Electrophoretic deposited graphene based functional coatings for biocompatibility improvement of Nitinol

Abstract

Shape memory effects and superelastic nature of Nitinol alloy has been exploited in various biomedical applications. However, prolonged usage of this implant material is restricted due to the toxic effect of released Ni-ions (Ni2+) into tissue environment. This reduces the biocompatibility of the material. The purpose of this study was therefore to assess the effect of graphene coatings on the biocompatibility of Nitinol wires used as dental braces. The graphene coating was prepared on Nitinol substrate through cathodic electrophoretic deposition (EPD). Isopropyl alcohol and magnesium nitrate hexahydrate (Mg(NO3)2•6H2O) were used as a dispersion medium and charging agent for the EPD process respectively. The surface morphology, structure, surface roughness and corrosion resistance behavior of as-deposited and annealed graphene coatings were investigated. As-deposited sample presents a rough surface with visible microcracks and fine pores, while annealed sample possesses a smooth surface with no visible microcracks and pores. The annealed sample showed better results than the as-deposited sample in terms of mechanical strength as observed from Nanoindentation test. Furthermore, Potentiodynamic polarization test was conducted in a simulated body fluid environment to evaluate the corrosion resistance behavior of bare Nitinol substrate and graphene coatings. From the test results, we have found that corrosion potential of a bare Nitinol substrate, as-deposited and annealed sample is around -510 mV, -375 mV and -261 mV respectively. This positive shift in corrosion potential highlighted the improvement in corrosion resistance property of Nitinol substrate by graphene coatings. This indicates the suitability of EPD coated graphene as an effective coating material for biomedical applications.