Physics of unravelling and micromechanics of hagfish threads

Hagfish slime is a unique biological material composed of mucus and protein threads that rapidly deploy into a cohesive network when deployed in seawater. The forces involved in thread deployment and interactions among mucus and threads are key to understanding how hagfish slime rapidly assembles into a cohesive, functional network. Despite extensive interest in its biophysical properties, the mechanical forces governing thread deployment and interaction remain poorly quantified. Here, we present the first direct in situ measurements of the micromechanical forces involved in hagfish slime formation, including mucus mechanical properties, skein peeling force, thread-mucus adhesion and thread-thread cohesion. Using a custom glass-rod force sensing system, we show that thread deployment initiates when peeling forces exceed a threshold of approximately 6.8 nN. To understand the flow strength required for unravelling, we used a rheo-optic setup to impose controlled shear flow, enabling us to directly observe unravelling dynamics and determine the critical shear rate for unravelling of the skeins, which we then interpreted using an updated peeling-based force balance model. Our results reveal that thread-mucus adhesion dominates over thread-thread cohesion and that deployed threads contribute minimally to bulk shear rheology at constant flow rate. These findings clarify the physics underlying the rapid, flow-triggered assembly of hagfish slime and inform future designs of synthetic deployable fibre-gel systems.