Microstructure refinement and in vivo evaluation of biodegradable magnesium metal matrix composites

The incorporation of bioactive ceramic particles can enhance the bioactivity of bioabsorbable magnesium (Mg) implants. In this study, a melt shearing process was implemented using a rotor-stator assembly to effectively disperse bioactive glass-ceramic (BG) nanoparticles in the Mg melt during casting. The microstructural, electrochemical, and mechanical properties, as well as in vitro cell behavior of the Mg-5wt.%BG composite prepared using the melt shearing process were compared to a Mg-5wt.%BG composite that was manually mixed (Mg-5BG manual) and to pure Mg. Two metallic wire meshes with pore sizes of 250 μm and 2000 μm were mounted on a wide-opening stator to investigate the effect of pore size on stator performance. The grain size distribution revealed a mixture of an equiaxed rosette-like dendritic structure (rosette-like) and equiaxed grains with a non-dendritic (globular) structure for Mg-5BG shear 250 (76.4 ± 15.7 μm) and a globular structure for Mg-5BG shear 2000 (136.9 ± 44.1 μm) compared to elongated grains in pure Mg (234.1 ± 163.8 μm) and rosette-like structure in Mg-5BG manual (121.4 ± 37.6 μm). Additionally, SEM/EDS analysis confirmed a uniform and narrower distribution of BG particles in the Mg-5BG shear 2000 (∼2–4 μm) compared to the unevenly distributed particles in the Mg-5BG manual (∼7–40 μm). The addition of 5 wt% of BG to Mg melt resulted in the appearance of monticellite (CaMgSiO₄), magnesium oxide (MgO), and magnesium silicide (Mg₂Si). Mg-5BG shear 2000 exhibited smaller pits after the removal of corrosion products compared to Mg-5BG manual. Mg-5BG shear 2000 and Mg-5BG manual showed slightly increased ultimate compressive strength values (177 ± 7.22 MPa and 172.9 ± 23.96 MPa, respectively) compared to pure Mg (151.8 ± 4.90 MPa, p > 0.05). Electrochemical testing revealed that Mg-5BG shear 2000 had a higher corrosion current density (i_corr = 47.2 ± 18.7 μA/cm²) and a less negative corrosion potential (E_corr = −1.47 ± 0.03 V) than both pure Mg (i_corr = 5.24 ± 1.60 μA/cm², E_corr = −1.56 ± 0.02 V) and Mg-5BG manual (i_corr = 6.87 ± 1.14 μA/cm², E_corr = −1.62 ± 0.04 V). In vitro cytocompatibility assay indicated high cell viability with 10 % extracts, although reduced viability was observed with 100 % extracts. In vivo implantation in a rat femur model was conducted with four groups: intact (without surgery), sham (drilled without an implant), pure Mg implant, and Mg-5BG shear 2000 implant. The Mg-5BG shear 2000 implant revealed superior bone regeneration and callus formation, as assessed by Nano-CT and X-ray imaging, alongside complete or partial implant resorption without observable histological signs of systemic toxicity in muscle, liver, or kidney tissues at 28 days. These findings suggest that Mg-5BG shear 2000 composites could be a promising candidate for bioactive, absorbable bone implants.