Synthesis and bioactivity of highly ordered TiO2 nanotube arrays

The highly ordered TiO₂ nanotube arrays were fabricated by potentiostatic anodization of Ti foils in fluorinated dimethyl sulfoxide (DMSO). TiO₂ nanotube arrays are formed using a 40 V anodization potential for 24 h, with a length of 12 μm, diameter of 170 nm and aspect ration of about 70. The as-prepared nanotubes are amorphous, but can be crystallized as the heat treatment temperature increases. Anatase phase appears at a temperature of about 300 °C, then transforms to rutile phase at about 600 °C. After heat treatment at 500 °C and soaking in SBF for 14d, a thick apatite layer of about 13 μm covers the whole surface of TiO₂ nanotube arrays, indicating their excellent in vitro bioactivity, which is mainly attributed to their high specific surface area and the anatase phase.     

Mechanical performance of apatite/TiO2 composite coatings formed on Ti and NiTi shape memory alloy

To make metals bioactive for orthopaedic applications, apatite/TiO₂ composite coatings were formed on Ti and NiTi shape memory alloy (SMA) using a H₂O₂-oxidation and hot water aging technique and the subsequent accelerated biomimetic process. In the current investigation, nanoindentation, scratch testing and frictional testing were employed to assess mechanical properties and the adhesion of apatite/TiO₂ composite coatings formed on Ti and NiTi SMA. Nanoindentation testing conducted on cross-sections of composite coatings indicated that there was no significant difference in nanohardness and elastic modulus between apatite/TiO₂ composite coatings formed on Ti and NiTi SMA samples. The enhancement of the adhesion between the apatite layer and the metal substrates arose from the TiO₂ intermediate layer in the composite coating. The highest values of coating adhesion strength for Ti and NiTi SMA samples, as measured by scratch tests, were 22.58 N and 19.07 N, respectively. However, compared to corresponding Ti samples, NiTi SMA samples had better tribological properties.     

Characteristics, apatite-forming ability and corrosion resistance of NiTi surface modified by AC anodization

NiTi samples were anodized in the non-sparking regime using AC voltage in a solution containing calcium and phosphate ions (solution Ca-P). The as-anodized samples were subsequently treated hydrothermally in water (sample A-W-NiTi) or in solution Ca-P (sample A-CaP-NiTi). Thin-film X-ray diffractometry (TF-XRD) analysis confirmed the existence of anatase in the hydrothermally treated samples, but not in the as-anodized sample, while hydroxyapatite (HA) was detected only in sample A-CaP-NiTi. Cross-sectional micrograph by scanning-electron microscopy (SEM) revealed that the thickness of the modified surface layer formed on sample A-CaP-NiTi was ∼200 nm. X-ray photoelectron spectroscopy (XPS) analysis showed that the Ni concentrations at the surface of sample A-W-NiTi and sample A-CaP-NiTi were in the order of 0.4 and 0.3 at.%, respectively, which were about an order of magnitude lower than that for bare NiTi. Both Ca and P were present in the surface layer on as-anodized NiTi and sample A-CaP-NiTi, but negligible on sample A-W-NiTi, as determined from XPS composition depth profiling. Immersion tests in a conventional simulated body fluid (SBF) of the Kokubo type to study apatite-forming ability showed that growth of apatite was induced on A-W-NiTi and much more abundantly on A-CaP-NiTi, but not on bare NiTi and as-anodized NiTi, suggesting that the presence of anatase and HA is favorable for apatite growth. The apatite-forming ability of the samples in the present study may be ranked in ascending order as: bare NiTi < As-anodized NiTi < A-W-NiTi < A-CaP-NiTi. Polarization tests in Hanks’ solution recorded significant increase in corrosion resistance due to anodization and further increase was obtained via hydrothermal treatment. The present study thus shows that anodization followed by hydrothermal treatment is a simple method to form a potentially bioactive and at the same time corrosion resistant surface layer on NiTi.     

