Nitinol Substrates Research Articles

In-situ laser metal deposition of nitinol and the effect of Ni4Ti3 phase concentration on its pseudoelasticity

Pseudoelasticity is one of the major properties of nitinol. It is highly altered by the presence of nickel-rich Ni4Ti3 phase in the nitinol matrix. Hence it is significant to make an attempt to study the connection between Ni4Ti3-concentration and the matrix-pseudoelasticity. In this work, the connection of pseudoelasticity of nitinol with the concentration of Ni4Ti3 in the matrix was investigated. A 0.5 mm thick preplaced layer of a powder mixture of nickel and titanium (55 % and 45 % by weight, respectively) on a 4 mm thick nitinol substrate was laser scanned with a laser power of 1000 W, scan speeds of 200, 400, 600, 800 and 1000 mm/min and a laser spot diameter of 3 mm under a shielding of argon gas stream. The various phases formed during the process at different scan speeds were quantified by Rietveld quantitative phase analysis. Further, load-displacement curves were obtained with nano-indentation to estimate the pseudoelastic recovery of the matrix and to show the pseudoelastic connection of the matrix with its constituent Ni4Ti3 concentration. The laser scanned tracks (with the scan speeds of 600, 800 and 1000 mm/min) having a higher concentration of Ni4Ti3 showed a higher amount of pseudoelastic recovery. On the other hand, the tracks (with the scan speeds of 200 and 400 mm/min) having a lower concentration of Ni4Ti3 showed a lower amount of pseudoelastic recovery.

Catechol-functionalized sulfobetaine polymer for uniform zwitterionization via pH transition approach

The study aims to develop a modification strategy to facilitate uniform catechol-assisted zwitterionization on nitinol alloy for bio-compatibility and fouling resistance. Catechol-functionalized polysulfobetaine methacrylate (pSBMA/DA) is synthesized via dopamine-initiated photo-polymerization. Under UV irradiation, semiquinone radicals from dopamine (DA) can be generated, and prevented loss of one electron to intramolecular cyclization and intermolecular dimerization in a solution at pH 2. Pseudo-first-order polymerization kinetics, and relations of apparent rate constant and number average molecular weight with the molar ratio of DA in photopolymerization for pSBMA/DA are unveiled. In a solution at pH 3, PSBMA/DA begins aggregation, kept catechol moieties from premature oxidization, and enabled even deposition on the nitinol substrate. After pH regulation to 8.5, pSBMA/DA extends, and concurrently catechol moieties are activated to interact with the nitinol surface via the formation of bidentate binding. X-ray photoelectron spectroscopy (XPS) analysis revealed that a shorter pSBMA/DA chain with higher catechol content provides more anchoring sites to enhance zwitterionic moieties coverage on substrates. Interestingly, atomic force microscopy (AFM) images revealed a smooth and uniform deposition of pSBMA/DA using the pH-transition method. Strong ionic hydration of pSBMA/DA coating on nitinol surface repels non-specific adsorption of bio-foulants, permitting excellent antifouling properties. Zwitterion-modified nitinol achieved a reduction rate of 99.9% against Escherichia coli and Staphylococcus aureus attachment. In addition, pSBMA/DA exhibits a robust antifouling performance to NIH 3T3 mouse fibroblasts in culture media after incubation for 24 h. Overall, the pSBMA/DA coating via pH transition approach opens up a promising strategy to facilitate uniform surface functionalization for antifouling and coating technology.

Porous thermosensitive coating with water-locking ability for enhanced osteogenic and antibacterial abilities.

Preferable antibacterial property and osteogenesis are the permanent pursuit for metallic implants. However, it is difficult to satisfy both the properties. In fact, implants may be contaminated with bacteria during storage and surgery, leading to inflammation. Therefore, the antibacterial property of biomaterial surfaces is required not only in the human environment but also at room temperature. In this study, porous structures loaded with a thermosensitive poly (N-isopropylacrylamide) (PNIPAM) hydrogel on a nitinol (NiTi) substrate were constructed. When the temperature is 25 ​°C, almost all bacteria cannot adhere to the sample surface due to the abundant hydration layer of the PNIPAM hydrogel. Meanwhile, when the temperature is 37 ​°C, the structure of the PNIPAM hydrogel collapses and the hydration layer disappears due to the temperature change. However, the porous structures lock water in the pores, which results in a high-hydration-rate sample surface. This surface has few bacterial adhesion sites; nevertheless, the adhesion of larger cells to the surface is not impeded by the porous structure. In addition, the PNIPAM hydrogel is soft and biocompatible, so the sample can have better cell adhesion and proliferation than a bare NiTi alloy. Based on these results, it can be concluded that the porous NiTi sample loaded with the thermosensitive PNIPAM hydrogel has the antibacterial property before implantation and the dual function of inhibiting bacterial adhesion and promoting cell adhesion and proliferation after implantation, which shows promising applications in the biomedical field such as orthopedic implantation.

