José Pérez-Rigueiro

Mechanical properties of single-brin silkworm silk

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 ± 1 GPa) and yield strength (230 ± 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 fibers 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/ 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 = 3.0 ± 0.8 GPa and E/G = 5.3 ± 0.3 for brin. The latter value indicates a mechanical, and therefore microstructural, anisotropy comparable to that of nylon.

Mechanical properties of silkworm silk in liquid media

Abstract Tensile tests have been performed on silkworm silk fibres submerged in liquid environments (water, acetone, ethanol and isopropanol). Liquid media were initially chosen in order to weaken non-covalent interactions specifically. However, only immersion in water leads to a decrease in the mechanical properties of silk, indicating the weakening of hydrogen bonds. Immersion in acetone, ethanol and isopropanol leads to an increase in the stiffness of the fibre. In addition, all three organic solvents produce similar force–displacement curves, which can be explained by the desiccating effect that these solvents exert on silk. These results indicate that water disrupts hydrogen bonds initially present in the amorphous phase, while the other solvents eliminate water and contribute to the formation of new hydrogen bonds in the amorphous phase of silk. This interpretation was developed through the shear lag model of the elastic modulus ( E ) of silk, and a good agreement has been found between the model and the experimental values of E .

Controlled supercontraction tailors the tensile behaviour of spider silk

The interest in the production of fibres that mimic the behaviour of natural silks has been boosted by the first successful attempts of spinning fibres based on spider drag line silk proteins. However, both the processing of biomimetic silk fibres and the basic studies on silk are hampered by the large variability of the fibre properties. Here we show that the tensile behaviour of spider silk can be predictably and reproducibly tailored by controlling the supercontraction effect, a large shrinkage of the longitudinal dimension of the fibre if unrestrained by its ends and immersed in water. This procedure allows to reproduce the tensile behaviour of natural drag line fibres and offers the possibility of obtaining silk fibres with predictable tailored properties in large quantities for experimental use. These results can be interpreted in the frame of the molecular model of drag line silk, as the result of re-orientation of the protein chains, leading to an explanation for the observed variability of natural drag line fibres.

Fractographic analysis of silkworm and spider silk

An investigation is presented of the fracture surfaces of three different silks produced by two silkworms (Attacus atlas (A. atlas) and Bombyx mori (B. mori)) and one spider (Argiope trifasciata (A. trifasciata)). Tensile tests up to fiber failure were performed at a strain rate of 0.0002 s � 1 , and the fracture surfaces of the broken fibers were analyzed through a scanning electron microscope. The nominal relative humidity during the tests was 60% and the average temperature was 20 C. A. atlas silk was formed of bunches of microfibrils of � 1 lm in diameter embedded in a soft matrix, which were pulled out from the matrix during fracture. B. mori fibers were made up of two brins of irregular shape embedded in a proteinaceous coating. Failure occurred by fracture of the brins, whose fracture surface presented a fine globular structure corresponding to the ends of the nanofibrils of 1–2 lm in length and � 100 nm in diameter, which form the B. mori silk brins according to the analysis of the brins by atomic force microscopy. A. trifasciata fibers were circular and exhibited a defined core-skin structure. The skin fracture surface was featureless while the core showed a globular structure similar to that of B. mori although slightly shallower. The fractographic observations were discussed in the light of current knowledge of the microstructure of each fiber and the corresponding mechanical properties. 2002 Elsevier Science Ltd. All rights reserved.

Stretching of supercontracted fibers: a link between spinning and the variability of spider silk.

SUMMARY The spinning of spider silk requires a combination of aqueous environment and stretching, and the aim of this work was to explore the role of stretching silk fibers in an aqueous environment and its effect on the tensile properties of spider silk. In particular, the sensitivity of the spider silk tensile behaviour to wet-stretching could be relevant in the search for a relationship between processing and the variability of the tensile properties. Based on this idea and working with MAS silk from Argiope trifasciata orb-web building spiders, we developed a novel procedure that permits modification of the tensile properties of spider silk: silk fibers were allowed to supercontract and subsequently stretched in water. The ratio between the length after stretching and the initial supercontracted length was used to control the process. Tensile tests performed in air, after drying, demonstrated that this simple procedure allows to predictable reproduction of the stress-strain curves of either naturally spun or forcibly silked fibers. These results suggest that the supercontracted state has a critical biological function during the spinning process of spider silk.

