Abstract text (incl. figure legends and references)

INTRODUCTION: Multi scale analysis of functional materials combining different imaging techniques for the same region of interest (ROI) is key for the development of digital twins[1]. These numerical models of the material and its behaviour accelerate the development of functional materials like biodegradable implants. Magnesium based alloys are gaining high interest for implant applications due to their biocompatibility and biodegradability.[2] Understanding the underlying degradation processes and their interdependencies during the biodegradation is necessary to control and tailor them and thus for the development of digital twins. In situ synchrotron radiation nano computed tomography (SRnanoCT) can be used as a non-destructive 3D imaging method to investigate dynamic degradation processes with nominal resolutions below 100nm. The investigation of the chemical compositions and morphology down to atomic-scale is possible with (scanning) transmission electron microscopy ((S)TEM). The correlation of the two techniques enable to achieve a complementary picture of the degradation process, but, at the same time, it brings a number of challenges, such as sample transfer, identification of ROI (e.g. precipitates) and image registration.

OBJECTIVES: The goal of this study is to develop a correlative workflow enabling to understand the biodegradation of Mg-based alloys at different length scales. For this purpose, a three-step workflow was developed (see Fig.1). Firstly, in situ SRnanoCT providing information on degradation rates, homogeneity, degradation layer (DL) formation and presence of larger precipitates within the alloy, and therewith enabling identification of degradation-relevant ROIs for further in-depth characterisation. Secondly, the transfer into a focused ion beam (FIB) and identical-location lamella preparation for the third step, i.e. correlated, in-depth (S)TEM investigation of structural defects and degradation layer.

MATERIALS & METHODS: A flow-cell setup is used for in situ SRnanoCT. Mg-x-wt.%Ag and Mg-y-wt.%Gd wires (80μm diameter) have been degraded in simulated body fluid (SBF) under physiological conditions (37 °C, pH 7.4, flow rate 1 ml/min) resulting in tomographic scans every 13-30 minutes. FIB lamellas for identical-location TEM characterization were prepared by a lift-out method in a dual-beam FIB FEI Helios NanoLab 400S. STEM imaging and EDX analyses were conducted with the aid of probe-corrected FEI Titan G2 80-200 at 200kV [3].

RESULTS: Figure 1 shows the correlative workflow. First results from SR tomograms (A) of Mg-4wt.%Ag yield a degradation rate of 9.4 mm/year after approx. 5 hours. A FIB lamella (B) depicts the preparation and the EDX analysis (C) shows Mg-rich bulk material , an inner and outer degradation layer, Ag-rich precipitates and FIB protection layers as well as a voids.

Figure 1: Mg-based alloy degraded in SBF: (A) SR nano tomogram (B) lamella preparation in FIB and (C) elemental map with EDX.

CONCLUSION: Preliminary results from tomograms show inhomogeneous degradation behaviours. Further studies are necessary to understand the influences on biodegradation processes, as the precipitates do not seem to act as micro galvanic elements, as no enhanced degradation can be observed in their vicinity.

REFERENCES: ¹M. Abdalla, et al., Corrs. Mater. Degrad. (2020), 219-248. ²M. P. Staiger, et al., Biomaterials (2006), 1728–1734. ³FEI Titan G2 80-200 CREWLEY (2016) A43.