Nitinol Surface Research Articles

Corrosion resistance, chemistry, and mechanical aspects of Nitinol surfaces formed in hydrogen peroxide solutions

Ti oxides formed naturally on Nitinol surfaces are only a few nanometers thick. To increase their thickness, heat treatments are explored. The resulting surfaces exhibit poor resistance to pitting corrosion. As an alternative approach to accelerate surface oxidation and grow thicker oxides, the exposure of Nitinol to strong oxidizing H(2)O(2) aqueous solutions (3 and 30%) for various periods of time was used. Using X-Ray Photoelectron Spectroscopy (XPS) and Auger spectroscopy, it was found that the surface layers with variable Ti (6-15 at %) and Ni (5-13 at %) contents and the thickness up to 100 nm without Ni-enriched interfaces could be formed. The response of the surface oxides to stress in superelastic regime of deformations depended on oxide thickness. In the corrosion studies performed in both strained and strain-free states using potentiodynamic and potentiostatic polarizations, the surfaces treated in H(2)O(2) showed no pitting in corrosive solution that was assigned to higher chemical homogeneity of the surfaces free of secondary phases and inclusions that assist better biocompatibility of Nitinol medical devices.

Influence of sodium hypochlorite treatment of electropolished and magnetoelectropolished nitinol surfaces on adhesion and proliferation of MC3T3 pre-osteoblast cells

The influence of 6 % sodium hypochlorite (NaClO) treatment on adhesion and proliferation of MC3T3 pre-osteoblast cells seeded on electropolished (EP) and magnetoelectropolished (MEP) nitinol surfaces were investigated. The chemistry, topography, roughness, surface energy, wettability of EP and MEP nitinol surfaces before and after NaClO treatment were studied with X-ray photoelectron spectroscopy (XPS), profilometry, and contact angle meter. In vitro interaction of osteoblast cell and NaClO treated EP and MEP nitinol surfaces were assessed after 3 days of incubation by scanning electron microscopy. The XPS analysis shows that NaClO treatment increases oxygen content especially in subsurface oxide layer of EP and MEP nitinol. The changes of both basic components of nitinol, namely nickel and titanium in oxide layer, were negligible. The NaClO treatment did not influence physico-morphological surface properties of EP and MEP nitinol to a big extent. The osteoblast cells show remarkable adherence and proliferation improvement on NaClO treated EP and MEP nitinol surfaces. After 3 days of incubation they show almost total confluence on both NaClO treated surfaces. The present study shows that NaClO treatment of EP and MEP nitinol surfaces alters oxide layer by enriching it in oxygen and by this improves bone cell-nitinol interaction.

Nanocoating of Diazoresin/Polysaccharides Multilayer Boosts Biocompatibility of Nitinol Biomedical Devices

The surface of a biomaterial and a biological environment are key factors determining the biocompatibility of the material. Determining biocompatibility of medical implants is a critical step toward approval and clinical application. In this study, the biocompatibility of a covalently attached nanocoating of alginate(Alg)/diazoresin(DR)/heparin(Hep) multilayer has been evaluated by thrombin inactivation, protein adsorption, leukocyte adhesion, cytotoxicity in vitro and subcutaneous implantation in vivo. It was found that the outermost layer of the coating played a key role on anticoagulant activity. No significant difference was seen in albumin adsorption between uncoated and coated one. However, the fibrinogen adsorption on DR/polysaccharides coatings was lower than the uncoated Nitinol. Compared with the uncoated Nitinol surface, DR/polysaccharides multilayer coating presented a drastically reduced adhesion in vitro of polymorphonuclear neutrophil leukocytes (PMN) and peripheral blood mononuclear cells (PBMC). Cell biocompatibility tests indicated that the nanocoating of DR/polysaccharides multilayer did not lead to an increase in the cytotoxicity of the Nitinol and the adherent cells maintained good viability. Subcutaneous implantation in C57BL/6 mice suggested that the coated Nitinol had good tissue biocompatibility. The use of alginate interlayer for heparin immobilization instead of proteins such as albumin, collagen, and gelatin may avoid the risk of contamination with pathogenic agents. Hence, the nanocoating of covalently attached DR/polysaccharide multilayer boosts the biocompatibility of Nitinol biomedical devices. Keywords: heparin, thrombin, Nitinol, alginate, biocompatibility, Layer-by-layer, Nitinol Biomedical Devices, medical implants, Diazoresin

