Magnesium (Mg) has attracted great interest as a biodegradable metallic implant due to the formation of bioresorbable corrosion products during its degradation in the body. Yet, a challenge in the reliable prediction of degradation mechanism is that currently performed in-vitro experiments typically induce the formation of corrosion products with different chemical structures and transport properties than those observed in animal studies (in-vivo). From a materials perspective, another complexity level is generated by the low solubility of alloying and impurity elements in magnesium, resulting in the formation of cathodic secondary phases. To investigate the influence of these cathodic secondary phases on the Mg corrosion behavior in a physiological mimicking environment, low-purity Mg containing a high amount of Fe-rich intermetallic phases and ultra-high purity Mg without any secondary phases were exposed to our new formulation of simulated interstitial body fluid (SIBF). The composition of SIBF is based on the measured values of inorganic ions and organic species in human body fluid. The pH of SIBF was regulated with dynamic bicarbonate buffering and flown over Mg. Electrochemical and surface characterizations showed the significantly reduced cathodic reactivity of the intermetallic phases on low-purity Mg, where the obtained values became comparable to those of ultra-high purity. Furthermore, corrosion products formed in solutions with different ions were studied on both purity degrees, revealing that the levels of free calcium, phosphate, and carbonate ions have opposite effects on the electrochemical reactivity of Mg depending on the presence of Fe-rich intermetallic phases. The results indicate that any deviation from physiological concentrations of these ions might be responsible for microstructure-dependent variations seen between in-vivo and in-vitro studies.

INTRODUCTION: The potential of biodegradable magnesium implants for treating bone fractures is gaining attention due to their excellent biocompatibility and mechanical properties. However, despite the increasing importance of understanding the regeneration process and bone tissue response, there are limited studies on the ultrastructure reaction in bone. As the implant degrades, its by-products form, potentially affecting the ultrastructure [1-3]. We aimed to analyze how the ultra-structure changes might affect the mechanical properties of bone. METHODS: Sheep bone explants with Ti and ZX00 Mg alloy screws (3.5 mm diameter, 16 mm length) were examined after 6, 12, and 24 weeks. high-resolution SAXS/WAXS experiments and nanoindentation to reveal differences in the strain distribution in the bone tissue were conducted at the P03 beamline at PETRA III, Hamburg. We investigated the ultrastructure (lattice spacing, crystallinity, orientation and the HAp platelet thickness) of the bone surrounding the implants, see Figure 1. Additionally, bone maturity and osteon density were studied using histology. DISCUSSION & CONCLUSIONS: The HAp (002) plane's d-spacing and platelet thickness decrease in the presence of the of ZX00 implants, with crystallinity and platelet thickness correlating to the bone remodeling and maturity. This is further supported by larger strains and lower stiffness observed in the bone surrounding ZX00 implants. Histological findings, such as lower osteon density in the presence of ZX00 implants, align with the hypothesis that bone maturation is slower around ZX00 than Ti implants.

Due to the wide variety of possible in vitro corrosion tests, it is difficult to directly compare reported corrosion rates for various materials across the literature. The entire field would benefit from a single, repeatable method which can allow for direct comparison of different materials in different laboratories. Recent work by the ASTM F04.15.03 task group has sought to establish such a test method. Three interlaboratory studies have taken place over the past 6 years, with substantial refinements to the method between each study. These data have been collected in support of a revision to ASTM F3268 which will likely include this method and data.