As Randy Lewis makes his morning rounds in the laboratory, he swats at a few stray flies--the ones that got away during the night. Then, bending over a glass cage, he cups his hands and gently lifts the occupant to eye level. Hello, lady, he says, his affection undiminished by the sight of this lady’s eight hairy legs and eight eyes, and the knowledge that she just spent the night hanging upside down devouring live flies.
It’s silking time in Room 255 of the University of Wyoming’s department of molecular biology. In a few minutes the spider in Lewis’s hand--oops, make that the one skittering up his arm--will be flat on its back under a microscope, legs restrained by Scotch tape, spinning silk for science. For the spider, it’s 20 minutes of hard labor, 100 yards of silk, and then back to the cage for another fly. For the tweezers-wielding researchers, the hard part is grasping the first wisp of silk extruded by the spider. After that it’s easy: they just wrap the thread around a spool mounted on a quarter-inch electric drill and reel off the silk, yards at a time.
The faint of heart make wide detours around Lewis’s lab these days, warned off by the foot-long rubber spider in the doorway. Anyone else soon comes face to face with the real thing. Several glass tanks house Lewis’s supplies of fist-size female Nephila clavipes, or golden orb weavers (males of the species don’t spin silk). But here, amid the clutter of beakers and petri dishes, the spiders are celebrities, revered for their mastery of 380 million years of protein chemistry and the ease with which they spin their astonishingly light, tenacious, stretchy fibers.
Lewis and his team of researchers are unraveling the molecular mysteries of these fibers, figuring out what makes them so strong and elastic. They are painstakingly finding and deciphering the genes that enable Nephila to produce its silken protein threads, in the hope that one day it will be possible to mass-produce and perhaps even improve on them. Without question, spider silks are some of the most extraordinary materials produced in nature. Lewis recites a litany of attributes. By weight, the spider silk he’s studying is five times stronger than steel, he explains. That’s not so unusual. Some synthetic fibers are too, like nylon and Kevlar, which is widely used in bulletproof vests. But spider silk, unlike synthetics, is also highly elastic. It can be stretched to 130 percent of its original length--that’s twice as stretchy as nylon. It’s chemically inert. It’s stable, even at high temperatures. It’s waterproof. And it’s nonallergenic. In many ways, it’s everything you could want a fiber to be.
Nature has endowed other creatures, including silkworms and butterflies, with the ability to make silk, but none are quite like the spider. Spiders are nature’s specialists in silk, says Lewis. They have been perfecting the technology for millions of years. The more we find out about their silk, the more we appreciate its wonderful characteristics.
In fact, orb weavers like Nephila produce a variety of silks-- each secreted by a different set of glands and extruded from spinnerets, small structures studded with spigots on the underside of the abdomen. One set of glands produces dragline silk, the lifeline that many spiders constantly trail behind them, enabling them to drop through the air to safety. Other glands make capture silks, sticky threads that form the bulk of the web and hold a struggling insect in place until the web’s owner arrives. Still other glands produce tough fibers for swathing prey, or make reinforcing threads for the web, or spin silken threads for cocoons, which are so good at trapping air that their insulating properties are on a par with goose down. Once you realize the enormous diversity of the silks that spiders produce, it’s easy to become entangled in the subject, says Mike Hinman, one of ten researchers working in Lewis’s laboratory.
So far, most research has concentrated on dragline silk. This fiber is so strong that a single strand would have to be nearly 50 miles long before it would break under its own weight. Yet a thread of dragline silk stretching around the world would weigh a mere 15 ounces.
The prospect of such a fiber has long interested the Office of Naval Research and the Army Research Office. Over the last five years they’ve invested half a million dollars in Lewis’s studies. But when Lewis looks at his fibers he’s apt to see a lot more than new parachute cords for the military--these days all sorts of folk are getting wind of what the spiders in Room 255 are up to. Women’s Wear Daily called recently to inquire about possible spider-silk fabrics, no doubt envisioning a whole new line of slinky arachnid haute couture. The medical equipment giant U.S. Surgical, seeing a potential for stronger, nonallergenic sutures, artificial tendons, and implantable medical devices, has an option on some of Lewis’s research. His mountaineering friends--always looking for the latest in lightweight climbing gear-- check in with him from time to time, too.
Raw material for Lewis’s research is seldom a problem. During the summer months he and his 11-year-old son, Brian, sometimes collect their own specimens--the brown orb-weaving cat face spider, Araneus gemmoides-- from the eaves of their two-story home outside Laramie. Occasionally Lewis advertises in the local paper, offering a dollar for each spider delivered alive to the lab. Most of the time, however, he extracts his silk from the huge golden orb spiders that build elaborate four-foot webs. The size of these semitropical arachnids makes them easier to work with. Every few weeks a new shipment arrives, air express, from a collector in the Florida Keys.
Spiders, as anyone who has battled cobwebs can attest, are able to produce prodigious quantities of silk. In nature adult females can spin 5 to 6 feet of silk a minute--and up to 20 feet a minute when their silk is mechanically teased out of them in the lab. The average garden spider’s web, consisting of three or four kinds of silk and up to 1,500 connecting points, usually takes less than an hour to construct. In some parts of Europe spider silk is so abundant in late summer that it literally rains from the sky--the ballooning of newly hatched spiders, which use streamers of dragline silk like para-chutes to ride the wind, becomes so heavy that the time when the silken threads shower down is known as the gossamer season. Even here in Wyoming, when you go out on the prairie, you find silk threads all over the place, says Lewis. You can just see the silk glistening in the sun.
