Assessing the degradation performance of ultrahigh-purity magnesium in vitro and in vivo
Graphical abstract
Introduction
Magnesium (Mg) offers great potential as biodegradable and biocompatible material for medical applications [1], [2], [3], [4], [5], [6]. Its corrosion performance, however, is often insufficient and thus limits its use. Within the living body, the generation of gaseous H2 and hydroxides (OH−) upon Mg degradation can influence the healing process of an injured tissue due to the formation of hydrogen pockets and localised increase in pH [1], [7], [8]. Therefore, slow and homogeneous degradation of Mg implants is required.
The degradation behaviour of pure Mg is influenced by impurity elements such as Fe, Ni, Cu and Co. These impurities are detrimental to corrosion resistance because of their low solid-solubility in α-Mg and due to their tendency to form cathodic sites for micro-galvanic corrosion in corrosive media [7], [8]. The corrosion rate of Mg increases drastically when the concentration of Fe in Mg castings exceeds its tolerance limit of 180 ppm. This drastic acceleration is due to the precipitation of an Fe-enriched body centered cubic (BCC) Mg–Fe phase, which forms from the melt before final solidification [9], [10]. Liu et al. [11] demonstrated that the corrosion rate in pure Mg increases with increasing Fe content. They also observed a significant increase in degradation after heat treatment, which is a necessary processing step for the production of Mg-wrought alloys. The tolerance limit was found to be much smaller in heat-treated pure Mg than in the as-cast material, due to the fact that above a limit of 5–10 ppm Fe the BCC Fe-rich phase may precipitate upon annealing [11]. Although high-purity (HP) Mg already exhibits very low amounts of impurities (see Table 1) and corrodes more slowly than conventional pure (CP) Mg, its alloys still corrode too fast for specific medical applications. To explore a solution, ultrahigh-purity (XHP) Mg [12] was produced in-house via a distillation process to significantly lower the impurity level (see Table 1). One of the investigation’s main goals was to test the degradation performance of this new XHP Mg in vitro and in vivo, and thus to understand its degradation in more detail.
The in vitro degradation of the XHP Mg was analysed by the hydrogen (H2) evolution method [13] and compared to HP Mg. Current H2-evolution setups, however, show several limitations as to their accuracy and ability to determine the corrosion rate, especially when testing very slow-corroding high-purity Mg alloys [13], [14], [15]. Therefore we designed within this work a new testing device, which enables precise determination of the evolved H2 volume, including a regulation of the pH via NaHCO3/CO2-buffering. In this context, we present solutions to the drawbacks of current H2-evolution setups, whose limitations were also recently described by Kirkland et al. [16]. To test the reliability of our setup, we also compare the in vitro degradation rate of XHP Mg with the in vivo degradation of XHP Mg pins implanted in the femurs of rats for 12 weeks.
In summary, the research aims of this study are to analyse and compare the in vitro and in vivo performance of ultrahigh-purity (XHP) and high-purity (HP) Mg to understand Mg corrosion in more detail. To achieve these goals, the in vitro degradation was studied with a newly designed hydrogen evolution testing setup.
Section snippets
Materials and methods
HP Mg (99.99%, CHEMCO Germany) and three batches of XHP Mg were used in the as-cast, annealed and as-extruded states to study the influence of impurity contents on in vitro degradation performance. The degradation rate of extruded XHP Mg was also evaluated in vivo. The chemical compositions of the tested materials are given in Table 1.
The XHP Mg batches were produced via a purification process in a vacuum distillation apparatus [12]. In order to avoid contamination, high-purity graphite
Microstructure
For the first series of experiments, HP Mg and XHP#1 Mg were cast to rods to obtain samples of similar microstructure. These exhibit a typical cast microstructure with a grain size of approx. 500 μm. The annealing treatment at 400 °C for 48 h did not result in significant changes in the microstructural features observable by optical microscopy. However, by means of scanning electron microscopy a few intermetallic particles with a size of typically 150 nm were detected in the annealed HP400
Mg of different purities and conditions
The hydrogen amount released by the XHP Mg samples with extremely low amounts of impurities is clearly smaller than that released by HP Mg. Lee et al. [26] analysed the effect of impurities on the corrosion behaviour of pure Mg. They stated that the corrosion behaviour of pure Mg is dependent on the content ratio of two specific impurity elements (Fe/Mn), rather than the absolute value of each. This fact was also observed earlier in Mg–Al alloys [27]. Our Fe/Mn ratio is ≈4 for HP and <1 for
Conclusions
Measurements of degradation rates of as-extruded ultrahigh-purity (XHP) Mg with Fe impurities of 0.2–2.2 ppm measured in vitro in NaHCO3/CO2-buffered SBF agree well with in vivo tests performed by implanting XHP Mg pins in rat femurs. In both cases very low and homogeneous degradation at an average rate of roughly 10 μm per year was evaluated. This very slow and homogeneous biodegradation can be explained by very low amounts of impurities, such as Fe, Si, Mn, Cu, Ni, and Al (see Table 1), which
Acknowledgements
The authors appreciate support via the K-project OptiBioMat (Development and optimization of biocompatible metallic materials), FFG – COMET program, Austria, and the Laura Bassi Centre of Expertise Bioresorbable Implants for Children (BRIC), FFG, Austria. We are also grateful to Christian Wegmann and Manuel Spielhofer for fruitful discussions and experimental assistance.
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