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Corrosion characterization of a novel ceramic stent
Conferencia; AMBA Advanced Materials for Biomedical Applications; 2014
Metallic coronary stents are placed in a high chloride environment that plays an important role on the corrosion behavior of the implant. Stents corrosion presents two main risks: release of metallic ions into the tissue and deterioration of the mechanical properties that could lead to the stent fracture. Release of heavy metal ions could affect the local tissue leading to inflammatory mediators and promote in-stent restenosis. MATERIALS AND METHODS 316L stainless steel cardiovascular stents were previously characterized by SEM (scanning electron microscope), AES (Auger electron spectroscopy), and EDS (Energy Dispersive X-ray Spectrometry). A Ceramic stent (CS) was developed by a simple and inexpensive surface treatment that it cannot be disclosed (patent in process). In vitro corrosion testing was performed as following: each stent was immersed in PBS medium (pH 7.4) and incubated at 37°C. Micrographic pictures were taken in different periods of time to assess the type of corrosion. Potentiodynamic polarization studies were performed on unexpanded stents and after balloon expansion at 12 bars. The corrosion potential (Ecorr) was measured under a virtual open circuit condition, with respect to a reference electrode; and the corrosion current (icorr) by a potentiodynamic polarization curve measurement, that involved potential (mV) scanning at a rate of 0.5 mV/s to a final potential of 1.4 V. Micrographic pictures were taken before and after the electrochemical analysis. To make a comparison, this test was performed also on different type of commercial stents: gold coated stent, carbon coated stent and chromium-cobalt stent. Corrosion rate was obtained according to: CR=Icorr.K.EW/dA; where Icorr is the corrosion current, K constant that defines the units, EW the equivalent weight in grams/equivalent, d is density in grams/cm3 and A is sample area in cm2. Statistical analysis: every assay was done in triplicate; ANOVA and a multiple range test by Duncan´s variation were performed. RESULTS AND DISCUSSION According to the in vitro corrosion, SST exhibited appreciable signs of corrosion during the first two weeks of incubation, whereas that the CS showed an early stage of corrosion at week eight of incubation. The potentiodynamic curves of CS exhibited a favorably anodic current density, significantly lower than other commercial stent. The corrosion rate (CR) is ten thousand times lower than value from regular stainless steel stent (SST). Micrographic pictures showed only light signs of corrosion on the CS, viewed as a changed on the surface color due to oxide deposition. CrCo micrographic study exhibited a clear intergranular corrosion. The gold and carbon coated stent showed extreme delamination leaving the stainless steel expose to the corrosive environment showing a tendency for pitting and intergranular corrosion. SST stents also showed a high tendency for pitting corrosion with loss of entire portions of stent struts. CONCLUSIONS Ceramic stents showed a better corrosion behavior when compared with different types of commercial stents. This treatment has the advantage that no coating is applied so there is no delamination during implantation and they can be produced at low cost.