Introduction: Release of potentially toxic/carcinogenic ions, degradation of mechanical properties, and delamination of the polymeric coating in drug-eluting stents are some of the problems cause by localized corrosion in medical grade stainless steel (SS316L) devices. To reduce these complications, a series of plasma etching followed by plasma oxidizing (PO) were used to replace the native oxide layer on SS316L with a new and denser one. Results showed that the corrosion resistance increased more than ten times[1]. Moreover, PO generated a 6-10 nm thick amorphous oxide layer on SS316L. This layer was not yet mechanically stable and needed optimization. In this work different oxide layers were produced by changing the process parameters (e.g. PO time), and characterized mainly through electrochemical methods and surface analyses.
Methods: The samples were cleaned and electropolished (EP) before entering the plasma reactor. The plasma surface modification was done in two steps: 1) removal of the native oxide layer 2) growing the new oxide layer by PO[1]. Electrochemical tests including potentiodynamic and galvanostatic tests, and electrochemical impedance spectroscopy (EIS) were applied to evaluate the properties of the oxide layer. Changes in the surface chemical composition were investigated by x-ray photoelectron spectroscopy (XPS). EP samples were used as controls.
Results: Tafel analyses from potentiodynamic curves showed the corrosion rate for a variety of samples. The corrosion rate for the 10 and 30 min PO samples (PO-10 and PO-30, respectively) were the lowest and almost equal (about 0.1 µm year-1). However, the amorphous oxide layer on PO-10 was considerably more stable according to galvanostatic results. Extrapolation of the Nyquist partial circles produced circles with different diameters, the diameter for PO-30 > PO-10 > EP, which implies increased impedance of the oxide layer in the same order (Fig. 1). In the Bode plots, the broadening plateau occurred in the middle frequency region for EP and PO-10 samples indicating an intact oxide layer[2], which was not the case for the PO-30 sample. The equivalent circuit model in Fig. 1 is the best agreement between experiment and fitting results.
Discussion and Conclusions: Presence of a dense amorphous oxide layer on SS316L surface could make it more resistant to leaching of potentially toxic materials in biological environment. PO for 10 min produced the most efficient protective layer. The methods applied in this project provided a technically straightforward approach to tune the surface properties of biomaterials and will facilitate surface characterizations for future surface modifications in nano-scale. Complementary analyses showed that the adhesion of a polymeric coating to this oxide layer strongly improved[3]. This gives new hope for modulating the interfacial properties between a metallic substrate and a polymeric coating using PO.
This work was partially supported by NSERC-Canada, CIHR-Canada, CFI-Canada, FRQ-NT-Quebec, and MRI-Quebec
References:
[1] M. Cloutier, et al (2011) Adv Mat Res 409:117-122.
[2] L. Jinlong, et al (2014) J Nucl Mater 452: 469-473.
[3] F. Lewis, et al (2011) ACS Appl Mater Inter 3(7): 2323-2331.