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Scientists Strengthen Spider Silk by Mixing In Metal

By David Brown
Washington Post Staff Writer
Monday, April 27, 2009

It's probably good that Charlotte isn't around to learn that her marvelous web has been improved upon -- and by people, no less.

Spider silk is one of nature's engineering triumphs, stronger on a per-weight basis than steel. Scientists reported last week that they had made it three to 10 times as strong (depending on how strength is measured) by infiltrating it with atoms of metals such as titanium, aluminum and zinc.

The process turns out to recapitulate something that locusts and marine worms -- animals not known for intelligence -- do. It is helping illuminate some fine points of protein chemistry. It might eventually lead to new types of strong and light material similar to carbon fiber.

The discovery, made by Seung-Mo Lee, a graduate student in engineering at the Max Planck Institute in Germany, occurred largely by chance.

"This is how it often is," said his supervisor, Mato Knez, a chemist at the research center. "You work toward some special goal that you hopefully can reach, but on the way you frequently find something different. That is often the most fascinating science."

Knez's research interest is nanotechnology -- creating infinitesimally tiny mechanical structures by manipulating small numbers of atoms. One of the techniques he uses is "atomic layer deposition" (ALD), in which substances are thinly coated with another material, often a metal.

When Lee, a native of Korea, came to Knez's laboratory, one of his projects was to investigate what ALD would do when applied to biological materials. Normally, it is used on hard, silica-based semiconducting materials, such as those in computer chips. But what would happen if the materials were soft and relatively fragile?

One way to get an answer would be to synthesize some sort of organic compound -- perhaps a long chain, called a polymer -- and subject it to ALD. But as a first pass at the question, Lee took an easier route. This was because he was pretty sure the process would destroy a soft material.

An Unexpected Discovery

The ALD machinery he used is housed in the basement of a building. The place is cluttered and, as it turns out, home to many spiders. So one day two years ago, Lee got the idea of testing spider silk as a model "soft biomaterial."

Lee found an Araneus arachnid with a dragline, the thread it generates from two glands on its abdomen. He reeled the thread in over a paper clip and put it in the ALD chamber.

When he took it out, it was almost a different material. He could hold the thread with a pair of tweezers -- dragline thread is somewhat less sticky than web thread -- and bounce the paper clip up and down without breaking the strand.

He showed this trick to Knez, who could hardly believe his eyes. Like Lee, he expected that ALD -- a process that involves heating something up to about 160 degrees Fahrenheit -- would destroy the biomaterial.

"We expected to see it breaking down, and indeed it happened the opposite," Knez said.

Knez initially thought the process had coated the spider silk like a ceramic sheath. But Lee, the engineer, knew the thread would not remain springy and easily bendable if that were the case. He went looking for a better explanation.

He found that, unlike with hard materials, ALD infiltrated soft materials with metal atoms in addition to depositing atoms in a thin surface film. The result, however, wasn't a structure in which the metal atoms were packed together and touching one another. Instead, the titanium and other elements were sprinkled through the silk like raisins in a loaf of bread.

Actually, spider silk is considerably more complicated than bread.

Like all proteins, it is made of long strands of beadlike molecules called amino acids. The strands can lie next to each other in sheets or twist apart in helixes. When a section of a protein molecule is dominated by sheets or helixes, it takes on a regular, or "crystalline," structure. In sections where the strand of amino acids folds or bunches up irregularly, the structure is said to be "amorphous."

A crystalline section is a bit like a zipped zipper. An amorphous section is like a zipper with one (or both) ends open, so the two halves can take on numerous shapes. Spider silk can stretch up to 40 percent of its length without breaking. The mixture of crystalline and amorphous regions is what gives the strand this elasticity.

The whole mixture is held together in part by many "hydrogen bonds." These are not like the "covalent bonds" in chemical compounds, in which two atoms overlap and share electrons. Hydrogen bonds are not nearly as strong. They are more like mild magnetic attractions.

Lee's research suggests that when the spider silk was heated, many of the hydrogen bonds broke. The metal-containing compounds used in the ALD process are highly reactive, and they sought out sections of the protein that used to be part of the hydrogen bond dalliances but were suddenly available for more serious covalent attachments.

The result, Lee believes, was a strand that is a bit more amorphous overall than native spider silk but whose various regions are more tightly locked together. The thread is denser and stronger.

Other Possibilities

The researchers are testing other biomaterials to see how they fare under metal-infiltration.

Among the first they have tried is collagen, which is basically nature's version of plastic, found in such things as tendons, ligaments and the inner membrane of eggs (which is what the researchers used).

The protein structure in collagen is dominated by helixes, not sheets, and is much less amorphous than spider silk. Metal infiltration was "shown to work," the scientists said, but provided no details.

In the process of writing up their work for publication in the journal Science last week, they learned that marine polychaete worms enhance the hardness of their jaws by incorporating small amounts of zinc into the protein matrix. Same with the mandibles of leaf-cutter ants and locusts.

Knez said he doubts there will be a practical application for the process in spider silk. But he suspects it may be useful with other biomaterials -- man-made or possibly natural.

"If we can transfer the process, there are a huge amount of potential applications, things like artificial bones and artificial skin," he said. "There could be new materials for the automobile industry or aerospace. There is always the need for lightweight and mechanically stable material."

The Max Planck Institute is looking into patenting the process.

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