Example of microprocessing in a natural polymeric fiber: Role of reeling stress in spider silk
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pider silk fibers were obtained by the monitored forced silking method. This procedure allows measurement of the silking force during the process and retrieving the fibers so their tensile behavior can be characterized. Silking conditions, including the reeling speed and the use of an anaesthetising gas, were varied to ascertain their influence on the tensile properties of the silk. In all cases, it was found that the tensile properties are determined by the silking stress, obtained by dividing the silking force by the diameter of the fiber. This suggests that the sophisticated spinning system of the spider can be characterized essentially by a single parameter, which controls the properties of spider silk almost independently of the reeling conditions.
I. INTRODUCTION
Spider silk represents a benchmark of biomimetics, a research field whose goal is to ascertain the rules that nature follows in the design and production of materials and apply them to the solution of technological problems.1 The excellent mechanical properties of spider silk rely on a combination of tensile strength and strain at breaking unrivalled by any other material, natural or artificial.2–4 These properties depend in turn on a complex microstructure5–9 and a sophisticated spinning process.10–12 A better knowledge of both microstructure and processing conditions is a pressing demand if artificial fibers are to be produced by genetic engineering techniques.13 The spider spinning process proceeds under extremely benign conditions in terms of temperature and pH, even though the dope is composed of extremely large proteins.14,15 Handling silk proteins during processing is made possible by a highly sophisticated glandular system that favors the formation of a liquid crystalline phase.11 In addition, it has been shown that the spinning system endows the spider with the ability to modify the silk properties over a wide range to adapt them to its immediate environment.16,17 Consequently, it was assumed that the spinning of spider silk is an extremely involved process, which was likely to depend on a large number of parameters both external and internal. This hypothesis has originated a number of works to ascertain the influence of a number of spinning conditions on the properties of spider silk, in a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0240 J. Mater. Res., Vol. 21, No. 8, Aug 2006
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particular the influence of the silking speed18 and the effect of anesthesia.19 The study of the influence of the silking force on the properties of spider silk has revealed its significant role in determining the mechanical behavior of silk fibers.20 Here we expand this initial study and show that the tensile properties of spider silk are determined not by the silking force, but by the silking stress, defined as the ratio between silking force and the diameter of the fiber. It is also shown that the control exerted on the properties of the fiber by the silkin
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