Evaluating the morphology of the degradation layer of pure magnesium via 3D imaging at resolutions below 40 nm

Abstract

Magnesium is attractive for the application as a temporary bone implant due to its inherent biodegradability, non-toxicity and suitable mechanical properties. The degradation process of magnesium in physiological environments is complex and is thought to be a diffusion-limited transport problem. We use a multi-scale imaging approach using micro computed tomography and transmission X-ray microscopy (TXM) at resolutions below 40 nm. Thus, we are able to evaluate the nanoporosity of the degradation layer and infer its impact on the degradation process of pure magnesium in two physiological solutions. Magnesium samples were degraded in simulated body fluid (SBF) or Dulbecco's modified Eagle's medium (DMEM) with 10% fetal bovine serum (FBS) for one to four weeks. TXM reveals the three-dimensional interconnected pore network within the degradation layer for both solutions. The pore network morphology and degradation layer composition are similar for all samples. By contrast, the degradation layer thickness in samples degraded in SBF was significantly higher and more inhomogeneous than in DMEM+10%FBS. Distinct features could be observed within the degradation layer of samples degraded in SBF, suggesting the formation of microgalvanic cells, which are not present in samples degraded in DMEM+10%FBS. The results suggest that the nanoporosity of the degradation layer and the resulting ion diffusion processes therein have a limited influence on the overall degradation process. This indicates that the influence of organic components on the dampening of the degradation rate by the suppression of microgalvanic degradation is much greater in the present study.

Computational modelling of magnesium degradation in simulated body fluid under physiological conditions

“…As previous experiments have shown there is no easily quantifiable correlation between the electrochemical driven anodic dissolution related degradation of Mg and the ionic composition of the degradation medium [12] . Thus, we are mathematically uncoupling the formation of precipitates and the degradation of Mg. As the degradation behaviour shown in [20,21] followed a linear trend following an initial period of low degradation the degradation of metallic Mg ( Eq. ( 1) ) is modelled as:…”

“…The porosity of the degradation layer evolving in SBF is determined using transmission X-ray microscopy, as published in [20] . In brief, pure magnesium (99.92%) discs were degraded in SBF in a semi-static immersion test for 1-4 weeks.…”

“…The calibration of the model is performed by evaluating the mean degradation depth of the Mg specimen using both data from μCT imaging and weight loss data. Data for 1-4 weeks is taken from [20] , while additional μCT data for 6-10 days is taken from [12] and from newly measured data. For a description of the data acquisition for the previously unpublished data please see the appendix.…”

“…For a description of the data acquisition for the previously unpublished data please see the appendix. Moreover, the data from [20] and [12] differ, as discs of 99.92% purity were used and wires of 99.95% purity in the experiments, respectively. By calibrating the model to the data based on both geometries and materials simultaneously, it is possible to assess the generality of the model with respect to metal impurity within a certain range.…”

Degradation Analysis of Thin Mg-xAg Wires Using X-ray Near-Field Holotomography

“…Therefore, higher resolution X-ray imaging techniques, such as transmission X-ray microscopy (TXM) and near-field holotomography (NFHT) should be utilized. For example, TXM was previously used to analyse the porosity of the degradation layer of pure Mg degraded in simulated body fluid at resolutions down to 40 nm [20]. However, TXM is still limited to field of views (FOVs) of less than 100 µm.…”