Spider dragline silk is of practical interest because of its excellent mechanical properties. However, the structure of this material is still largely unknown. In this article, we report what we believe is a new model of the hierarchical structure of silk based on scanning electron microscope and atomic force microscope images. This hierarchical structure includes beta-sheet, polypeptide chain network, and silk fibril. It turns out that an exceptionally high strength of the spider dragline silk can be obtained by decreasing the size of the crystalline nodes in the polypeptide chain network while increasing the degree of orientation of the crystalline nodes. Based on this understanding, how the reeling speed affects mechanical properties of spider dragline silk can be understood properly. Hopefully, the understanding obtained in this study will shed light on the formation of spider silk, and consequently, on the principles for the design of ultrastrong silk.

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We have studied the effect of the lipophilicity of the counterions Xon the rheological properties of aqueous solutions of cetyltrimethylammonium-based surfactants (CTAX): cetyltrimethylammonium n-hexane sulfonate (CTAC6SO3), cetyltrimethylammonium n-heptane sulfonate (CTAC7SO3), and mixtures of cetyltrimethylammonium 3-hydroxy-2-naphthalenecarboxylate (CTAHNC) and cetyltrimethylammonium bromide (CTAB) in the molar ratio 1:1 and 2:1. Both linear and nonlinear viscoelasticity properties have been investigated as a function of surfactant volume fraction and temperature. For the system CTAHNC + CTAB (1:1), the effect of added salt (NaBr) was also studied. In the concentration and temperature ranges used in this study, all the systems exhibit a viscoelastic behavior characteristic of wormlike micellar solutions. By changing the lipophilic character of the counterion, one varies the net electrical charge carried by the micelle, which allows us to present a general description of the effect of the electrostatic interactions on the viscoelasticity of micellar systems.