Scientists learn from nature's spinners

Spiders are nature’s master silk makers, and over millions of years of evolution have developed silks that could be useful to people — from sticky toothpastelike mush to strong and stretchy draglines.

“There’s not just one kind of material we’re talking about,” said Cheryl Hayashi, who studies the evolutionary genetics of spider silk at the University of California, Riverside. “You can look in nature, and there are a lot of solutions already made. You want a glue? There’s a silk that’s already a glue.”

For years there has been talk of the bright promise of spider silk: that it might one day be used to make cables that are stronger than those of steel, for example, or bulletproof vests that are more effective than those made of Kevlar.

There has been a big fly in the ointment, however: spiders cannot spin enough of the stuff. Although a typical spider can produce five types of silk, it does not make much of any of them. Obtaining commercial quantities is a practical impossibility — spiders are loners and require a diet of live insects; some are cannibals. In other words, spider ranching is out of the question.

Researchers have worked to overcome this fundamental limitation by trying to unlock the secrets of the spider’s silk-making abilities so silk could be made in the laboratory, or by genetically transferring those abilities to other organisms that could produce silk in quantity. But so far the materials produced lack the full strength, elasticity and other qualities of the real thing.

Some scientists are making an end run around the spider problem and working on reinventing the one silk that is plentiful — that of silkworms. They are reconstituting it to make materials that have the potential to go far beyond the dream of bulletproof vests.

Among these researchers are David Kaplan and others at Tufts University, whose creations have potential applications in medicine and other fields. “Here’s a material that’s been around for 5,000 years and used in sutures for about that long,” Kaplan said. “Yet there’s this untapped territory.”

Kaplan’s group and colleagues at the University of Illinois and University of Pennsylvania have recently produced electrode arrays, for example, that are printed on flexible, degradable films of silk. The arrays — so thin they can conform to the nooks and crannies of the surface of the brain — may one day be used to treat epilepsy or other conditions without producing the scarring that larger implanted electrodes do.

For centuries, beginning in China, commercial silk has been produced by cultivating silkworms — the larvae of a moth, Bombyx mori — which, unlike spiders, are content to loll about cheek by jowl, munching on mulberry leaves, spinning the material in quantities large enough to be harvested.

Silk is a fibrous protein, produced in glands within the spider or silkworm and some insects. What these creatures do is something no laboratory has been able to achieve: control the chemistry so exquisitely that the silk, which is a liquid inside the organism, becomes a solid upon leaving it.

Chief among the advantages of natural silk is the way the proteins are organised. They are folded in complex ways that help give each silk its unique properties. Scientists have not been able to replicate that intricate folding.

Producing spider-silk proteins in other organisms — bacteria, goats, plants and, most recently, silkworms themselves are among those that have been genetically engineered — has limitations because the process of reconstituting the proteins ruins any folding pattern. “As soon as you extract the silk, you basically randomise the protein structure,” Porter said. “You destroy all the capacity of that material to do what it wants.”

Harvest like cotton

At Tufts, Kaplan thinks that eventually, genetically modified plants will produce useful spider-based silk that could be harvested like cotton. Until then, however, he is working with reconstituted silkworm silk, making novel films and other materials.

Kaplan has been researching silk for 21 years — “sad but true,” he joked — and spent much of the first decade learning about the fundamental mechanisms by which silk assembles.

Over the past decade, Kaplan’s group has focused on biomedical applications in fields like tissue engineering. In 2005, a postdoctoral researcher in his laboratory developed a water annealing process, reconstituting the silks slowly in a humid environment. “We got these films that were crystal-clear,” Kaplan said. “No one had ever seen this before with silk.”

That led to thoughts about how to make an artificial cornea from silk. But a cornea has to be permeable, so Kaplan got the idea to involve a laser scientist down the hall, Fiorenzo Omenetto.

It also led to a long collaboration with Omenetto, who has developed ways to pattern silk films, making diffraction gratings and other structures. The grating can act as a substrate for other proteins or compounds, raising the possibility that silk films could be used for implantable biosensors or in drug delivery, with the silk dissolving in the body at a controlled rate to release the drug.

One advantage with silk, Omenetto said, is that the process of making films or other structures is ‘green’ — water-based and at low temperatures. “You can make incredibly sophisticated diffraction gratings out of glass or plastic,” he said. “But those are made at high temperatures or in a very harsh chemical environment,” conditions that would make it difficult to incorporate drugs or other compounds.

Researchers elsewhere have further developed the idea of using silk films for medical applications. At the Georgia Institute of Technology, Eugenia Kharlampieva experimented with depositing silver nanoparticles on films of silk as a way of strengthening them.

“Silk is a wonderful material because it’s biocompatible,” said Kharlampieva, who is continuing her research at the University of Alabama, Birmingham. “The main drawback is it’s soft. If you want to use it for optical applications, you need to reinforce it.”

The films she uses are extremely thin, and she layers them. “We make this nanocomposite which is flexible, still soft, but mechanically stronger.”

Because the films remain flexible, Kharlampieva is experimenting with fashioning them into tiny capsules that could contain minute quantities of drugs. Potentially as small as blood cells, they could be used to deliver drugs through the bloodstream.

Omenetto’s work on patterning silk has led to even more exotic potential applications. Among the latest, developed with colleagues at Boston University, is the idea of using silk as the basis for metamaterials, which can manipulate light or other electromagnetic radiation in ways that nature ordinarily cannot. By producing intricate structures in the films and depositing metal on them, metamaterial antennas may be produced that could be used inside the body as a means of monitoring health — the signal from the antenna changing as conditions inside the body change.

Such applications may be far off, Omenetto said, but the potential is vast — a fact he realised when he was first asked to poke holes in silk. “It looked like a cool optical material,” he said. “And I haven’t been sleeping that much ever since.”