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First published online December 15, 2004
Journal of Experimental Biology 208, 25-30 (2005)
Published by The Company of Biologists 2005
doi: 10.1242/jeb.01344

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Stretching of supercontracted fibers: a link between spinning and the variability of spider silk

G. V. Guinea, M. Elices^*, J. Pérez-Rigueiro and G. R. Plaza

Departamento de Ciencia de Materiales, Universidad Politécnica de Madrid, ETS de Ingenieros de Caminos, Ciudad Universitaria, 28040 Madrid, Spain

View larger version (44K): [in a new window]   Fig. 1. Stress-strain curves of spider silk (MAS from A. trifasciata, see inset). FS, forcibly silked fibers. NS, naturally spun fibers, i.e. fibers retrieved either from the web or from the safety line. MS, maximum supercontracted fibers, i.e. fibers subjected to supercontraction and subsequent drying. All tests in air were performed at 20°C, 35%RH and strain rate 0.0002 s-1. SCW, supercontracted fibers tested in water at 20°C and strain rate 0.0002 s-1.

View larger version (24K): [in a new window]   Fig. 2. Procedure for stretching supercontracted fibers. The fiber is allowed to fully supercontract in water at 20°C for 10 min down to a length LSC (A). The fiber is stretched up to length LA with a force FA and their ends are clamped in this position (B). Water is removed after 10 min and the fiber is allowed to dry overnight. Stresses are built in up to a force FB as a result of drying (C), and are allowed to relax by unloading to a final length LC (D). The length LC is considered as the base length to calculate strain. Stresses are calculated under the assumption that the volume remains constant throughout the process. The evolution of the forces during the process are sketched in the inset as an F-L plot. FA, force required to stretch to length LA; FB, force built up in the fiber after drying.

View larger version (29K): [in a new window]   Fig. 3. Stress-strain curves of supercontracted fibers subjected to stretching in water and drying (tensile testing conditions 20°C, 35%RH, strain rate 0.0002 s-1). The stretching process is characterized by the alignment parameter, , as defined in the text. At least two samples were tested for each alignment parameter, showing the remarkable reproducibility of the process. Stress-strain curves of forcibly silked (FS) fibers are shown for comparison. The inset shows the whole range of curves produced by the naturally spun (NS) silk when building the web, or from the safety line.