No abstract

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.

Over the last few years, Nitinol (NiTi) has become one of the most attractive alloy materials for industrial applications. However, its implementation is still problematic due to its surface nickel content, making it sensitive to pitting corrosion. In applications, it is often necessary to modify NiTi surfaces by using organic coatings, which provides new physico-chemical properties as well as functionalities and often contributes to a reinforcement of the alloy corrosion resistance. In this work, we assess the differences between the molecular layers made out of methylphosphonic acid (C1P) and the bi-phosphonic acid derivatives: (methylimino)dimethylene-bisphophonic acid (MIP2) and 1-hydroxyethylidene-1,1-diphosphonic acid (HEP2) using conventional (CG) and electro-assisted (EG) graftings. The surface modifications with the bi-phosphonic derivatives (MIP2) and (HEP2) carried out with the EG process lead to denser layers and a reinforced NiTi corrosion resistance.