The hype over spider silk has been building since 1710. That was the year François Xavier Bon de Saint Hilaire, president of the Royal Society of Sciences in Montpellier, France, wrote to his colleagues, "You will be surpriz'd to hear, that Spiders make a Silk, as beautiful, strong and glossy, as common Silk." Modern pitches boast that spider silk is five times stronger than steel yet more flexible than rubber. If it could be made into ropes, a macroscale web would be able to snare a jetliner.
The key word is "if." Researchers first cloned a spider silk gene in 1990, in hopes of incorporating it into other organisms to produce the silk. (Spiders can't be farmed like silkworms because they are territorial and cannibalistic.) Today, Escherichia coli bacteria, yeasts, plants, silkworms, and even goats have been genetically engineered to churn out spider silk proteins, though the proteins are often shorter and simpler than the spiders' own. Companies have managed to spin those proteins into enough high-strength thread to produce a few prototype garments, including a running shoe by Adidas and a lightweight parka by The North Face. But so far, companies have struggled to mass produce these supersilks.
Some executives say that may finally be about to change. One Emeryville, California-based startup, Bolt Threads, says it has perfected growing spider silk proteins in yeast and is poised to turn out tons of spider silk thread per year. In Lansing, Michigan, Kraig Biocraft Laboratories says it needs only to finalize negotiations with silkworm farms in Vietnam to produce mass quantities of a combination spider/silkworm silk, which the U.S. Army is now testing for ballistics protection. "There has been huge progress since … the '90s, when both function and commercial scale seemed far away," says My Hedhammar, a biochemist at the KTH Royal Institute of Technology in Stockholm.
Yet many biotech and research observers remain cautious about prospects for mass-produced spider silk rope and fabric. "It's not quite there yet," says Cheryl Hayashi, a spider silk geneticist at the American Museum of Natural History in New York City. "We're still not seeing these things on store shelves."
The dream is to mimic the toughest of spiders' seven types of silks: the dragline that spiders hang from, which incorporates several kinds of silk proteins. The protein molecules are massive—up to 600 kilodaltons (kDa) each, nearly double the size of the average-sized human protein—making them difficult to produce in engineered organisms. Companies have mostly opted to produce 50-kDa to 200-kDa versions that are more readily expressed. Those smaller proteins make silks that are typically less strong and flexible than their natural counterparts. "At some point, you lose the mechanical properties as the protein gets smaller," says Randy Lewis, a chemist at Utah State University in Logan, whose group cloned the first spider silk gene.
Mimicking the way spiders spin concentrated protein solutions into fibers has also proved to be difficult. Lewis can now spin spider silk proteins into fibers out of a water solution, eliminating the need for expensive and potentially toxic organic solvents. And researchers led by Daniel Söderberg, a fluid physicist at the KTH institute, have found a way to use the linearly oriented cellulose fibers in wood and paper to align spider silk proteins to form fibers. But spinning the fibers on a commercial scale is another matter. "It has been a huge challenge to scale up," says Lin Römer, chief scientific officer of AMSilk, a spider silk startup in Munich, Germany.
Spider silk proteins are already making their retail debut—but in cosmetics and medical devices, not high-strength fibers. AMSilk grows spider silk proteins in E. coli and dries the purified protein into powders or mixes it into gels, for use as additives for personal care products, such as moisture-retaining skin lotions. The silk proteins supposedly help the lotions form a very smooth, but breathable, layer over the skin. Römer says the company now sells tons of its purified silk protein ingredients every year.
AMSilk is also finishing up a clinical trial of silicone breast implants coated with the company's spider silk proteins. Because the proteins consist of a highly repetitive sequence of small amino acids, they are said to be virtually invisible to the immune system. The coating also shrugs off bacteria better than Teflon or stainless steel. Those same advantages, Römer says, should allow the company to improve other implants, from artificial hips to catheters, with spider silk coatings. Kraig Biocraft Labs and Spiber Technologies in Stockholm say they are pursuing similar applications.
In another medical use, Hedhammar and colleagues reported in 2015 that they had engineered spider silk to include peptides that promote cell adhesion, then produced the altered silk in bacteria. They wove it into a mesh, with which they captured and cultured insulin-producing islet cells in vitro. The result was an engineered tissue that Hedhammar hopes might one day replace the mechanical insulin pumps used by people with diabetes. Thomas Scheibel, a biochemist at the University of Bayreuth in Germany and one of AMSilk's founders, used a similar approach to culture cardiac cells, aiming to create a matrix that could replace cardiac tissue damaged by heart attacks.
Bolt Threads says it is now poised to use spider silk as nature intended—as a high-strength fiber. David Breslauer, Bolt's co-founder and chief scientific officer, says company scientists have engineered thousands of recombinant spider silk genes into yeast, in search of the highest performing materials. They have also plowed through a vast amount of trial and error to optimize the conditions for growing and purifying the proteins and spinning them into fibers. "It has been hard," Breslauer says. But he insists that Bolt Threads has finally succeeded, though he declines to detail the exact procedures.
Earlier this month, the company, which has raised some $90 million from investors and research grants, announced a partnership with fashion designer Stella McCartney to produce a line of spider silk fashions. Commercial production will begin next year, Breslauer says. In addition to being lightweight and strong, Breslauer says he expects McCartney's designs to attract customers interested in "vegan" fabrics, made without the need for animals.
Still, similar promises have come and gone. In October 2015, for example, a Japanese spider silk startup, called Spiber Inc., announced that The North Face outlets in Japan would soon begin selling $1000 golden "Moon Parkas" made from the company's synthetic spider silk. But in September 2016, the company pulled back, saying "more time is required to further improve our production process for more stability of material quality."
It was one more reminder that spider silk, for all its allure, can also be a snare.