Comparison of Physical Properties of Three Silks

Scientific Reports, 2014

Spider silk fibers were produced through an alternative processing route that differs widely from natural spinning. The process follows a procedure traditionally used to obtain fibers directly from the glands of silkworms and requires exposure to an acid environment and subsequent stretching. The microstructure and mechanical behavior of the so-called spider silk gut fibers can be tailored to concur with those observed in naturally spun spider silk, except for effects related with the much larger cross-sectional area of the former. In particular spider silk gut has a proper ground state to which the material can revert independently from its previous loading history by supercontraction. A larger cross-sectional area implies that spider silk gut outperforms the natural material in terms of the loads that the fiber can sustain. This property suggests that it could substitute conventional spider silk fibers in some intended uses, such as sutures and scaffolds in tissue engineering. T he essential traits of spider silk fibers 1 and of their constituent elements, spidroin proteins 2 , have been fixed in a series of sequential steps that started with the production of solid threads and culminated with the spinning of high performance fibers 3 . During this process, protein composition and spinning route evolved to yield the unique microstructure and properties of spider silk 4 . However, disentangling the individual contributions of the chemical structure of the proteins 5 and of the physiological and anatomical features related to fiber processing 6 , has proven an extremely involved task that severely hampers a possible biomimetic 7 approach based on these materials . An alternative to the natural spinning process exists for the production of silkworm silk fibers. This processing route is based on a traditional procedure that allows obtaining fibers directly from the glands of the worms after being exposed to an acid environment and subsequently stretched , yielding the so-called silkworm silk gut fibers. Following this technique, it is shown that high-performance spider silk gut fibers can also be produced directly from the major ampullate gland of orb-web weaving spiders. Major ampullate gland as dissected from the spider shows a mucus-like texture and breaks up readily if stretched while being held with tweezers. Similar behavior is observed if the gland is incubated in neutral or slightly basic solutions prior to stretching. In contrast, if incubated in an acid environment (acetic acid/water), a solid fiber is obtained upon stretching. The morphology of spider silk guts, including fracture surfaces of tensile tested samples, is shown in Figure . These images do not show any remains of the gland tissues on the fiber, which are assumed to play no significant role in the process. A wide range of diameters from 30 mm to 240 mm were found in the fibers, and significant variability was observed even along individual fibers. The diameter in the middle region of each fiber was systematically greater than those at the ends. Different diameters were typically measured at both ends of a given fiber, possibly reflecting initial variations in the diameter of the original gland. Longer incubation times usually led to increased values of the average diameter of the fiber. Fracture surfaces appeared flat with no evidence of the presence of a fibrillar microstructure at the micrometer scale.

Journal of Experimental Biology, 2005

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 pr...

Journal of Arachnology, 2005

Spider silk has attracted the attention of many scientists because of its desirable physical properties. Most of this attention has been devoted to dragline silk, a thread that has high tensile strength, high strain and ultra-low weight. To help understand structure-property relationships in spider silks, the tensile behavior of egg sac (cylindrical gland) silk of Araneus diadematus Clerck 1757 was compared with dragline (major ampullate gland) and silkworm silks. In addition, stress-strain curves of egg sac silk were simulated by a spring-dashpot model, specifically a Standard Linear Solid (SLS) model. The SLS model consists of a spring in series with a dashpot and in parallel with another spring, resulting in three unknown parameters. The average stress-strain curve of fibers from five different egg sacs could be accurately described by the model. Closer examination of the individual stress-strain curves revealed that in each egg sac two populations of fibers could be distinguished based on the parameters of the SLS model. The stress-strain curves of the two populations clearly differed in their behavior beyond the yield point and were probably derived from two different layers within the egg sac. This indicates that silks in the two layers of A. diadematus egg sacs probably have different tensile behavior.

Scientific Reports, 2012

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.

Journal of evolutionary biology, 2018

While phylogenetic studies have shown covariation between the properties of spider major ampullate (MA) silk and web building, both spider webs and silks are highly plastic so we cannot be sure whether these traits functionally co-vary or just vary across environments that the spiders occupy. Since MaSp2-like proteins provide MA silk with greater extensibility, their presence is considered necessary for spider webs to effectively capture prey. Wolf spiders (Lycosidae) are predominantly non-web building, but a select few species build webs. We accordingly collected MA silk from two web building and six non-web building species found in semi-rural ecosystems in Uruguay to test whether the presence of MaSp2-like proteins (indicated by amino acid composition), silk mechanical properties, and silk nanostructures, were associated with web building across the group. The web building and non-web building species were from disparate subfamilies so we estimated a genetic phylogeny to perform ...