Spontaneous Hollow Coacervate Transition of Silk Fibroin via Dilution and Its Transition to Microcapsules

Polymeric microcapsules are useful for drug delivery, microreactors, and cargo transport, but traditional fabrication methods require complex processes and harsh conditions. Coacervates, formed by liquid-liquid phase separation (LLPS), offer a promising alternative for microcapsule fabrication. Recent studies have shown that coacervates can spontaneously form hollow cavities under specific conditions. Here, we investigate the spontaneous hollow coacervate transition of silk fibroin (SF). SF coacervates, induced by mixing SF with dextran, calcium ions, and copper ions, transition to hollow coacervates upon dilution. Adding ethylenediaminetetraacetic acid (EDTA) further transforms them into vesicle-like capsule coacervates, which solidify into microcapsules. As a proof-of-concept, we successfully loaded a high-molecular-weight polymer cargo into the hollow cavity and bioactive enzyme cargo into the capsule layer by simply mixing the cargo with the coacervate solution. Our results demonstrate a facile, organic-solvent-free approach for fabricating SF-based microcapsules and provide insight into the mechanisms driving hollow coacervate formation.