Phase transition-driven multistage self-assembly of silk fibroin into ordered nanofibrils

Bombyx mori silk fiber has been utilized as a high-performance natural material for millennia, yet the hierarchical self-assembly mechanisms governing silk fibroin (SF) organization, particularly the regulatory role of silk sericin (SS), remain enigmatic. Here, we investigate the assembly pathway across incubation and post-treatment conditions using a regenerated SF (RSF) and regenerated SS (RSS) system. Under salt-assisted conditions, RSS facilitated liquid-liquid phase separation and formed dynamic compartments (coacervates) with RSF. Cryo-EM confirms that these coacervates retained their hydrated spherical state. In salt-free mixtures, RSS promoted rapid composite aggregation of RSF nanoparticles. After 6 d, RSF/RSS assemblies matured into ordered nanofibrils with increased β-sheet content (from ∼11.2% to 27.1%) and crystallinity. Moreover, these SS-mediated compartments regulate RSF viscosity and shear-responsive alignment, enabling the formation of long nanofibrils. Collectively, SS functions beyond lubrication by: (1) locally concentrating and stabilizing SF through compartmentalization, preventing premature formation of branched nanofibrils, and (2) synergistically guiding the oriented arrangement of SF nanofibrils through compartment-driven maturation. This work provides a mechanistic framework for silk protein storage, phase transition, and spinning, and offers design principles for programmable bioinspired materials.