Selective Laser Melting of Pre-Alloyed NiTi Powder: Single-Track Study and FE Modeling with Heat Source Calibration.

Unique functional properties such as the low stiffness, superelasticity, and biocompatibility of nickel–titanium shape-memory alloys provide many applications for such materials. Selective laser melting of NiTi enables low-cost customization of devices and the manufacturing of highly complex geometries without subsequent machining. However, the technology requires optimization of process parameters in order to guarantee high mass density and to avoid deterioration of functional properties. In this work, the melt pool geometry, surface morphology, formation mode, and thermal behavior were studied. Multiple combinations of laser power and scanning speed were used for single-track preparation from pre-alloyed NiTi powder on a nitinol substrate. The experimental results show the influence of laser power and scanning speed on the depth, width, and depth-to-width aspect ratio. Additionally, a transient 3D FE model was employed to predict thermal behavior in the melt pool for different regimes. In this paper, the coefficients for a volumetric double-ellipsoid heat source were calibrated with bound optimization by a quadratic approximation algorithm, the design of experiments technique, and experimentally obtained data. The results of the simulation reveal the necessary conditions of transition from conduction to keyhole mode welding. Finally, by combining experimental and FE modeling results, the optimal SLM process parameters were evaluated as P = 77 W, V = 400 mm/s, h = 70 μm, and t = 50 μm, without printing of 3D samples.

Fabrication of rutile – TiO2 nanowire on shape memory alloy: A potential material for energy storage application

Herein, we reported a novel and a facile method to fabricated TiO2 nanowire arrays on Nitinol alloy substrate as electrode material for supercapacitors. The rutile TiO2 nanowire arrays on alloy substrates were developed using chemical vapor deposition technique with camphor as the source. A systematic study for the growth of nanostructure TiO2@NiTi has been carried out along with its exceptional physical and chemical properties for energy storage application. The growth of the nanostructure for different concentration at the elevated temperature has been studied through different characterization techniques like scanning electron microscopy, X-ray diffraction and Raman spectroscopy, which resulted in confirming the formation of rutile phase having length and diameter of 10 ± 3 µm and 80 ± 10 nm, respectively. In addition, the COMSOL simulation was carried out to determine the temperature gradient in the furnace tube at elevated temperature. The presence of nickel in the bi-metallic alloys played a vital role in the growth of TiO2 nanostructures array. The synthesized TiO2 nanowire arrays on Nitinol substrate exhibited the specific capacitance of ∼ 1F/g with long term stability, making it a potential candidate for supercapacitor applications.

The investigation of mechanical and thermal properties of super-hydrophobic nitinol surfaces fabricated by hybrid methods of laser irradiation and carbon ion implantation

Comparing with laser irradiation only, the laser ablation combined with chemical modification process is a widely used technique to obtain bio-inspired super-hydrophobic surface. However, the as-prepared surfaces may be polluted by toxic substance during chemical modification such as fluoroalkyl silane and stearic acid. The side effect of polluted functional surface on organisms and environment limited its application value. In this paper, a green and environmental-friendly super-hydrophobic surface was quickly fabricated on nitinol substrates through hybrid of nanosecond laser ablation and carbon ion implantation. The time that turning from super-hydrophilicity to super-hydrophobicity was only 16 h exhibiting high efficiency compared with pure laser processing. Surface morphology and chemical component were systematically investigated to reveal the formation mechanism of super-hydrophobicity in such short time. The mechanical abrasion tests implied that the mechanical properties of surface microstructure could be heightened after carbon ion implantation, showing the superior structure stability. It is noted that chemical modified super-hydrophobicity could be hardly destroyed under high temperature, and the thermal stability of this ion implanted super-hydrophobic surface was on a par with it. This hybrid method of laser irradiation and carbon ion implantation paves a green way for rapid fabrication super-hydrophobic surface on nitinol, which would have great application value in biomedicine and industry.

