Mg Ion Diffusion at in Vivo Bone-Mg-10Gd Implant Interface Revealed by Atom Probe Tomography

Besides their biocompatibility, biodegradable Mg-based implants are considered to promote new bone formation and influence its remodeling. Prior work has shown that ions from the implanted screw diffuse into the bone, yet the questions of how these ions are incorporated into the bone (ultra)structure remains open. Here, we investigate the nanoscale distribution of Mg in rat tibia following implantation of a Mg-10wt. % Gd screw after 18 weeks. Electron microscopy, combined with X-ray spectroscopy, reveals Mg-depleted threads with Gd, O, Ca, and P corrosion products, transitioning to porous degradation corrosion products adjacent to bone. Atom probe tomography analyses across bone-implant interfaces, up to 20-50 μm from the implant, show Mg enrichment up to 3 at. % in organic-rich regions, corresponding to collagen, in between the hydroxyapatite (HA) crystals, and 2-3 at. % within the crystals. In comparison, cortical bone located approximately 1.68 mm from the implant exhibit levels of ∼ 1 at. % Mg. Our observations indicate co-enrichment of Mg in both organic-rich and mineral phases of peri-implant bone, with equal or slightly higher atomic fractions in organic-rich regions. These findings demonstrate Mg ion short-range transport and incorporation into peri-implant bone primarily through organic phases. Later, substitution reactions and/or recrystallization cause progressive Mg enrichment in the inorganic phase, facilitating degradation-controlled bone regeneration. STATEMENT OF SIGNIFICANCE: Atom probe tomography (APT) can characterize the 3D microstructure of materials at a near-atomic level, combined with high elemental sensitivity - making it a powerful technique for analyzing biomaterials and bio-interfaces. Here, we investigated the interface between bone/biodegradable Mg implant. Our results demonstrate co-enrichment of Mg²⁺ in both organic-rich and mineral phases, even at locations hundreds of µm away from the initial implanted screw. These results provide new insights into the active role of Mg in the remodeling process. To optimize the design of future orthopedic Mg-alloys and their degradation behavior, it is essential to understand how alloying elements affect the remodeling process and their incorporation into the bone structure at an atomic level.