Insight into role and mechanism of Li on the key aspects of biodegradable ZnLi alloys: Microstructure evolution, mechanical properties, corrosion behavior and cytotoxicity.

ZnLi based alloys have been proved as desirable candidates for biodegradable materials accounting for its high mechanical performance and great biocompatibility. However, effects of Li on microstructure and comprehensive properties of Zn alloys are seldom investigated and need to be addressed. Herein, Zn-(0.1-1.4 wt%)Li alloys are fabricated and systematically analyzed. Lath-like Zn precipitates are observed in the primary β-LiZn4 (β) phase of Zn-(0.5-1.4 wt%)Li alloys, leading to the formation of dense β/Zn lamellar structure with an inter-spacing of 0.8 μm. Mechanical tests show that the strengths of the ZnLi alloys have at least tripled due to the formation of dense β/Zn lamellar structure. Early degradation behaviors of the ZnLi alloys in simulated body fluid (SBF) reveal a competitive releasing of Li+ and Zn2+. As the priority of Li+ releasing becomes more obvious with increasing Li content in the alloys, aqueous insoluble Li-rich corrosion products containing LiOH and Li2CO3 form a passivation film on Zn-(0.5-1.4 wt%)Li alloys. Consequently, corrosion rate decreases significantly from 45.76 μm/y of pure Zn to 14.26 μm/y of Zn-1.4Li alloy. Importantly, observations of white light interferometer microscope and transmission electron microscope demonstrate that β phase degrades prior to Zn in the alloys, suggesting that biomedical implants made of ZnLi alloys are likely to degrade completely in human body. Cytotoxicity tests of the alloys exhibit no cytotoxicity in 10% extracts. The most tolerated Zn2+/Li+ concentrations of the alloy extracts to L-929 cells are calculated, which provides guidance for future design of Zn alloys containing Li.