Modification of a Nitinol Surface by Phosphonate Self-Assembled Monolayers

Nitinol, as a shape memory alloy, is attractive material for medical implants and devices. Contrary to titanium, corrosion by releasing Ni2+ ions occurs during a long-term contact of Nitinol with (physiological) solutions containing Cl− ions. In order to develop chemically/electrochemically stable surfaces and interfaces, Nitinol was modified by dodecylphosphonate (−OH and −CH3 terminated) self-assembled monolayers (SAMs), which were characterized by XPS, PM-IRRAS and contact angle measurements. Strongly bounded well-ordered films of high homogeneity and resistance were synthesized. An innovative method that allows in situ study of influence of thermal annealing, following SAM formation, on their protecting properties in simulated (physiological) solutions is presented. Changes of structural sensitive impedance parameters were correlated with the changes in the interfacial layer. Effective thermal annealing greatly enhances the quality of the self-assembled alkyl-phosphonate films, which behave as non-ideal dielectrics, i.e., the solid|liquid interfaces formed represent the blocking contact preventing charge-transfer reactions.

Functionalization of Nitinol surface toward a versatile platform for post-grafting chemical reactions

Nitinol is a material of great interest given its many possible applications, especially in the biomedical field. However, due to its high nickel content and the cytotoxic nature of nickel (II), a great attention has been paid to the improvement of Nitinol corrosion resistance and the limitation of Ni 2+ ions release but also to the improvement of the material–biological environment interaction. In this work, we report on the modification of Nitinol by electrografting of 1,4-carboxybenzene diazonium and by formation of a 11-phosphoundecanoic acid monolayer as well as on the assessment of the efficiency of these coatings to act as a platform for post-grafting chemical reactions. The electrografting of 1,4-carboxybenzene diazonium leads to the formation of a non-compact hydrophilic multilayer with some solvent molecules and electrolyte cations entrapped in it and has a negative impact on the Nitinol corrosion resistance properties. The grafting of 11-phosphoundecanoic acid on Nitinol leads to the formation of a monolayer showing less pronounced hydrophilic properties than the electrografted carboxybenzene layer does and a less negative impact on the Nitinol corrosion resistance properties. The post-grafting esterification reaction has been shown to be more efficient when carried out on an 11-phosphoundecanoic acid monolayer than on the carboxybenzene electrografted layer.

Cytotoxicity of Ni from Surface-Treated Porous Nitinol (PNT) on Osteoblast Cells

The leaching of nickel from the surface of porous Nitinol (PNT) is mainly dependent on its surface characteristics, which can be controlled by appropriate surface treatments. In this investigation, PNT was subjected to two surface treatments, namely, water-boiling and dry-heating passivations. Phosphate buffer saline (PBS) solutions obtained from cyclic potentiodynamic polarization tests on PNT were employed to assess the cytotoxicity of Ni contained therein on osteoblast cells by Sulforhodamine B (SRB) assay. In addition, similar concentrations of Ni were added exogenously to cell culture media to determine cytotoxic effects on osteoblast cells. The morphologies of the untreated and the surface-treated PNTs were examined using SEM and AFM. Furthermore, growth of human osteoblast cells was observed on the PNT surfaces.

Polymeric coating of surface modified nitinol stent with POSS-nanocomposite polymer

Stent angioplasty is a successful treatment for arterial occlusion, particularly in coronary artery disease. The clinical communities were enthusiastic about the use of drug-eluting stents; however, these stents have a tendency to be a contributory factor towards late stage thrombosis, leading to mortality in a significant number of patients per year. This work presents an innovative approach in self-expanding coronary stents preparation. We developed a new nanocomposite polymer based on polyhedral oligomeric silsesquioxanes (POSS) and poly(carbonate-urea)urethane (PCU), which is an antithrombogenic and a non-biodegradable polymer with in situ endothelialization properties. The aim of this work is to coat a NiTi stent alloy with POSS-PCU. In prolonged applications in the human body, the corrosion of the NiTi alloy can result in the release of deleterious ions which leads to unwanted biological reactions. Coating the nitinol (NiTi) surface with POSS-PCU can enhance surface resistance and improve biocompatibility. Electrohydrodynamic spraying was used as the polymer deposition process and thus a few experiments were carried out to compare this process with casting. Prior to deposition the NiTi has been surface modified. The peel strength of the deposit was studied before and after degradation of the coating. It is shown that the surface modification enhances the peel strength by 300%. It is also indicated how the adhesion strength of the POSS-PCU coating changes post-exposure to physiological solutions comprised of hydrolytic, oxidative, peroxidative and biological media. This part of the study shows that the modified NiTi presents far greater resistance to decay in peel strength compared to the non-modified NiTi.

