Spinning conditions affect structure and properties of Nephila spider silk

Abstract

Raman spectroscopy is used to elucidate the effect of spinning conditions upon the structure and mechanical properties of silk spun by Nephila spiders from the major ampullate gland. Silk fibers produced under natural spinning conditions with spinning rates between 2 and 20 mm s−1 differed in microstructure and mechanical properties from fibers produced either more slowly or more rapidly. The data support the “uniform strain” hypothesis that the reinforcing units in spider silk fibers are subjected to the same strain as the fiber, to optimize the toughness. In contrast, in the case of synthetic high-performance polymer fibers, the both units and the fiber experience uniform stress, which maximizes stiffness. The comparison of Nephila major and minor ampullate silks opens an intriguing window into dragline silk evolution and the first evidence of significant differences between the two silks providing possibilities for further testing of hypotheses concerning the uniform strain versus uniform stress models. Impact statement It is well established that the microstructure and mechanical properties of engineering materials are controlled by the conditions employed to both synthesize and process them. Herein, we demonstrate that the situation is similar for a natural material, namely spider silk. We show that for a spider that normally produces silk at a reeling speed of between 2 and 20 mm s−1, silk produced at speeds outside this natural processing window has a different microstructure that leads to inferior tensile properties. Moreover, we also show that the silk has a generic microstructure that is optimized to respond mechanically to deformation such that the crystals in the fibers are deformed under conditions of uniform strain. This is different from high-performance synthetic polymer fibers where the microstructure is optimized such that crystals within the fibers are subjected to uniform stress. Graphic abstract

Impact of physio-chemical spinning conditions on the mechanical properties of biomimetic spider silk fibers

“…7). This suggests, that the internal molecular structure is indeed affected by the reeling speed, which is in agreement with what has been found for native silk 46 .…”

“…Second, with increasing reeling speed there is a trend toward lower strain at break (Fig. 5), a correlation which was also reported in the literature for native silk 43,46 , but since there is an opposite trend for the tensile strength, the toughness values are fairly constant for reeling speeds ≥29 cm/s and ≤58 cm/s. Again, excluding NaCl from the spinning buffer in this set-up results in fibers with better mechanical properties (Supplementary Fig.…”

Carbon Nanotubes Facilitate Silk Hierarchical Assembly by Dry Drawing

“…The above findings confirm that higher applied strain rates lead to an increased amount and degree of alignment of high ED structures, which is attributable to proteins in a silk II conformation [ 44 ] (S3, Supporting Information), which is supported by numerous bulk spectroscopic measurements. [ 45,46 ] Taken together, these observations provide evidence of the process in which nanocompartments convert into silk II nanofibrils aligned with the fiber axis as a result of imposed mechanical stress (a more detailed hypothesis is discussed in S8, Supporting Information).…”