The Amazing Spider-Man may be fictional, but his most formidable weapon—the spider web—is no less amazing in real life.

The spider silk produced by the superhero's nonfiction namesake is extraordinarily strong, fine and tough—a knockout combination of properties that has scientists and entrepreneurs racing to develop medicine, defense and leisure applications. Researchers across the world are not only studying the silk's unique makeup, but also copying it—attempting to mass-produce a fiber that is similarly super-strong and elastic.

Spiders actually manufacture up to seven different types of silk, each one geared towards a certain function. Researchers have been most interested in "dragline" silk—which forms the arachnids' safety line and frame for their webs. Weight-for-weight, dragline silk actually surpasses steel in strength. What's more, it's finer than human hair, has greater resiliency than any synthetic fiber and is totally biodegradable. In short, it's a super-material.

Possible applications for dragline silk include medical uses—surgical sutures and ligament repair—and military gear—parachute straps and flexible body armor. The material's potential is so enticing that it has ensnared entrepreneurs, military personnel and university researchers in a web of intrigue.

One such researcher is Helen Hansma, who, along with colleagues from the Department of Physics, University of California, Santa Barbara (UCSB), is trying to figure out why spider silk is so extraordinary. To analyze the material's structure, Hansma's team studied a solution of spider dragline silk protein.

The UCSB team found that the protein solution self-assembles into nanofibers—with bumps along the nanofibers indicating that they are made up of segments. Furthermore, each segment is composed of a stack of molecules, forming nanocrystals. Thus the nanofiber makeup can be described as a series of nanocrystals within an amorphous structure.

The UCSB researchers are also collecting data on how the protein reacts when you pull on the silk. They are using this information to improve models of the structure and to better understand how silk molecules and fibers perform.

Physicist Carl Michal and a team of graduate students at the University of British Columbia (UBC) in Vancouver are also creating advanced models, which Michal hopes can assist those who are intent on mass-producing materials based on dragline silk.

Michal's methods of harvesting the silk, however, will be of little use to such commercial-minded groups. Even though his team uses relatively big spiders—the nephila clavipes—the researchers are only able to obtain about 1.5 mg of silk a day, and that's on a good day.

So how do you mass-produce spider silk, when these eight-legged creatures tend to be hostile and territorial and thus impossible to farm?

For Jeffrey Turner, CEO of Nexia Biotechnologies, Inc. the answer is biomimicry. Turner has chosen to copy the material, relying not on spiders for production, but on goats—their mammary glands to be precise.

Nexia researchers first inserted genes from spider dragline silk into goats' mammary gland cells, which make milk. "And it was really the 'Eureka!' moment," says Turner. "The mammary cells took the spider silk genes in all their complexity and made a beautiful, water-soluble, authentic spider silk. That was a really incredible time."

And to turn the watery solution into fibrous threads, the company sought the help of researchers at the U.S. Army Natick Research and Development Center in Massachusetts. Turner recalls what transpired next: "The protein came zipping out of the syringe tip and when the water was removed, it self-assembled into a beautiful silk."

Finally, to establish a reliable production line, Nexia researchers transferred individual spider silk genes into single cell goat eggs, making transgenic goats (or goats whose genomes have been altered).

These special goats produce milk with the coveted water-soluble silk proteins. According to Turner, each animal is capable of making "literally miles" of this spider silk-based material, otherwise known as BioSteel®. The company's total herd size of over 1,500 can produce enough silk for the medical devices industry, though it would have to be 10 times that size to meet industrial demands.

The first Nexia product is expected to be a biodegradable fishing line, which will probably hit the market in two years. Meanwhile in the medical field, a BioSteel® microsuture will likely mount a challenge to nylon-based threads for surgery by the end of 2004. And further into the future, the spider silk-based material may be promoted as a crucial ingredient in tendon and ligament repair because it doesn't tire when frequently flexed and can take regular impact and great pressure.

The material's ability to dissipate energy also makes it ideal for body armor for the military and the police. Compared to the standard issue bullet-proof vest, BioSteel® ballistic protection would be superior at energy management—more effectively spreading out and thus reducing the force of a gunshot or an explosion. It would also be more comfortable—lighter and more flexible.

But before such products even make their way to consumers, the material must address some concerns. For one, these synthetic materials have only passed laboratory tests, and such trials do not reflect real-life rigors, says Chris Viney, head of materials chemistry at Heriot-Watt University, Edinburgh. For example, he says, the silks are only made to carry loads for less than an hour, when a weeks-long or even years-long trial would be more realistic.

In addition, the natural material on which the synthetic silks are based was not built to endure. "Spiders produce their dragline and webs to do a specific job, which is accomplished in minutes," says Viney. "Webs are repaired or replaced daily. These silks are not designed to last!" This casts serious doubts on the synthetic version's ability to weather moisture changes, temperature fluctuations and exposure to ultraviolet radiation—all real-life conditions.

What's more, all bets are off when spider silk gets wet. Viney's experiments have revealed that dragline silk "supercontracts" when wet—shrinking by half in length and fattening up to almost twice its diameter. It also loses its load-supporting abilities and tends to creep quickly and change in shape.

But maybe the trick is not to manufacture an exact replica of natural spider silk—a material tailor-made for arachnids' needs—but to produce a copy that is geared towards satisfying our needs. And this is feasible, says Frank Ko, director of the Fibrous Materials Research Center at Drexel University in Philadelphia. He thinks engineers can make synthetic versions that can fulfill specific functions by adding nanoparticles to a solution of spider silk protein.

Indeed, we still have a long way to go to take advantage of spider silk. Even Nexia's Turner admits it will take nearly superhero-level efforts. "People say: 'Oh, spider silk. Everybody's working on it. That's easy to make.' But it isn't," he says. "We've got a group of really talented, serious people doing the development, and big partners, and it's still taking time. That's an important message."

Sources: Exploiting Spiders' Silk
Paula Gould
Materials Today, Dec. 2002
http://www.materialstoday.com/