Titanium nanostructures for biomedical applications.

Titanium and titanium alloys exhibit a unique combination of strength and biocompatibility, which enables their use in medical applications and accounts for their extensive use as implant materials in the last 50 years. Currently, a large amount of research is being carried out in order to determine the optimal surface topography for use in bioapplications, and thus the emphasis is on nanotechnology for biomedical applications. It was recently shown that titanium implants with rough surface topography and free energy increase osteoblast adhesion, maturation and subsequent bone formation. Furthermore, the adhesion of different cell lines to the surface of titanium implants is influenced by the surface characteristics of titanium; namely topography, charge distribution and chemistry. The present review article focuses on the specific nanotopography of titanium, i.e. titanium dioxide (TiO2) nanotubes, using a simple electrochemical anodisation method of the metallic substrate and other processes such as the hydrothermal or sol-gel template. One key advantage of using TiO2 nanotubes in cell interactions is based on the fact that TiO2 nanotube morphology is correlated with cell adhesion, spreading, growth and differentiation of mesenchymal stem cells, which were shown to be maximally induced on smaller diameter nanotubes (15 nm), but hindered on larger diameter (100 nm) tubes, leading to cell death and apoptosis. Research has supported the significance of nanotopography (TiO2 nanotube diameter) in cell adhesion and cell growth, and suggests that the mechanics of focal adhesion formation are similar among different cell types. As such, the present review will focus on perhaps the most spectacular and surprising one-dimensional structures and their unique biomedical applications for increased osseointegration, protein interaction and antibacterial properties.

H2 mapping on Pt-loaded TiO2 nanotube gradient arrays.

We describe a rapid screening technique for determining the optimal characteristics of nanophotocatalysts for the production of H2 on a single surface. Arrays of TiO2 nanotubes (NTs) with a gradient in length and diameter were fabricated by bipolar anodization, and a perpendicular gradient of Pt nanoparticles (NPs) was generated by the toposelective decoration of the TiO2 NTs. Photocatalytic hydrogen evolution was locally triggered with a UV laser beam, and the arrays were screened in the x and y directions for spatially resolved kinetic measurements and the mapping of the optimal hydrogen production. By using this technique, we demonstrate the time-efficient and straightforward determination of the tube dimensions and Pt loading for optimized H2 production. The concept holds promise for generally improving the study of many photoreactions as a function of the physicochemical characteristic of nanophotocatalysts, which renders it highly attractive for the optimization of various important chemical processes.

MFI-type (ZSM-5) zeolite-filled TiO2 nanotubes for enhanced photocatalytic activity.

The present work demonstrates enhanced photocatalytic activity for zeolite-filled TiO2 nanotubes. ZSM-5 zeolite nanocrystals were grown on and into a TiO2 nanotubular skeleton (TiNT/ZSM-5) by multi-step hydrothermal synthesis consisting of in situ seeding and multiple in situ crystallization (MISC). The resulting zeolite nanocrystals were in the range of a few nanometers and they adhere well to the nanotubular inner walls. After crystallization, the photocatalytic activity of this zeolite-filled nanotube catalyst system was compared with neat anatase TiO2 nanotube (TiNT) and with calcined ZSM-5 powder. The results show for TiNT/ZSM-5 a highly enhanced efficiency for the decomposition of acetophenone (used as an aromatic model organic pollutant).

Photo-induced effects on self-organized TiO2 nanotube arrays: the influence of surface morphology.

Self-organized TiO(2) nanotubes with packed, vertically aligned morphology and different lateral characteristics were grown on Ti metal substrates by controlled electrochemical anodization in phosphate/HF and ethylene glycol/HF electrolytes. The wetting, photo-induced superhydrophilicity, and photocatalytic activity of the nanotubular materials were investigated under ultraviolet irradiation. The photoactivity of the TiO(2) nanotube arrays was analysed in terms of their morphological characteristics that were determined by means of scanning electron microscopy and atomic force microscopy in conjunction with geometrical modelling. The wetting and the UV-induced superhydrophilicity could be accordingly modelled by the Cassie-Baxter mode arising from the large scale roughness of the nanotubular arrays in combination with the Wenzel mode due to the small scale roughness induced by ridges at the outer tube surface. The photocatalytic activity of the TiO(2) nanotube arrays was further found to correlate quantitatively with the variation of the geometric roughness factor, verifying the strong impact of morphology on the photo-induced properties of the vertically oriented TiO(2) tubular architecture.

