Iowa State Spiderman Discovers Super Heat Conducting Properties of Spider Silk

© Golden silk orbweaver photo courtesy of the Xinwei Wang research group/Photo of X. Wang and team in lab by Bob Elbert courtesy of ISU

They call him Spiderman since he began coaxing 8 golden silk orbweavers (pictured) to spin webs, and harvesting the different silks within each web for thermal conductivity testing. Xinwei Wang, an associate professor of mechanical engineering at Iowa State University, had a hunch about the ability of spider silk to conduct heat.

His hunch paid off. Using techniques that require "time and patience," Xinwei Wang and his team have proven that spider silk conducts heat better than even copper, and hundreds of times better than other organic tissues. But that was not the only surprise.

Spider Silk Vs. Typical Heat Conductors

Two types of materials typically conduct heat well: metals and crystals. Each has its own trick. Metals conduct heat well for the same reason they conduct electrical current, due to the relatively free movement of electrons. Crystals convey heat via vibrations in the structured lattice.

Spider silk is neither metallic nor crystalline, but has a complex structure consisting of crystalline and semi-crystalline nano-segments separated by amorphous (non-crystalline) regions. This complex structure gives spider silk amazing properties, like the strength of steel combined with incredible ductility (ability to stretch like elastic).

Spider Silk Heat Highways

It turns out that the complex nano-structure of spider silk also creates heat superhighways. The spider silk draglines which anchor webs in place conduct heat at 416 Watts per meter Kelvin. Compare this with copper (401W/m-K) or skin (0.6W/m-K). According to Wang:
For organic material, this is the highest ever. There are only a few materials higher - silver and diamond.

Wang believes the spider's biologically generated defect-free molecular structure contributes to the incredible heat conducting properties, but notes that more research is needed to fully describe the mechanisms that work to carry heat so efficiently in spite of the amorphous materials interspersed in the silk.

Another Surprise

Further enhancing spider silk's reputation as an amazing material, it turns out that spider silk conducts heat even better when it is stretched. Most materials become more insulating when stretched.

Polymers designed to biomimic spider silk's great heat dissipating capacity, even greater when stretched, could be used by engineers as a "soft" option for keeping electronics parts cool as well as for bandages that do not trap heat, or high-tech clothing. Wang's research is partially supported by the Army Research Office.

The research -- which was assisted by Xiaopeng Huang, a post-doctoral research associate in mechanical engineering; and Guoqing Liu, a doctoral student in mechanical engineering -- has been published online by the journal Advanced Materials - "New Secrets of Spider Silk: Exceptionally High Thermal Conductivity and its Abnormal Change under Stretching."

Tags: Biomimicry | Cooling | Heating | Insects

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