Hirofumi Hidai

The effect of micronscale anisotropic cross patterns on fibroblast migration

Cell movement on adhesive surfaces is a complicated process based on myriad cell-surface interactions. Although both micron and nanoscale surface topography have been known to be important in understanding cell-materials interactions, typically only simple patterns (e.g., parallel lines or aligned posts) have been used in studying cell morphology, migration, and behavior. This restriction has limited the understanding of the multidirectional aspects of cell-surface response. The present study was performed to investigate cell morphology and motility on micronscale anisotropic cross patterns and parallel line patterns having different aspect ratios (1:2, 1:4, and 1:infinity), grid size (12-, 16-, and 24-mum distance neighboring longer side ridges), and height of ridges (3- and 10-mum). The movement characteristics were analyzed quantitatively with respect to cell migration speed, migration angle, persistence time (P) and motility coefficient (mu). A significant effect of the 1:4 grid aspect ratio cross patterns and parallel line patterns on cell alignment and directionality of migration was observed. Cell motility was also dependent on the patterned surface topography: the migration speed was significantly enhanced by the 1:2 and 1:4 cross patterns when the grid size was smaller than the size of individual cells (i.e., approximately 16 microm). In addition, the migration speed of cells on lower patterns was greater than on higher ridges. Overall, cell morphology and motility was influenced by the aspect ratio of the cross pattern, the grid size, and the height of ridges.

Picosecond optical vortex pulse illumination forms a monocrystalline silicon needle

The formation of a monocrystalline silicon needle by picosecond optical vortex pulse illumination was demonstrated for the first time in this study. The dynamics of this silicon needle formation was further revealed by employing an ultrahigh-speed camera. The melted silicon was collected through picosecond pulse deposition to the dark core of the optical vortex, forming the silicon needle on a submicrosecond time scale. The needle was composed of monocrystalline silicon with the same lattice index (100) as that of the silicon substrate, and had a height of approximately 14 μm and a thickness of approximately 3 μm. Overlaid vortex pulses allowed the needle to be shaped with a height of approximately 40 μm without any changes to the crystalline properties. Such a monocrystalline silicon needle can be applied to devices in many fields, such as core–shell structures for silicon photonics and photovoltaic devices as well as nano- or microelectromechanical systems.

Fabrication of arbitrary polymer patterns for cell study by two‐photon polymerization process

Topographically patterned surfaces are known to be powerful tools for influencing cellular functions. Here we demonstrate a method for fabricating high aspect ratio ( approximately 10) patterns of varying height by using two-photon polymerization process to study contact guidance of cells. Ridge patterns of various heights and widths were fabricated through single laser scanning steps by low numerical aperture optics, hence at much higher processing throughput. Fibroblast cells were seeded on parallel line patterns of different height ( approximately 1.5-microm, approximately 0.8-microm, and approximately 0.5-microm) and orthogonal mesh patterns ( approximately 8-microm and approximately 4-microm height, approximately 5-microm and approximately 5.5-microm height, approximately 5-microm and approximately 6-microm height). Cells experienced different strength of contact guidance depending on the ridge height. Our results demonstrate that a height threshold of nearly 1 microm influences cell alignment on both parallel line and orthogonal mesh patterns. This fabrication technique may find wide application in the design of single cell traps for controlling cell behavior in microdevices and investigating signal transduction as influenced by surface topology.

Metal particle manipulation by laser irradiation in borosilicate glass

We propose a new technique of manipulating a metal particle in borosilicate glass. A metal particle that is heated by laser illumination heats the surrounding glass by radiation and conduction. A softened glass enabled metal particle migration. A 1-µm-thick platinum film was deposited on the back surface of a glass plate and irradiated with a green CW laser beam through the glass. As a result, the platinum film was melted and implanted into the glass as a particle. Platinum particles with diameters of 3 to 50 μm migrated at speeds up to 10 mm/s. In addition to platinum particles, nickel and austenitic stainless steel (SUS304) particles can be implanted.

Moving force of metal particle migration induced by laser irradiation in borosilicate glass

We optically manipulated a metal particle in borosilicate glass. The glass in the neighborhood of the laser-heated metal particle softened; hence, the metal particle was able to migrate in the glass. In this letter, the driving force of the metal particle toward the light source in the glass provided by laser illumination was investigated. The variation in the surface tension of the glass at the interface between the glass and the metal particle induced by the temperature gradient was calculated via a numerical temperature calculation. It was found that the temperature at the laser-illuminated surface of a stainless-steel particle with a radius of 40 μm was ~320 K higher than that on the nonilluminated side. The force applied to the metal particle from the surrounding glass was calculated to be ~100 μN, which was approximately equal to the viscous resistance force. In addition, the experimental and numerically calculated speeds of the moving particle, which was measured while varying the laser power, are discussed.

