Nitinol is a very attractive material for biomedical applications due to its shape memory and superelasticity properties. However, its high nickel content makes it a potentially toxic material because of nickel biotoxicity. Among the numerous ways explored to tune Nitinol surface properties and improve its corrosion resistance, the formation of alkylphosphonic acids self-assembled monolayers (SAMs) is a versatile and attractive approach. Recently, electroassisted adsorption of surfactant molecules on active metals has been investigated. In this paper we compare the electroassisted formation (EG) of alkylphosphonic acids (n-dodecyl-and n-octadecylphosphonic acids) SAMs to the direct adsorption method (direct adsorption CG) on Nitinol surfaces initially submitted to a hydrothermal treatment. XPS, contact angle and polarization curves measurements show that electroassisted grafting gives nearly as good results as the ones obtained by the conventional immersion method, but in a much shorter time. There is no significant impact of grafting time. Grafting potential, on the contrary, appears to influence positively the layers' density and the substrate resistance to corrosion. Alkyl chain length effects on the resulting SAMs properties have also been discussed. Electroassisted grafting is thus confirmed to be a very promising method for the grafting of phosphonic acids derivatives on oxidized metallic surfaces.
Nitinol, a nickel and titanium alloy in quasi-equiatomic proportions, is used in numerous biomedical applications due to its remarkable mechanical characteristics, its resistance towards corrosion and good biocompatibility. Further research to reinforce osseointegrative abilities and barrier properties towards external aggressive agents are still needed. In this context, electrochemical methods can be used to generate protective and functional surface coatings on Nitinol substrates with a good level of control and versatility. Specifically here are considered the elaboration of a structured thin layer on Nitinol plates by electrodeposition of tantalum and its subsequent modification by the self-assembly of phosphonic acid derivatives to consolidate corrosion protection and bioactivity features. Electrochemical techniques are also used to assess and study the different deposited layers. Cyclic voltammetry, polarization curves, free potential measurements, and scanning electrochemical microscopy experiments are performed to qualitatively and quantitatively evaluate electron and mass transfers at the interface, as well as corrosion resistance properties. Spectroscopic (XPS) and microscopic (SEM) analyses complete the characterization process regarding chemical nature and morphology aspects.
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