Understanding the flow behaviour of native silk proteins

This thesis furthers our collective understanding of the flow behaviours exhibited by the silk proteins fibroin and sericin in their native state. This knowledge is both necessary for the creation of biomimetic spinning devices, and highly advantageous to those in need of bio-inspiration. The multiplicity of evolutionary paths which have found silk to be the solution to a given problem is testament to the versatility of this wonderful material. Evolution offers an unrivalled testing ground for new ideas, and when the convergence on fibre production is as broad and strong as it is for silk, it becomes an enticing target—first for understanding and later replication. However, much of the previous work on silk has opted to focus on more accessible areas of the field—studying fibres as a finished product—rather than the mechanisms by which they are formed. Thus, exploring the liquid precursors to solid silk fibres provides substantial academic freedom. In this work, I use bulk computational flow models, supported by experimental data, to provide the first tangible evidence that spinning silk is a process dominated by pultrusion, and that typical soft bodied silk producers such as B. mori are incapable of spinning fibres in any other way. I use this evidence to explore the oft-touted suggestion that sericin provides lubrication to fibres and reduces the energy required for spinning. As a result, I present the first rheological characterisation of native, unadulterated sericin, which offers proof that contrary to previous work, it behaves in a non-Newtonian manner; and that although it is capable of lubricating fibroin flow, it is far too viscous to support the notion that silk is spun via extrusion, which allows me to conclude that sericin evolved primarily as an adhesive rather than a lubricant. I also present a new model for silk spinning, built by exploring the difference in energetic requirements for solidification and gelation. I argue that fibre spinning is the result of controlled energetic application and work accumulation—again an important take home principle for bio-inspiration. Finally, I reflect on past research in our field, discuss the drawbacks of the biomimetic ideal, and offer suggestions on where our collective efforts might be best directed. In summary, this thesis provides a valuable resource for further exploration of the flow properties of biopolymers for fibre spinning applications, and substantial alternative views on the conventional definition of what the silk community’s goal should be.