Research progress and prospect of biodegradable Zn-Fe alloys

Zinc has become a promising biodegradable metal after magnesium and iron because of its good biocompatibility, suitable degradation rate and strong antibacterial properties. The strength of pure Zn is low, and addition of nutrient element Fe has a strengthening effect. In this paper, research progress of Zn-Fe based alloys is reviewed from four aspects, i.e., microstructure, mechanical properties, degradation behavior and biocompatibility. The main second phase in Zn-Fe based alloys is FeZn13 phase with a bottom-centered monoclinic structure, which can form 110<-1, 1, -2.81> type I twins. During solidification of the alloy melt, the twinning plane 110 is the preferred growth interface. The hardness of FeZn13 is 208 HV, which is about 4 times that of pure Zn, and the ultimate compressive strain of FeZn13 is only 0.5%. Addition of a small amount of Fe can form a higher volume fraction of FeZn13 phase. When Fe content reaches 2.6 wt.%, volume fraction of FeZn13 phase reaches 50%. Addition of Mg, Si, Mn or RE in Zn-Fe alloy can improve its strength. Mn addition leads to formation of (Fe, Mn)Zn13/MnZn13 core/shell structured second phase. At present, the Zn-Fe based alloy with the highest comprehensive mechanical properties is 'BCWC + rolled' Zn-0.3Fe alloy. Its yield strength (YS) is 218 MPa, ultimate tensile strength (UTS) is 264 MPa, and elongation to failure (EL) is 24%. Mechanical properties of biodegradable alloys for orthopedic implants are required to be YS > 230 MPa, UTS > 300 MPa, and EL > 15%. By comparison, YS and UTS of the Zn-0.3Fe alloy are still 12 MPa and 36 MPa lower than the requirements respectively. Potential of FeZn13 is 317 mV higher than that of Zn, which promotes degradation of Zn phase, resulting in corrosion products of Zn(OH)2, ZnO, Zn3(PO4)2, ZnCl2, ZnCO3 and Ca3(PO4)2. Viability rates of various cells in Zn-Fe alloy extracts are more than 85%, including human umbilical vein endothelial cells and human osteosarcoma cells. Hemolysis rates of Zn-Fe alloys are less than 5%, showing good biocompatibility. Antibacterial rates of Zn-Fe alloys against S. aureus are near 100%. Implantation in rats reveals that Zn-Fe alloys are beneficial to promote mineralization of osteoid bone tissue into new bone tissue and have good osseointegration ability. In the future, development of Zn-Fe based alloys should solve the key problems of low strengthening effect and low degradation uniformity caused by coarse FeZn13 phase, and carry out long-term research on implant devices made of this kind of alloys in large animals to promote clinical application.