2, Marche Polytechnic University, Ancona, AN, Italy
3, Milan Polytechnic University, Milan, MI, Italy
For cardiovascular applications, and especially for the fabrication of stents, alloys such as AISI316L and Co-Cr based ones, are preferred for their mechanical properties, overall resistance to corrosion and acceptable biocompatibility. High ductility and toughness are in fact required when the stent is deployed in the damaged vessel. The release of Ni is a major concern for the cytocompatibility properties of the considered materials. A way to improve the surface properties, and create a surface which effectively can both withstand the material plastic deformation and act as a barrier for toxic ion release is the use of plasma treatments. This work copes with the characterization of oxygen plasma ion immersion implanted (PIII) L605; several parameters were investigated in this study, such as for example received dose (proportional to implantation time, t) and substrate bias (Ubias).
Co-20Cr-15W-10Ni square samples after implantation were studied by angle resolved X-ray photoelectron spectroscopy (AR-XPS) was used to assess the surface chemistry. Transmission electron microscopy (TEM) provided information related to the oxidized layer thickness, structure and interface. Glowing discharge optical emission spectroscopy (GD-OES) was used to evaluate the chemical composition of the investigated layer. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used respectively to analyze the surface morphology and roughness. The sample were also subjected to static immersion tests. The viability of human umbilical vein endothelial cells (HUVECs) and smooth muscle cells (HUASMCs) were evaluated by resazurin test.
The implantation process modified from a chemical point of view the sample surface, forming a mixed Co and Cr oxides. Other elements that were present in traces, such as for example Ni, were no more present after implantation. The relative amount of Co and Cr changed with depth, with Cr amount increasing with increasing depth. The thickness of the layer varied, from around 25 nm for the highest Ubias and implantation time t (respectively -10 kV and 120'), to around few nanometers for lower Ubias and t. The presence of oxide nanocrystals in an amorphous matrix was detected, for almost all the investigated conditions; the formation of an oxide layer increased the corrosion potential in modified Hanks' solution, compared to that one of the electropolished samples. Surface roughness analysis was performed on different scales; Ra was in the range of few nanometers, and it did not vary relevantly for the studied parameter ranges. Static contact angle showed an increase for the oxidized samples. Cytocompatibility studies, performed by means of direct contact tests, showed an increasing viability trend of endothelial cells for oxidized samples. PIII surface treatment, in the range of the studied parameter, proved to be an efficient way to increase the corrosion potential and modify alloy surface cytocompatibility.