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’.
Highlights
Silk fibre mechanical properties are attributed to the development of a multi-scale hierarchical structure during spinning
In order to address these gaps, we previously proposed a comparative framework for silk fibre production[4,5]
The focus of this study is to ascertain the mechanistic origin of these features and to attempt to place them within a continuum of structure development as silk undergoes a flow-induced phase transition during spinning. This would be probed in vivo, and while this is becoming a reality for nanoscale structures via spectroscopy[6,24], at the mesoscale there are currently significant practical and technological challenges that make this an impossibility. Previously this has been approached bottom-up, by looking at the structures formed by an aqueous silk feedstock as it subjected to a range of thermal, chemical and mechanical stress fields which serve to initiate protein self-assembly and denaturation[25,26]
Summary
In order to investigate structure transition in silk spinning, we replicate the natural process by subjecting a regenerated B. mori silk feedstock to an extensional flow field in order to partially draw fibres[14,28,29,30] and viewing the structures formed as the silk proteins self-assemble using low-voltage SEM (SE and BSE modes) on cryo-sectioned uncoated samples (Fig. 1). This cross-section of a partially spun regenerated silk fibre (Fig. 1a) clearly contains features that have been seen in natural silkworm silk fibres that are an order of magnitude smaller in diameter (Fig. 1b–e), such as fibrillar structures on the surface[13] and the presence of nanoscale voids which we propose are more appropriately referred to as ‘pockets’[31]. Towards the centre of the fibre, the fracture surface becomes smoother and raised, which suggests a crosssectional gradient in the mechanical properties, caused by the internal structure deforming in line with pultrusion orientation[30,41] Upon magnification, this raised section reveals the presence of pockets, which are more concentrated at the centre of the fibre (Fig. 2d), akin to the distribution seen in.
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