Biocorrosion of iron based alloys for bioresorbable coronary stent applications

Details

Supervisors

Jacques, Pascal

Faculty

Ecole polytechnique de Louvain

Degree label

Master [120] : ingénieur civil en chimie et science des matériaux

Abstract

Cardiovascular diseases, such as blood vessels obstructions are common. They are the main cause of death in Belgium and worldwide. Bad diets can cause these atherosclerosis plaques to build up. In such cases, percutaneous coronary intervention (PCI) needs to be performed, during which a stent is implanted. Different stent families exist. Permanent bare-metal stents, drug-eluting and bioresorbable stents are differentiated. Complication due to stenting include restenosis and thrombosis. The foreign body can induce an unwanted cell proliferation, resulting in a re-narrowing of the blood vessels. By making stents bioresorbable, late-thrombosis can be avoided, as the foreign body will degrade and not alter the blood environment anymore. The mechanical support should be present for about 6 month-1 year, after which the stent is obsolete and should degrade. Investigated bioresorbable stents are polymer based, or metal based. The former category does not present good mechanical properties and the latter category can be distinguished between magnesium based and iron based stents. The magnesium based stents degrade too fast, whereas iron stents degrade too slowly. The aim of this Master’s thesis is to investigate the corrosion behaviour of iron based alloys. The two materials that will be tested are Armco iron for comparison reasons and Fe-22Mn-0.6C. Armco iron is used as a reference material, which corrosion properties are widely investigated in the literature. Therefore, the experimental protocol is validated on this material. Fe-22Mn-0.6C is a twinning induced plasticity (TWIP) alloy, presenting excellent work-hardening properties. This is a critical feature for the mechanical support of the injured vessel. Its corrosion properties have not been studied a lot and will be investigated and compared to the ones of iron. The impact of deformation on these samples has also been investigated. Indeed, the stent undergoes compression to be placed onto the balloon and expansion to be put in place in the injured vessel. Locally the deformation is supposed less than 30%. The different deformations tested were 0%, 10%, 20%, 30%, and 40%. Corrosion properties will be studied by the use of electrochemical tests and quasi static immersion tests. Electrochemical tests were performed on iron and TWIP samples using simulated body fluid (SBF) with or without Hepes buffer. Once the experimental protocol was put in place, the tests were performed on TWIP samples (in SBF containing 50mM buffer). Quasi static immersion were also performed on these TWIP samples (SBF containing 50mM buffer). The electrochemical results highlighted a higher corrosion rate for the non TWIP samples compared to Armco iron. Inside the TWIP samples, the differences in corrosion rates are minimal. The non deformed samples exhibit the highest corrosion rate. The 40% samples seem to corrode slightly faster than 30% and 20% families and the 10% family seems to have the slowest corrosion rate. SEM and EDX analysis showed the presence of a cracked layer on top of iron samples containing Ca, P, Na, and Mg. The TWIP samples showed the presence of agglomerates on a surface layer composed mainly of the metal constituents and some SBF elements (Ca, and Mg especially). The corrosion products contain P, Ca, Na, Mg, and also Cl. The immersion tests (in SBF containing 50mM Hepes buffer) on Fe-22Mn-0.6C showed that the corrosion rate was highest after one day immersion and decreased significantly between day 1 and day 7, followed by a slighter decrease from day 7 to day 21. The samples tested were non deformed samples and 40% deformed ones. The differences between the different deformation families for one week immersions (non-deformed, 10%, 20%, 30%, and 40%) were slight and the standard deviations too high to conclude any dependence. The higher buffer concentration (100mM), however showed an increased corrosion rate. The corrosion mechanism is similar to ones already explained in the literature: the metal gets attacked by oxygen reduction, forming a rust layer. Chlorine then attacks the matrix creating metal chlorides. A final layer is made of Ca/P deposits (and some Na, Mg, and and K). A qualitative idea of the sublayer compositions was found by ToF-SIMS to correspond to this mechanism (expect Cl that was present on the top layer aswell). XPS showed the presence of hydroxides (corrosion products), phosphorous, calcium, and sodium (all three are SBF deposits).