Challenges in the use of zinc and its alloys as biodegradable metals: Perspective from biomechanical compatibility

To date, more than fifty articles have been published on the feasibility studies of zinc and its alloys as biodegradable metals. These preliminary in vitro and in vivo studies showed acceptable biodegradability and reasonable biocompatibility in bone and blood microenvironments for the experimental Zn-based biodegradable metals and, for some alloy systems, superior mechanical performance over Mg-based biodegradable metals. For instance, the Zn-Li alloys exhibited higher UTS (UTS), and the Zn-Mn alloys exhibited higher elongation (more than 100%). On the one hand, similar to Mg-based biodegradable metals, insufficient strength and ductility, as well as relatively low fatigue strength, may lead to premature failure of medical devices. On the other hand, owing to the low melting point of the element Zn, several new uncertainties with regard to the mechanical properties of biomedical zinc alloys, including low creep resistance, high susceptibility to natural aging, and static recrystallization (SRX), may lead to device failure during storage at room temperature and usage at body temperature. This paper comprehensively reviews studies on these mechanical aspects of industrial Zn and Zn alloys in the last century and biomedical Zn and Zn alloys in this century. The challenges for the future design of biomedical zinc alloys as biodegradable metals to guarantee 100% mechanical compatibility are pointed out, and this will guide the mechanical property design of Zn-based biodegradable metals. STATEMENT OF SIGNIFICANCE: Previous studies on mechanical properties of industrial Zn and Zn alloys in the last century and biomedical Zn and Zn alloys in this century are comprehensively reviewed herein. The challenges for the future design of zinc-based biodegradable materials considering mechanical compatibility are pointed out. Common considerations such as strength, ductility, and fatigue behaviors are covered together with special attention on several new uncertainties including low creep resistance, high susceptibility to natural aging, and static recrystallization (SRX). These new uncertainties, which are not significantly observed in Mg-based and Fe-based materials, are largely due to the low melting point of the element Zn and may lead to device failure during storage at room temperature and clinical usage at body temperature. Future studies are urgently needed on these topics.