Black TiO2 nanotubes: Efficient electrodes for triggering electric field-induced stimulation of stem cell growth.

TiO2 nanostructures represent a key platform for biomedical applications, due to the combination of biocompatibility and high surface area. Especially TiO2 nanotube layers have been widely investigated due to controllable nanotopographic effects as well as for electrodes in electrostimulation experiments. In the present work we produce Ar/H2-reduced 'black' TiO2 nanotube arrays with a strongly enhanced electrical conductivity and explore their interaction with mesenchymal stem cells when used as electrodes to apply electric fields (EF) across the cells. While we observe no significant change in cell adhesion and their focal contact formation on these high conductivity nanotubes, we do observe a rapid stem cell response when EF is engaged using the 'black' TiO2 nanotube arrays as electrodes. Compared to as-formed nanotube arrays, a faster stem cell growth was observed and a lower EF intensity caused an intracellular calcium level elevation. Our results indicate that the increased conductivity in TiO2 nanotubes significantly enhances the early stem cell response to minimal electric field stimuli. STATEMENT OF SIGNIFICANCE: The use of TiO2 nanostructures in biomedical applications is widely investigated, especially considering the nanostructured surface influence on the biomaterial-cell interactions. We have previously shown that an applied electric field (EF) on stem cells grown on TiO2 nanotubes leads to synergistic osteogenic stimulation in the absence of biochemical bone-inducing supplements. Here we report that black (i.e. highly conductive nanotubes obtained by reduction treatments) TiO2 nanotubes enable short-time EF effects on stem cells: we observe a faster stem cell growth and a significantly enhanced early stem cell response to minimal EF stimuli. The application of such nanostructures under electric field is promising for therapeutic interventions for bone regeneration and tissue engineering approaches.

Electrochemical formation of self-organized anodic nanotube coating on Ti-28Zr-8Nb biomedical alloy surface.

In recent years, Ti-Zr-Nb alloys have become increasingly attractive as biomedical implant materials. In the present communication, we report the formation of self-organized nanotube oxide layers on a Ti-28Zr-8Nb biomedical alloy surface in 1M (NH4)2SO4 containing 0.25M NH4F. The morphology of the nanotube layers (the diameter and the length) is affected by the electrochemical conditions used (applied potential and time). Under specific conditions oxide layers consisting of highly ordered nanotubes with a wide range of diameters and lengths can be formed, varying, respectively, from approx. 50 to 300nm and from approx. 500nm to 22microm. The present results are highly promising for this biomedical alloy, as the large surface area and the tunable nanoscale geometry of the surface oxide provide novel pathways for the interaction of the materials with biorelevant species, such as cells and proteins.

Enhanced performance of dye-sensitized solar cells based on TiO2 nanotube membranes using an optimized annealing profile.

We use free-standing TiO2 nanotube membranes that are transferred onto FTO slides in front-side illuminated dye-sensitized solar cells (DSSCs). We investigate the key parameters for solar cell arrangement of self-ordered anodic TiO2 nanotube layers on the FTO substrate, namely the influence of the annealing procedure on the DSSC light conversion efficiency. The results show that using an optimal temperature annealing profile can significantly enhance the DSSC efficiency (in our case η = 9.8%), as it leads to a markedly lower density of trapping states in the tube oxide, and thus to strongly improved electron transport properties.

Transparent TiO2 nanotube electrodes via thin layer anodization: fabrication and use in electrochromic devices.

In the present work, we describe an anodization process that is able to fully transform a thin Ti metal layer on a conductive glass into a TiO(2) nanotubular array. Under optimized conditions, nanotube electrodes can be obtained that are completely transparent and defect-free and allow electrochromic switching. These electrochromic electrodes show remarkable properties and can be directly integrated into devices.

Bioactivation of titanium surfaces using coatings of TiO(2) nanotubes rapidly pre-loaded with synthetic hydroxyapatite.