In vitro evaluation of bioactivity of CaO-SiO2-P2O5-Na2O-Fe2O3 glasses

Glasses with compositions 41CaO(52 − x)SiO₂4P₂O₅·xFe₂O₃3Na₂O (2 ≤ x ≤ 10 mol.%) were prepared by melt quenching method. Bioactivity of the different glass compositions was studied in vitro by treating them with simulated body fluid (SBF). The glasses treated for various time periods in SBF were evaluated by examining apatite formation on their surface using grazing incidence X-ray diffraction, Fourier transform infrared reflection spectroscopy, scanning electron microscopy and energy dispersive spectroscopy techniques. Increase in bioactivity with increasing iron oxide content was observed. The results have been used to understand the evolution of the apatite surface layer as a function of immersion time in SBF and glass composition.     

Bioactivity of SiO2-CaO-P2O5-Na2O glasses containing zinc-iron oxide

Glasses with composition x(ZnO,Fe₂O₃)(65 − x)SiO₂20(CaO,P₂O₅)15Na₂O (6 ≤ x ≤ 21 mol%) were prepared by melt-quenching technique. Bioactivity of the glasses was investigated in vitro by examining apatite formation on the surface of glasses treated in acellular simulated body fluid (SBF) with ion concentrations nearly equal to those in human blood plasma. Formation of bioactive apatite layer on the samples treated in SBF was confirmed by using Fourier transform infrared reflection (FTIR) spectroscopy, grazing incidence X-ray diffraction (GI-XRD) and scanning electron microscope (SEM) equipped with energy dispersive X-ray spectrometer. Development of an apatite structure on the surface of the SBF treated glass samples as functions of composition and time could be established using the GI-XRD data. FTIR spectra of the glasses treated in SBF show features at characteristic vibration frequencies of apatite after 1-day of immersion in SBF. SEM observations revealed that the spherical particles formed on the glass surface were made of calcium and phosphorus with the Ca/P molar ratio being close to 1.67, corresponding to the value in crystalline apatite. Increase in bioactivity with increasing zinc-iron oxide content was observed. The results have been used to understand the evolution of the apatite surface layer as a function of glass composition and immersion time in SBF.     

Study on the surface bioactivity of novel magnetic A-W glass ceramic in vitro

Novel magnetic A-W glass ceramic (M GC) in the system MgO-CaO-SiO₂-P₂O₅-CaF₂-MnO-ZnO-Fe₂O₃ was synthesized by doping Mn-Zn ferrite to apatite-wollastonite glass ceramic. The phase composition was investigated by XRD. The magnetic property was measured by VSM. The in vitro bioactivity was tested by immersion in simulated body fluid. The result shows apatite, wollastonite, fluorapatite and Zn_0.75Mn_0.75Fe_1.5O₄ are the main phases of M GC. Under a magnetic field of 10,000 Oe, the saturation magnetization and coercive force of the material are 6 emu g⁻ and 180 Oe, respectively. After soaking in SBF for 14 days, the surface of M GC is coated by a hydroxycarbonate apatite layer.     

Apatite formation on Poly (vinyl alcohol)-coated Poly (?-caprolactone) films by incubation in simulated body fluids

In this study, biodegradable poly (?-caprolactone) (PCL) films were coated with poly (vinyl alcohol) (PVA) and then incubated in a simulated body fluid 1.5SBF to prepare an apatite (HA)/PCL composite. It was found that the bone-like apatite formability of PCL was enhanced by PVA coating. The changes of surface properties induced by PVA coating were effective for apatite formation. The apatite formability increased with increasing coating amount. After 24 h incubation, apatite was formed on PVA-coated PCL film but hardly any apatite was found on uncoated PCL plate. The surface chemistry of the specimens was examined using XPS, FT-IR-ATR. The apatite formed was characterized by using SEM, TF-XRD, FT-IR, EDS. The apatite formed was similar in morphology and composition to that of natural bone. This indicated that simple PVA coating on PCL substrate could serve as a novel way to accelerated apatite formation via biomimetic method.     

Hydrogen peroxide treatment on Ti-6Al-4V alloy: A promising surface modification technique for orthopaedic application

Ti-6Al-4V alloy was treated with various concentrations (5 wt.%, 15 wt.% and 25 wt.%) of hydrogen peroxide (H₂O₂) and then heat treated to produce an anatase titania layer. The surface modified substrates were immersed in simulated body fluid (SBF) solution for the growth of an apatite layer on the surface and the formed apatite layer was characterized using various surface characterization techniques. The results revealed that titania layer with anatase nature was observed for all H₂O₂ treated Ti-6Al-4V alloy, irrespective of the H₂O₂ concentrations. Ti-6Al-4V alloy treated with 15 wt.% and 25 wt.% of H₂O₂ induced apatite formation, however 5 wt.% of H₂O₂ treated Ti-6Al-4V failed to form apatite layer on the surface. The electrochemical behaviour of H₂O₂ treated specimens in SBF solution was studied using potentiodynamic polarization and electrochemical impedance spectroscopy. Ti-6Al-4V alloy treated with 25 wt.% of H₂O₂ solution exhibited low current density and high charge transfer resistance values compared to specimens treated with other concentrations of H₂O₂ and untreated Ti-6Al-4V alloy.     

