Reproducibility of the Tensile Properties of Spider (Argiope trifasciata) Silk Obtained by Forced Silking GUSTAVO V. GUINEA, MANUEL ELICES, * JOSE ´ IGNACIO REAL, SARA GUTIE ´ RREZ, and JOSE ´ PE ´ REZ-RIGUEIRO Departamento de Ciencia de Materiales, Universidad Polite´cnica de Madrid, ETS de Ingenieros de Caminos, Ciudad Universitaria, 28040 Madrid, Spain ABSTRACT A modified forced silking procedure was developed to allow an accurate study of the tensile properties of spider (Argiope trifasciata) silk, especially the characterization of the variability of the tensile properties of forcibly silked fibers. The procedure involves an immobilization technique that does not require anesthetization of the spider, a mode of collection that allows immediate access to any silk sample with a minimum manipulation, and a technique to measure the diameters of the spider silk fibers systematically. The forcibly silked fibers obtained by this procedure show reproducible tensile properties in terms of force–displacement curves as well as stress–strain curves. Furthermore, reproducibility also extends to forcibly silked fibers obtained from different spiders when stress–strain is considered. J. Exp. Zool. 303A:37–44, 2005. r 2005 Wiley-Liss, Inc. The growing interest in the production of artificial fibers based on natural silks, particularly spider silk from the major ampullate gland (MAS), has been stimulated by the possibility of mimick- ing the extraordinary combination of properties of the natural material (Kaplan et al., ’91; ’94; Vollrath, ’99; Viney, 2000; Lazaris et al., 2002). A proper characterization of the natural material is a first step toward clarifying the relationship between microstructure, tensile properties, and processing conditions, so a procedure to obtain spider silk fibers of reproducible properties in sufficient quantities appears to be a prerequisite for any experimental work on spider silk. The large variability of stress–strain curves displayed by MAS silk was recognized in some of the earliest studies (Work, ’76; Griffiths and Salinatri, ’80), although the values obtained for the tensile strength (E1 GPa) compared favorably in all cases with those of high-performance fibers, such as polyaramide (E3 GPa) and high-strength steel (E3 GPa) fibers. The strain at breaking of spider silk (E30%) greatly exceeds that of other high-performance fibers (polyaramide E1%; high- strength steel E0.5% ). The large variability of the tensile properties of spider silk was interpreted in biological terms as a contribution to the survival ability of the spider (Madsen et al., ’99): the possibility of varying the mechanical properties of silk would allow the spider to adapt it to immediate requirements. This variability is one of the major drawbacks in the experimental study of silk because it prevents drawing reliable conclusions: the scatter of the control samples blurs the possible relationship between the experi- mental conditions and tests results (Work, ’76; Dunaway et al., ’95; Madsen et al., ’99). Work with artificial silk (Lazaris et al., 2002) indicates that these fibers are also affected by a wide variability in their tensile properties, despite the use of carefully controlled processing techniques. Efforts to obtain reproducible spider silk fibers have relied on establishing a correlation between silking conditions and the tensile properties of silk fibers. Consequently, fibers spun by spiders under carefully controlled conditions have been retrieved and tested, and fibers spun naturally either to build the web or as safety lines have been Grant sponsor: Ministerio de Ciencia y Tecnologı ´a (Spain); Grant number: MAT 2003-04906; Grant sponsor: Comunidad de Madrid; Grant number: 07N/0001/2002 * Correspondence to: Manuel Elices. Departamento de Ciencia de Materiales, Universidad Polite´cnica de Madrid, ETS de Ingenieros de Caminos, Ciudad Universitaria, 28040 Madrid, Spain. E-mail: melices@mater.upm.es Received 8 March 2004; Accepted 21 June 2004 Published online in Wiley InterScience (www.interscience.wiley. com). DOI: 10.1002/jez.a.111 r 2005 WILEY-LISS, INC. JOURNAL OF EXPERIMENTAL ZOOLOGY 303A:37–44 (2005)