Biomimetic Silk Fibre Assembly: Mimicking Nature's Pultrusion Process

Abstract

Among the best natural structural materials, silks have remarkable properties due to their hierarchical structure. The silk proteins from spiders or caterpillars, despite being distinct Classes, are produced by similar mechanisms with conserved features. They are stored as aqueous liquid solutions that undergo irreversible liquid-to-solid transformations driven by different stimuli, primarily pH and shear strain. This transformation has attracted the attention of many researchers aiming to replicate this apparently facile process. However, most biomimetic assembly processes that have been developed rely on extrusion-based technologies or flow-focusing microfluidic devices, typically using coagulating baths with unnatural solvent conditions. These synthetic processing strategies differ substantially from natural, all-aqueous, pultrusion-based fibre production and increase the overall energy input required to drive the transformation. In contrast, we observe that native-like silk fibroin (NLSF) rapidly forms a highly viscoelastic film at the air–water interface. This phenomenon is then exploited by applying an extensional strain field to produce multimeter silk-like fibres with observable coaligned nanofibrillar bundles. Our studies showed that the proteins undergo stress-induced denaturation, consistent with a model of hexagonal packing of β-solenoid units, at low pulling speeds, at which point the proteins switch to a β-sheet-rich structure as the speed increases. Moreover, the produced fibres showed optimal mechanical properties when the pulling speeds were near the maximum physiologically relevant speeds (ca. 30 mm/s). s pulled at 26.3 mm/s had an elastic modulus of 8 ± 1 GPa and a toughness of 8 ± 5 MJ/m2, which is commensurate with the mechanical performance of natural fibres. Moreover, the method demonstrated here is readily compatible with complex material fabrication under ambient conditions, opening up the possibility of facile incorporation of cells and biomolecules. Overall, the developed method replicates the natural pultrusion process entirely water-based and offers great potential for the future development of novel fibre-based composite materials.