Development of a high-strength Zn-Mn-Mg alloy for ligament reconstruction fixation.

Although various biodegradable materials have been investigated for ligament reconstruction fixation in the past decades, only few of them possess a combination of high mechanical properties, appropriate degradation rate, good biocompatibility, and osteogenic effect, thus limiting their clinical applications. A high-strength Zn-0.8Mn-0.4Mg alloy (i.e., Zn08Mn04Mg) with yield strength of 317 MPa was developed to address this issue. The alloy showed good biocompatibility and promising osteogenic effect in vitro. The degradation effects of Zn08Mn04Mg interference screws on the interface between soft tissue and bone were investigated in anterior cruciate ligament (ACL) reconstruction in rabbits. Compared to Ti6Al4V, the Zn alloy screws significantly accelerated the formation of new bone and further induced partial tendon mineralization, which promoted tendon-bone integration. The newly developed screws are believed to facilitate early joint function recovery and rehabilitation training and also avoid screw breakage during insertion, thereby contributing to an extensive clinical prospect.

Enhancement in mechanical and corrosion resistance properties of a biodegradable Zn-Fe alloy through second phase refinement.

Biodegradable Zn alloys containing Fe suffer from a common problem that FeZn13 second phase particles are coarse. This problem roots thermodynamically from the negligible solid solubility of Fe in Zn and priority of FeZn13 solidification over Zn. In this paper, bottom circulating water-cooled casting method is successfully developed to significantly refine FeZn13 particles in Zn-0.3Fe alloy, owing to its cooling speed about 8 times of that of conventional casting. The second phase refinement alleviates brittleness of the alloy, increases the ultimate tensile strength by about 62%, and decreases electrochemical corrosion rate (CR) by about 19%, but immersion CR by only about 4% due to barrier effect of corrosion products. Viability of human umbilical vein endothelial cells maintains at a high level over 95% in 25-100% extracts. A great potential is shown for improving comprehensive properties of biodegradable Zn alloys without changing its chemical compositions through such a physical method.

Design biodegradable Zn alloys: Second phases and their significant influences on alloy properties.

Alloying combined with plastic deformation processing is widely used to improve mechanical properties of pure Zn. As-cast Zn and its alloys are brittle. Beside plastic deformation processing, no effective method has yet been found to eliminate the brittleness and even endow room temperature super-ductility. Second phase, induced by alloying, not only largely determines the ability of plastic deformation, but also influences strength, corrosion rate and cytotoxicity. Controlling second phase is important for designing biodegradable Zn alloys. In this review, knowledge related to second phases in biodegradable Zn alloys has been analyzed and summarized, including characteristics of binary phase diagrams, volume fraction of second phase in function of atomic percentage of an alloying element, and so on. Controversies about second phases in Zn-Li, Zn-Cu and Zn-Fe systems have been settled down, which benefits future studies. The effects of alloying elements and second phases on microstructure, strength, ductility, corrosion rate and cytotoxicity have been neatly summarized. Mg, Mn, Li, Cu and Ag are recommended as the major alloying elements, owing to their prominent beneficial effects on at least one of the above properties. In future, synergistic effects of these elements should be more thoroughly investigated. For other nutritional elements, such as Fe and Ca, refining second phase is a matter of vital concern.