Surface modifications of 316 stainless steel for the improvement of its interface properties with RFGD-deposited fluorocarbon coating

Metallic endovascular stents are used as medical devices to scaffold biological lumen, most often diseased arteries, after balloon angioplasty. They are commonly made of 316L stainless steel or Nitinol, two alloys containing nickel, an element classified as potentially toxic and carcinogenic by the International Agency for Research on Cancer (IARC). Although they are largely implanted, the long-term safety of such metallic elements is still controversial, since the corrosion processes may lead to the release of several metallic ions, including nickel ions in a number of oxidation states. To avoid metallic ion release in the body, the strategy behind this work was to develop a process aiming the complete isolation of the stainless steel device from the body fluids by a thin, cohesive and strongly adherent coating of plasma-polymerized fluoropolymer. Nevertheless, prior to the plasma-polymer film deposition in a Radio Frequency Glow Discharge (RFGD), an essential aspect was the development of two pre-treatment procedures on the 316 stainless steel surface, which consists of mechanical and electrochemical polishing, ultrasonic cleaning, and dipping in alkaline and acidic solutions. The aim was to obtain a smooth and homogeneous substrate surface as well as the removal of any fragile interlayer, particularly the metallic oxide layer, in order to optimize the adhesion between the plasma-deposited film and the metallic substrate. In this work, the effects of these pre-treatments on surface roughness and topography were investigated by atomic force microscopy (AFM). The modifications induced on the surface atomic concentrations were studied by X-ray photoelectron spectroscopy (XPS). The improvements in terms of the chemistry, the roughness and the topography of the surface after the pre-treatments were emphasized. The chemical composition of the interface between an ultra-thin RFGD-deposited fluorocarbon coating and the pre-treated metallic surfaces was also investigated.