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Spider silk as a novel high performance biomimetic muscle driven by humidity
Ingi Agnarsson, Ali Dhinojwala, Vasav Sahni, Todd A. Blackledge


The abrupt halt of a bumble bee's flight when it impacts the almost invisible threads of an orb web provides an elegant example of the amazing strength and toughness of spider silk. Spiders depend upon these properties for survival, yet the impressive performance of silk is not limited solely to tensile mechanics. Here, we show that silk also exhibits powerful cyclic contractions, allowing it to act as a high performance mimic of biological muscles. These contractions are actuated by changes in humidity alone and repeatedly generate work 50 times greater than the equivalent mass of human muscle. Although we demonstrate that this response is general and occurs weakly in diverse hydrophilic materials, the high modulus of spider silk is such that it generates exceptional force. Furthermore, because this effect already operates at the level of single silk fibers, only 5 μm in diameter, it can easily be scaled across the entire size range at which biological muscles operate. By contrast, the most successful synthetic muscles developed so far are driven by electric voltage, such that they cannot scale easily across large ranges in cross-sectional areas. The potential applicability of silk muscles is further enhanced by our finding that silkworm fibers also exhibit cyclic contraction because they are already available in commercial quantities. The simplicity of using wet or dry air to drive the biomimetic silk muscle fibers and the incredible power generated by silk offer unique possibilities in designing lightweight and compact actuators for robots and micro-machines, new sensors, and green energy production.


  • We thank Linden Higgins and Claire Ritschoff for supplying spiders. Cecilia Boutry and Victoria Sain assisted with collection of data for non-spider silk fibers. Cheryl Hayashi, Alan Gent, John Gosline, Hans Thewissen, and an anonymous reviewer provided helpful comments on early drafts of the manuscript. This work was supported by National Science Foundation awards DBI-0521261 (T.A.B.), IOS-0745379 (T.A.B.), DEB-0516038 (T.A.B.) and DMR-0512156 (A.D.). Additional funding was provided by a Slovenian Research Agency research fellowship ARRS Z1-9799-0618-07 (I.A.), and from the Integrated Bioscience Program at University of Akron.

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