Enhanced Osseointegration of Zn-Mg Composites by Tuning the Release of Zn Ions with Sacrificial Mg-Rich Anode Design

Relatively slow degradation rate and delayed osseointegration induced by excessive release of Zn2+ ions are two main disadvantages of the use of pure Zn ion bioabsorbable orthopedic implants. In light of this, we designed a cathodic protection strategy by incorporating Mg, acting as a sacrificial anode, into Zn to form Zn-Mg composites. The performance of novel Zn-Mg composites with regard to degradation behavior and biocompatibility was evaluated systematically under in vitro and in vivo conditions. Macro-galvanic coupling that formed between the Mg-rich phase (anode) and the Zn matrix phase (cathode) accelerated the degradation of Zn-Mg composites as compared to that of pure Zn. Composition analysis revealed ZnO as the dominant product of Zn-Mg composites, followed by calcification matrix formation during the bone healing process. Cytotoxicity assay showed prominently improved cell viability after addition of Mg. Histological analysis manifested delayed osseointegration for the pure Zn group. In contrast, direct contact between new bone and Zn-5Mg composite in multiple locations and increased bone bonding areas were found over time. The synergic biological effect of co-releasing Zn2+ and Mg2+ ions by preferential corrosion of sacrificial Mg-rich phase contributed to the ameliorated bone integration. Thus, introducing sacrificial Mg-rich anode is an effective design strategy to increase the degradation rate of pure Zn while simultaneously improving its bone integration ability.