Surface-Initiated ATRP of (hydroxyethyl)Methacrylate on Nitinol Modified By in Situ Generated Diazonium from Its Nitro Precursor

Nitinol (NiTi) is a shape memory alloy that has become a material of great interest given its many applications especially in the biomedical field ¹. However, one of the major problems of this alloy is its high nickel content (often more than 50%) and the cytotoxic nature of nickel(II) ². Its biocompatibility properties result from the presence of a thin protective TiO₂ surface layer which enhances corrosion resistance and prevents Ni(II) release ¹. Therefore, in recent years, a great deal of attention has been paid to the surface modification of NiTi to impart and/or improve surface properties such as biocompatibility ¹, bioactivity ³ or corrosion resistance ⁴.

Since its simultaneous discovery by K. Matyjaszewski ⁵ and by M. Sawamoto ⁶, the atom transfer radical polymerisation (ATRP) is one of the most used method for the polymerization of polymer with controlled length and composition. The ATRP mechanism is briefly described on fig. 1 (right part). this technique is effective for the polymerisation of hydrophilic monomer in aqueous media and soft experimental conditions ⁷. The use of such conditions is quite valuable economically and environmentally in the current framework of the sustainable development but also for a final use in a biomedical application. Polymer obtained by ATRP are used notably for cardiovascular application ⁸ and for orthopaedic ⁹ application, two of the main application of the NiTi ¹. Curiously, there is only one publication covering ATRP polymerisation and NiTi ⁸.

Moreover, a particular domain of ATRP is surface initiated ATRP (SI-ATRP) where the initiator is chemically grafted to the surfaces and allows a covalent grafting between the surface and the polymer as well as the formation of densely packed polymer brushes ⁷. In that context, diazonium salts are of interest to develop a layer of initiator covalently grafted to the surface for SI-ATRP.

Since the beginning of the past decade, a one-step procedure including in situ generation of diazoniums from their amine precursor and their subsequent grafting in the same solution is actively investigated ¹⁰. It is an economical way to synthesize a diazonium salt from its amine precursor. More recently, the in situ generation of the diazonium salt has gone further thanks to Cougnon et al ¹¹ with the use of a nitro precursor reduce at the electrode into the amine precursor which then allow the diazotation reaction to form the diazonium salt (as illustrated on fig. 1 left part). This diazotation reaction is known to follow a 2 equivalent (eq.) acid/aniline ratio mechanism^12,13. Yet, in practice, in situ generated diazonium grafting is generally described in the literature with a large excess of acid (at least 8 eq.)¹⁴.

However, these harsh conditions can be detrimental in the frame of active metals surface modification. In particular, this might result in the alteration and/or dissolution of the protective thin oxides layer of Nitinol thereby reducing its biocompatibility and corrosion resistance properties. Recently, the use of gentle conditions for the in situ generation of diazonium have been reported ^12,13,15.

The present work is an investigation of the electrografting of in situ generated 2-bromoisobutyrate p-nitrophenyldiazonium salt (PD-Br) after the reduction of this nitro precursor (A-NO₂Br) into amine on NiTi followed by SI-ATRP polymerization of (hydroxyethyl)methacrylate (HEMA) (Fig. 1).

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