The role of zinc in the biocorrosion behavior of resorbable Mg‒Zn‒Ca alloys.

Zinc- and calcium-containing magnesium alloys, denominated ZX alloys, excel as temporary implant materials because of their composition made of physiologically essential minerals and lack of commonly used rare-earth alloying elements. This study documents the specific role of nanometric intermetallic particles (IMPs) on the in vitro and in vivo biocorrosion behavior of two ZX-lean alloys, Mg‒Zn1.0‒Ca0.3 (ZX10) and Mg‒Zn1.5‒Ca0.25 (ZX20) (in wt.%). These alloys were designed according to thermodynamic considerations by finely adjusting the nominal Zn content towards microstructures that differ solely in the type of phase composing the IMPs: ZX10, with 1.0 wt.% Zn, hosts binary Mg2Ca-phase IMPs, while ZX20, with 1.5 wt.% Zn, hosts ternary IM1-phase IMPs. Electrochemical methods and the hydrogen-gas evolution method were deployed and complemented by transmission electron microscopy analyses. These techniques used in concert reveal that the Mg2Ca-type IMPs anodically dissolve preferentially and completely, while the IM1-type IMPs act as nano-cathodes, facilitating a faster dissolution of ZX20 compared to ZX10. Additionally, a dynamically increasing cathodic reactivity with progressing dissolution was observed for both alloys. This effect is explained by redeposits of Zn on the corroding surface, which act as additional nano-cathodes and facilitate enhanced cathodic reaction kinetics. The higher degradation rate of ZX20 was verified in vivo via micro-computed tomography upon implantation of both materials into femurs of Sprague DawleyⓇ rats. Both alloys were well integrated with direct bone‒implant contact observable 4 weeks post operationem, and an appropriately slow and homogeneous degradation could be observed with no adverse effects on the surrounding tissue. The results suggest that both alloys qualify as new temporary implant materials, and that a minor adjustment of the Zn content may function as a lever for tuning the degradation rate towards desired applications. STATEMENT OF SIGNIFICANCE: In Mg‒Zn‒Ca (ZX)-lean alloys Zn is the most electropositive element, and thus requires special attention in the investigation of biocorrosion mechanisms acting on these alloys. Even a small increase of only 0.5 wt.% Zn is shown to accelerate the biodegradation rate in both simulated body conditions and in vivo. This is possible due to Zn's role in influencing the type of intermetallic particles (IMPs) in these alloys. These IMPs in turn, even though minute in size, are shown to govern the biocorrosion behavior on the macroscopic scale. The deep understanding gained in this study on the role of Zn and of the IMP type it governs is crucial to ensuring a safe and controllable implant degradation.

In vivo and in vitro degradation comparison of pure Mg, Mg-10Gd and Mg-2Ag: a short term study.

The purpose of this study was to compare short term in vitro and in vivo biodegradation studies with low purity Mg (> 99.94 %), Mg-10Gd and Mg-2Ag designed for biodegradable implant applications. Three in vitro testing conditions were applied, using (i) phosphate buffered saline (PBS), (ii) Hank's balanced salt solution (HBSS) and (iii) Dulbecco's modified eagle medium (DMEM) in 5 % CO2 under sterile conditions. Gas evolution and mass loss (ML) were assessed, as well as the degradation layer, by elemental mapping and scanning electron microscopy (SEM). In vivo, implantations were performed on male Sprague-Dawley rats evaluating both, gas cavity volume and implant volume reduction by micro-computed tomography (µCT), 7 d after implantation. Samples were produced by casting, solution heat treatment and extrusion in disc and pin shape for the in vitro and in vivo experiments, respectively. Results showed that when the processing of the Mg sample varied, differences were found not only in the alloy impurity content and the grain size, but also in the corrosion behaviour. An increase of Fe and Ni or a large grain size seemed to play a major role in the degradation process, while the influence of alloying elements, such as Gd and Ag, played a secondary role. Results also indicated that cell culture conditions induced degradation rates and degradation layer elemental composition comparable to in vivo conditions. These in vitro and in vivo degradation layers consisted of Mg hydroxide, Mg-Ca carbonate and Ca phosphate.