Alloy design strategies for biodegradable magnesium implants: Linking composition, microstructure, and degradation control

Biodegradable magnesium (Mg) alloys are among the most promising materials for temporary orthopaedic implants due to their bone-matched mechanical properties, intrinsic bioactivity, and physiological resorption. However, achieving clinically reliable performance requires precise control of alloy chemistry to balance mechanical retention, corrosion kinetics, and biological compatibility within complex physiological environments. This review adopts a composition-centred, degradation-controlled perspective, treating alloy chemistry as the primary design variable linking microstructural evolution, phase stability, galvanic interactions, and degradation behaviour. Quantitative insights from pure, binary, ternary, and emerging multicomponent Mg alloy systems are synthesised to establish composition–microstructure design windows that minimise galvanic mismatch, stabilise surface films, and maintain mechanical integrity during healing while supporting biologically safe resorption. Key strengthening and degradation mechanisms, including solid-solution strengthening, grain refinement, second-phase effects, and micro-galvanic corrosion, are systematically analysed. The roles of physiologically acceptable alloying elements, particularly Zn, Ca, Mn, and Si, are detailed with respect to their thermodynamic and electrochemical influences on intermetallic formation, corrosion-film stability, hydrogen evolution, ion release, and cytocompatibility. Rather than identifying a single optimal composition, this review defines alloy-specific trade-offs and compositional safety windows that enable predictable and clinically compatible degradation. By integrating metallurgical principles, corrosion mechanisms, and preclinical performance within a unified design framework, this review provides practical guidance for the rational design and clinical translation of next-generation biodegradable magnesium implants.