S. A. Shabalovskaya, Katholieke Universiteit Leuven, Leuven, Belgium; G. Rondelli, CNR-IENI, Milano, Italy; J. Anderegg, Ames Laboratory, Ames, IA; J. Van Humbeeck, Katholieke University Leuven, Leuven, Belgium
Nitinol surface oxides prepared with chemical etching, electropolishing and heat treatments and modified using cyclic polarization were studied in connection with biological responses induced in vitro. XPS, GXRD, AFM, OCP, polarization resistance and cyclic potential polarization under strain were employed to characterize surface oxides and clarify the nature of the biological responses. Prepared surfaces in their original states ranged significantly in Ti and Ni concentrations and produced either mild or no toxic effect on biological cells, but they caused variable fibrinogen adsorption (from 130 to 300 ng/cm2) and platelet behaviour. This pointed at the possibility of manipulating Nitinol thrombogenicity to adjust it to the requirements for medical devices (stents versus defect closures). Surface oxides differed in structure from amorphous to nano-crystalline, topography and electrochemical properties. Open circuit potentials varied from -450 mV to -70 mV (vs SCE) and polarization resistance Rp from < 1 MµÙ to 8 MµÙ depending on preparation procedure. Upon cyclic polarization oxide thickness increased, all Ni on the surface was oxidized, and the resulting Rp could be as high as 67 MµÙ indicating ceramic-like nature of modified surfaces. The corrosion resistance of studied surfaces did not deteriorate under ~6% strain in tension mode. Analysis of the results indicates that surface chemistry and charge rather than electron exchange rate on the biological interfaces govern fibrinogen adsorption tendencies of Nitinol surfaces.
Summary: The results of this complex study indicate that bare Nitinol surfaces prepared appropriately do not release Ni or exert toxic effects. Their properties vary in a wide range providing various chemistry, topographies, surface charge and conductivity from metallic to ceramic-like that can be utilized for various medical applications.
