Formation Mechanism of Lotus-root-shaped Nanostructure during Two-step Anodization

A comparative study of two-step anodization with one-step anodization at constant voltage

Two-step anodization has been widely used because it can produce highly self-organized anodic TiO₂nanotubes, but the differences in morphology and current-time curve of one-step anodization and two-step anodization are rarely reported. Here, one-step anodization and two-step anodization were conducted at different voltages. By comparing the FESEM image of anodic TiO₂nanotubes fabricated by one-step anodization and two-step anodization, it was found that the variation of morphology characteristics is same with voltage. The distinction of morphology and current-time curve between one-step anodization and two-step anodization at the same voltage were analyzed: the nanotube average growth rate and porosity of two-step anodization are greater than that of one-step anodization. In the current-time curve, the duration of stage I and stage II in two-step anodization are significantly shorter than one-step anodization. The traditional field-assisted dissolution theory cannot explain the three stages of the current-time curves and their physics meaning under different voltages in the same fluoride electrolyte. Here, the distinction between one-step anodization and two-step anodization was clarified successfully by the theories of ionic current and electronic current and oxygen bubble mould.

Surface and Tribological Properties of Oxide Films on Aluminium Alloy through Fly-Ash Reinforcement

Hard anodizing has proven to be a helpful surface treatment for aluminium alloy and typically accompanied by the growth of a porous and highly flawed oxide layer. The presence of pores on the oxide surface can be taken as an advantage in improving the surface properties. Fly-ash particles are high in SiO2 and Al2O3 content and can be utilized as inexpensive strengthening particles, which can increase the wear resistance and microhardness of composite material. It was noticed that limited research had been carried out in utilizing fly-ash as reinforcement on composite oxide coating as a wear resistance candidate. Thus, this study focused on reinforcing fly-ash on oxide coating and investigating its tribological performance. The composite oxide coating was grown on AA2017 aluminium alloy through anodizing process. To understand the effect of anodizing time and fly-ash content, the parameters were varied from 5–60 min and 0–50 g/L, respectively. The findings suggested that 60 min of anodizing time provides the highest thickness and surface roughness at 35 µm and 6.5 µm, respectively. Interestingly, composite oxide coating with 50 g/L fly-ash provides the highest coating thickness but has the lowest roughness at 52 μm and 8.2 μm, respectively. The composite oxide coatings are observed to reduce friction only for a limited time, despite their potential in significantly reducing the wear rate. The wear mechanism observed was adhesion, micro-crack, and delamination. The findings of this study are believed to provide insight on the potential of fly-ash to be a reinforcement for wear-reduction materials.

The Implication of Spatial Statistics in Human Mesenchymal Stem Cell Response to Nanotubular Architectures

INTRODUCTION

In recent years there has been ample interest in nanoscale modifications of synthetic biomaterials to understand fundamental aspects of cell-surface interactions towards improved biological outcomes. In this study, we aimed at closing in on the effects of nanotubular TiO2 surfaces with variable nanotopography on the response on human mesenchymal stem cells (hMSCs). Although the influence of TiO2 nanotubes on the cellular response, and in particular on hMSC activity, has already been addressed in the past, previous studies overlooked critical morphological, structural and physical aspects that go beyond the simple nanotube diameter, such as spatial statistics.

METHODS

To bridge this gap, we implemented an extensive characterization of nanotubular surfaces generated by anodization of titanium with a focus on spatial structural variables including eccentricity, nearest neighbour distance (NND) and Voronoi entropy, and associated them to the hMSC response. In addition, we assessed the biological potential of a two-tiered honeycomb nanoarchitecture, which allowed the detection of combinatory effects that this hierarchical structure has on stem cells with respect to conventional nanotubular designs. We have combined experimental techniques, ranging from Scanning Electron (SEM) and Atomic Force (AFM) microscopy to Raman spectroscopy, with computational simulations to characterize and model nanotubular surfaces. We evaluated the cell response at 6 hrs, 1 and 2 days by fluorescence microscopy, as well as bone mineral deposition by Raman spectroscopy, demonstrating substrate-induced differential biological cueing at both the short- and long-term.

RESULTS

Our work demonstrates that the nanotube diameter is not sufficient to comprehensively characterize nanotubular surfaces and equally important parameters, such as eccentricity and wall thickness, ought to be included since they all contribute to the overall spatial disorder which, in turn, dictates the overall bioactive potential. We have also demonstrated that nanotubular surfaces affect the quality of bone mineral deposited by differentiated stem cells. Lastly, we closed in on the integrated effects exerted by the superimposition of two dissimilar nanotubular arrays in the honeycomb architecture.

DISCUSSION

This work delineates a novel approach for the characterization of TiO2 nanotubes which supports the incorporation of critical spatial structural aspects that have been overlooked in previous research. This is a crucial aspect to interpret cellular behaviour on nanotubular substrates. Consequently, we anticipate that this strategy will contribute to the unification of studies focused on the use of such powerful nanostructured surfaces not only for biomedical applications but also in other technology fields, such as catalysis.

Simulation of anodizing current–time curves and the morphology evolution of TiO2 nanotubes obtained in phosphoric electrolytes

The simulation and separation of anodizing current density–time curves obtained in mixed electrolytes.