Attempts to exploit spider silk are not new. In New Guinea the silk used to be twisted into small gill nets and fishing lines. Primitive peoples in Vanuatu (the group of South Pacific islands once known as the New Hebrides) employed silk from large web-building spiders to make pouches for tobacco and arrowheads, and even to fashion conical smothering caps for the purpose of suffocating adultresses. No one, however, has ever been able to produce commercial quantities of spider silk.
Unlike silkworms--docile insects that are easily reared in confined quarters, like feedlot cattle--spiders are agile, independent predators. In the early 1700s a Frenchman, Bon de Saint-Hilaire, managed to clean and card enough spider silk to knit a few pairs of gray stockings. He presented his hosiery to the French Academy, but practical considerations kept spider products on a back burner. Apart from the difficulties of rearing large numbers of the normally diffident arachnids, spider silk is so fine that it would take the lifework of 5,000 of them to produce enough fabric for one dress.
In the 1960s the U.S. Army, among others, got interested in spider silks and began studying their chemistry, initially with the goal of constructing synthetic supertenacity fibers for flak jackets, parachutes, and other military items. Thanks to such early work, materials scientists realized that the silks are pretty special stuff. Each type of silk starts out as a soluble protein secreted in one of the spider’s abdominal silk glands. But as the protein is extruded, it passes through a tubular duct on the way to the spinnerets. This narrow tube forces all the protein’s molecules to align in the same direction, turning it into a solid, rodlike quasi-crystalline thread.
When Lewis began looking at spider silks five years ago, however, he decided that the answer to mass-producing the proteins was not to try to copy them using conventional chemistry. He wanted to make real dragline silk, not a synthetic, and that meant finding the actual spider gene that orchestrates the protein’s production. In the process, Lewis and his team discovered what makes the silk both so tough and so flexible. In fact, they were initially stymied by its resilience. We now understand why spider silks resist degradation so well in nature, says Lewis. The enzymes usually used to cleave proteins into small pieces for analysis just didn’t work. To fragment them and get them into solution we literally had to boil the stuff in acid.
In 1989 Lewis’s team managed to fish out the gene from the gland where this silk is produced. They were then able to decipher the sequence of the gene’s DNA building blocks, which gave them the code for assembling the protein. Finally, a computer translated this code into the sequence of amino acids that actually compose the protein. After all that, the researchers discovered that the protein they’d sweated over, which they named spideroin 1, wasn’t even the whole story. To make dragline silk, they found, you need a second protein, spideroin 2, that acts as a scaffold for the first protein.
It was the structure of these two interacting proteins that helped explain the silk’s unusual combination of strength and flexibility. In their extruded form, it turned out, these proteins have not only stiff, rodlike regions but regions that curve back and forth in hairpin turns. When the fiber is stretched, these kinked regions straighten out. But when the fiber is released, they snap back into their original shape--hence the fiber’s elasticity. In any given length of silk, these two interspersed regions function a little like a chain with alternating links of steel and rubber. It’s the combination of both that gives spider silk its unique characteristics, says Lewis.
The researchers’ next step will be to engineer the genes for both proteins into an accommodating organism and turn it into a spider-silk factory. Now that we know the DNA that codes for spideroin 1 and 2, explains Lewis, we should be able to find some simple form of life, like lab bacteria or yeast, give them the genes, and persuade them to produce large quantities of the silk.
In fact, one fledgling San Diego biotech company, Protein Polymer Technologies, is already doing something like this, although not with spider proteins. They are using genetically modified bacteria to grow large batches of a silklike protein called ProNectin, inspired by one produced by silkworms. It’s used as a coating on implanted heart valves and vein grafts and helps promote cell attachment. The company will soon be marketing another of its protein creations, BetaSilk, as a finishing touch to make textiles softer and more breathable. Using other characteristics of natural silk as its model, the company is developing a whole portfolio of feels- like-silk designer proteins.
So far, though, Lewis has had only limited success in inserting his genes for spider silk into common bacteria. Unfortunately, he says, his lowly organisms seem to recognize spider DNA as distinctly foreign and get rid of it. He is now thinking of inserting the genes into yeasts, which he thinks will be more tolerant. Meanwhile, researchers at Monsanto, one of the country’s leading chemical and fiber companies, are also trying to mass-produce a spider-silk protein from a gene reportedly isolated and cloned by U.S. Army researchers.
The technological challenge is enormous, but the more we learn about spider silk, the more interesting it becomes, says Stephen Brewer, manager of Monsanto’s bioproducts chemistry research. It’s an incredible, beautiful product and we have developed the deepest admiration for the amazing way it is produced in nature.
Lewis says it may be five years or more before biotech spider silk is produced on anything like a commercial scale. And in the beginning, he suspects, it will be used only for products whose price is of secondary importance--for skin wrappings to treat burn victims, for artificial tendons and ligaments, for coatings on implants, and perhaps for some shimmering $10,000 high-fashion original by Christian Lacroix or Azzedine Alaïa.
When it comes to making large quantities of natural silk, it may always be more efficient and economical to stuff mulberry leaves into a bunch of silkworms, Lewis speculates. Silkworms, of course, don’t produce fibers that are as strong or as stretchy as spider silk, at least not yet. But Lewis likes to imagine that they will one day. Someday it may be possible to insert a spider-silk gene into a virus that infects silkworms, he says. The virus will ferry the gene into their silk glands, reprogramming their cells--and you’ll produce silkworms that spin superior, spiderlike silk. Or spider-silk genes might even be spliced into the cells of cotton plants, he muses, making it possible to grow one of nature’s strongest and stretchiest fibers by the bale.
