Anisotropic nanomechanical properties of Nephila clavipes dragline silk
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Spider silk is a material with unique mechanical properties under tension. In this study, we explore the anisotropic mechanical properties of spider silk using instrumented indentation. Both quasistatic indentation and dynamic stiffness imaging techniques were used to measure the mechanical properties in transverse and longitudinal sections of silk fibers. Quasistatic indentation yielded moduli of 10 ± 2 GPa in transverse sections and moduli of 6.4 ± 0.5 GPa in longitudinal sections, demonstrating mechanical anisotropy in the fiber. This result was supported by dynamic stiffness imaging, which also showed the average reduced modulus measured in the transverse section to be slightly higher than that of the longitudinal section. Stiffness imaging further revealed an oriented microstructure in the fiber, showing microfibrils aligned with the drawing axis of the fiber. No spatial distribution of modulus across the silk sections was observed by either quasistatic or stiffness imaging mechanics.
I. INTRODUCTION
Spider silk is a natural biomaterial that is both strong and extensible, providing it with a toughness unmatched by other natural or synthetic fibers.1 Some orb weaver spiders have up to seven silk glands, which can produce silks that perform different functions in the web.1–3 The dragline silk fibers produced in the major ampullate gland have the highest combination of strength and toughness and have been the focus of the majority of spider silk research. These are the fibers that support the spider’s weight as it hangs in midair and form the framework for the web, and hence they have evolved good load-bearing capabilities. Over the past decade, researchers have been working to discover how silk structure and composition contribute to its desirable mechanical properties. The sequences and secondary structures of silk fibroin proteins from many spider and silkworm species have been extensively studied, and common motifs and protein conformations have been identified.4–6 Silk proteins contain short repeating sequences of small side chain amino acids (e.g. GAGAGS in Bombyx mori silkworm fibroins and poly-A in Nephila clavipes dragline silk fibroins) that have been shown to form highly structured -sheet crystals in silk fibers.6 These crystals are thought to contribute to silk’s high strength.6 The organization of silk proteins within the silk fiber is a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0246
also likely to influence the fiber mechanical properties. Studies using wide angle x-ray scattering have shown that both the -sheet crystals and a fraction of the lessstructured silk fibroin protein segments tend to be aligned with the drawing axis of the fiber, suggesting an anisotropic fiber structure.7 At a larger scale, freezefracture and microscopy studies have suggested that spider dragline silk fibers have hierarchical structures made up of a core of many microfibrils aligned with the drawing axis of the fiber,8,9 possibly surrounded by a skin of diff
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