Surface Modification of Nitinol by Biocompatible Passive Films

Nitinol, near-equiatomic nickel-titanium alloy, exhibits unique properties, shape memory effect and superelasticity, and is therefore of a huge importance in biochemical engineering for medical devices. The high corrosion resistivity and biocompatibility are required in application of Nitinol as implant material for cardiovascular stents and orthodontic wires. Thus, a main goal of this work was designing and optimizing corrosion resistant and biocompatible coatings on Nitinol. Electrochemical methods were used for the functionalization of the Nitinol surface with anodically formed oxide films in acetic acid and investigation of their barrier and semiconducting properties.

Synergistic Effect of Cyclic Mechanical Strain and Activated Macrophage Cells on Nitinol Corrosion

Several previous studies have documented that self-expanding (SE) Nitinol stents implanted in highly mobile peripheral arteries exhibit a fracture rate as high as 30%. The hypothesis of this study is that repetitive mechanical stresses interact synergistically with the atherosclerotic lesion environment to induce corrosive events that ultimately lead to nitinol stent material failure. Using a novel in vitro chamber, electro-polished nitinol coupons were subjected to cyclic defined stress regimens in the presence or absence of adherent acetylated lipoprotein-activated macrophage-like (THP-1) cells over a 5 day period. Nitinol surface corrosion was monitored in real time using open circuit potential (OCP) measurements. Changes in surface chemistry and topography were measured post-stress exposure using X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The combination of defined bending in the presence of adherent activated macrophages resulted in a significant increase in nitinol surface corrosion as compared to nitinol subjected to either treatment or media exposure alone. Further, a deterioration of the protective oxide layer and an increase in the surface roughness were observed in nitinol specimens exposed to the combined influence of surface-adherent macrophages and repetitive loading. These results demonstrate a significant interaction between the biological and mechanical environment relative to nitinol corrosion potential and provide a basis for design of future studies to improve the long-term performance of peripheral nitinol stents.

Influence of diamond‐like carbon coating on the corrosion resistance of the NITINOL shape memory alloy

AbstractThe aim of this study is to modify the surface of Nitinol (Nickel Titanium Naval Ordnance Laboratory) alloy by covering with diamond‐like carbon to improve the corrosion resistance and biocompatibility, and to prevent the toxic nickel ion release inside the human body. This alloy has unique properties, including shape memory effect and super elasticity, which make it an attractive candidate for biomedical applications. Diamond‐like carbon (DLC) films were deposited by d.c. magnetron sputtering using a graphite target and argon as the discharge gas. The electrochemical characteristics of the DLC coatings were investigated by electrochemical techniques (potentiodynamic polarization test and electrochemical impedance spectroscopy) at 37 °C in Fetal Bovine Serum (FBS). Surface analysis of the DLC coatings was conducted by means of X‐Ray Diffraction (XRD), SEM and Vickers microhardness, and the thickness of DLC layer was measured using interferometry technique. The metallic ions release in bulk solution was studied by inductive coupling plasma‐mass spectrometry (ICP‐MS). In the case of uncoated samples, the release of metallic ions in bulk solution was much higher than in the case of DLC coated samples. Also, the corrosion resistance and Vickers microhardness of DLC‐coated samples were improved in comparison with the uncoated samples. Copyright © 2010 John Wiley & Sons, Ltd.

Improved biocompatibility of thrombo-resistant iron-polysaccharides multilayer coatings on Nitinols

Biocompatibility of two multilayer coatings of (Fe 3+/Hep) 10 and (Fe 3+/DS/Fe 3+/Hep) 5 was comparatively analyzed with respect to protein adsorption, leukocyte adhesion and cell–material interaction. Both of them showed significantly high albumin-to-fibrinogen adsorption ratio, suggesting good biocompatibility. Furthermore, the (Fe 3+/DS/Fe 3+/Hep) 5 coating was found to exhibit the lowest non-specific protein adsorption due to the incorporation of dextran sulfate. Compared with uncoated Nitinol surfaces, iron-polysaccharide multilayer coating presented no deformation of leukocytes, indicating no signs of inflammatory reactions. Cell growth, cell adhesion and cell metabolic activity were all in good condition, verifying both (Fe 3+/Hep) 10 and (Fe 3+/DS/Fe 3+/Hep) 5 coatings had good cytocompatibility. Therefore, iron-polysaccharides multilayer coatings had greatly improved the biocompatibility of Nitinols.

