Study on the mechanical properties of magnetron sputtered W-based degradable radiopaque coatings for tiny biodegradable metallic endovascular implants

Abstract

Bioresorbable metals constitutes a new class of biomaterials, and raised increasing interest over the last decade to fabricate tiny endovascular metallic implants. However, the lack of visibility especially when tiny bioresorbable implants are implanted is a major concern in clinics, affecting their implantation and limiting their clinical follow-up. Among the radiopaque elements, tungsten (W) is a promising candidate to be used as a radiopaque resorbable agent due to its high radiopacity and high corrosion rate. Therefore, this research aimed to produce a W-based radiopaque coatings providing mechanical properties close to Fe–Mn–C alloys. The mechanical mismatch between the substrate and the coating was also investigated and optimized. A magnetron sputtering was used to deposit Fe–Mn–C–W coatings (A and B) with 38 and 79 at. % of W, respectively, and at different deposition temperatures, i.e. 25, 300 and 600 °C. TEM and XRD analyses evidenced that coatings A, ∼38 at.% of W, at all deposition temperature, exhibited an amorphous structure. The amorphous structure of coating A was composed of a matrix of Fe with tiny α-W nanocrystals embedded, while coating B (W: 73–79 at. %) displayed the presence of a pyramidal-like β-W phase, observed only at low deposition temperature (25 °C). The presence of this β-W phase significantly increased the coating roughness, and increased its corrosion rate. Mechanical properties, assessed by nanoindentation tests, confirmed that samples containing β-W exhibited higher elastic modulus and hardness when compared to other samples. X-ray studies by CT scanning demonstrated that 1 μm of coatings A and B deposited at high temperature could increase significantly the radiopacity of the samples by 109% and 62%, respectively. On the basis of all these results, the coating A deposited at 600 °C appears to be the most promising coating for X-ray enhancement as well as providing the mechanical properties matching those of Fe–Mn alloy.