Statement of purpose:
Zinc (Zn) has been proposed as a novel biodegradable metal thanks to its essential physiological and biological roles and its promising in vivo degradation rate [1, 2]. Compared to Mg alloys and Fe alloys, the degradation behaviour of Zn alloys is more likely in line with clinical demand. One of the main factors limiting the extensive clinical application of Zn and its alloys is their lower mechanical strength. Another concern about Zn as degradable metals is its local and systemic toxicity . Thus, in this study, several beneficial alloying elements were used, and the resulting alloys were tested for mechanical and biocompatibility analysis.
A series of Zn-0.5X and Zn-3X alloys (X = Cr, Fe, Sr, Zr, V, at.%) were prepared and extruded from Φ28 mm down to Φ10 mm cylinder. Mechanical property was measured using a universal material test machine according to ASTM-E8M-09 and ASTM E9-89a (2000) standards. Cells were seeded onto different alloys in 24 well plate at a density of 104/well and cultured at 37 °C for 3 days. The cell morphology was observed by SEM after being fixed and dehydration. A rat femur model was used for in vivo animal test. The explanted tissue with the wire implants was fixed, dehydrated, PMMA embedded, sectioned, and then stained with Masson and immunofluorescent staining agents (CD11b and CD68).
All the alloys showed improved mechanical strengths when compared to pure Zn (Fig. 1). The Zn-0.5Cr, 0.5Zr, 0.5 and 3V alloys have significantly improved mechanical strengths and elongation, which make them good candidates for the load bearing implants. The in vitro and in vivo degradation tests of different materials showed similar corrosion behaviors to pure Zn (data not shown), which is helpful for their potential clinical applications. The in vitro immersion test has higher degradation rates than in vivo data. It showed slighter inflammation responses from femur tissue to Zn-0.5Sr than that of pure Zn, while the Zn-3Sr alloy had more serious inflammation after 3 months of implantation. The slight inflammation is common for these metallic implants in the early stage of implantation, which is consistent to that of stainless steel, Zn–Li and Zn–Al alloys in previous studies [4-6]. The stronger expression of CD68 marker for Zn-3Sr indicates its enhanced M2 macrophage polarization, which is helpful to promote the surrounding tissue regeneration.
Collectively, these data clearly illustrate the improved mechanical property and biocompatibility of these Zn based alloys when compared to pure Zn. This provides various choices and extends the Zn-based biomaterials for different biomedical applications.
 Bowen PK, et al. Adv. mater., 2013, 25(18): 2577.
 Su Y., et. al. Trends biotechnol., 2019, 37(4): 428.
 Trumbo P. et al. J. Am. Diet. Assoc. 2001, 101 (3): 294.
 Fu J. et al., Biomaterials 2019: 119641.
 Bowen PK, et al. J. Biomed. Mater. Res. B, 2018, 106 (1): 245.
 Zhao S. et al. Mat. Sci. Eng. C, 2017, 76: 301.
This work was supported by National Institutes of Health [R01HL140562].