Electropolished NiTi Research Articles

High precision electrolytic polishing of Ni–Ti shape memory alloy for biomedical vascular stents

There is a continuous demand for multi-disciplinary design approaches for the development of new implant materials. A potential biomaterial that is suitable for vascular stents and catheters is Ni–Ti alloy (Nitinol), which exhibits excellent biocompatibility, superelasticity, and shape memory effects. Corrosion resistance of Ni–Ti vascular stents in body fluids is significantly dependent on its surface finishing characteristics. In the study, as-drawn Ni–Ti wires were electropolished from a methanolic sulfuric acid electrolyte. The effect of various electropolishing (EP) parameters, i.e., current density, electropolishing time, and electrode gap, on the surface properties of Ni–Ti wires were examined. The surface morphology and topography of the Ni–Ti wires were characterized via scanning electron microscopy (SEM) and atomic force microscopy (AFM). The root mean square (Ra) surface roughness of electropolished Ni–Ti wires was also measured using AFM. The results indicated that the surface finishing of Ni–Ti alloy exhibited maximum sensitivity to electropolishing current density. An optimum combination of surface morphology and roughness was obtained at a parameter set with current density of approximately 0.5 A/cm2, polishing time of 10 s, and electrode gap of 1.0 cm.

The influence of Desulfovibrio desulfuricans bacteria on a Ni-Ti alloy: electrochemical behavior and surface analysis

The effect of the sulfate-reducing bacteria (SRB, Desulfovibrio desulfuricans) on the surface properties and corrosion resistance of an electropolished NiTi alloy was assessed. NiTi samples were exposed to abiotic and biotic media (Postgate medium+artificial saliva) for different exposure periods (24, 168 and 720h).Electrochemical impedance spectroscopy (EIS) measurements were carried out in physiological Ringer’s solution to assess the effect of biofilms on the electrochemical properties of the NiTi alloy.Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) were employed to analyze the properties and chemical compositions of the surface that was modified with biofilm.Evident influence of SRB on the surface of the NiTi alloy was observed, and the most intense effect was observed after 168h of bioexposure. Biofilm-covered NiTi alloy was characterized by reduced resistance (Ri) and higher capacitance (Ci) in the barrier region of the oxide. However, some restoration of the protective properties of the passive film was observed in samples that had been exposed to bacteria for 720h. XPS studies showed that no significant changes from the surface chemistry of the abiotic NiTi samples were induced by biofilm formation.

Interface of Nanoparticle-Coated Electropolished Stents

Nanostructures entail a high potential for improving implant surfaces, for instance, in stent applications. The electrophoretic deposition of laser-generated colloidal nanoparticles is an appropriate tool for creating large-area nanostructures on surfaces. Until now, the bonding and characteristics of the interface between deposited nanoparticles and the substrate surface has not been known. It is investigated using X-ray photoelectron spectroscopy, Auger electron spectroscopy, and transmission electron microscopy to characterize an electropolished NiTi stent surface coated by laser-generated Au and Ti nanoparticles. The deposition of elemental Au and Ti nanoparticles is observed on the total 3D surface. Ti-coated samples are composed of Ti oxide and Ti carbide because of nanoparticle fabrication and the coating process carried out in 2-propanol. The interface between nanoparticles and the electropolished surface consists of a smooth, monotone elemental depth profile. The interface depth is higher for the Ti nanoparticle coating than for the Au nanoparticle coating. This smooth depth gradient of Ti across the coating-substrate intersection and the thicker interface layer indicate the hard bonding of Ti-based nanoparticles on the surface. Accordingly, electron microscopy reveals nanoparticles adsorbed on the surface without any sorption-blocking intermediate layer. The physicomechanical stability of the bond may benefit from such smooth depth gradients and direct, ligand-free contact. This would potentially increase the coating stability during stent application.

Cytocompatibility and haemocompatibility of Zr, ZrC and ZrCN films

The cytocompatibility and haemocompatibility of three zirconium based films, i.e. Zr, ZrC and ZrCN, deposited on NiTi shape memory alloy by magnetron sputtering are investigated and compared to those of electropolished NiTi shape memory alloy. The Zr(C,N) series films have deteriorated wettability but have positive effects on the blood compatibility and cytocompatibility of NiTi. Better haemolysis resistance and thromboresistant properties are observed. There are more living cells on the Zr(C,N) series films, and the cells show a higher relative growth rate value than those on the electropolished NiTi. The Zr(C,N) series films act as barrier layers and promote the proliferation of fibroblasts by blocking the leaching of toxic nickel ions from NiTi.

