Spider silk: Material of the future?

You can’t fight Mother Nature. But you sure can copy her. That’s an engineering approach called biomimetics – the quest to exploit the three billion-year evolutionary process that has perfected structures and materials as strong, spare and sophisticated as the hawk’s eye and mother-of-pearl. Now we read about progress in the effort to make artificial silk – the light, ultra-tough fiber produced by spiders and silkworms. Like plastic, silk is a polymer – a series of repeated structures that can be altered to produce different results. But unlike plastic, the sub-units in silk are proteins. And silk can’t be made in the lab – yet. In fact, it’s not yet clear how silk is made inside silkworms and spiders. As silk is forming, its proteins are so dense that they should glom together before the animal can spin the silk fiber. Because a glance at a spider’s web proves that silk is possible, biologists and engineers are exploring the chemistry and physics of silk production. By controlling the acidity and flow of the liquid pre-silk, and using mechanisms that are presently mysterious, spiders and silkworms create a fiber that shames even Kevlar, the fiber that is blended with polymer for lightweight canoes and bullet-proof vests.

Strong, ‘n silky?

In terms of tensile (pulling) strength, silk approaches high-tensile steel, and is one-quarter as strong as Kevlar. But if you bend Kevlar, it “will fail immediately,” says David Kaplan, a professor of biomedical engineering at Tufts University. In contrast, silk excels in a quality called toughness – the Rambo factor, which combines tensile strength and flexibility. “Silk is really good at tensile strength and toughness, and you can’t emulate that with a synthetic material,” Kaplan says. Silk has many other desirable properties, adds Kaplan, co-author of a review on silk technology being published in tomorrow’s Science. The silkworm’s silk cocoon must protect the developing moth against rain and other environmental perils, yet the moth must digest the cocoon as it emerges. Silk can also be highly elastic. “To catch prey, the spider can throw the silk like a lasso, and it sticks so the spider can reel the prey back in.” Courtesy of what Kaplan calls “a glue-like feature that holds the fibers together through a protein-protein interaction,” spider-web silk can also adhere to itself, and to vegetation. Because spiders and silkworms are only distantly related, the genes for silk must have evolved several times, Kaplan says. “That’s a vote for the simplicity and utility of the system, which clearly provides an important survival function.” Finally, these remarkable materials are made with the ultimate green chemistry, with neither heat nor toxic byproducts, and using only water as the solvent.

Medical miracle?

Silk has been used for surgical suturing since Egyptian times. But Kaplan and others envision using this ultra-tough, biodegradable material as a * scaffold to hold stem cells to regenerate diseased tissues, such as bone, kidney and cartilage; * container to introduce cells, drugs or growth factors; and * an injectable goop of silk precursors and the appropriate drugs or cells which would transform into a gel state and deliver its cargo before slowly degrading. In 2009, Serica Technologies, Inc., got Food and Drug Administration approval for a silk-based material to be used as a supportive mesh in soft-tissue repairs. (Serica has since been acquired by Allergan, Inc.) If silk is so slick, can it be made in larger quantities with traditional, in-glass chemistry? Perhaps, but Kaplan is more excited about moving the silk genes into plants or animals, so biology can make the precursors, or possibly a finished silk fiber. As mentioned, the study of silk illustrates how engineers can be inspired by biology. Seventy-five percent of silk is composed of just two amino acids, Kaplan says, yet “this material is unique. It can make incredibly strong, tough, interesting materials, and do it through a green process. I can’t imagine where you can get more interesting properties from a simpler system.”