Spatiotemporally Adaptive Bilayer Silk Fibroin–Polyvinyl Alcohol–Metformin Composite Coating on Biodegradable Zinc for Orthopedic Applications

ABSTRACT

Biodegradable zinc (Zn)‐based materials are considered promising candidates for orthopedic applications because of their favorable mechanical properties, controllable biodegradability, and good biosafety profiles. However, their clinical translation is limited by early‐stage corrosion‐induced cytotoxicity and inflammation, as well as insufficient osteogenesis and osseointegration at later stages. To address these challenges, we developed a spatiotemporally adaptive and dynamically responsive bilayer composite coating tailored for bone repair. The coating consists of a dense silk fibroin inner layer combined with an electrospun polyvinyl alcohol (PVA) nanofiber membrane loaded with metformin as the outer layer. The coating is engineered to deliver stage‐adapted functions. In the early stage, the dense silk fibroin inner layer suppresses the burst release of Zn ²⁺ ions and mitigates the formation of an alkaline microenvironment, thereby providing crucial corrosion control. In the early‐to‐mid stage, the outer PVA nanofiber layer enables time‐controlled release of metformin, which alleviates inflammatory responses and establishes an immunomodulatory milieu conducive to healing. Progressing to the mid‐to‐late stage, the coating evolves into a hybrid organic–inorganic interface through mineral entrapment within the swollen PVA network. This interface actively facilitates cell adhesion, promotes mineralization, and enhances osseointegration. These findings highlight a spatiotemporally adaptive surface‐engineering strategy that enhances the clinical potential of biodegradable Zn‐based orthopedic implants and underscores the need for long‐term in vivo validation.