Formation of a buried silver nanowire network in borosilicate glass by solid-state ion exchange assisted by forward and reverse electric fields

Using electric-field-assisted solid-state ion exchange, we formed a buried silver nanowire network in borosilicate glass. This procedure had two stages: a silver doping stage by applying voltage with silver as the anode (referred to as forward) and a silver precipitation stage by applying voltage in the opposite direction (referred to as reverse). Microscopic observations revealed many needle-like precipitates (100–300 nm in diameter) linked to each other, forming a thin layer at the bottom of the silver-doped area. The configuration of the layer formed in the glass matrix was precisely transferred from that of the dopant, silver foil in the present study. The embedded electrical wiring in the glass slide was tested using a patterned circuit-like silver foil as a dopant. Measuring the electrical resistance between two ends of the formed wire, we found that the embedded layer had high conductivity and acted as an electrical circuit.

Laser micro-machinability of borosilicate glass surface-modified by electric field-assisted ion-exchange method

In order to improve the laser micro-machinability of borosilicate glass, the glass surface was doped with metal (silver or copper) ions by an electric field-assisted ion-exchange method. Doped ions drifted and diffused into the glass substrate under a DC electric field. The concentration of metal ions within the doped area was approximately constant because the ion penetration was caused by substitution between dopant metal and inherent sodium ions. Nanosecond ultraviolet laser irradiation of metal-containing regions produced flat, smooth and defect-free holes. However, the shapes of holes were degraded when the processed hole bottoms reached ion penetration depths. A numerical analysis of ionic drift-diffusion behaviour in glass material under an electric field was also carried out. The calculated results for penetration depth and ionic flux showed good agreement with the measured values.

Experimental and theoretical study on the driving force and glass flow by laser-induced metal sphere migration in glass

Light is able to remotely move matter. Among various driving forces, laser-induced metal sphere migration in glass has been reported. The temperature on the laser-illuminated side of the sphere was higher than that on the non-illuminated side. This temperature gradient caused non-uniformity in the interfacial tension between the glass and the melted metal as the tension decreased with increasing temperature. In the present study, we investigated laser-induced metal sphere migration in different glasses using thermal flow calculations, considering the temperature dependence of the material parameters. In addition, the velocity of the glass flow generated by the metal sphere migration was measured and compared with thermal flow calculations. The migration velocity of the stainless steel sphere increased with increasing laser power density; the maximum velocity was 104 μm/s in borosilicate glass and 47 μm/s in silica glass. The sphere was heated to more than 2000 K. The temperature gradient of the interfacial tension between the stainless steel sphere and the glass was calculated to be −2.29 × 10−5 N/m/K for borosilicate glass and −2.06 × 10−5 N/m/K for silica glass. Glass flowed in the region 15–30 μm from the surface of the sphere, and the 80-μm sphere migrated in a narrow softened channel.

Laser-induced nickel sphere migration and nanoparticle precipitation in silica glass

Techniques to control the colors and properties of glasses based on doping of the glasses with various metals and nanoparticles are widely used. In this paper, we demonstrate the migration of a nickel sphere in silica glass caused by laser illumination accompanied by nickel nanoparticle precipitation in the sphere migration trajectory. During migration, the diameter of the nickel sphere decreased. Precipitated nanoparticles with diameters of several hundred nanometers were observed in areas of up to 50 μm in radius and these nanoparticles formed four cylindrical coaxial layers with stripes at 10–20 μm intervals in the migration direction.

Selective component nanoparticle precipitation from Invar 42 during metal particle migration by laser irradiation in silica glass

ABSTRACT Laser illumination on a nickel sphere in silica glass has shown to migrate the sphere towards the light source with a nickel nanoparticle precipitated around the sphere. The selective nanoparticle precipitation by Invar 42 sphere migration in silica glass is reported. An Invar sphere is implanted into glass by laser illumination of an Invar foil. In addition to the sphere migration in glass, stripes are formed along the trajectory of the sphere. The stripes consist of metal nanoparticles with a diameter of several hundred nanometres. Nanoparticles precipitated on the laser-illuminated side are composed only of nickel and those on the non-illuminated side are composed of nickel and iron alloys. The reason for the difference in components is discussed.