Compositional "plainification" in biodegradable magnesium-rare earth alloys - Achieving well-balanced performance in an ultra-lean Mg-Pr alloy.

Contrary to the traditional strengthening route by adding multiple & high-dosage alloying elements, we here explored extremely compositional and phase-constituent "simplification" in rare earth (RE) containing biodegradable magnesium alloys for better biocompatibility. An ultra-lean Mg-0.1Pr alloy with a multiscale microstructure has been developed through casting and extrusion, which showed well-balanced performances that match the commercial Mg-based orthopedic products. Compared to the first clinically approved WE43 magnesium alloy, the complex RE addition (7 wt.%) in WE43 is simplified into one single RE (0.1 wt.%) with its content having a 70-fold reduction in the newly developed alloy, while without impairing mechanical properties (tensile strength: 285.6 MPa). This also leads to a mild degradation rate, a good cell & tissue compatibility and osteogenic capacity both in vitro and in vivo. The simplified Mg-0.1Pr alloy exhibits excellent local and systemic biocompatibility in the femoral condyle of rats during a 16-month long-term follow-up, with this period approximately covering the entire life cycle of the implant. In addition, the Mg-0.1Pr alloy exhibits a significantly lowered RE transfer and accumulation in rats compared to other Mg-RE alloys, which possibly alleviates the RE metabolic burden to main organs. Pr is neither accumulated in the corrosion product layer nor in the surrounding bones. This work provides a new solution to maintain the high performance of Mg-RE alloys with extremely compositional "plainification", which should further benefit for the safety and reliability of RE-strengthened biodegradable metals in the long-term.