Nitinol Surfaces for Implantation

Nitinol, a group of nearly equiatomic Ni-Ti alloys, steadily conquers new areas of application. Because of the need to keep a low profile of miniature implant devices, and considering the lack of compatibility between Nitinol superelasticity and the mechanical properties of traditional coatings, bare surfaces are of interest. In this article, an overview of our studies of bare Nitinol surfaces is presented, and the performance of coated surfaces is outlined. Together dense and porous Nitinol offer a wide array of surface topographies, suitable for attachment and migration of biological cells and tissue ingrowth. Native Nitinol surface oxides vary from amorphous to crystalline and exhibit semiconducting properties associated with better blood compatibility. Nitinol surfaces are analyzed with regard to high and lasting nickel release in vitro. Surface oxide thickness and Nitinol intermetallic particulates are discussed in relation to corrosion resistance and mechanical performance of the material.

FUNCTIONALITY OF BARE NITINOL SURFACES

The results of pilot in situ studies of the responses of Nitinol surfaces to deformation are presented. It is shown that the mechanical behavior of Nitinol surfaces differs, depending on oxide thickness and its chemical composition. The corrosion resistance of the surfaces evaluated in strain-free and strained states using potentiodynamic and potentiostatic cyclic polarization at the body potentials demonstrated quite stable behavior.

Effect of Nitinol surface treatments on its physico-chemical properties

The main purpose of this work is the study of different physicochemical treatments on Nitinol slabs and wires, with the aim of inducing the formation of a TiO(2) surface film capable of increasing the corrosion resistance of the material and of reducing the release of Ni when the Nitinol samples were immersed in simulated body fluid (SBF). To this end, a battery of measurements (surface roughness, contact angle, electrochemical corrosion, chemical analysis as a function of depth, and Ni release to SBF) has been used to characterize Nitinol commercial samples, as received, and also after the different treatments performed. The results clearly indicate the effectiveness of the passivation TiO(2) layer as a barrier against Ni leaching, and the detrimental effects of any processes (such as polishing or cutting) that result in exposure of areas not coated by the TiO(2) film. Chemical methods such as oxidation in nitric acid or hydrothermal treatment of the samples (by prolonged immersion in boiling water) seem to provide simple and efficient ways of forming TiO(2) films of adequate thickness on the Nitinol surface.

Investigation of surface endothelialization on biomedical nitinol (NiTi) alloy: Effects of surface micropatterning combined with plasma nanocoatings

Plasma nanocoated films with trimethylsilane-oxygen monomers showed outstanding biocompatibility in our previous studies. In this study, endothelialization on biomedical nitinol alloy surfaces was systematically investigated. Our study focuses on elucidating the effects of surface micropatternings with micropores and microgrooves combined with plasma nanocoating. Plasma nanocoatings with controlled thickness between 40 and 50 nm were deposited onto micropatterned nitinol surface in a direct current plasma reactor. Bovine aortic endothelial cells were cultured in vitro on these nitinol samples for 1, 3 and 5 days. It was found that rougher surfaces could enhance cell adhesion compared with the smoother surfaces; the surfaces patterned with micropores showed much more endothelialization than microgrooved surface after a 3 days culture. The cell culture results also showed that plasma nanocoatings significantly further increased cell proliferation and cell adhesion on the micropatterned nitinol surfaces, as compared with non-plasma nanocoated surface of nitinol samples. The surface micropatternings combined with plasma nanocoatings could improve the cell adhesion and accelerate surface endothelialization after implantation of intravascular stents, which is expected to reduce in-stent restenosis.

The electrochemical characteristics of native Nitinol surfaces

The present study explored the avenues for the improvement of native Nitinol surfaces for implantation obtained using traditional procedures such as mechanical polishing, chemical etching, electropolishing and heat treatments for a better understanding of their electrochemical behavior and associated surface stability, conductivity, reactivity and biological responses. The corrosion resistance (cyclic potential polarization, open circuit potential and polarization resistance) of Nitinol disc and wire samples were evaluated for various surface states in strain-free and strained wire conditions. The surface response to tension strain was studied in situ. Surface chemistry and structure were explored using XPS and Auger spectroscopy and photoelectrochemical methods, respectively. It was found that the polarization resistance of the Nitinol surfaces varied in a range from 100 kΩ to 10 MΩ cm 2 and the open circuit potentials from −440 mV to −55 mV. The surfaces prepared in chemical solutions showed consistent corrosion resistance in strain-free and strained states, but mechanically polished and heat treated samples were prone to pitting. Nitinol surface oxides are semiconductors with the band gaps of either 3.0 eV (rutile) or 3.4 eV (amorphous). The conductivity of semiconducting Nitinol surfaces relevant to their biological performances is discussed in terms of oxide stoichiometry and variable Ni content. Such biological characteristics of Nitinol surfaces as Ni release, fibrinogen adsorption and platelets behavior are re-examined based on the analysis of the results of the present study.

