from which the study of human diseases would later benefit. Even the tiny dusty scales of their wings have been a source of fascination. These structures became specialized through evolution to absorb sound energy from predatory bats attempting to echolocate them. As each scale on each wing, vibrating to a bat’s sound waves, all with different frequencies, creates a cloak against every soundscape that might be used for hunting, it allows moths to seem to simply disappear from the night sky. Those frequencies span some two hundred kilohertz of sound that the moths transform instead into energy and heat. Under these astonishing fore and hind wings, a furry thorax is covered in setae also lie tiny hairs so wondrously light and porous that they absorb almost half their sound.

The threads of these silk moths are around one thousand time thinner than a human hair, and yet, because of their extremely repetitive molecular structures, possess a remarkable tensile strength. These facts have long been known: silks have been used in military applications from the body armor of Mongol and Chinese soldiers in the middle ages, to 19th century bullet proof vests in Chicago, to fire retardant Nazi parachutes. Studying both domesticated and wild silk moths has brought incredible scientific insight; but beyond these animals traditionally used for silk are others, whose productions science has desperately sought to emulate. These include the extrusions of a small shrimplike animal called Crassicorophium bonellii, studied because it spins a thread which in strength and in elasticity, somewhere between the kind of cement barnacles use to affix themselves to rocks and ships’ hulls—and spider silk, which has been the holy grail of silks for scientific applications; and the ‘sea silk’ of the Pinna nobilis mollusk and its mussel-like relatives—whose threads were woven into bronze-colored cloths in the ancient Mediterranean—are not spun but blow-molded and flipped out from the foot of the fan-shaped animal. The Pinna nobilis has been studied by scientists at the University of California, Santa Barbara, as the inspiration for creating a new polymer that could not only repair itself—“self-heal”—if damaged but do so underwater, under conditions in which adhesives would normally struggle to set.

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Of all of these, spider silk has been the most sought after, and the most elusive. Theirs are fibers that combine incredible tensile strength with resilient elasticity, required by spiders for a number of applications, including so that their webs to hold prey while being tough enough not to break upon impact. It is said, that if a spider were the size of a person, the web made would be strong enough to stop a jetliner in its path. Over the centuries, the fact that spiders’ webs were made out of more than one type of silk meant that the best strategy for collecting it was not from webs but extracting it directly from the spider, immobilized on a kind of medieval-style rack. But, unlike the Bombyx mori, time and time again spiders have shown scientists and weavers that they are not willing to be collected or farmed for their silk. It is nearly impossible to gather enough of these animals, due to their territorial nature, and because they are likely to eat each other. For all its potential for novel applications both in medicine and in engineering, the eternal problem has been acquiring, affordably, enough of their silk to work with. That led scientist Prof Randy Lewis, while at universities in Wyoming and Utah, to create synthetic genes to augment the properties of natural silks, as well as to attempt to generate quantities of silks like those made by spiders, but without spiders: using genetically modified bacteria, yeast, then goats (milked for the silk), and even Bombyx mori caterpillars, created with genetic changes that would allow them to generate a type of hybrid moth-spider silk.

In England’s University of Oxford, Prof Fritz Vollrath experimented with materials made from silk, and destined for the repair of damaged structures inside the human body. Having tested them in the laboratory, and finding them to be effective for such repair, Vollrath’s silk innovations went into the clinic, at least as far as human trials, in which his silk was tested for use in the repair of cartilage. As an alternative to spider silk, Vollrath also worked with the silk of the wild moth, the Antheraea. He chose the Antheraea rather than the Bombyx because wild silkworms evolved independently from the domesticated Bombyx mori silkworm. While the Bombyx lost much that it would need to survive in the wild along the road to its domestication, wild moths continued adapting to cope with the trials of their natural environments: adaptations to protect their delicate, succulent silkworm pupae as they metamorphosed, defenseless, against the threats of physical attack from animals; from bacteria and other organisms that might bring disease; and from the endless other dangers to which they might be exposed. As a result, those silk fibers of the Antheraea display breaking stress and toughness of the same magnitude as spider major ampullate silks—that is, the coveted properties of the dragline silk of spiders that had so diligently avoided being farmed. For him, it was the new, promise of silks that represented the future. “I think the whole question is not just of using silk as a sustainable material in textiles,” Vollrath says. “It’s always been that, in India since the Indus Valley, and in China, it’s been around as a key material for a very long time. But I think it has a future both in biomedical and in smart materials.”

Both of these have also been the objectives of Prof Fiorenzo Omenetto’s SilkLab at Tufts University, Boston. Using reverse-engineered silk, but once again, of the Bombyx mori, some of their silk innovations produced include temperature-stabilized vaccines that require no refrigeration; penicillin that has been stabilized, using silk, for many months; and a chemotherapy drug that has also remained stable and fully functional at the Mayo Clinic for over a decade. Omenetto’s lab has also developed a soft-tissue filler based on a silk protein. That is in clinical trials for cosmetics use, to fill wrinkles. And it is already approved for the treatment of vocal cord paralysis. “Medically right now they reconstruct vocal cords,” Omenetto says, “that’s one of the things that I think I can chalk up as having accomplished. Because patients walk into a room for the procedure, and they can’t speak, and then they walk out and they can. So this is actually a product that is available for therapy today… The fact that we start from a naturally based material drives us to put tech where tech normally doesn’t go really brings biology and technology together.”

From “silk plastic” cups and bags to biodegradable silk-based electronics—for electronic waste remains an intractable scourge on our planet—sustainability is a major reason for working with silk. Vollrath makes the point that—apart from humans, who have catastrophically broken the mold—nature tries to be as energy efficient as possible, be- cause, as he says, “why waste energy on something that you could use for reproduction? The currency of life is energy.”

And our problem is that our currency has ceased to be energy. “It was cheap. We just burn it. We are not husbanding it. We are not looking after it well enough.” It has been by understanding the efficiency of energy use—the lack of obscene wastage in the production of silk—that the scientists working toward its future applications place a huge emphasis on learning from nature. In the natural world, everything is recycled, and that’s the way it works.” Each of the animals that make silk evolved many very specific applications as a response to environmental pressures or availability of proteins from their food. “Therefore, we have a lot of historical research and development—not by humans, as they did in the case of the silkworm, which was domesticated for six or seven thousand years, but in the case of all the other creatures, for millions of years. So there’s a lot of embedded knowledge there about the protein, which is a biopolymer. And we have issues with polymers. We’ve got to get away from the bloody hydrocarbons.”

And that, to these scientists, is a very good reason to look to silk, because at least when it comes to the cocoons, or even the liquid silk from silkworms, they can be harvested from nature for diverse applications. The polymer chemistry and polymer physics that have become so well established, and upon which we have become so dependent— “Because humans now have been making plastics for a hundred years,” as Vollrath put it—also mean that science understands exactly how to make a plastic and, therefore, potentially, how silk polymers can be constructed. It is an apt transition back to an ancient material, because although traditional plastics, which are synthetic polymers, have been enormously useful, their continued production from un- sustainable hydrocarbons is increasingly unconscionable.

It is in this kind of bigger picture problem-solving that silk- manipulating scientists see real potential for the future of silk—what Omenetto describes as more profound social implications than anything that happens in American or European curiosity-driven research bubbles. That is pertinent, because silk has never had only one source, and the animals that make it can be found in the wild on nearly every continent.

From the book SILK by Aarathi Prasad. Copyright © 2024 by Aarathi Prasad. Reprinted by permission of HarperCollins Publishers.