Mesoscale structure development reveals when a silkworm silk is spun

Abstract

Silk fibre mechanical properties are attributed to the development of a multi-scale hierarchical structure during spinning. By careful ex vivo processing of a B. mori silkworm silk solution we arrest the spinning process, freezing-in mesoscale structures corresponding to three distinctive structure development stages; gelation, fibrilization and the consolidation phase identified in this work, a process highlighted by the emergence and extinction of ‘water pockets’. These transient water pockets are a manifestation of the interplay between protein dehydration, phase separation and nanofibril assembly, with their removal due to nanofibril coalescence during consolidation. We modeled and validated how post-draw improves mechanical properties and refines a silk’s hierarchical structure as a result of consolidation. These insights enable a better understanding of the sequence of events that occur during spinning, ultimately leading us to propose a robust definition of when a silkworm silk is actually ‘spun’.

Nanovoid formation induces property variation within and across individual silkworm silk threads

“…The finding that tensile properties can be improved by repeated mechanical tests on unbroken sections of silkworm silks 27 suggest that some aspect of the silk's molecular structure prevents the fibers from achieving a plastic phase. Various experiments have reported the presence of nano-scale voids, cavities, or other structural defects in Tasar and mulberry silkworm silk, 32–37 with their location, shape, and significance in inducing fiber formation well reported by these and other authors. 34,38,39 Whilst it is conceivable, from these accounts, that nanovoids may play a role in influencing the material properties of silks under tensile deformation, 37,40,41 their impact on silk mechanical and thermal properties are yet to be elucidated for different silkworm silks.…”

“…The squeezing action during pressing in the duct during spinning could be responsible for some of the variation in void sizes within the different species silks. 37,50 Confirming or refuting this will require further investigations to be made. Nevertheless, such actions are unlikely to explain any of the within thread variations in void formation we observed.…”

“…51 In certain instances, processes such as thin layer yielding of nanovoid-modulated crazing and core flattening can serve to enhance the properties of void containing polymers. 37,40 However, voids generally serve as nucleation points for certain macrostructural defects and local failures that can ultimately lead to fiber breakage. 52 This means that the voids observed within the silkworm silks were likely a major source of tensile property variation.…”

“…48 An alternative explanation postulates that the voids are formed from the elongation of vacuoles that are observed within the viscous fibroin dope in the lumen of the silk gland. 37 To investigate these proposals for the origin of the nanovoids we ran 1 H- 13 C two-dimensional heteronuclear correlation (HETCOR) NMR experiments to measure the hydrogen bonding lengths within the crystalline region of the silks since this would serve as an indicator of the relative change in free volume.…”

A silk composite fiber reinforced by telechelic-type polyalanine and its strengthening mechanism

“…Especially in the form of fibers, the desire to pursue high-performance artificial fibers regenerated from abundant silkworm silks has never faded. Despite intensive investigation of the silk assembly mechanism to inspire spinning, 1–3 two central scientific issues remain challenging for the artificial spinning process. One is how to largely retain the molecular weight.…”