Fretting and Flow Assisted Corrosion Effects on Nitinol Stents for Biomedical Use, 18-R8282
James F. Dante
Inclusive Dates: 01/01/12 – Current
Background — Nickel-titanium alloys, also referred to as nitinol, have been used for many years as a biomedical stent material to repair damaged vessels. Although there have been marked improvements in the design of stents and nitinol processing, the number of stent failures has remained high (1.7 percent to 32 percent). Failure of a stent is considered to be a fracture of the stent support system and/or corrosion on the stent that results in nickel-ion release, which is toxic to the body. A common procedure is to overlap stents in order to accommodate a longer damaged vessel length. This introduces a fretting scenario. A combination of fretting along with flow-assisted corrosion, pulsatile effects, as well as pH of the local wound site, are potential performance issues in the in vivo condition that have not been accounted for in the literature. The proposed research looks to understand these combined effects on nitinol stents. The project has three major objectives:
- Determine the effect of fretting on the corrosion behavior of nitinol stent material under biological fluid flow and under different pH conditions,
- Compare the effects of pulsing flow conditions, stagnant conditions, fretting and its relation to corrosion resistance of nitinol stent material, and
- Assess the biocompatibility of nitinol stents using endothelium cells after fretting/flow testing.
Approach — The proposed research project set out to construct a test apparatus to simulate the combined effects of fretting and flow conditions in simulated biological solutions. The corrosion potential during fretting and flow are to be recorded for up to three months. Lower pH solution tests shall be performed to determine how the corrosion behavior will change when the material is near inflamed tissue or crevice areas. Also, pulsatile testing will be performed at rates that allow a comparison between resting and high heart-rate condition. After testing, the stent surface morphology will be examined by micro-CT, scanning electron microscopy (SEM), and auger spectroscopy. Cyclic polarization testing will be performed to determine general and localized corrosion effects. Nickel-ion concentration measurements will be obtained from the test fluid during the exposure. In addition, biocompatibility testing will be performed to see if there is cell adhesion and proliferation on the nitinol stents after exposure.
Accomplishments — A four-channel flow apparatus was constructed and baseline testing (flow testing only) has been completed. For the flow tests, the open circuit potential was monitored for 14 days, 30 days, 60 days and 90 days with phosphate buffered saline (PBS) solution flowing at a rate of 250 mL per min. In all four test cells, the open circuit starts very low and then increases to a value of approximately 90 mV after approximately 4 to 5 days. There were also small perturbations in the open circuit potential during testing for all four test cells. During this time the potential showed a sudden drop. These could indicate a potential breech in the surface oxide where corrosion may have occurred. Subsequent micro-CT and SEM performed on the sample surfaces, however, did not show any major surface degradation due to the flow on the samples. Haemocytolysis testing was performed on the stents after flow testing. Haemocytolysis refers to the destruction/ dissolution of red blood cells (RBCs). It was determined that flow testing has no negative effects on haemocytolysis. In addition, metabolic assay and cell culture testing using human umbilical vein endothelial cells were performed on the stents after flow testing. All stents exhibited excellent cytobiocompatibility. Cells were able to attach and proliferate on each stent quite well.