Silks are remarkable materials with desirable mechanical properties, yet the fine details of natural production remain elusive and subsequently inaccessible to biomimetic strategies. Improved knowledge of the natural processes could therefore unlock development of a host of bio inspired fibre spinning systems. Here, we use the Chinese silkworm Bombyx mori to review the pressure requirements for natural spinning and discuss the limits of a biological extrusion domain. This provides a target for finite element analysis of the flow of silk proteins, with the aim of bringing the simulated and natural domains into closer alignment. Supported by two parallel routes of experimental validation, our results indicate that natural spinning is achieved, not by extruding the feedstock, but by the pulling of nascent silk fibres. This helps unravel the oft-debated question of whether silk is pushed or pulled from the animal, and provides impetus to the development of pultrusion-based biomimetic spinning devices.

Natural silk spinning has undergone strong selection for resource efficiency and thus presents a biomimetic ideal for fiber production. Industrial replication of natural silk fibers would enable access to low energy, cost-efficient processing, but is hampered by a lack of understanding surrounding the conversion of liquid feedstock into a solid fiber as a result of flow. Previously, shear stress, shear rate, or time have been presented as criteria for silk fiber formation, but here it is proposed that spinning requires carefully balancing all three, and is a result of controlled energy accumulation in the feedstock. To support this hypothesis, rheology is used to probe the energy required for conversion, compare differences between amorphous solid and ordered fiber production and explain the energetic penalty the latter demands. New definitions of what constitutes an artificial silk fiber are discussed, along with methods to ensure that each spinning criterion is met during biomimetic spinning.

This study addresses one of the major gaps in our knowledge regarding natural silk spinning by providing rigorous rheological characterisation of the other major protein involved - sericin. This allows progress in silk flow modelling, biomimetic system design, and in assessing the quality of bioinspired and waste sericin materials by providing a better understanding of the native, undegraded system.