Second Nature
More and more, innovative scientists are turning to the natural world for inspiration...and design solutions
- By Jim Robbins
- Smithsonian magazine, July 2002, Subscribe
(Page 3 of 4)
But there may be no better example of nature’s elegance and efficiency than the silk in a spider’s web. In his cubbyhole office at the University of Wyoming in Laramie, molecular biologist Randy Lewis displays a computerized anatomical drawing of the golden orb weaver spider. Each of six separate sets of glands in the spider’s abdomen produces a different protein solution, or dope, which the spider forces through spinnerets to produce six kinds of silk: one for wrapping eggs; another to secure prey; three for building a web; and, strongest of all, dragline silk, which a spider uses to hang from a ceiling or branch and for the structure of its web. “Dragline silk is the strongest material ever made by an animal,” says Lewis, a leading expert on spider genes. In theory, a braided spider silk rope the diameter of a pencil could stop a fighter jet landing on an aircraft carrier. And it is as elastic as nylon. The combination of strength and elasticity allows it to withstand an impact five times more powerful than can Kevlar, the synthetic fiber used in bulletproof vests.But harvesting spider silk is not a job for Little Miss Muffet. “Everyone has tried to farm spiders,” says Nexia’s Turner, “but no one has been successful.” Put spiders together and they end up eating one another. In 1998, Turner learned that Lewis and others had isolated the genes for spider silk. Aware that researchers had used the lactation system of goats to produce medicines, he wondered why a goat couldn’t also make spider silk in its milk. After all, the gland that produces silk in a spider is similar to that which produces milk in a goat. “So I called up Randy to help us with the golden orb weaver genes,” Turner recalls.
Nexia technicians began by removing hundreds of fertilized eggs from several dozen goats. The researchers then inserted spider silk genes into the fertilized eggs and returned them to the goats. As the first of the resulting females become mothers themselves this summer, Nexia technicians will skim and concentrate their milk, which at this stage will look like maple syrup. Up to this point, Nexia will not have done anything so revolutionary. “Mimicking what the spider does is the hard part,” says Turner. In its spinnerets, a spider somehow turns the liquid dope into silk of a perfect consistency—not too wet or brittle but strong and super elastic.
Turner volunteers only the barest details about how Nexia and its collaborator, the U.S. Army Soldier Biological Chemical Command in Natick, Massachusetts, force the spider dope through a syringelike apparatus to create long monofilament fibers that can be braided or woven. In preliminary tests of silk produced by isolated cells in the lab, Nexia has created silk with many properties comparable to natural spider silk. But, Turner admits, it has only 30 percent of the natural fiber’s strength. Still, he is optimistic that he can make the fiber stronger, and he says he will patent the process in the next several years.
Such fundamental tinkering is not without controversy, of course. “There’s this simpleminded notion that you’re dealing with Lego blocks and if you pull one out and put it somewhere else, you know exactly what you are doing,” says Ruth Hubbard, a professor emerita of biology at Harvard and a founder of the Council for Responsible Genetics. “But it’s unpredictable, and it’s naive to think you can predict exactly what will happen. I always ask ‘Why are we doing this?’ If there’s a good reason for doing it, and it’s done carefully, it’s OK. If it’s just another way of making money, I don’t think it’s worth taking the chances.”
But there’s more than one way to skin a cat—or mimic spider silk. David Kaplan, a professor of chemical and biological engineering at Tufts who has studied biological silk for years, has high hopes for braided silkworm silk. While it is not as strong as spiders’, he says, it can be used in biomedical materials relatively quickly and is commercially available in large quantities.
In his Tufts laboratory, Kaplan shows me a large, shallow metal box, called a winding tray, that looks something like the inside of a grand piano and holds a dozen tiny electrical motors. Taut four-foot-long fibers, each composed of ten strands of silkworm silk, are affixed to the motors at each end of the tray. A computer programs each motor to wind fibers with a different number of twists per inch, giving each fiber a different strength and elasticity. “You can get whatever properties you need if you bundle it and cable it the right way,” Kaplan says.
He believes that human tissue will grow around the fibers to create new ligaments. Though he is concentrating on making an artificial alternative to the anterior cruciate ligament, the knee tissue that is a problem for many athletes, “in theory it could be used for any tendon or ligament, as well as other tissues,” he says. Human trials are between one and three years away.
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