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Nickelimmersion from Nitinol Thin-Film Heart Valves
Constantin Fuehner, Saskia Pokorny, Irma Haben, Katharina Huenges, Jochen Cremer, Georg Lutter.
University Hospital Schleswig-Holstein, Department of Cardiovascular Surgery, Kiel, Germany.

OBJECTIVE: The development of heart valve leaflets manufactured from a thin film nitinol is focus of recent research. A nitinol heart valve would be efficient, durable, very thin heart valve with low thrombogenity and would hence be able to decrease the minimal implantation diameter of valved stents. However, nickel has a high allergic potential and heart valves are exposed to permanent deformation. In this study the release of nickel from nitinol valves within a permanent pulsatile flow was evaluated.
METHODS: Thereto an in vitro test set up was developed, simulating the dynamic blood flow in vivo and enabling the analysis of nickel release. The experimental set up was a closed system composed of a roll pump, causing a continuous, pulsatile flow, a specimen mounting and a reservoir. Ultrapure water was used as test medium. Five heart valve specimens were tested following a standardized protocol. Samples of the test medium were taken at defined points of time (1, 72, 144, 216, 288 and 360 hours) and the nickel concentration was determined with a mass spectrometer. Within this testing period, the specimens were loaded with 70 bpm, resulting in 1.515.000 cycles in total. Furthermore, the surface of the heart valves was analyzed using a scanning electron microscope, to detect corrosion and abrasion.
The amount of nickel released was determined by a mass spectrometer and normalized to the surface area and the number of hours within the test set up. The released nickel was highest within the first 72 hours with 2.9±1.2 ng/cm2/d after 1 hour and 2.1±0.5 ng/cm2/d after 72 hours and decreased thereafter to 1.0±0.5 ng/cm2/d after 12 days (288 hours). After 15 days the nickel immersion slightly increased to 1.8±0.8 ng/cm2/d.
The scanning electron microscope showed signs for kinks in restricted areas.
CONCLUSIONS: A test set up was developed to evaluate the nickel immersion from nitinol thin-film heart valves within pulsatile flow conditions. The nickel immersion in six specimens was analysed over a period of 15 days under pulsatile flow loading. A very low nickel immersion was detected, being below the level associated with allergic responses.

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