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First published online June 12, 2009
Journal of Experimental Biology 212, 1981-1989 (2009)
Published by The Company of Biologists 2009
doi: 10.1242/jeb.028944

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How super is supercontraction? Persistent versus cyclic responses to humidity in spider dragline silk

Todd A. Blackledge^1,*, Cecilia Boutry¹, Shing-Chung Wong², Avinash Baji², Ali Dhinojwala³, Vasav Sahni³ and Ingi Agnarsson^1,4

¹ Department of Biology, University of Akron, Akron, OH 44325, USA
² Department of Mechanical Engineering, University of Akron, Akron, OH 44325, USA
³ Department of Polymer Science, Integrated Bioscience Program, University of Akron, Akron, OH 44325, USA
⁴ Department of Biology, University of Puerto Rico, PO Box 23360, San Juan, PR 00931, USA

^* Author for correspondence (e-mail: blackledge{at}uakron.edu)

Accepted 15 April 2009

Spider dragline silk has enormous potential for the development of biomimetic fibers that combine strength and elasticity in low density polymers. These applications necessitate understanding how silk reacts to different environmental conditions. For instance, spider dragline silk `supercontracts' in high humidity. During supercontraction, unrestrained dragline silk contracts up to 50% of its original length and restrained fibers generate substantial stress. Here we characterize the response of dragline silk to changes in humidity before, during and after supercontraction. Our findings demonstrate that dragline silk exhibits two qualitatively different responses to humidity. First, silk undergoes a previously unknown cyclic relaxation–contraction response to wetting and drying. The direction and magnitude of this cyclic response is identical both before and after supercontraction. By contrast, supercontraction is a `permanent' tensioning of restrained silk in response to high humidity. Here, water induces stress, rather than relaxation and the uptake of water molecules results in a permanent change in molecular composition of the silk, as demonstrated by thermogravimetric analysis (TGA). Even after drying, silk mass increased by 1% after supercontraction. By contrast, the cyclic response to humidity involves a reversible uptake of water. Dried, post-supercontraction silk also differs mechanically from virgin silk. Post-supercontraction silk exhibits reduced stiffness and stress at yield, as well as changes in dynamic energy storage and dissipation. In addition to advancing understanding supercontraction, our findings open up new applications for synthetic silk analogs. For example, dragline silk emerges as a model for a biomimetic muscle, the contraction of which is precisely controlled by humidity alone.

Key words: biomimetic muscle, humidity, major ampullate fibroin, spider silk, supercontraction

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