Five years ago, the graduate students behind a secretive startup called Bolt Threads set out to replicate the unique chemical properties of spider silk, an almost magically flexible and durable material that's in some ways as strong as steel. One of the first things they did was buy a batch of Nephila spiders-the common golden silk orb- weavers-from an insect dealer in Florida. Then they let the spiders spin their webs all over the company's first office at the University of California at San Francisco. One day a well- known UCSF molecular biologist walked in, saw a spider hanging in a doorway, and ran away screaming.

Scientists, at least those who aren't arachnophobes, have tried to mass-produce spider silk for decades with little success. Spiders are territorial and cannibalistic-try to farm them, and they end up eating each other. But scientists have long believed that if spiders would only cooperate, fabric made from their silk would be well-suited for use in military and medical equipment, like wound sutures or artificial tendons, as well as in high-performance athletic clothing and other garments.

Bolt Threads has ditched the live spiders but held on to this goal. The company has developed a synthetic alternative to spider silk by engineering proteins identical to the natural threads stretched across the nooks in your basement.

It has raised $40 million from Silicon Valley venture capital firms Foundation Capital, Formation 8, and Founders Fund to commercialize its technology and turn those proteins into fabric. "Over the past few decades, as clothing companies squeezed on price, they've taken the innovation out of apparel," says Dan Widmaier, a graduate of the UCSF Ph.D. program in chemical biology and Bolt's chief executive officer.


Widmaier and co-founders Ethan Mirsky, Bolt's vice president for operations, and David Breslauer, its chief scientific officer, are genetically modifying yeast, single-cell organisms that convert simple carbohydrates to proteins through fermentation, and getting them to excrete silk-like proteins. "What would have been done in cells of spiders is now being done by yeast in our lab," Widmaier says.

Investors are betting this could revolutionize an apparel industry coasting on the decades-old invention of petroleum- based fibres such as polypropylene (lightweight and breathable but a breeding ground for stinky bacteria) and lycra (thin and stretchy, maybe too much so after repeated wear).

Bolt says that by slightly modifying the genetic makeup of yeast cells and tweaking the way it spins the secreted proteins into thread, it can engineer fabrics to specified levels of softness, durability, and strength. "Our investing hypothesis was to make tunable silk that is hyperelastic and machine- washable," says Steve Vassallo, a general partner at Foundation Capital, who discovered Widmaier and Breslauer during the summer of 2011, when they were in their spider-infested UCSF office.

Back then, Bolt Threads was called Refactored Materials and had scientific rather than commercial goals. Widmaier was studying how to make proteins from yeast cells when he met Breslauer, a Ph.D. student at UC Berkeley, who was looking at how to process spider silk into fibres. They were operating on grants from the U.S. Army and the National Science Foundation to research yeast-made proteins and evaluate uses in ballistics-possibly to replace Kevlar in bulletproof vests-and materials such as surgical sutures. The most lucrative opportunity was right in front of them; Widmaier's wife is a fashion designer at Old Navy, and the founders realized they could pitch their synthetic silk as an alternative to petroleum-based textiles such as polyester or cheap but non- eco-friendly staples like cotton.

Bolt Threads occupies offices across the San Francisco Bay, in Emeryville. A copy of Charlotte's Web is in the lobby, and a chandelier made of silkworm cocoons hangs in the conference room. The real action is in a series of interlinked labs, where billions of bioengineered yeast cells ferment, excreting their protein fibers into slurries of water, salt, and sugar. In another room, centrifuges filter out the silk from the liquid. A larger room will hold 200-liter fermentation units for producing silk in greater quantities. Bolt is working with manufacturing partners such as the Michigan Biotechnology Institute in Lansing, which will do larger-scale fermentation in 4,000-liter tanks using Bolt's process, and Unifi, a yarn manufacturer based in Greensboro, N.C., which will spin Bolt's fibers into apparel- ready yarn and textiles.

The company's aiming at the market for high-performance apparel such as sports shirts and bras and expects to have products by late next year. It hired Sue Levin, the founder and former CEO of athletic wear retailer Lucy Activewear, to lead merchandising and marketing.

Bolt Threads must decide whether it wants to make its own clothes or provide the fabric to other apparel makers, or both. The co-founders say they're talking to a major apparel brand about using the fabric Bolt will make but declined to disclose the company. There's plenty of competition: Rivals and researchers at Massachusetts Institute of Technology are studying bioengineered yeast for similar uses in apparel and medicine, like making artificial heart tissue. Bolt's fabrics may be expensive until the company can boost production over the next few years and drive down costs. "Almost all technology starts as a premium product," Widmaier says. At least there won't be spiders eating each other.

The bottom line: Bolt Threads expects products made with its yeast cell-based silk to be available in 2016.