Electrophoretic deposited graphene based functional coatings for biocompatibility improvement of Nitinol

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.

Silver/Selenium/Chitosan-doped Hydroxyapatite coatings on biological NiTi alloy: In vitro biodegradation analysis

Nitinol (NiTi) substrates were bioceramic coated by single layer Hydroxyapatite (HA), double layer HA/Silver (Ag) and double-layered HA/Selenium (Se)–Chitosan (Cht) substitutions by sol–gel method. Surface morphologies and adhesion strengths of the obtained coatings were characterized by means of SEM, EDX, XRD, and adhesion tests. Crack-free coatings were obtained in single layer-HA (S1), double layer HA/Ag (S2), and double-layered HA/Selenium–Cht (S3) coatings on NiTi alloys executed via the sol–gel method. Degradation behavior was analyzed in via simulated body fluid (SBF) environment by means of weight loss (%) and ion release rates for 1–4 weeks of immersion periods. The degraded coating particles were measured and metallic ions were detected quantitatively by an inductively coupled plasma–mass spectrometer (ICP–MS). The results show that such coatings could minimize and/or prevent the degradation of metallic ions and weight loss (%) from the coated substrates. From the overall results, HA/Ag double layer biocomposite (S2)-coated NiTi substrates revealed the highest adhesion strength and lowest weight loss and degradation. It was also shown in all groups that the Ni ion release from the NiTi substrates was prevented by the coatings.

Semiconductor-to-Metal Transition in Rutile TiO2 Induced by Tensile Strain

We report the first observation of a reversible, degenerate doping of titanium dioxide with strain, which is referred to as a semiconductor-to-metal transition. Application of tensile strain to a ∼50 nm film of rutile TiO2 thermally grown on a superelastic nitinol (NiTi intermetallic) substrate causes reversible degenerate doping as evidenced by electrochemistry, X-ray photoelectron spectroscopy (XPS), and conducting atomic force microscopy (CAFM). Cyclic voltammetry and impedance measurements show behavior characteristic of a highly doped n-type semiconductor for unstrained TiO2 transitioning to metallic behavior under tensile strain. The transition reverses when strain is removed. Valence band XPS spectra show that samples strained to 5% exhibit metallic-like intensity near the Fermi level. Strain also induces a distinct transition in CAFM current–voltage curves from rectifying (typical of an n-type semiconductor) to ohmic (metal-like) behavior. We propose that strain raises the energy distribution of o...

PEEK (polyether-ether-ketone)-coated nitinol wire: Film stability for biocompatibility applications

High quality biocompatible poly-ether-ether-ketone (PEEK) coatings were produced on NiTi shape memory alloy wires using dipping deposition from colloidal aqueous PEEK dispersions after substrate surface treatment. The surface morphology and microstructure were investigated by Scanning Electron Microscopy at every step of the process from the as-received Nitinol substrate to the ultimate PEEK-coated NiTi wire. Nanoscratch tests were carried out to access the adhesive behavior of the polymer coated film to the NiTi. The results indicate that the optimum process conditions in cleaning, chemical etching, and electropolishing the NiTi, were the most important and determining parameters to be achieved. Thus, high quality PEEK coatings were obtained on NiTi wires, straight or curved (even with a U-shape) with a homogeneous microstructure along the wire length and a uniform thickness of 12μm without any development of cracks or the presence of large voids. The biocompatibility of the PEEK coating film was checked in fibrobast cultured cells. The coating remains stable in biological environment with negligible Ni ion release, no cytotoxicity, and no delamination observed with time.

On the Electrodeposition of Ca-P Coatings on Nitinol Alloy: A Comparison Between Different Surface Modification Methods

In this study, a combination of surface modification process and the electrochemical deposition of Ca-P coatings was used for the modification of the Nitinol shape memory alloy. DSC, SEM, GIB-XRD, FT-Raman, XPS, and FTIR measurements were performed for the characterization of the samples. Results indicated that chemical etching and boiling of the samples in distilled water formed TiO film on the surface. After the chemical modification, subsequent aging of the sample, at 470 °C for 30 min, converted the oxide film to a stable structure of titanium dioxide. In that case, the treated substrate indicated a superelastic behavior. At the same electrochemical condition, the treated substrate revealed more stable and uniform Ca-P coatings in comparison with the abraded Nitinol substrate. This difference was attributed to the presence of hydroxyl groups on the titanium dioxide surface. Also, after soaking the sample in SBF, the needle-like coating on the treated substrate was completely covered with the hydroxyapatite phase which shows a good bioactivity of the coating.

