Optimisation of the corrosion rate of iron-based alloys for bioresorbable stent applications by surface acidification
(2018) 10th Symposium on Biodegradable Metals for Biomedical Applications — Location: Oxford, Royaume-Uni (26.August.2018)
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- INTRODUCTION: Biodegradable materials progressively become of interest for stent applications. Indeed, a stent is not needed anymore after 6-12 months due to artery remodelling. Bioresorbable stents could prevent late complications such as late thrombosis [1]. Stents need to be mechanically strong, making steel a good candidate. But steel does not corrode sufficiently fast. Furthermore, during corrosion in the blood environment, different layers form on top of the metal. This hinders the oxygen diffusion towards the metal, slowing down the corrosion [2]. Therefore, a way to increase the corrosion rate needs to be found. The pH of the environment greatly influences the presence of these layers; an acidic pH favours their dissolution [2,3,4], which could help in activating the corrosion. However, the blood environment needs to keep a pH close to physiological pH (7.4) and thus only the near stent surface can undergo an acidification without affecting the blood system too much. The present work investigates different methods that can influence the pH on Fe-based alloys surfaces; especially hydrogen addition in the steel and polymer coating. METHODS: Hydrogen charging was conducted either electrochemically, thermally or chemically on Fe-based alloys. The influence of the hydrogen concentration on the corrosion mechanism in SBF (simulated body fluid) is assessed by means of immersion tests. These tests allow to mimic closely what happens in vivo. Potentiodynamic polarisation tests were also conducted to highlight the influence of the chemical composition on the corrosion rate. Tensile tests were also carried out. The polymer was coated on the steel by dip coating and the influence of this polymer addition on the corrosion rate was assessed by immersion tests as well as potentiodynamic polarisation tests. RESULTS: Samples that were electrochemically charged with hydrogen show an increased corrosion for higher hydrogen contents after one day of immersion. However, after three days of immersion, the corrosion rate is similar for different hydrogen contents (Figure 1). On Figure 2, one can see the amount of hydrogen present at time 0 in the steel and after three days at room temperature. Polymer coated samples lead to an increased corrosion rate during immersion tests. DISCUSSION & CONCLUSIONS: Electrochemically charged hydrogen disappears in between three days of charging. This explains the similar corrosion rate after three days immersion. The presence of a polymer (PLA) coating leads to an increased corrosion rate due to the polymer hydrolysis that acidifies the surface [2]. Bioresorbable stents made of steel could become a solution to avoid late complications due to the presence of a foreign body in the blood environment. In order to accelerate the corrosion of steel, surface acidification can be implemented, leading to a dissolution of the corrosion layers that hinder the oxygen transport and thus decrease the corrosion rate. REFERENCES: 1 T. M. Jeewandara, et al. (2014) Materials 7(2):769–786. 2 A. H. Md. Yusop, et al. (2015) Scientific Reports 5:11194. 3 M. Z. Yang, et al. (1999) Journal of the Electrochemical Society 146(6):2107-2112. 4 J. Flis, et al. (1999) Electrochimica acta 44(23):3989-3997. ACKNOWLEDGEMENTS: The FRIA and ARC are thanked for funding this research.
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Reuter, S., Georges, C., Magnin, D., Demoustier, S., & Jacques, P. (2018). Optimisation of the corrosion rate of iron-based alloys for bioresorbable stent applications by surface acidification. 10th Symposium on Biodegradable Metals for Biomedical Applications, Oxford, Royaume-Uni. https://hdl.handle.net/2078.5/53569