Microstructure Regulates Corrosion Behavior and Systemic Aluminum Fate in Biodegradable Mg–Al Alloys: Integrated In-Vitro and In-Vivo Insights

Magnesium–aluminum (Mg–Al) alloys are promising candidates for biodegradable implants, offering a favorable balance of mechanical strength and corrosion resistance. However, the release and systemic fate of aluminum (Al) during alloy degradation remain poorly understood, especially in relation to their microstructure. In this study, we investigate the role of β-Mg ₁₇ Al ₁₂ precipitates in governing Al release from Mg–9Al alloys under both in-vitro and in-vivo conditions. In-vitro immersion testing revealed that peak‑aged (PA) samples, containing a high density of β-Mg ₁₇ Al ₁₂ precipitates, exhibited accelerated pitting corrosion and higher total Al release compared to solution‑treated (ST) samples. In contrast, in-vivo subcutaneous implantation demonstrated the opposite trend regarding Al release with ST implants yielding higher systemic Al ion levels, whereas PA implants retained more particulate β-Mg ₁₇ Al ₁₂ and corrosion products at the implantation site. This behavior is attributed to differences in Al speciation and mobilization, with Al from ST alloys releasing primarily in solute form and Al from PA alloys predominantly present as highly elongated precipitate fragments that remained localized and resisted systemic transport. These findings underscore that microstructure influences not only corrosion kinetics but also the bioavailability and physiological distribution of Al degradation products. This work provides a framework for designing Mg‑based alloys that balance mechanical performance with favorable physiological clearance, advancing the development of safe and effective biodegradable implants.