Copying What Comes Naturally

By J. MADELEINE NASH CHICAGO

A cobweb glistening with dew seems as fragile as it is lovely. But one day soon, predicts University of Wyoming biologist Randy Lewis, man-made analogues of spider silk will be put to an astonishing variety of heavy-duty uses, from reinforcing fibers in aircraft doors to body-hugging suits for downhill skiers. Over the past four years, Lewis has played the attentive host to dozens of fist-size spiders called golden orb weavers, housing them in Plexiglas condominiums, feeding them a daily diet of flies and, every now and then, flipping them on their backs to unravel yards of gossamer thread. The ambitious goal of all this effort: to unravel the secrets of spider silk, a family of materials stronger than steel, stretchier than nylon and tougher than Kevlar, the stuff used to make bulletproof vests.

What gives spider silk its impressive array of qualities? What, for that matter, lends crack resistance to horses' hooves and adhesiveness to the secretions of mussels and barnacles? What makes rats' teeth sharp and insect cuticle hard? By answering such questions, Lewis and other researchers hope to usher in an exciting new era in materials science, one based not on petroleum products like nylon and plastic but on proteins synthesized by living, growing things. "Why go to an organic chemist for new materials," asks University of Mississippi biochemist Steven Case, "when nature has already produced some beauties?"

Fine-tuned by 4 billion years of evolution, protein chemistry has a lot to recommend it. To produce Kevlar, for instance, requires vats of concentrated sulfuric acid that must be maintained at high pressure. But spiders produce silk in the open air using water as a solvent. "I am absolutely fascinated," says University of Washington materials scientist Christopher Viney, "that such an incredible material starts out as a solution in water, and all the spider does is squirt it out through a small hole. In the process, proteins that were soluble turn into insoluble fibers. Now, isn't that amazing?" Just as amazing is Viney's discovery that spider silk in its soluble phase forms a liquid crystal rather like the displays on digital wristwatches.

Biological wizardry of a different sort is responsible for the ruggedness of abalone shells, which under high-powered microscopes resemble elaborately constructed stone walls. In this case, crystals of calcium carbonate, siphoned from seawater, serve as the stones, while a slurry of protein and complex sugars acts as the mortar between them. "The ingredients themselves are not at all impressive," marvels Princeton University materials scientist Ilhan Aksay. "Yet the shell is as strong as the most advanced man-made ceramics." And if a simple stone-and-mortar design can turn an intrinsically chalky substance into a tough coat of armor, exclaims Aksay, just think of what it might do for materials like aluminum oxide and silicon carbide that are heat resistant but tend to fracture easily.

Metallurgist Ann Van Orden, for her part, is fascinated by the fibrous structure of rhinoceros horn. "What strikes me about rhino horn," says Van Orden, "is that it is a natural composite. Really, it looks just like the material used to make the wings of a Stealth aircraft!" The benefits that might flow from such an insight can only be guessed at. Perhaps most intriguing is the fact that rhino horn is self-healing: capable of repairing the tiny cracks that come from jousting matches with other rhinos. "Now imagine a car that could self-heal after a fender bender," grins Van Orden mischievously. "There would definitely be a market for something like that."

Of course, no car of the future will be made of rhino horn, just as no silk spun by spiders is likely be woven into designer clothes. For starters, it would take 500 to 1,000 spiders to spin out enough silk for one necktie. "And you probably wouldn't want to wear a necktie made of spider silk anyway," laughs zoologist John Gosline of the University of British Columbia. Reason: when wet, spider silk contracts 50%, a property that, in a necktie at least, might prove decidedly unpleasant on damp days. Armed with the tools of molecular biology, however, scientists can learn how spiders construct their silk and then apply those lessons to the design of other fibers. "After all," says Gosline, "we do not aim to copy nature directly, but to adapt her designs and processes to our own purposes."

An inkling of what the future may hold comes from Protein Polymer Technologies, a small San Diego firm that is attempting to transform this notion of biomimicry into commercial technology. The company's first product, intended for use in medical research, is a hybrid composed of silkworm protein and fibronectin, a blood protein that promotes cell adhesion. When painted onto plastic sheets, the hybrid provides a high-quality medium for growing cells in the lab. Soon the company hopes to add to its product line other protein-based coatings, including ones that give cheap polyester the luxurious feel of silk.

Biomimetic materials hold particular promise as coatings and wrappings that increase the body's tolerance of implanted devices. Eventually these substances may be put to work as nearly natural replacements for injured ligaments and arteries. University of Alabama molecular biophysicist Dan Urry, for example, has succeeded in turning a key segment of the protein elastin, present in many body tissues, into a material whose expansive and contractile properties closely approximate those of arterial walls. The material can be fashioned into tubes that feel, uncannily, like real blood vessels and also into sheets for encasing mechanical devices like pacemakers. Tests of this new material in animals are already under way. At the University of Utah, for example, veterinary surgeons are preparing to wrap sheets of synthetic elastin around the artificial hearts they plan to implant into calves.

Producing elastin through chemical synthesis is a tedious process consuming the better part of three months. But eventually, colonies of genetically engineered bacteria will be harnessed as factories to churn out protein building blocks for all sorts of weird and wonderful materials. Right now, in fact, molecular biologists are struggling to create lines of bacteria capable of producing "dragline" silk -- the sturdy strands that orb weavers use as struts to frame their webs. Dragline proteins actually contain alternating stiff and soft regions, says the University of Wyoming's Lewis, "sort of like a series of Lego blocks with Slinkys in between." By tinkering with the spider's genes, Lewis believes, it may be possible to alter the ratio of Legos to Slinkys, creating a line of designer silks customized for different uses.

Like what, for instance? Well, spiderlike silk might be turned into a dandy rip-resistant parachute. It might also be fashioned into high-strength cable for temporary suspension bridges. "Anything that is light and strong and flexible is potentially of interest to us," observes David Kaplan, a materials expert at the U.S. Army's research center in Natick, Massachusetts. Kaplan is also intrigued by resilin, a springy protein found in cockroach cuticle. Unlike synthetic rubber, Kaplan notes, resilin does not swell on contact with organic solvents. Gloves that incorporate this quality would certainly come as a boon to soldiers who have to handle large quantities of gasoline and other fuels.

To date, it must be acknowledged, biomimicry remains long on promise and short on accomplishment. "Hype," erupts Pennsylvania State University materials scientist Rustum Roy. "The whole damn thing is hype! What does your mother use when she needs a hip replacement? Titanium. Technology has helped out biology far more often than the other way around." But Paul Calvert, a materials scientist at the University of Arizona, believes that the tide is beginning to be reversed. Man-made ceramics, Calvert notes, are notoriously brittle and prone to cracking, whereas biological ceramics like teeth are not. Calvert is trying to duplicate in titanium oxide the crisscross molecular structure that gives a rat's tooth its toughness and durability. Whether or not he succeeds seems almost beside the point. Already, designs from nature's sketch pad have enlarged the range of useful materials and enriched the imagination of those working to improve them.