Fretting and Flow Characteristics Of Nitinol Stents

Nitinol is a Nickel-Titanium alloy known for its excellent biocompatibility and superelastic properties.  It has been utilized by the biomedical industry as cardiovascular stent material.  In this study, overlapping Nitinol stents were electrochemically monitored during fretting and under flow conditions.  Two different conditions were compared; one with the flow of phosphate buffered saline solution (PBS) at a pH of 7.4, and the second with a PBS solution with a pH of 5.5.  The lower pH condition would simulate a wound scenario.  Four sets of overlapping Nitinol stents were tested in each of the two solutions for 14 days, 30 days, 60 days and 90 days.  The samples were fretted utilizing a four point loading device and observed for changes in open circuit potential.  The time to recover the open circuit change was also noted.   SEM and MicroCT were then performed on the ID of the outer stent to determine if the effects of fretting could be observed.  Localized and general corrosion behavior was determined using cyclic polarization after the exposures.  Nickel ion release was also quantified during fretting/flow testing.  Biocompatibility studies were also performed on the inner stent samples after exposure.  Haemocytolysis rates were determined on the 14 day and 30 day exposures in each of the two solutions.  In addition, human umbilical vein endothelial cells were cultured and seeded on the stents to determine if the fretting/flow condition affected the cell growth and proliferation of the stent.