Oxidized nitinol substrate for interference enhanced Raman scattering of monolayer graphene

We prepared a controllable and reproducible active NiTi alloy substrate with tunable thickness of titanium oxide capping layer for surface enhanced Raman scattering (SERS) of monolayer graphene using a facile oxidation method.

Mechanical behavior and blood compatibility of copper-containing films as potential biomaterials

Surface modification is one approach to enhance the biocompatibility of implanted cardiovascular devices. In this work, a copper-containing film used to blood contacted biomaterials was prepared by vacuum arc deposition. The phase composition of the films was investigated via X-ray diffraction, and the adherence strength of the films was evaluated with conventional deformation tests. Blood compatibility of the films was characterized by hemolysis ratio, clotting time and platelet adhesion etc. The surface of inferior vena cava filters were smooth and uniform, no cracks or delaminations were observed on the deformed surface. These results indicate that the mechanical behavior of the films is suitable for withstanding deformation stresses as operation in clinic. Good blood compatibility of the copper-containing films was identified through experiment in vitro, the activated partial thromboplastin times (APTTs) of Cu/Ti films were similar to that of the uncoated substrate, and Cu/Ti films were also found to inhibit platelet adhesion comparing to the nitinol substrate. However, with increasing ratio of Cu/Ti, the hemolysis ratio increased, resulting in platelet damage. These results indicate that the copper-containing film has potential application on blood contacted devices.

Electrodeposition of tantalum coatings for corrosion protection of nitinol articles

Electrodeposition of tantalum coatings from molten salts onto nitinol substrates was studied in steady and pulsed modes. The adhesion of the tantalum coatings, their porosity, and the corrosion resistance, of the nitinol-tantalum composite were determined.

Electrochemical Investigation of Nitinol/Tantalum Hybrid Surfaces Modified with Phosphonic Acid Derivatives

Nitinol, a nickel and titanium alloy in quasi-equiatomic proportions, is used in numerous biomedical applications due to its remarkable mechanical characteristics, its resistance towards corrosion and good biocompatibility. Further research to reinforce osseointegrative abilities and barrier properties towards external aggressive agents are still needed.In this context, electrochemical methods can be used to generate protective and functional surface coatings on Nitinol substrates with a good level of control and versatility. Specifically here are considered the elaboration of a structured thin layer on Nitinol plates by electrodeposition of tantalum and its subsequent modification by the self-assembly of phosphonic acid derivatives to consolidate corrosion protection and bioactivity features. Electrochemical techniques are also used to assess and study the different deposited layers. Cyclic voltammetry, polarization curves, free potential measurements, and scanning electrochemical microscopy experiments are performed to qualitatively and quantitatively evaluate electron and mass transfers at the interface, as well as corrosion resistance properties. Spectroscopic (XPS) and microscopic (SEM) analyses complete the characterization process regarding chemical nature and morphology aspects.

Fabrication of bioactive titania coating on nitinol by plasma electrolytic oxidation

Surface modification was attempted on Nitinol (NiTi) by plasma electrolytic oxidation (PEO) in aqueous solutions of sodium sulphate and sodium hydroxide (Na2SO4-NaOH) using an AC power supply. A thick and porous oxide layer with micron-sized pores was formed on the Nitinol substrate, with the thickness of the oxide layer ranging from a few μm to over 10μm, depending on the processing time. X-ray diffraction (XRD) analysis confirmed that the oxide formed was anatase. Potentiodynamic polarization tests in Hanks’ solution showed that the corrosion resistance of PEO-coated Nitinol was much higher than that of the substrate. More importantly, the apatite-forming ability of the PEO-treated NiTi was found to be enhanced. This could be attributed to the anatase crystalline structure of the titanium oxide and the porous structure that facilitates the anchorage of the hydroxyapatite particles.