Low-temperature biomimetic formation of apatite/TiO2 composite coatings on Ti and NiTi shape memory alloy and their characterization

Through a low temperature process, a bilayer composite coating was formed on Ti and NiTi shape memory alloy (SMA). The composite coating consisted of a layer of titania, which was formed using a H₂O₂-oxidation and hot water aging technique, and a layer of apatite, which was formed through an accelerated biomimetic process by immersing as-oxidized metals in a high-strength simulated body fluid (5SBF). Various techniques including X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy were used to characterize the surfaces of samples at different stages of coating formation and the coatings formed. Bioactive apatite/TiO₂ coatings could be formed on NiTi SMA and firmly bonded to the metal substrate. But there were differences for the formation of the composite coating on Ti and NiTi SMA substrates. The composite coatings formed will render both metals bioactive and hence bone-bonding.     

The effect of chemical treatment on apatite-forming ability of the macroporous zirconia films formed by micro-arc oxidation

Macroporous and nano-crystalline zirconia film was prepared by micro-arc oxidation (MAO) of zirconium, and the effect of chemical treatment in H₂SO₄ or NaOH aqueous solutions on the microstructure and apatite-forming ability of the film was investigated. Compared with the MAO film, the chemically treated films do not exhibit apparent changes in phase component, morphology and grain size, however, have more abundant basic Zr-OH groups. The films treated with H₂SO₄ and NaOH solutions can induce apatite formation on their surfaces in simulated body fluids (SBF) within 1 day, whereas no apatite was detected on the untreated ZrO₂ surface by 30 days. It is believed that the enhanced apatite-forming ability of the chemically treated ZrO₂ films is related to the abundant basic Zr-OH groups on their surface.     

Biomimetic apatite induction on Ca-containing titania

To understand the apatite induction mechanism in SBF, Ca-containing titania film without CaTiO₃ phase was fabricated by micro-arc oxidation (MAO) at low voltage (230 V) in an electrolytic solution containing calcium acetate monohydrate. Macro-porous, Ca-containing titania film was formed on the titanium substrate and the oxidized layer was composed of anatase and rutile phase. When immersed in 1.5SBF, no apatite was induced in the MAO specimen similar to the CaTiO₃-containing titania. However, after hydrothermal treatment at 250 °C for 2 h, numerous precipitates, presumably calcium phosphates, were formed on the surface of the titania after 7 day immersion and titania surface was entirely covered with apatite after 14 days of immersion. This study clearly showed that Ca-containing titania has the capability to induce apatite in SBF and hydrothermal treatment plays a decisive role in apatite induction, particularly producing surface hydroxyl groups such as Ca–OH or Ti–OH.     

Enhanced in vitro biocompatibility of ultrafine-grained biomedical NiTi alloy with microporous surface

Bulk ultrafine-grained Ni_50.8Ti_49.2 alloy (UFG-NiTi) was successfully fabricated by equal-channel angular pressing (ECAP) technique in the present study, and to further improve its surface biocompatibility, surface modification techniques including sandblasting, acid etching and alkali treatment were employed to produce either irregularly roughened surface or microporous surface or hierarchical porous surface with bioactivity. The effect of the above surface treatments on the surface roughness, wettability, corrosion behavior, ion release, apatite forming ability and cytocompatibility of UFG-NiTi alloy were systematically investigated with the coarse-grained NiTi alloy as control. The pitting corrosion potential (E_pit) was increased from 393 mV (SCE) to 704 mV (SCE) with sandblasting and further increased to 1539 mV (SCE) with following acid etching in HF/HNO₃ solution. All the above surface treatment increased the apatite forming ability of UFG-NiTi in varying degrees when soaked them in simulated body fluid (SBF). Meanwhile, both sandblasting and acid etching could promote the cytocompatibility for osteoblasts: sandblasting enhanced cell attachment and acid etching increased cell proliferation. The different corrosion behavior, apatite forming ability and cellular response of UFG-NiTi after different surface modifications are attributed to the topography and wettability of the resulting surface oxide layer.     