Bioactivity and Platelet Adhesion Study of a Human Thrombomodulin-Immobilized Nitinol Surface

Nitinol is a newly developed biomaterial that is gaining popularity in many biomedical applications. It has been reported that nitinol would not induce an inflammatory response and repulsion by the immunization after implantation in the human body. Besides, nitinol is a kind of shape memory alloy, which can memorize shapes at different temperatures. This can improve the convenience in surgery. However, nitinol has poor blood compatibility, so that further modification was needed to improve the antithrombogenicity. Human thrombomodulin (hTM), an endothelial-cell-associated glycoprotein, can be considered as a natural potent anticoagulant by converting thrombin from a procoagulant protease to an anticoagulant. In this study, the surface of nitinol was pre-activated by utilizing silanization with amino-terminated silane. The incorporated amino groups were available for the subsequent covalent immobilization of hTM by 2,4,6-trichloro-1,3,5-triazine (TCT), the coupling reagent. The surface density of immobilized hTM was determined by the Bradford method. The bioactivity of immobilized hTM and blood compatibility of various nitinol substrates were evaluated by the protein C activation assay and platelet adhesion test. It was observed that the immobilized hTM still had the ability to enhance protein C activation, though its activity was lower than the free hTM in solution. Furthermore, the platelet adhesion test showed that only a few platelets were adhered on the hTM-immobilized nitinol substrate. Therefore, the immobilization of thrombomodulin onto nitinol substrate could improve the blood compatibility of nitinol and might have the potential of application in antithrombogenic medical applications.

Surface Modifications of Nitinol

Nitinol (an acronym for the Nickel-Titanium Naval Ordnance Laboratory) has been extensively explored as an implant material for the medical industry. The potential problem with Nitinol implant devices is the release of Ni in the human body, which has stimulated a great deal of research on surface modifications and the application of coatings. This paper presents a comprehensive review of various treatments to modify the surface of Nitinol in an effort to inhibit Ni release and to render improved biocompatibility. We discuss the important in-service properties of Nitinol, such as biocompatibility, corrosion resistance, stability, uniformity, and the nature of passivating oxides produced by passivation, electropolishing, magnetoelectropolishing, ion beam implantation, sterilization, and artificial coatings.

Nitinol surface finishing by magnetoelectropolishing

A new advanced technology is proposed to obtain bare metal surfaces for peripheral stents, endodontic files and another medical devices. The effect of an externally applied magnetic field during electropolishing (magnetoelectropolishing) of Nitinol on its surface properties was investigated. The results are compared with standard electropolished Nitinol in the same electrolyte under the same process conditions. To evaluate surface properties, the following techniques were used: SEM and EDX, AFM, contact angle measurement, Auger electron spectroscopy, electrochemical impedance spectroscopy and fatigue resistance testing. All of the above analyses showed significant differences between magnetoelectropolished and conventional electropolished Nitinol surfaces. The two most prominent differences, which are likely to have the most profound implications on increased use of this intermetallic material in medical application are improved wettability and fatigue resistance.

Evolution of the passive film on mechanically damaged nitinol

The corrosion behavior of Nitinol-based medical implants is critical to their success in vivo. Contemporary Nitinol-based medical implants are typically chemically passivated or electrochemically polished to form a protective passive film. However, mechanically formed surfaces caused by handling damage, fretting, or fatigue fracture may also be present on a device in vivo. In this study, mechanically polished surfaces are used to simulate mechanically damaged surfaces such that analytical techniques, including electrochemical impedance spectroscopy, open circuit potential monitoring, X-ray photoelectron spectroscopy (XPS), and Mott-Schottky analysis may be used to monitor the evolution of the passive film on mechanically damaged Nitinol. These mechanically polished Nitinol surfaces are compared with chemically passivated and electrochemically polished Nitinol surfaces and mechanically polished titanium surfaces in phosphate buffered saline solution. The mechanically polished Nitinol exhibits lower impedance at low frequencies, empirically modeled to a thinner film with lower film resistance than chemically passivated and electrochemically polished Nitinol and mechanically polished titanium. Moreover, the passive film on mechanically polished Nitinol continues to develop over time, increasing in its thickness and film resistance. This characterization demonstrates that mechanically formed surfaces may be initially less protective than chemically passivated and electrochemically polished Nitinol surfaces, but continue to become thicker and more resistant to electrochemical reactions with exposure to saline solution.