Degradation of biodegradable stents : evaluation and optimization of microCT for the characterization of the degradation rate and the roughness evolution

Stent technology has been introduced in the 80s to overcome the limitations of balloon angioplasty. However, complications can still occur after the implantation of permanent metallic stents such as prolonged physical irritations, chronic inflammation, stent fracture, thombosis and particularly in-stent restenosis. Nowadays, to overcome those limitations, the most promising type of stents is the biodegradable stent. As the name suggests, the stent remains in situ only for a limited period of time before being completely degraded and disappearing, allowing reendothelialization to take place. So far, two main materials have been mostly investigated : magnesium-based alloys and iron-based alloys. This master thesis is focused on the latter. Pure iron shows adapted mechanical properties. However, its degradation is too low and need to be increased by alloying. TWIP was created to meet this need. To select the best candidate material for biodegradable stent applications, a detailed characterization is required. In this master thesis, the degradation and roughness of pure iron and TWIP samples was analysed using different physical and microCT-based methods. Studying degradation and roughness of candidate materials for biodegradable stent ap- plications and gathering a 3D reconstruction of the samples before and after immersion thanks to microCT is something that has not been done yet. Therefore, the use of microCT needs to be assessed and optimised for the characterization of the degradation rate and roughness evolution over time. For this purpose, the roughness and degradation rate of wires made of TWIP and pure iron was analysed during dynamic immersion tests. Results highlighted the limitations of microCT to compute the degradation rate with the mass loss method. Several assumptions have been done to explain those bad results. An aliasing effect may occur at the ends of the scan, wires may be curved during degradation and handling, a protective layer may form on the surface or spatial image resolution may be too low to detect such small volume differences. Inductively coupled plasma mass spectrometry results has allowed us to observe an increase in the degradation rate of TWIP after 5 hours of immersion. For microCT-based roughness analysis, two different Matlab routines were used, both using linear baselines. The first one rapidly demonstrated limitations as it is not able to take into account the curvature of the wire. The second one gave better results. It enabled us to detect the presence of outliers. It showed an higher initial roughness for TWIP caused by the manufacturing process. It also allowed us to demonstrate a low correlation for both materials between the degradation rate and the initial roughness, meaning that alloying has effectively improved the corrosion rate of TWIP. Finally, we were able to observe the roughness evolution of pure Fe over time. To conclude, the use of microCT seems limited to compute the degradation rate. Its use for roughness analysis seems more promising. However, additional work should be done to evaluate the roughness with polygonal baselines instead of linear baselines. Ideally, the roughness should be analysed with a routine that could compute the roughness parameters in 3D, taking into account any outliers. Comparison should also be made between microCT and standard profile measuring systems results. Finally, no conclusion could be drawn concerning the use of TWIP for biodegradable stent applications since the number of samples was limited and its roughness and degradation rate evolution over time could not be observed.