While silk exhibits high values of tensile strength and stiffness, these properties are compromised by their poor reproducibility. We present the results of experiments aimed at characterizing the variability of tensile properties exhibited by cocoon silk from Bombyx mori silkworms. Scanning electron microscopy is used to measure an average diameter for individual test specimens; the interspecimen variability of diameter is quantified and found to be inadequately represented by standard deviation. When load-extension data are converted into stress-strain curves, a marked improvement in reproducibility is realized if each specimen cross-section is calculated from diameter measurements specific to that specimen. Nevertheless, a significant variability in fracture stress remains; a Weibull analysis reveals that silkworm silk has a failure predictability comparable with that of glass and nonengineering ceramics.Unloading/reloading tests demonstrate that stiffness is not significantly affected by cumulative deformation, and the stress-strain relationship is not sensitive to strain rate.

the supercontracted state has a critical biological function during the spinning process of spider silk.

Major ampullate (MA) dragline silk supports spider orb webs, combining strength and extensibility in the toughest biomaterial. MA silk evolved ~376 MYA and identifying how evolutionary changes in proteins influenced silk mechanics is crucial for biomimetics, but is hindered by high spinning plasticity. We use supercontraction to remove that variation and characterize MA silk across the spider phylogeny. We show that mechanical performance is conserved within, but divergent among, major lineages, evolving in correlation with discrete changes in proteins. Early MA silk tensile strength improved rapidly with the origin of GGX amino acid motifs and increased repetitiveness. Tensile strength then maximized in basal entelegyne spiders, ~230 MYA. Toughness subsequently improved through increased extensibility within orb spiders, coupled with the origin of a novel protein (MaSp2). Key changes in MA silk proteins therefore correlate with the sequential evolution high performance orb spider silk and could aid design of biomimetic fibers.

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Spider silk is considered as the basis of a new family of high performance fibers that would reproduce the excellent mechanical properties of the silk, in particular its extreme toughness. However, it has been observed that the mechanical properties of spider silk are severely influenced by humid environments that give rise to significant decreases in its length and elastic modulus. The change from stiff to compliant tensile properties is associated with the glass transition from a glassy to a rubbery state; here, we have found that it depends on both temperature and relative humidity. The glass transition was identified at different temperatures and relative humidities by monitoring the variation of the elastic modulus and observing the emergence of supercontraction. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 994–999, 2006

ABSTRACT:Mechanical tests were performed on single brins of Bombyx mori silkworm silk, to obtain values of elastic modulus (E), yield strength, tensile breaking strength, and shear modulus (G). Specimen cross-sectional areas, needed to convert tensile loads into stresses, were derived from diameter measurements performed by scanning electron microscopy. Results are compared with existing literature values for partially degummed silkworm baves. The tensile modulus (16 Ϯ 1 GPa) and yield strength (230 Ϯ 10 MPa) of B. mori brin are significantly higher than the literature values reported for bave. The difference is attributed principally to the presence of sericin in bave, contributing to sample cross-section but adding little to the fiber's ability to resist tensile deformation. The two brins in bave are found to contribute equally and independently to the tensile load-bearing ability of the material. Measurements performed with a torsional pendulum can be combined with tensile load-extension data to obtain a value of E/ ͌ G that is not sensitive to sample cross-sectional dimensions or, therefore, to the presence of sericin. The value of E measured for brin can be used together with this result to obtain G ϭ 3.0 Ϯ 0.8 GPa and E/G ϭ 5.3 Ϯ 0.3 for brin. The latter value indicates a mechanical, and therefore microstructural, anisotropy comparable to that of nylon.

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The relationship between microstructure and mechanical properties has been investigated in Argiope trifasciata dragline silk fibers (major ampullate silk, MAS) by X-ray diffraction, Raman spectroscopy and tensile testing. We have analyzed three fractions of the material, i.e. amorphous, highly oriented nanocrystals and weakly oriented material, for different values of the macroscopic alignment parameter a, calculated as the relative difference between the length of the fiber and its length when supercontracted. Two distinct regimes have been identified: for low values of the alignment parameter a, microstructural changes are dominated by the reorientation of the nanocrystals; however, at high values (a > 0.5) of the alignment parameter, an increase in the fraction of the crystalline phase is revealed. The two regimes are also reflected in the mechanical behaviour, which can be explained by microstructural changes. This finding of the two distinct regimes in the microstructural evolution, which separates the reorientation and the increase in the crystalline phase, will be valuable to develop and validate molecular models of natural and artificial silk fibers, as well as to deepen our present knowledge of the origin of the outstanding properties of MAS fibers. In addition, we have analyzed the characteristics of the crystal lattice, and discussed the relationship between the percentage of short sidechain residues and the unit cell dimensions in different silks.