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Posted: Mar 15, 2012

New secrets of spider silk - its high thermal conductivity beats most materials

(Nanowerk Spotlight) Researchers and material scientists have been fascinated by spider silks for a long time – ultra-strong and extensible self-assembling biopolymers that outperform the mechanical characteristics of many synthetic materials, including steel. Atomistic studies have contributed to a better understanding of the source of the strength and toughness of this amazing biological material (see our previous Nanowerk Spotlight: "Atomistic model of spider silk nanostructure paves way for new class of synthetic materials").
Now, researchers have come up with another set of very surprising findings: The highly periodical structure of spider silk can sustain super fast thermal transport that surpasses those of most organic and inorganic materials.
"In the past, people realized that for organic materials, their thermal conductivity usually is very low," Xinwei Wang, an associate professor of mechanical engineering at Iowa State University, tells Nanowerk. "So they are not preferred for thermal transport and heat conduction. Our discovery transforms this traditional ideal and points out that highly organized organic materials can feature extremely high thermal conductivity."
golden silk orbweavers
This is one of the golden silk orbweavers spinning webs for Xinwei Wang's research project. (Photo: Xinwei Wang research group)
In the March 5, 2012 online edition of Advanced Materials ("New Secrets of Spider Silk: Exceptionally High Thermal Conductivity and Its Abnormal Change under Stretching"), Wang's team reports two new significant discoveries: 1) the dragline silk of N. clavipes spider has an exceptionally high thermal conductivity that beats most materials; 2) contrary to normal materials, its thermal conductivity increases with strain.
For a lot of materials like silicon or carbon nanotubes, their thermal conductivity will go down if the material is stretched. But Wang and his team asked themselves: Are there materials whose thermal conductivity will go up under stretching?
"Spider silk came to our attention because mechanically it is very strong, it is stretchable, and little is known about its thermal transport capability," says Wang. "Also we have some very unique technologies that can measure the thermal conductivity and specific heat of micro/nanoscale wires. So we bought spiders, put them in cages and fed them, and collected the silks they produced for our research."
Using their lab techniques, the team discovered that spider silk conducts heat at the rate of 416 watts per meter Kelvin. Copper measures 401 W/m·K. And skin tissue measures 0.6 W/m·K.
"This is very surprising because spider silk is organic material," says Wang. "For organic material, this is the highest ever. There are only a few materials higher – silver and diamond. Our discoveries will revolutionize the conventional thought on the low thermal conductivity of biological materials."
He points out that it is even more surprising that when spider silk is stretched, thermal conductivity also goes up. According to the researchers, stretching spider silk to its 20 percent limit also increases conductivity by 20 percent. Most materials lose thermal conductivity when they're stretched.
Xinwei Wang shows the instruments they used to study the thermal conductivity of spider silk
Xinwei Wang shows the instruments they used to study the thermal conductivity of spider silk.
It can be expected that these findings will inspire new ideas to produce natural or synthetic polymer fibers of very high thermal conductivity. This kind of materials are always sought in electronics for heat dissipation, in biomedical applications for therapy and device enclosure, and in energy areas as the light-weight materials to meet the demand of heat dissipation.
The Iowa State team has their work cut out for them. "This is a completely new research area, and there a large number of aspects that need more research," says Wang. "At present we are actively seeking funding from agencies to ensure the research will continue in future. Our next step research will focus on exploring the physics behind the ultra-high thermal conductivity of spider silks, and how the spider silk type, and spider species affect the silk's thermal conductivity."
The particular challenge facing future research will be the large variety and complexity of spider silks. A same spider could produce different types of silks. Even for the same spider and same type of silk it produces, the structure and properties can vary with the spider's age, food, health, and living environment. So future research will be very time-consuming and challenging.
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