Hidetaka Asoh

Bioactivation of titanium surfaces using coatings of TiO2 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.

Ordered hexagonal array of Au nanodots on Si substrate based on colloidal crystal templating

We report two types of site-selective metal deposition methods based on colloidal crystal templating. We discuss in particular the controllability of the morphology and crystallinity of Au nanodots depending of the choice of method.

Pt–Pd-Embedded Silicon Microwell Arrays

Si microwell arrays containing Pt–Pd thin film were fabricated by the chemical etching of a Si substrate through a polystyrene honeycomb mask using a patterned metal catalyst. The honeycomb mask, which was formed by the utilization of binary colloidal crystals composed of large silica spheres and small polystyrene spheres, acted as a mask for metal deposition. After immersing a locally Pt–Pd-coated Si substrate into a solution containing hydrofluoric acid and hydrogen peroxide, an ordered Si microwell array with a hexagonal arrangement could be obtained by site-selective metal-assisted chemical etching. Moreover, the notable feature of this process is that an isolated circular Pt–Pd film used as a catalyst remained at the bottom of each well after chemical etching.

Site-Selective Metal Patterning/Metal-Assisted Chemical Etching on GaAs Substrate Through Colloidal Crystal Templating

The fabrication of nano-/microsized columns and hole arrays on an n-GaAs substrate was carried out using a combination of colloidal crystal templating, electroless plating/two-step catalyzation, and subsequent metal-assisted chemical etching. Using self-assembled polystyrene spheres as a mask, metal particles, specifically Ag and Pd, were selectively deposited at the interspaces between the polystyrene spheres, resulting in the formation of metal honeycomb patterns on GaAs. Ordered GaAs column arrays were then fabricated by the chemical etching of the GaAs originating from the honeycomb-patterned Ag and Pd metals, which acted as etching catalysts. Each metal catalyst resulted in the formation of a characteristic etched structure and etching depth. Pd-etched column arrays exhibited conical structures, while Ag-etched specimens showed slender column structures owing to the anisotropic etching of the GaAs. Moreover, patterned-GaAs slender grooves exhibiting triangular cross sections were fabricated by Ag-assisted chemical etching. To obtain the groove structures, honeycomb-structural polystyrene spheres were used as an etching mask. After the chemical etching of the GaAs, patterned slender grooves were formed.

Periodic GaAs Convex and Hole Arrays Produced by Metal-Assisted Chemical Etching

Periodically ordered GaAs convex and hole arrays were fabricated through a combination of colloidal crystal templating and metal-assisted chemical etching using an ion-sputtered Pt–Pd catalyst. A change in the hydrofluoric acid concentration in the etchant results in different morphologies of hole arrays, such as circular and hexagonal holes. Pt–Pd-assisted chemical etching realizes an etching structure having an aspect ratio that is better than those realized by Ag-, Pd-, and Au-assisted chemical etching. GaAs undergoing Pt–Pd-assisted chemical etching exhibited anisotropy of the etching rates with respect to the two-dimensional crystallography of the substrate.

Formation of Periodic Microbump Arrays by Metal-Assisted Photodissolution of InP

The metal-assisted chemical etching of an InP substrate combined with UV irradiation was investigated. Microbump arrays with ordered intervals were fabricated by the site-selective photodissolution of an InP substrate using patterned noble-metal films as catalysts. The etching rate of the InP substrate using noble-metal catalysts was drastically accelerated by UV irradiation. The etching speed of the present metal-assisted photodissolution increased in the order of Au < Pd < Pt, corresponding to the order of the magnitude of the work function of each metal used in this study.

Triangle pore arrays fabricated on Si (111) substrate by sphere lithography combined with metal-assisted chemical etching and anisotropic chemical etching.

The morphological change of silicon macropore arrays formed by metal-assisted chemical etching using shape-controlled Au thin film arrays was investigated during anisotropic chemical etching in tetramethylammonium hydroxide (TMAH) aqueous solution. After the deposition of Au as the etching catalyst on (111) silicon through a honeycomb mask prepared by sphere lithography, the specimens were etched in a mixed solution of HF and H2O2 at room temperature, resulting in the formation of ordered macropores in silicon along the [111] direction, which is not achievable by conventional chemical etching without a catalyst. In the anisotropic etching in TMAH, the macropores changed from being circular to being hexagonal and finally to being triangular, owing to the difference in etching rate between the crystal planes.

Hexagonal geometric patterns formed by radial pore growth of InP based on Voronoi tessellation

To fabricate ordered geometric patterns consisting of InP nanoporous structures, a photoresist mask with periodic opening arrays was prepared by sphere photolithography. The diameter and interval of the openings of the photoresist mask could be controlled independently by adjusting the diameter of silica spheres used as a lens and the exposure time. Through this resist mask with a two-dimensional (2D) hexagonal array of openings, the pore growth of InP during anodic etching was investigated. The isolated openings could act as initiation sites for the radial growth of pores, resulting in the formation of hexagonal geometric patterns based on Voronoi tessellation in 2D space. With further anodic etching, inside the substrate, the growth direction of the pores changed from radial to perpendicular relative to the substrate. Moreover, by removing domains consisting of nanopores by anisotropic chemical etching, the fabrication of InP microhole arrays with circular and triangular cross sections was also achieved.

Sub-100-nm ordered silicon hole arrays by metal-assisted chemical etching

Sub-100-nm silicon nanohole arrays were fabricated by a combination of the site-selective electroless deposition of noble metals through anodic porous alumina and the subsequent metal-assisted chemical etching. Under optimum conditions, the formation of deep straight holes with an ordered periodicity (e.g., 100 nm interval, 40 nm diameter, and high aspect ratio of 50) was successfully achieved. By using the present method, the fabrication of silicon nanohole arrays with 60-nm periodicity was also achieved.

Anisotropic chemical etching of silicon through anodic oxide films formed on silicon coated with microspheres

Periodic inverted pyramid arrays in silicon were fabricated using a nonlithographic technique combining the localized anodization of the substrate and the subsequent anisotropic chemical etching in an alkali solution. A silicon oxide layer with a honeycomb pattern, which was produced by the anodization of a silicon substrate coated with self-assembled microspheres, served as a passivation layer for alkaline etching. After the removal of microspheres, the unoxidized silicon region was etched selectively by chemical etching in tetramethyl ammonium hydroxide, resulting in the formation of ordered inverted pyramid arrays in silicon. The interval between inverted pyramid holes could be controlled by changing the diameter of the spheres used as an initial mask for silicon anodization.