Mesoscale structure development reveals when a silkworm silk is spun

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’. Studying the development of silk structure is important for understanding material properties and biomimetics. Here, the authors use ex vivo processing to characterise the stages of silk spinning and identify a consolidation phase where transient water pockets emerge due to multiple factors leading to nanofibril coalescence.