Surface Nanomechanical Behavior of ZrN and ZrCN Films Deposited on NiTi Shape Memory Alloy by Magnetron Sputtering

Surface nanomechanical behavior under nanoindentation of ZrN and ZrCN film on NiTi substrate was studied. The surface hardness and modulus of the films increase initially with larger nanoindentation depths and then reach their maximum values. Afterwards, they diminish gradually and finally reaching plateau values which are the composite modulus and composite hardness derived from the ZrN/ZrCN film and NiTi substrate. They are higher than those of electropolished NiTi SMA due to the properties of ZrN and ZrCN. In comparison, the surface nanomechanical properties of electropolished NiTi exhibit a different change with depths.

Fabrication, properties, and cytocompatibility of ZrC film on electropolished NiTi shape memory alloy

The microstructural characteristics, mechanical properties, and cytocompatibility of ZrC films deposited on electropolished NiTi shape memory alloy (SMA) by magnetron sputtering are investigated by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), nanoindentation, and MTT test. The deposition rate of the ZrC film is smaller than the pure Zr film. Although ZrC is the only phase in the film, the pure ZrC film with small oxygen content is non-stoichiometric. The hardness and modulus increase initially with larger nanoindentation depths, reach maximum values, and then gradually decrease afterwards as a result of the composite effects of the ZrC film and NiTi substrate. Deposition of the ZrC film promotes proliferation of fibroblasts revealing enhanced cytocompatibility compared to uncoated NiTi.

Surface chemistry and topographical changes of an electropolished NiTi shape memory alloy

AbstractElectropolishing and its mechanism for NiTi shape memory alloy in methanolic sulfuric acid have been investigated recently. In the current study, X‐ray photoelectron spectroscopy was carried out to characterize the chemical nature of the electropolished surface. The electropolished surface is covered by a thin layer of TiO2. For an electropolished specimen, the Ni:Ti ratio at surface (∼1 nm) was 0.31, whereas for a mechanically polished specimen the ratio was 0.70. The second aspect of this work is the attempt to characterize the salt film formed during electropolishing. The characterization of salt‐film showed sulfide and sulfate compounds of Ni and/or Ti. The crystallographic and topographical changes caused due to thermally induced phase transformation on an electropolished NiTi was followed by in situ XRD and AFM. AFM studies revealed significant changes in the surface features due to phase transformation. The surface roughness value increased by a factor of 2.

Microstructure, mechanical properties, and blood compatibility of zirconium nitride deposited on nickel–titanium shape memory alloy

ZrN is deposited on electropolished NiTi shape memory alloy to enhance the surface properties. The microstructure, mechanical properties, and blood compatibility are evaluated by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), nanoindentation, hemolysis analysis, and blood platelet adhesion test. The ZrN film has a fine fibrous structure composed of the desirable stable ZrN phase together with a small amount of a second ZrO 2 phase. The hardness and modulus values increase initially with larger nanoindentation depths and after reaching maximum values, gradually decrease finally attaining stable values of about 3.4 GPa and 68 GPa, respectively which reflect the composite hardness and modulus values of the system. Our study reveals that the ZrN film may degrade the surface wettability, but the hemolysis resistance and thromboresistance can be improved thereby making the surface treated materials more suitable in biomedical implants.

Surface structure and biomedical properties of chemically polished and electropolished NiTi shape memory alloys

The surface structure and biomedical properties of NiTi shape memory alloy (SMA) samples after undergoing electropolishing and chemical polishing are determined and compared employing scanning electron microscopy, X-ray photoelectron spectroscopy, inductively-coupled plasma mass spectrometry, hemolysis analysis, blood platelet adhesion test, and MTT test. The results indicate that after chemical polishing, there is still a high Ni concentration on the surface of the NiTi SMA. On the other hand, electropolishing can form a thin surface titanium oxide film (about 10 nm thickness) with depleted Ni. In addition to the TiO 2 phase, some titanium suboxides (TiO and Ti 2O 3) are found in the surface film. Compared to chemical polishing, electropolishing can more effectively mitigate out-diffusion of Ni ions and the wettability, blood compatibility, and thromboresistance are also better. However, no difference on the cytocompatibility can be observed from samples that have been chemically polished or electropolished.

Human gingival fibroblast response to electropolished NiTi surfaces

In the present study the in vitro biocompatibility of electropolished NiTi sheets is investigated. The assessment of cytotoxic effects due to potential Ni leaching from metal sheets was performed in direct contact with primary human fibroblast cultures using the 5-bromo-2'-deoxyuridine cell proliferation assay and morphologic studies via light microscopy and scanning electron microscopy. To assess toxic effects related to Ni-ions release, cells cultured in the presence of increasing concentrations of Ni(2+) (NiSO(4).6H(2)O) served as positive controls. It is shown that while the addition of NiSO(4) caused severe proliferation decrease (approximately 80%) and morphologic damage at a concentration of 50 mg/L Ni(2+) no negative effects were observed in fibroblasts cultured in the presence of electropolished NiTi sheets. The results are discussed in terms of surface topography effects on the biocompatibility of NiTi shape memory alloys.