Electroassisted Assembly of Alkylphosphonic Acids Monolayers on Nitinol

Nitinol alloys have been widely studied as biomaterials for medical implants since they have very interesting mechanical properties, such as shape memory effect and superelasticity as well as a relatively good biocompatibility ¹. The biocompatibility of Nitinol implants obviously depends on their surface properties, their corrosion resistance but also on the release of Ni ions after implantation, this chemical element being known to be allergenic and toxic, though essential for the human body. This problematic being of crucial importance for Nitinol biomedical applications, numerous works have been published on the study of Ni release from Nitinol ^2-4 and its corrosion resistance compared to other commonly used alloys ^5-11. 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 versatile and attractive approach.

SAMs are typically formed using a passive adsorption method by exposing the surface either to a surfactant solution or its vapors for relatively long periods of time. Recently, electroassisted adsorption of surfactant molecules started to be investigated. With this approach, high-quality organothiol SAMs have been generated in much shorter periods of time on gold ^12-14 as well as on oxidizable metals and alloys such as nickel ¹⁵, carbon steel ¹⁶ and copper ^17,18. More recently, Metoki et al. have investigated the electroassisted formation of organophosphonic SAMs on titanium alloy surfaces ¹⁹. They showed that this grafting method significantly reduces the time formation of the SAMs. This is attributed to the combined effects of oriented amphiphiles in the vicinity of the electrode, increased surface energy and underpotential oxide structure.

In the present work, we investigate and compare the passive (adsorption) and active (electroassisted) grafting of alkylphosphonic acids SAMs on mechanically polished Nitinol surfaces. The effects of the applied potential and modification time on the monolayers and their properties are assessed by X-ray photoelectron spectroscopy, polarization modulation infrared reflection absorption spectroscopy, contact angle measurements, cyclic voltammetry and polarization curves measurements. It is shown that electroassisted method leads to a strong increase of the grafting kinetics compared to the passive method while preserving a low Ni surface concentration which is a key parameter for biomedical applications of Nitinol. Increasing the applied potential appears to improve the quality of the obtained SAMs while time has little influence, especially for grafting at low potential values.

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