Influence of trace impurities on the in vitro and in vivo degradation of biodegradable Mg–5Zn–0.3Ca alloys
Graphical abstract
Introduction
Magnesium alloys are of special interest in the context of structural lightweight applications in the transport and aerospace industries, and for temporary implants in medicine because of their biocompatibility and biodegradability [1], [2]. One of the major obstacles to widespread application of magnesium alloys, however, is their high corrosion rate [3], [4], which is often attributed to the presence of specific impurity elements. Elements such as Fe, Ni, Cu and Co can significantly accelerate Mg corrosion when their concentrations are above their tolerance limits [5], [6], [7], [8], [9]. Fe, for example, is the most common impurity element; its tolerance limit is reported as 170 ppm in unalloyed as-cast Mg [3] and roughly 5 ppm in wrought Mg [6], [9]. Below this limit, when no Fe-rich particles are formed and thus no electrochemically active cathodic sites exist to accelerate corrosive attack, the corrosion rate decreases dramatically. Recent studies have added silicon (Si) to the above-mentioned reactive impurity elements detrimental to corrosion. Si plays a critical role in promoting the formation and growth of Fe-rich particles and should thus also be considered as corrosion-provoking element [9], [10].
For osteosynthesis implants and stent applications, not only electrochemical and biocompatible requirements are crucial, but also mechanical properties [1]. Recent studies have demonstrated that the alloying system Mg–Zn–Ca offers simultaneous high strength and high ductility, with values of Rp0.2 > 250 MPa, Rm > 300 MPa and Af > 20%. These attractive large values were achieved within a composition window of 5–6 wt.% Zn and 0.2–0.4 wt.% Ca [11], [12]. Recently the in vivo behavior of an alloy with 5 wt.% Zn and 0.25 wt.% Ca (ZX50) of conventional purity (CP) was investigated, and undesired rapid degradation was observed [13]. In the present study, the alloy ZX50 was chosen again, but this time raw materials of two different purities were used. The first alloy was made of CP Mg identical to that used in [13], but the second was synthesized using ultrahigh-purity (XHP) Mg [14]. The research aim of this study was to compare the in vitro and in vivo behavior of the two alloys CP ZX50 and XHP ZX50, and to clarify the influence of trace elements on degradation characteristics and mechanisms. The in vitro corrosion was determined via the hydrogen evolution method [15], deploying a newly designed testing device [9]. For the in vivo examination pins of each alloy were implanted in the femoral shafts of rats and their degradation was studied over a period of 12 weeks (see also Ref. [13]). A few pins were also explanted after two weeks to investigate their surface topography and elucidate the degradation mechanisms.
Section snippets
Materials and methods
CP and XHP ZX50 were each processed to extruded rods, from which specimens for the in vivo and in vitro tests were machined. Conventional pure Mg (99.95%), Zn (99.5%), and Ca (99.5%) were used as raw materials for CP ZX50. Mg and the alloying elements were melted in an electric furnace at a temperature of approximately 690 °C, treated with MnCl2 to reduce the Fe-content [16], and then cast by vertical direct chill casting on an industrial scale (billet diameter: 185 mm) at a speed of
Microstructure
Fig. 1 shows the microstructure of the alloys tested. The grain size D of CP ZX50 is slightly smaller (D ≈ 6 μm) than that of XHP ZX50 (D ≈ 7.5 μm). Both alloys contain second-phase particles, examples of which are shown in SEM images of CP ZX50 in Fig. 2. These intermetallic particles (IMPs) contain Zn and Ca, and in the case of CP ZX50 also Fe and Mn. Si- and Ca-containing IMPs were also detected very sporadically in CP ZX50. In XHP ZX50, however, only Zn and Ca were found to be present in the IMPs
Discussion
The results of the investigation reveal clearly the influence of the trace impurity level (see Table 1) on the degradation rate of ZX50 alloys. The alloy made with XHP Mg degrades more slowly in vitro and in vivo than the alloy with an increased impurity level, but primarily during the first period after implantation. The degradation difference between CP and XHP ZX50 is most significant in the first period of immersion, as can be observed from the optical microscopy cross-sections of the in
Conclusion
Bioresorbable Mg alloys of type ZX50 (Mg–5Zn–0.3Ca) exhibit rapid and inhomogeneous in vitro and in vivo degradation. A reduction in the impurity level to very low values decreases the degradation rate, but mostly only during the first period after implantation. The ZX50 alloys are characterized by the presence of Zn-rich IMPs. These IMPs are nobler than the Mg matrix and therefore act as cathodic sites. Local attack and galvanically accelerated dissolution of the matrix cause the formation of
Acknowledgements
The authors gratefully acknowledge financial support by the Swiss National Science Foundation (SNF Grant No. 200021-157058) and by the Laura Bassi Center of Expertise BRIC (Bioresorbable Implants for Children), FFG, Austria.
References (37)
- et al.
In vivo corrosion of four magnesium alloys and the associated bone response
Biomaterials
(2005) - et al.
Biodegradable metals
Mater. Sci. Eng.
(2014) - et al.
Calculated phase diagrams and the corrosion of die-cast Mg–Al alloys
Corros. Sci.
(2009) - et al.
Assessing the degradation performance of ultrahigh-purity magnesium in vitro and in vivo
Corros. Sci.
(2015) - et al.
Effect of traces of silicon on the formation of Fe-rich particles in pure magnesium and the corrosion susceptibility of magnesium
J. Alloys Compd.
(2015) - et al.
High-strength magnesium alloys for degradable implant applications
Mater. Sci. Eng. A
(2011) - et al.
Magnesium alloys for temporary implants in osteosynthesis: in vivo studies of their degradation and interaction with bone
Acta Biomater.
(2012) - et al.
Preparation of SBF with different content and its influence on the composition of biomimetic apatites
Acta Biomater.
(2006) - et al.
Critical assessment and thermodynamic modeling of Mg–Ca–Zn system supported by key experiments
Calphad
(2014) - et al.
Coherent nanoscale ternary precipitates in crystallized Ca4Mg72Zn24 metallic glass
Scr. Mater.
(2013)
Relationship between the corrosion behavior and the thermal characteristics and microstructure of Mg–0.5Ca–xZn alloys
Corros. Sci.
Mechanical and bio-corrosion properties of quaternary Mg–Ca–Mn–Zn alloys compared with binary Mg–Ca alloys
Mater. Des.
Mechanical properties, degradation performance and cytotoxicity of Mg–Zn–Ca biomedical alloys with different compositions
Mater. Sci. Eng. C
Understanding corrosion behavior of Mg–Zn–Ca alloys from subcutaneous mouse model: effect of Zn element concentration and plasma electrolytic oxidation
Mater. Sci. Eng. C
The corrosion of magnesium in aqueous solution containing chloride ions
Corros. Sci.
Influence of microstructure on the corrosion of die cast AZ91D
Corros. Sci.
The role of Si and Ca on new wrought Mg–Zn–Mn based alloy
Mater. Sci. Eng. A
Corrosion mechanisms of magnesium alloys
Adv. Eng. Mater.
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