Significantly improving the degradation performance of low-alloyed Mg-1Zn-0.3Ca-1.0MgO composite in vitro and in vivo

Abstract

The in vitro and in vivo degradation behavior of a low-alloyed Mg-1Zn-0.3Ca-1.0MgO (wt.%) composite with different grain sizes were investigated to understand the effect of high-density grain boundaries (GBs) and high-volume-fraction second phases on its degradation behavior. The results indicated that the ultra-fine-grained (UFG, 0.5 μm) composite had plentiful nano-sized Ca₂Mg₆Zn₃ phase, while coarse grained (CG, 8 μm) composite possessed very few minor Ca₂Mg₆Zn₃ phases due to the high solid solubility of solute atoms in Mg matrix. The immersion test in simulated body fluids (SBF) suggested the UFG composite had a low corrosion rate of 0.68 mm/y, which was only half of the CG composite (1.39 mm/y). This improved corrosion resistance was attributed to the quick formation of a compact and stable corrosion product, resulting from the preferential corrosion of uniformly distributed GBs and second phases in the UFG matrix during the early stage (first 12 hours). This can shield the Mg matrix from further corrosion over the long term. The in vivo implantation results after 24 weeks also showed that the corrosion rate of UFG composite was extremely low at 0.047 mm/y, while it was 0.252 mm/y for CG composite. Such higher corrosion resistance of UFG promoted luxuriant new bone growth and a more tightly bonded interface between the bone and the sample. The matched material degradation-bone maturation rate of UFG provided a better environment for osteoblast attachment and differentiation. Furthermore, the UFG composite maintained approximately 50 % residual yield strength even after 24 weeks post-implantation, which provided excellent mechanical support during service, especially in the first three months post-implantation. These results provide insights into the composition and microstructural design, which could be a promising avenue for exploring the manufacturing of high strength and highly corrosion resistant magnesium-based materials to enhance the security as bone implant instruments.