EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Magnesium (Mg) is investigated as a powerful alternative to state-of-the-art titanium implants for temporary bone support, due to its biodegradability, biocompatibility and mechanical properties. However, certain processes occurring during Mg degradation are still unclear, in particular the relative importance of transport phenomena vs. micro-galvanic corrosion. Mechanistic and computational models consider the degradation as a diffusion-limited transport problem, yet the diffusivity of the medium in the degradation layer is mostly unknown.

We will present a multi-scale imaging approach using micro computed tomography (µCT) and transmission X-ray microscopy (TXM) that enables studying the degradation of Mg at resolutions down to 40 nm. We have degraded pure Mg (>99.92%) discs in simulated body fluid (SBF) or Dulbecco’s modified Eagle’s medium (DMEM) with 10% fetal bovine serum (FBS) for one to four weeks. The immersion tests were conducted under physiological conditions (37°C, 5% CO2). The discs were then imaged using µCT to determine the degradation rate of the material. Subsequently, focused-ion-beam milling was used to extract small cylindrical samples (Ø25µm) for imaging with TXM. Following tomographic reconstruction, we can observe the interconnected pore network within the degradation layer, as well as remnants of micro-galvanic cells, and quantify the network morphology. Additional scanning electron microscopy and energy-dispersive X-ray spectroscopy, as well as X-ray diffraction measurements of the bulk samples were performed to enrich the observed degradation layer morphology with information on its composition. Based on the obtained results, we are able to infer the importance of the nanoporosity of the degradation layer on the overall degradation process of pure Mg in the immersion media. The results can be used directly to inform a computational model of Mg implant degradation, as will be presented in the outlook.