The Biology of . . . Spider Silk

The race to synthesize the world's strongest fiber

By Robert Kunzig|Saturday, September 01, 2001
RELATED TAGS: UNUSUAL ORGANISMS


Up on the roof of the zoology building, Fritz Vollrath pushes open the door of a small greenhouse and walks in. Tupperware containers full of maggots and decomposing fruit are scattered on every available surface, and the thick and sickly smell of rot fills Vollrath's nostrils. But he ignores these signs that all is well: "It's too hot," he says. Not for the palm by the door or the tall cactus, but for the distinguished architects who are hanging out in the upper corners. Most of them are a species of Nephila, the golden silk spider. The species has an inch-long abdomen, greenish-black with yellow markings, and eight long, delicate legs. Nephila are orb weavers, and the silk orbs they have woven in Vollrath's "spider house" are thick with the flies he intended for them. But the spiders themselves are looking a bit sluggish. Vollrath opens a window to cool things off. Right next to the window there is a web, and Vollrath brushes its owner into a little plastic jar. Come nightfall it will get a bit too chilly in that spot, and anyway he needs a spider to take to the synchrotron in Grenoble.

Vollrath is a handsome, muscular man with shaggy gray hair, a very slight German accent, and a face that broadens readily into a pleased grin when he sees he's amazing you with facts about spider silk. It is pretty amazing stuff. Nephila silk has a tensile strength almost as great as steel's per unit volume and far greater than steel's per unit weight. Kevlar is three times harder to break than Nephila silk, but the spider silk is five times more elastic. Kevlar stops bullets by brute force; Nephila silk stops flies by stretching without breaking. As is so often the case, human engineers trail the elegant solutions of nature.

Lately though, the humans have been trying hard to catch up. Spider silk not only has wonderful mechanical properties; it is made of biodegradable proteins, using water as a solvent, rather than nasty organic chemicals. Thus there are strong economic and environmental incentives for finding a way to make spider silk artificially. (You can't farm spiders as you would silkworms, because they eat each other.) Fragments of genes for a few spider-silk proteins have been cloned and inserted in goats, which secrete the proteins in their milk, and just this year in tobacco and potato plants, which secrete them in their leaves. Nexia Biotechnologies, the company that keeps the goats on a farm in upstate New York, has even announced the creation of a product called BioSteel— though all the company really seems to have at the moment are artificial proteins, not threads of artificial silk. "There is a lot of optimistic hype in this field," Vollrath says.

Vollrath began studying spiders in 1974 and concentrated on silk a decade ago. He soon found, he says, that "it was an extremely interesting material and far more complex than people gave it credit for." There are more than 34,000 known species of spider, each of which makes its own silk and some of which make different silks for different purposes. The toughest kind is called dragline silk: It forms the framework of the spider web, and it's also what the spider spins at frantic speeds of up to a meter per second when it jumps off a tree or house to escape a bird.

More research has been done on the dragline silk of Nephila than on any other kind. But it is a long way from being understood. A single thread of that silk is perhaps three to five micrometers across. When Vollrath started looking into it, people thought dragline silk was a relatively simple composite material, like fiberglass, consisting of stiff sheets of crystallized protein floating in an elastic rubbery matrix. But that, Vollrath has found, is not the structure of the whole thread; it's the structure of a single filament inside the thread— and there may be thousands of such filaments, each only a few nanometers across, too small to be seen with a typical microscope, and perhaps bundled in some way that has yet to be discerned.

"That's what gives it this incredible tensile strength, this whole microstructure," Vollrath says. "If you're jumping off a bridge, would you prefer I gave you a single rubber band or a thousand rubber bands with the same total diameter? It's intuitive— if you have a thousand, a few can snap and there are still enough to hold you." The spider's dragline is made even more snap-resistant, Vollrath thinks, by long, fluid-filled channels that are interspersed among the tightly packed filaments. Those channels may help distribute the tensile forces and so stop a nascent crack from ripping right across the thread.

A garden spider's silk, seen through a magnifying glass (top) and a microscope (middle and bottom), is coated in amino acids that attract water. As the water gathers in beads it reels in the line, keeping it taut.
Photograph courtesy of Fritz Vollrath
Of course, some of the strength of the silk must come from the protein molecules that make it up. Earlier this year, John Gatesy and Cheryl Hayashi of the University of California at Riverside and their colleagues reported that in all of the orb-weaving spiders they have studied, the long chain of amino acids that constitutes a silk protein is dominated by certain repetitive sequences— long stretches consisting only of the amino acid alanine, for example. Natural selection has apparently kept those repetitive sequences intact over the 120 million years since the orb weavers diverged into different species, which suggests the sequences are important. Alanine chains, for instance, are very good at binding to other alanine chains, allowing protein molecules to link up side by side like logs in a raft, forming the stiff sheet crystals that are the heart of each silk filament. "Silks with polyalanine regions are the strongest tested thus far," Hayashi says.

But strong proteins alone don't make strong silk, any more than good logs alone make a good raft: It has to be assembled well. "Silk is not a self-assembling material," says Vollrath. "It has extraordinary properties because it is spun in a very sophisticated device." The proteins that will become dragline silk are secreted by cells on the walls of a long, saclike gland, from which they are funneled as a watery solution into a long, looping duct. The protein molecules line up with the direction of flow, as logs do in a river— and they become a liquid crystal. In the tapering duct this dope is stretched long and thin and bathed in acid; water is extracted from it and recycled; and the proteins bind and solidify. Vollrath and his colleague David Knight have applied for a patent on a device, whose secrets they won't divulge, that reproduces part of this process. But Vollrath is the first to admit that no one understands in full detail how the complex structure of a spider thread emerges from a spider's abdominal machinery.

That's why he takes a nephila to the synchrotron in Grenoble every now and then. There he straps the spider to a little platform and gives her milk to keep her happy as she is manhandled. He then proceeds to pull silk from her bottom with a tiny reel, sometimes for eight hours at a stretch. By varying the speed of the reel he can vary the speed at which she spins, and by heating or cooling her platform he can vary the temperature. The synchrotron produces high-energy X rays that allow Vollrath to see how those and other variables affect the internal structure of the silk— the overall idea being to perfect a way of producing artificial spider silk before someone else beats him to it.

"It's a race now," Vollrath says. "People have tried before and given up, because they've used standard industrial technology scaled down. You can do a lot worse than copying the spider."









For more about Nexia's efforts to make a synthetic form of spider silk visit their Web site at nexiabiotech.com/HTML/technology/biosteel.shtml.


ADVERTISEMENT
Comment on this article
ADVERTISEMENT
ADVERTISEMENT
ADVERTISEMENT
DSC-CV1017web
+