Modification of a Nitinol Surface by Phosphonate Self-Assembled Monolayers

Abstract

Nitinol, as a shape memory alloy, is attractive material for medical implants and devices. Contrary to titanium, corrosion by releasing Ni 2 ions occurs during a long-term contact of Nitinol with (physiological) solutions containing Cl- ions. In order to develop chemically/electrochemically stable surfaces and interfaces, Nitinol was modified by dodecylphosphonate (-OH and -CH3 terminated) self-assembled monolayers (SAMs), which were characterized by XPS, PM-IRRAS and contact angle measurements. Strongly bounded well-ordered films of high homogeneity and resistance were synthesized. An innovative method that allows in situ study of influence of thermal annealing, following SAM formation, on their protecting properties in simulated (physiological) solutions is presented. Changes of structural sensitive impedance parameters were correlated with the changes in the interfacial layer. Effective thermal annealing greatly enhances the quality of the self-assembled alkyl-phosphonate films, which behave as non-ideal dielectrics, i.e., the solidliquid interfaces formed represent the blocking contact preventing charge-transfer reactions. © 2011 The Electrochemical Society.