Passage IV

NATURE SCIENCE: This passage is adapted from the book Biomimicry: Innovation Inspired by Nature by Janine M. Benyus (© 1997 by Janine M. Benyus)

It's a steamy 80 degrees Fahrenheit in Christopher Viney's lab, in deference to a six-inch-long golden orb weaver spider, who is dinning on crickets while being silked. A gossamer thread issues from her enormous abdomen at a steady clip, wound by a motor onto a revolving spindle. In this session alone, she will donate about 100 feet of "dragline," a specialty silk designed for rappelling from drop-offs and framing the spokes and perimeter of her web.

Compared ounce to ounce with steel, dragline silk is five times stronger, and compared to Kevlar (found in bulletproof vests), it's much tougher—able to absorb five times impact force without breaking. Besides being very strong and very tough, spider silk also manages to be highly elastic, a hat trick that is rare in any one material. If you suspend increasingly heavy weights from a steel wire and a silk fiber of the same diameter, their breaking points is about the same. But if a gale force wind blows, the strand of silk (five times lighter in weight) will do something the steel never could—it will stretch 40 percent longer than its original length and bounce back good as new. Up against our stretchiest nylon, spider silk bungees 30 percent further.

Another characteristic in spider silk's favor is that it has to get very, very cold before it becomes brittle enough to break easily. In the frigid temperatures that parachutes encounter, for instance, spider silk would make ideal lightweight lines. Other uses would be cable for suspension bridges, artificial ligaments, and sutures. But how would we fit so much function into such a small package?

Spider silk begins as a pool of raw liquid protein sloshing around in a gland. Viney hypothesizes that the raw liquid silk leaves the gland and travels through a thin duct just before entering the spinneret. As it squeezes through the duct, water is wrung out of the protein and calcium is added. The globules hook up in a pop-bead necklace, making the solution one thousand times less viscous, because the rodlike assemblies can now slide past one another. It's analogous to putting lanes of traffic on a highway sliding past one another, versus the mess that is a laneless, lawless, traffic jam.

Viney's model has a pleasing simplicity and completeness: The globular proteins line up into a pop-bead necklace, which squeezes through the spinneret to become a silk fiber. The final product is partly flexible and partly rigid, like a reinforced Slinky. The amorphous part gives, but the stiff crystalline domains don't give. When the fiber becomes notched, a crack or tear gets interrupted by the crystalline regions and can't propagate.

Some, like silk researcher Randy Lewis, don't agree with Viney's model. Lewis feels he has evidence that there are actually two proteins rather than just one that make up spider silk. "In the two-protein hypothesis, Viney's pop-bead model doesn't make sense," says Lewis. But other researchers are still not convinced of the existence of two proteins. While the jury is out and the debate is lively, all the investigator in spider silk research encourage one another to keep theorizing. When you think about what it could mean in terms of sustainable fiber manufacture, this research, tough as it is, is definitely worth it.

Consider: The only thing we have that comes close to silk in quality is polyaramid Kevlar, a fiber so tough it can stop bullets. But to make Kevlar, we pour petroleum-derived molecules into a pressurized vat of concentrated sulfuric acid and boil it at several hundred degrees Fahrenheit in order to force it into a liquid crystal form. We then subject it to high pressures to force the fibers into alignment as we draw them out. The energy input is extreme, the toxic by-products odious.

The spider manages to make an equally strong and much tougher fiber at body temperature, without high pressures, heats, or corrosive acids. Best of all, says Viney, spiders don't have to drill offshore for oil to produce the silk. They take files and crickets at one end and process a high-tech material at the other hand.

If we could learn to do what the spider does, we could take a soluble raw material that is infinitely renewable and make a super-strong water-insoluble fiber with negligible energy inputs and no toxic outputs. We could apply that processing strategy to any number of fiber industry, which is now heavily dependent on petroleum, both for raw material and processing! To break that dependency, says Viney, we have to become spider's apprentices.