The impact of structure and temperature on the mechanical properties and radiopacity of Ta-W coatings for tiny endovascular medical implants

Abstract

The trackability of tiny endovascular medical implants (TEMIs) during and after surgery is a crucial parameter for their accurate implantation and after surgery follow-ups. In this work, novel radiopaque Ta-W coatings were produced by magnetron sputtering deposition and further investigated with the aim to improve X-ray visibility of TEMIs under fluoroscopy imaging. Different deposition strategies, named micro-multilayer (Micro-ML), nano-multilayer (Nano-ML) and co-deposition (Co-dep), and different substrate temperatures (25 °C and 600 °C) were implemented which allowed to tune the morphology, mechanical properties and radiopacity of the coatings, while avoiding the formation of brittle phases, such as β-Ta and β-W. Ta-W coatings produced at high temperature (600 °C) contained stable phases such as α-Ta (bcc) and α-W (bcc), without brittle phases such as β-Ta and β-W. However, traces of β-W and β-Ta were detected in the group of samples produced at low substrate temperature (25 °C). The elastic modulus and the hardness of Micro-ML and Nano-ML were found to increase with increasing temperature, whereas they decreased in the case of samples produced by the Co-dep condition. The chemical state of Ta as well as the proportion of oxidized Ta (Ta-O) detected on all coatings were found to change as a function of the substrate temperature and deposition strategy. Samples with the highest amount of Ta-O at their surfaces showed the lowest corrosion potential. Finally, the samples produced by Micro-ML strategies performed on 600 °C substrates and irradiated according to radiographic imaging of conditions comparable to that of fluoroscopy, exhibited an X-ray contrast improvement of 82%/μm, which was higher than what was found in all other samples. Higher volume fractions of α-W in the coatings resulted in higher X-ray attenuation potential for photons in the energy range typical of fluoroscopy. Overall, the production of Ta-W coatings by the Micro-ML strategy and performed with a substrate temperature of 600 °C seems an optimal approach for the production of highly visible TEMIs in X-ray imaging.