Immersion behaviour of calcium polyphosphate in simulated body fluid

This work documents an investigation into the immersion behaviour of calcium polyphosphate (CPP) in simulated body fluid (SBF) for various periods. The results showed that with the increase of soaking time in SBF, a number of tiny particles were observed on the surface of samples by scanning electron microscope (SEM). The results of X-ray diffraction (XRD) indicated that the particles were contributed to apatite. And the changes of PO₄³⁻ and Ca²⁺ concentrations in SBF were detected by phospho-vanado-molybdate method and inductively coupled plasma atomic emission spectroscopy (ICP) methods respectively. The results indicated that the concentrations of PO₄³⁻ and Ca²⁺ in SBF reached the maximum value after 25 days of immersion, and then decreased with further increase of the soaking time. It is suggested that the degradation and ion exchange of CPP samples are dominant during the early stage of soaking, and then the precipitates begin to form and increase gradually as the soaking time increases. This study has demonstrated that apatite could be formed on the surface of CPP samples, and CPP would be used as bone substitution material.     

Correlation between bioactivity and structural properties of titanium dioxide coatings grown by atomic layer deposition

TiO₂ coatings were grown on Ti and Si by Atomic Layer Deposition (ALD) from titanium ethoxide and water at 300 °C in a wide range of the reaction cycles number N = 100-2000. TiO₂ coatings were found to be amorphous at low value of N < 300 while the coatings grown at N ≥ 300 revealed anatase polycrystalline structure. The TiO₂ coatings bioactivity was evaluated by hydroxyapatite forming ability by the technique of soaking in Simulated Body Fluid (SBF). Correlation between bioactivity and structural properties of TiO₂ was determined. X-ray diffraction and scanning electron microscopy with electron probe microanalysis showed that amorphous TiO₂ coating did not induce the hydroxyapatite growth whereas anatase resulted in the hydroxyapatite forming on the samples surfaces which confirmed TiO₂ anatase bioactivity.     

Enhanced corrosion resistance and cellular behavior of ultrafine-grained biomedical NiTi alloy with a novel SrO-SiO2-TiO2 sol-gel coating

NiTi alloy has a unique combination of mechanical properties, shape memory effects and superelastic behavior that makes it attractive for several biomedical applications. In recent years, concerns about its biocompatibility have been aroused due to the toxic or side effect of released nickel ions, which restricts its application as an implant material. Bulk ultrafine-grained Ni50.8Ti49.2 alloy (UFG NiTi) was successfully fabricated by equal-channel angular pressing (ECAP) technique in the present study. A homogeneous and smooth SrO-SiO₂-TiO₂ sol-gel coating without cracks was fabricated on its surface by dip-coating method with the aim of increasing its corrosion resistance and cytocompatibility. Electrochemical tests in simulated body fluid (SBF) showed that the pitting corrosion potential of UFG NiTi was increased from 393 mV(SCE) to 1800 mV(SCE) after coated with SrO-SiO₂-TiO₂ film and the corrosion current density decreased from 3.41 μA/cm² to 0.629 μA/cm². Meanwhile, the sol-gel coating significantly decreased the release of nickel ions of UFG NiTi when soaked in SBF. UFG NiTi with SrO-SiO₂-TiO₂ sol-gel coating exhibited enhanced osteoblast-like cells attachment, spreading and proliferation compared with UFG NiTi without coating and CG NiTi.     

In vitro remineralization of acid-etched human enamel with Ca3SiO5

Bioactive and inductive silicate-based bioceramics play an important role in hard tissue prosthetics such as bone and teeth. In the present study, a model was established to study the acid-etched enamel remineralization with tricalcium silicate (Ca₃SiO₅, C₃S) paste in vitro. After soaking in simulated oral fluid (SOF), Ca-P precipitation layer was formed on the enamel surface, with the prolonged soaking time, apatite layer turned into density and uniformity and thickness increasingly from 250 to 350 nm for 1 day to 1.7-1.9 μm for 7 days. Structure of apatite crystals was similar to that of hydroxyapatite (HAp). At the same time, surface smoothness of the remineralized layer is favorable for the oral hygiene. These results suggested that C₃S treated the acid-etched enamel can induce apatite formation, indicating the biomimic mineralization ability, and C₃S could be used as an agent of inductive biomineralization for the enamel prosthesis and protection.     

