Besides iron and magnesium alloys, zinc alloys are the most promising materials to be proposed for an application such as biodegradable implants. The development of biodegradable implants from Zn alloys is one of the main interests in the new generation of bioabsorbable alloys. The main drawbacks of Zn-based materials are their insufficient mechanical properties. Subjecting pure zinc and zinc alloyed with magnesium to hydrostatic extrusion improved their mechanical properties to the level acceptable for the application. In the presented studies, two other elements such as Ca and Sr were added to the alloy in order to improve the uniformity of corrosion properties and preserve good biocompatibility.

A quaternary alloy composed of zinc with 0.5 % wt. of magnesium and 0.5 wt. % of calcium and 0.5 wt. % of strontium (ZnMgCaSr) for investigation were prepared by gravity casting in an argon atmosphere followed by hot extrusion at 250 °C and then by hydrostatic extrusion. Obtained materials in the form of 5 mm rods were investigated. Microstructural characterization using SEM, TEM, and XRD phase analysis at each step of processing was performed for as cast, hot extruded, and hydrostatic extrusion material. The mechanical properties were examined, including microhardness and static tensile tests. Corrosion properties were analyzed by using two methods: electrochemical polarization and immersion tests. Both tests were carried out in Hanks’ solution, under conditions mimicking a human body (temperature = 37 °C, pH = 7.4). The corrosion rate was calculated based on the current density measured. Polished samples were immersed in Hanks’ solution (temperature = 37 °C, pH = 7.4) for 14 days. Samples were subjected to SEM observations to investigate the corrosive products by EDS technique. Sample surfaces without corrosion products were characterized to observe the evidence of uniform degradation (arrangement of pits). The weight loss of the samples was measured after cleaning the surface to remove corrosion products. On these measurements, the corrosion rate was also calculated. Cytotoxicity testing of samples was focused on assessing the level of lactate dehydrogenases (LDH) cytotoxicity analysis by spectrophotometry. Analysis of cell growth, proliferation, and in situ detachment mechanisms were applied. The efficiency of the bone and connective tissue function in the fluid and on the sample surface, including their adhesion strength, was analyzed using human fibroblasts and human osteoblasts cell species in the dynamic flow bioreactor setup. Investigated properties were compared to those obtained for ZnMg and ternary ZnMgSr.

Thanks to the synergy of the proper alloying ( by addition of Mg, Ca, and Sr) and using the accurate deformation method (hydrostatic extrusion preceded by hot extrusion), outstanding mechanical properties were obtained along with improvement of corrosion rate and acceptable biocompatibility.

Acknowledgments

This work was supported by Norwegian Financial Mechanism and National Centre for Research and Development (NCBR) - grant NOR/SGS/BioAbsMat/0096/2020