Microstructure and surface composition effects on the transpassivation of NiTi wires for implant purposes

Abstract

A influência da microestrutura e da composição da camada superficial na transpassivação de fios de NiTi utilizados em implantes endovasculares não tratados e tratados termicamente (500 ºC) foi estudada por métodos eletroquímicos (potencial de circuito aberto e curvas de polarização potenciodinâmica), difração de raios-X (DRX) e microscopia eletrônica de varredura (MEV). O potencial no qual o filme passivo rompe-se (E b ) depende da composição química da camada de óxidos que recobre o eletrodo, assim como da microestrutura do biomaterial. Para as amostras de NiTi não tratadas termicamente, a microestrutura é mais desordenada, porém altamente resistente à transpassivação (E b ~ 1,00 V/ECS). Entretanto, a presença de um filme nativo rico em oxigênio facilita a sua dissolução (E b ~ 0,00 V/ECS). Quando tratado termicamente, o biomaterial NiTi tornase estruturalmente mais ordenado, todavia menos resistente à dissolução (E b ~ 0,12 V/ECS), sendo que a presença do filme nativo e rico em oxigênio tem pouca influência no valor de E b (E b ~ 0,03 V/ ECS) neste caso.The microstructure and surface composition effects on the transpassivation behavior of untreated and heat treated at 500 ºC NiTi wires used for implant purposes were investigated by electrochemical techniques (open circuit potential and potentiodynamic polarization curves), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The potential at which the passive film breaks down (E b ) (transpassivation) was found to be strictly dependent on both the chemical composition of oxide covering layer and the microstructure of the biomaterial. It could be concluded that the disordered microstructure of untreated NiTi is highly resistant to transpassivation (E b ~ 1.00 V/SCE), but the presence of a native oxygen rich oxide film onto it makes the dissolution easier (E b ~ 0.00 V/SCE). Upon thermally treating, the microstructure becomes ordered and less resistant to dissolution (E b 0.12 V/SCE), but in this case the presence of an oxygen rich native film influenced E b in a lower extent (E b ~ 0.03 V/SCE).Keywords: NiTi alloy, electrochemical behavior, artificial saliva, microstructure and surface composition of NiTi alloy IntroductionNear equiatomic NiTi alloy has become ubiquitous among biomedical materials used as prosthesis due to its mechanical (shape memory and superelasticity) and corrosion resistance properties, and, most importantly, biocompatibility. An overview of the medical applications of NiTi alloys was recently given by Duerig et al. Nevertheless, a major concern on the dissolution properties of NiTi alloy still remains owing to the high nickel content, since nickel element has been shown to provoke several adverse biological effects, depending on the species and its concentration.2-7 Usually, NiTi electrodes present a superficial thin layer with TiO 2 as the main component, 1,8 protecting them from dissolution, as for pure Ti.9,10 The potential at which this passive film breaks down (E b ) is, however, sometimes very low for NiTi electrodes, lea...

In vivo human electrochemical properties of a NiTi‐based alloy (Nitinol) used for minimally invasive implants

“…Three simulated body fluids (AFNOR S90-701 artificial saliva, 19 artificial urine 32 and Ringer's solutions 32 ) were used as SBFs for in vitro tests. The solution pH was adjusted with phosphoric acid or sodium hydroxide when necessary.…”

“…These topics are currently the focus of many research groups. [15][16][17][18] According to previous investigations, 10,12,19,20 the lack of corrosion resistance is due to heat treatments imposed to Nitinol wires, and absolutely necessary not only to improve the mechanical properties of Nitinol, but also for the fabrication of stents. After heating, a porous oxide layer extremely susceptible to pitting corrosion is formed onto the NiTi surface.…”

Critical overview of Nitinol surfaces and their modifications for medical applications

“…An interesting development is that X-ray photoelectron spectroscopy (XPS) and Auger spectroscopy have become routine methods, and most of the recently published studies on Nitinol have been done with an impressive list of techniques, including even energy dispersive X-ray spectroscopy (EDS) [3,[9][10][11], unsuitable for surface analysis. Despite a great number of new publications, some of which include detailed XPS surface studies, their analyses are very difficult especially when relevant to biological responses.…”

Effect of Thermocycling on Nickel Release from Orthodontic Arch Wires: an In Vitro Study

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