Active control of spider silk strength: comparison of drag line spun on vertical and horizontal surfaces

Abstract There is widespread interest in producing high-performance fibers that mimic the behavior of natural silks, especially spider drag line. Given the multiple roles of drag line in nature, it is pertinent to explore whether spiders can tailor the tensile properties of this material to match its intended use. Here we distinguish between the ability of spiders to control the quality (intrinsic stress–strain response) versus the amount (load-bearing cross-section) of drag line. The mechanical characteristics of drag line spun during a vertical climb differ from those of drag line spun when the spider crawls on a horizontal surface. Also, the intrinsic stress–strain response of drag line spun during a vertical climb is significantly more reproducible (i.e. dependable) than when this fiber is produced under other conditions. Implications for biomimetic polymer science are discussed.

Self-tightening of spider silk fibers induced by moisture

Abstract Spider dragline silk has a unique combination of desirable mechanical properties—low density, high tensile strength and large elongation until breaking—that makes it attractive from an engineering perspective [Nature 410 (2001) 541]. Nevertheless, this outstanding performance is threatened by the way mechanical properties are affected by a wet environment, particularly if the stress of these fibers can relax when exposed to moisture. Tests on spider dragline silk ( Argiope trifasciata ) performed by the authors have shown that when the fiber is clamped and exposed to a wet enough environment non-vanishing supercontraction forces develop. When the moisture is removed the residual stresses increase, and this effect has proven long lasting, as the fiber remains stressed for hours. In addition, the tensile properties of the fiber remain unaffected by the residual stresses build up after removing the moisture or after a wetting and drying cycle. These tests give support to the thesis that supercontraction helps to keep the spider webs tight and opens new applications for synthetic analogs.

The effect of spinning forces on spider silk properties.

SUMMARY A new forced silking procedure has been developed that allows measurement of the low forces involved in the silking process and, subsequently, retrieval and tensile testing of the samples spun at the measured silking forces. A strong correlation between silking force and tensile behaviour of spider silk has been established. Fibres spun at high silking force – compared with the conventional yield stress – are stiff and show stress–strain curves previously found in forcibly silked fibres. By contrast, fibres spun at low and very low silking forces are more compliant, and their tensile behaviour corresponds to that of fibres naturally spun by the spider or to fibres subjected to maximum supercontraction, respectively. It has also been found that samples retrieved from processes with significant variations in the silking force are largely variable in terms of force–displacement curves, although reproducibility improves if force is re-scaled into stress. Fibres retrieved from processes with constant silking force show similar tensile properties both in terms of force–displacement and stress–strain curves.

The variability and interdependence of spider drag line tensile properties

There is considerable interest in producing fibres that mimic the impressive tensile properties of spider drag line silk. It must, however, be recognised that these properties have been assessed largely on the basis of their average values; there can be significant variability about these averages. The natural variability can also serve as a useful indicator of the range of values over which particular properties of biomimetic silk may be tailored. Here we quantify several tensile properties of drag line from Argiope trifasciata spiders. We distinguish between two groups of properties on the basis of their statistical coefficient of variation. There is significantly greater scope for tailoring the viscoplastic hardening aspects of drag line, compared to the variability of the initial elastic response or the yield strength. We also consider whether elastic modulus, yield strength and viscoplastic hardening can be controlled independently of one another.

The hidden link between supercontraction and mechanical behavior of spider silks.

The remarkable properties of spider silks have stimulated an increasing interest in understanding the roles of their composition and processing, as well as in the mass-production of these fibers. Previously, the variability in the mechanical properties of natural silk fibers was a major drawback in the elucidation of their behavior, but the authors have found that supercontraction of these fibers allows one to characterize and reproduce the whole range of tensile properties in a consistent way. The purpose of this review is to summarize these findings. After a review of the pertinent mechanical properties, the role of supercontraction in recovering and tailoring the tensile properties is explained, together with an alignment parameter to characterize silk fibers. The concept of the existence of a mechanical ground state is also mentioned. These behaviors can be modeled, and two such models-at the molecular and macroscopic levels-are briefly outlined. Finally, the assessment of the existence of supercontraction in bio-inspired fibers is considered, as this property may have significant consequences in the design and production of artificial fibers.