Graphene Coatings for Biomedical Implants

Atomically smooth graphene as a surface coating has potential to improve implant properties. This demonstrates a method for coating nitinol alloys with nanometer thick layers of graphene for applications as a stent material. Graphene was grown on copper substrates via chemical vapor deposition and then transferred onto nitinol substrates. In order to understand how the graphene coating could change biological response, cell viability of rat aortic endothelial cells and rat aortic smooth muscle cells was investigated. Moreover, the effect of graphene-coatings on cell adhesion and morphology was examined with fluorescent confocal microscopy. Cells were stained for actin and nuclei, and there were noticeable differences between pristine nitinol samples compared to graphene-coated samples. Total actin expression from rat aortic smooth muscle cells was found using western blot. Protein adsorption characteristics, an indicator for potential thrombogenicity, were determined for serum albumin and fibrinogen with gel electrophoresis. Moreover, the transfer of charge from fibrinogen to substrate was deduced using Raman spectroscopy. It was found that graphene coating on nitinol substrates met the functional requirements for a stent material and improved the biological response compared to uncoated nitinol. Thus, graphene-coated nitinol is a viable candidate for a stent material.

Graphene Coatings for Biomedical Implants

Atomically smooth graphene as a surface coating has potential to improve implant properties. This demonstrates a method for coating nitinol alloys with nanometer thick layers of graphene for applications as a stent material. Graphene was grown on copper substrates via chemical vapor deposition and then transferred onto nitinol substrates. In order to understand how the graphene coating could change biological response, cell viability of rat aortic endothelial cells and rat aortic smooth muscle cells was investigated. Moreover, the effect of graphene-coatings on cell adhesion and morphology was examined with fluorescent confocal microscopy. Cells were stained for actin and nuclei, and there were noticeable differences between pristine nitinol samples compared to graphene-coated samples. Total actin expression from rat aortic smooth muscle cells was found using western blot. Protein adsorption characteristics, an indicator for potential thrombogenicity, were determined for serum albumin and fibrinogen with gel electrophoresis. Moreover, the transfer of charge from fibrinogen to substrate was deduced using Raman spectroscopy. It was found that graphene coating on nitinol substrates met the functional requirements for a stent material and improved the biological response compared to uncoated nitinol. Thus, graphene-coated nitinol is a viable candidate for a stent material.

Study of the formation process and the characteristics of tantalum layers electrodeposited on Nitinol plates in the 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ionic liquid

Thanks to excellent mechanical and biochemical properties, the nickel–titanium shape memory alloy (Nitinol) constitutes an increasingly praised platform material in dental, cardiovascular and orthopedic biomedical devices. In order to strengthen their protective abilities toward corrosion, to reinforce their biocompatibility and to confer them specific osseointegrative capacities, Nitinol plates are covered with a thin tantalum layer by electrodeposition in the 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ionic liquid. XPS and SEM/EDX analyses highlight the chemical and morphological characteristics of the deposits: notably, these present an intrinsic dimpled nanometric structuration which is particularly remarkable considering the “soft” experimental conditions and very interesting for fundamental and applied bioactive perspectives. The present study investigates the specific and synergic effects of the Ni occurrence on the surface of the Nitinol substrates, the presence of fluorine species in the working bath, and the electrodeposition duration on the resulting formation process, morphology and chemical composition of the tantalum coating. Finally, samples are submitted to electrochemical characterizations and in vitro hydroxyapatite growth tests for a primary assessment of their corrosion resistance and osseoinductive features.

Investigating the feasibility of using a grit blasting process to coat nitinol stents with hydroxyapatite

This study investigates the feasibility of utilising a grit blasting process to coat three nitinol substrates (a planer 2D surface, a circular wire and a cardiovascular stent geometry) with a hydroxyapatite coating. Surface characteristics of the coating on the three substrates were determined and additionally the durability of the coating post fatigue testing was analysed. The coating process resulted in a consistent covering of the substrate that resulted in an extremely hydrophilic stent surface. The surface roughness was dependant on grit blasting particle size. A general trend of smaller particle size resulted in a lower surface roughness, while particle size did not have an effect on the hydroxyapatite coating thickness. Fatigue integrity tests that simulated 16 months implantation demonstrated minimal damage to the coating. In conclusion we demonstrated the initial feasibility of using a grit blasting method to produce a consistent, hydrophilic, and durable HAp stent coating that has the capability of incorporating a drug eluting function.