Tikalon Blog by Dev Gualtieri

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Spider Silk

March 12, 2012

Whenever a materials scientist gets a little too proud of his accomplishments, he just needs to look at the lowly spider to see that nature has provided it with a structural material that's better in some respects than his science provides. As I mentioned in a previous article (Scorpion Windbreakers, February 3, 2012), spider silk has nearly the same tensile strength (1.3 GPa ) as high grade steel , but at a fifth the density (1.3 g/cc). Not only that, but spider silk has very high mechanical toughness .

The admirable mechanical properties of spider silk derive from its hierarchical structure , as shown in the figure. Crystalline regions of alanine are joined by amorphous glycine linkages. It's the combination of a hard material, the crystalline segments, with the elastic amorphous material that gives spider silk its properties.

Hierarchical structure of spider silk

Hierarchical structure of spider silk. Crystalline regions are joined by amorphous linkages.

(Detail of an illustration by Chen-Pan Liao, via Wikimedia Commons )

Analogues of spider silk material have been spun, but their properties have fallen short of the real thing. A significant advance in the art was made in 2010 by a research team from Korea Advanced Institute of Science and Technology (KAIST), the Seoul National University , and Tufts University , who genetically modified E. coli bacteria to produce a 284.9 kDa protein of the spider, Nephila clavipes . They were able to spin this into a fiber with mechanical properties comparable to natural spider silk.[1]

Just recently, researchers at the University of Wyoming (Laramie, WY), Zhejiang University (Hangzhou, PRC) and Notre Dame University have published research on the creation of transgenic silkworms that produce a mixture of silkworm and spider silk. These fibers were mechanically stronger than silkworm silk, and nearly as strong as spider silk.[2]

Although it's difficult at present to make an artificial spider silk as good as nature's own, there are still ways to do some useful experiments with it. Just as the silk from silkworms can be harvested, silk from spiders can be harvested as well, although you can imagine the difficulty in getting large quantities of this material.

Shigeyoshi Osaki of the Nara Medical University in Japan , after ten years of labor, was able to develop a process to make spider silk violin strings and harvest enough spider silk to make some.[3-4] His spiders were of the species, Nephila maculata , which spins long, strong dragline fibers. He twisted these fibers into bundles, twisted these bundles into thicker bundles, and so on, until he was able to make violin strings. The fattest string (G ) has 15,000 filaments.[4]

It appears that the twisting into bundles was an important part of the process, since it changed the fiber's original circular cross-section into a polygonal cross-section so that the fibers were more tightly packed. New Scientist has audio that compares the sound of a spider silk violin to that for other types of strings.[4] As can be expected from the physics of strings that I reviewed in a previous article (Anharmonic Strings, May 16, 2011), the timbre of spider silk is unique.[4-5]

The mechanical properties of spider silk have been studied extensively, but what about its other properties? One important material property is thermal conductivity . As I reviewed in two previous articles (Boron Nanoribbons, January 11, 2012 , and Sound and Heat, August 23, 2011 ), the thermal conductivity of materials is mediated by electrons and phonons . For electrical insulators , only the phonons matter, so electrical insulators generally have low thermal conductivity.

Spider silk is an electrical insulator, so you would expect low thermal conductivity. In an interesting example of why theory always needs to be vindicated by experiment , Xinwei Wang , an associate professor of mechanical engineering at Iowa State University , post-doc , Xiaopeng Huang, and doctoral candidate , Guoqing Liu, have found that silk from the spider species, Nephila clavipes , also called golden silk orbweavers, is highly thermally conductive. They found that spider silk conducts heat hundreds of times better than woven silkworm silk and most other organic materials .[5-6]

Nephila clavipes, the golden silk orbweaver

Nephila clavipes, the golden silk orbweaver

Image courtesy of Prof. Xinwei Wang, Iowa State University.

Used with permission.

Experimental determination of thermal conductivity on such small specimens is difficult, and it required construction of some unique instrumentation, as described in their publication in Advanced Materials .[5] The dragline spider silk is just four micrometers in diameter.[6] The spider silk had a thermal conductivity of 416 watts per meter Kelvin (W/m-K). Copper's thermal conductivity is about the same, and that of skin is about five hundred times smaller.[6] Says Wang, "Our discoveries will revolutionize the conventional thought on the low thermal conductivity of biological materials."[6]

One other interesting property of the silk is that the thermal conductivity increases as it's stretched, contrary to what happens in most materials. The thermal conductivity increases by 19% under a 20% strain.[5] This is likely because the crystalline units come into closer proximity to each other,

Why is spider silk so thermally conductive? Wang suspects it's because the molecular structure of spider silk is defect-free.[6] Phonons will scatter from defects, thereby limiting thermal conductivity.

Figure caption

Left to right, Xiaopeng Huang,Guoqing Liu and Xinwei Wang, around the instruments used to study the thermal conductivity of spider silk.

Photograph by Bob Elbert, courtesy of Prof. Xinwei Wang, Iowa State University.
Used with permission.

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