Effects of water plasma immersion ion implantation on surface electrochemical behavior of NiTi shape memory alloys in simulated body fluids

Water plasma immersion ion implantation (PIII) was conducted on orthopedic NiTi shape memory alloy to enhance the surface electrochemical characteristics. The surface composition of the NiTi alloy before and after H₂O-PIII was determined by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) was utilized to determine the roughness and morphology of the NiTi samples. Potentiodynamic polarization tests and electrochemical impedance spectroscopy (EIS) were carried out to investigate the surface electrochemical behavior of the control and H₂O-PIII NiTi samples in simulated body fluids (SBF) at 37 °C as well as the mechanism. The H₂O-PIII NiTi sample showed a higher breakdown potential (E_b) than the control sample. Based on the AFM results, two different physical models with related equivalent electrical circuits were obtained to fit the EIS data and explain the surface electrochemical behavior of NiTi in SBF. The simulation results demonstrate that the higher resistance of the oxide layer produced by H₂O-PIII is primarily responsible for the improvement in the surface corrosion resistance.     

Synthesis and photocatalytic property of N-doped TiO2 nanorods and nanotubes with high nitrogen content

Nano N-doped TiO₂ nanotubes were fabricated by hydrothermally treating N-doped TiO₂ nanorods in a 8 M NaOH solution at 110 °C for 20 h. The N-doped TiO₂ nanorods were synthesized by a solvothermal process with precursor solution containing titanium sulfate, urea, and dichloroethane. The N-doped TiO₂ nanorods and nanotubes were characterized with X-ray diffraction, transmission electron microscopy, and UV-vis spectrophotometry. The nitrogen contents of the N-doped TiO₂ nanorods and nanotubes were reached to high values of 36.9 at.% and 25.7 at.%, respectively. The nitrogen doping narrowed the band gap of the N-doped TiO₂ nanorods and nanotubes and introduced indirect band gap to the powders, which respectively extended the absorption edge to visible light and infrared region. The nanotubes showed larger specific surface area and greater degradation efficiency to methyl orange than the nanorods.     

Fabrication of hydroxyapatite and TiO2 nanorods on microarc-oxidized titanium surface using hydrothermal treatment

AC-type microarc oxidation (MAO) and hydrothermal treatment techniques were used to enhance the bioactivity of commercially pure titanium (CP-Ti). The porous TiO₂ layer fabricated by the MAO treatment had a dominant anatase structure and contained Ca and P ions. The MAO-treated specimens were treated hydrothermally to form HAp crystallites on the titanium oxide layer in an alkaline aqueous solution (OH-solution) or phosphorous-containing alkaline solution (POH-solution). A small number of micro-sized hydroxyapatite (HAp) crystallites and a thin layer composed of nano-sized HAps were formed on the Ti-MAO-OH group treated hydrothermally in an OH-solution, whereas a large number of micro-sized HAp crystallites and dense anatase TiO₂ nanorods were formed on the Ti-MAO-POH group treated hydrothermally in a POH-solution. The layer of bone-like apatite that formed on the surface of the POH-treated sample after soaking in a modified simulated body fluid was thicker than that on the OH-treated samples.     

Effect of substrate temperature on the surface and interface oxidation of NiTi thin films

NiTi shape memory alloy thin films are deposited on pure Cu substrate at substrate ambient temperatures of 300 °C and 450 °C. The surface and interface oxidation of NiTi thin films are characterized by X-ray photoelectron spectroscopy (XPS). After a subsequent annealing treatment the crystallization behavior of the films deposited on substrate at different temperatures is studied by X-ray diffraction (XRD). The effects of substrate temperature on the surface and interface oxidation of NiTi thin films are investigated. In the film surface this is an oxide layer composed of TiO₂. The Ni atom has not been detected on surface. In the film/substrate interface there is an oxide layer with a mixture Ti₂O₃ and NiO in the films deposited at substrate temperatures 300 °C and 450 °C. In the films deposited at ambient temperature, the interface layer contains Ti suboxides (TiO) and metallic Ni.