Achieving Unique Interfacial Structure, Excellent Mechanical Performance and Desired Degradation Behavior in Novel Zn/Mg Shell-Core Biocomposites Via a Two-Step Co-Extrusion Process

Abstract

Herein, to address the clinical need for biodegradable materials that provide mechanical support through slow degradation in the early stages of implantation and rapid degradation after bone healing to reduce the implant's residence time in the body, a novel Zn/Mg shell-core biocomposite were developed via a precisely controlled two-step co-extrusion process. A unique interface structure, consisting of an MgZn2 intermetallic interlayer, a Mg-rich diffusion layer in the Zn matrix, and a Zn-rich diffusion layer in the Mg matrix, was confirmed in the experimental composites. Through low-temperature and low-speed secondary extrusion, the grains of the Mg core and Zn shell were significantly refined, with average grain sizes of 0.51 µm for the Mg core and 1.45 µm for the Zn shell. Thanks to the intimate interfacial connection and the significant grain refinement, an ultimate compressive strength (UCS) of 609.4 MPa, a yield strength (YS) of 331.3 MPa, and failure strain of 18.3% were achieved, indicating a good balance of strength and plasticity. After 4 months of immersion in simulated body fluid (SBF), the Zn/Mg bimetallic composite exhibited a degradation rate of 0.016 mm/y, demonstrating that the Zn shell effectively protects the Mg core from corrosion, maintaining mechanical integrity during the initial period. Through precise adjustment of Zn shell thickness, the composite's degradation period can be precisely tuned within a broad range of 5 to 72 months, enabling optimal matching with bone regeneration timelines. This investigation establishes a novel paradigm for designing biodegradable implants with degradation profiles tailored to physiological healing processes.

Keywords: Zn/Mg shell-core biocomposites, Two-step co-extrusion, interfacial structure, mechanical properties, Degradation behavior