Apatite depositions from simulated body fluid (SBF) have been widely used for the in vitro assessment of the bioactivity of bone- and dental-implant materials. In previous work, we reported that titanium-based implant materials can be coated with an anodic TiO(2) nanotube layer which can significantly stimulate apatite formation. In the present work, we demonstrate that the tubular nature of such coatings makes them highly suitable for the application of a treatment called "alternative immersion method (AIM)", which preloads the coatings with synthetic hydroxyapatite. This treatment is indeed found to additionally promote natural apatite formation significantly. To study the AIM effect, layers of nanotubes with various diameters and crystal structures (amorphous, anatase/rutile) were produced, AIM-treated, and the formation of apatite in SBF10 (10mmol1(-1) HCO(3)(-)) was evaluated. The results show a drastic enhancement of apatite deposition rates (in some cases >20-fold acceleration) for AIM-treated TiO(2) nanotube layers in comparison with non-treated TiO(2) surfaces.

The efficiency of nanotube formation on titanium anodized under voltage and current control in fluoride/glycerol electrolyte.

The formation of nanotubes on titanium is compared for anodizing under controlled voltage and controlled current in a fluoride/glycerol electrolyte. Rutherford backscattering spectroscopy and nuclear reaction analysis are employed to determine the film compositions. Film morphologies are examined by electron microscopy. The findings reveal films of approximate composition TiO(2).0.15TiF(4) that probably also contain derivatives of glycerol. Controlled voltage conditions resulted in more uniform final nanotube dimensions, for a particular charge density, and the highest efficiency of film growth, with the charge of the titanium in the film representing ∼48% of the charge passed during anodizing. Under current control, the efficiency decreased from ∼40% to ∼23% with increase of the current density from 0.1 to 0.5 mA cm(-2). Further, the thickness of the barrier layer was sometimes enhanced under current control, possibly due to a non-uniform current distribution and consequently elevated local temperature.

Dye-sensitized solar cells based on thick highly ordered TiO(2) nanotubes produced by controlled anodic oxidation in non-aqueous electrolytic media.

Dye-sensitized solar cells (DSSCs) were prepared using TiO(2) nanotubes, grown by controlled Ti anodic oxidation in non-aqueous media. Smooth, vertically oriented TiO(2) nanotube arrays, presenting a high degree of self-organization and a length of 20 µm, have been grown using ethylene glycol electrolyte containing HF. As-grown nanotubes exhibit an amorphous structure, which transforms to the anatase TiO(2) crystalline phase upon post-annealing in air at 450 °C. Atomic force microscopy (AFM) revealed the porous morphology together with high roughness and fractality of the surface. The annealed tubes were sensitized by the standard N719 ruthenium dye and the adsorption was characterized using resonance micro-Raman spectroscopy and adsorption-desorption measurements. The sensitized tubes were further used as active photoelectrodes after incorporation in sandwich-type DSSCs using both liquid and solidified electrolytes. The efficiencies obtained under air mass (AM) 1.5 conditions, using a back-side illumination geometry, were very promising: 0.85% using a composite polymer redox electrolyte, while the efficiency was further increased up to 1.65% using a liquid electrolyte.

Unexpected adsorption of oxygen on TiO2 nanotube arrays: influence of crystal structure.

We present kinetics data of O2, n/iso-butane, CO2, and CO adsorbed at ultrahigh vacuum conditions on TiO2 nanotube (TiNTs) arrays produced by electrochemical anodization; amorphous and polycrystalline (anatase and mixed anatase/rutile) TiNTs have been studied addressing structure-activity relationships. Oxygen distinctly interacts with the TiNTs, whereas this process is not observed on fully oxidized TiO2 single crystals. Both molecularly and atomically bonded oxygen have been observed. Variations in the binding energies of alkanes were also detected.

Selective high-resolution electrodeposition on semiconductor defect patterns.

We report a new principle and technique that allows one to electrodeposit material patterns of arbitrary shape down to the submicrometer scale. We demonstrate that an electrochemical metal deposition reaction can be initiated selectively at surface defects created in a p-type Si(100) substrate by Si (++) focused ion beam bombardment. The key principle is that, for cathodic electrochemical polarization of p-type material in the dark, breakdown of the blocking Schottky barrier at the semiconductor/electrolyte interface occurs at significantly lower voltages at implanted locations than for an unimplanted surface. This difference in the threshold voltages is exploited to achieve selective electrochemical deposition.