Exploring the fretting-corrosion mechanisms of NiTi in a simulated biological environment
Stents made from biomedical alloys such as cobalt-chromium-molybdenum (CoCrMo) and nickel-titanium (NiTi) are commonly used as interventions for cardiovascular disease, yet clinical limitations persist. In-stent restenosis (ISR) accounts for 20% of stent failures at 1 year. Damage from tribocorrosion and release of bioactive wear debris to tissue has been observed in vivo. To date, tribocorrosion in a cardiovascular context is comparatively underexplored despite its implication in clinical literature. This paper investigates the fretting-corrosion mechanisms of these alloys, the effect of experimental conditions on wear debris and cell response to debris.
Samples of CoCrMo or NiTi in a cross-cylinder contact underwent fretting in serum-free medium to simulate physiological biochemical environment. Tests were conducted for 80,000 cycles under free corrosion or applied potential (0V or 0.2V). Wear debris was characterised in terms of ion release to media (ICP-MS), particle composition and morphology and wear scar composition and morphology (VSI, SEM-EDX). Human smooth muscle cell (SMC) viability was evaluated using fluorescent LIVE/DEAD stain.
Results and Discussion:
Wear scar chemistry for both alloys showed complexing with components of the culture medium such as sulphur, phosphorus and chlorine. Increasing potential led to domination of release of cobalt ions for CoCrMo and nickel ions for NiTi to the media; at OCP alloys had stoichiometric ion release. Increased applied potential resulted in altered particle composition and morphology: from typically bulk metal at OCP to corrosion product particulates. The degree of applied potential altered the toxicity level of SMCs for both CoCrMo and NiTi.