Katsuo Mogi

Nanometer-level high-accuracy molding using a photo-curable silicone elastomer by suppressing thermal shrinkage

Although the so-called “labs-on-a-chip” or micro total analysis systems (micro TAS) fields hold high promise for applications in many fields, conventional fabrication processes based on the semiconductor industry such as photolithography have limitations in terms of productivity. Silicone elastomers are widely used for micromodeling and offer biocompatibility and chemical stability, but they are generally thermosetting and undergo unacceptable levels of shape deformation during curing. In this study, a photocurable silicone elastomer that has recently become commercially available was examined, and its basic optical, mechanical, and other related characteristics, along with its shape transfer capabilities, particularly its nanostructure replication characteristics, were measured in comparison with those of a representative existing thermosetting silicone elastomer. As a result, the photo-cured elastomer was shown to be superior to existing heat-cured silicone elastomers, having mechanical strength approximately three times greater, and was shown to have the same optical transmittance, extending from the near-IR to the near-UV regions. In addition, it was shown that the elastomer is sensitive to light in a wide range of wavelengths, from 254 to 600 nm, with no large difference in its curing characteristics, indicating that curing can be performed under a variety of common forms of illumination. Most importantly, the photocured elastomer provided extremely high replication accuracy due to its thermal shrinkage of less than 0.02%, compared to 2.91% in the heat-cured elastomer.

A Novel Fabrication Technique for Liquid-Tight Microchannels by Combination of a Paraffin Polymer and a Photo-Curable Silicone Elastomer

The development and growth of microfluidics has been mainly based on various novel fabrication techniques for downsizing and integration of the micro/nano components. Especially, an effective fabrication technique of three-dimensional structures still continues to be strongly required in order to improve device performance, functionality, and device packing density because the conventional lamination-based technique for integrating several two-dimensional components is not enough to satisfy the requirement. Although three-dimensional printers have a high potential for becoming an effective tool to fabricate a three-dimensional microstructure, a leak caused by the roughness of a low-precision structure made by a 3D printer is a critical problem when the microfluidic device is composed of several parts. To build a liquid-tight microchannel on such a low-precision structure, we developed a novel assembly technique in which a paraffin polymer was used as a mold for a microchannel of photo-curable silicone elastomer on a rough surface. The shape and roughness of the molded microchannel was in good agreement with the master pattern. Additionally, the seal performance of the microchannel was demonstrated by an experiment of electrophoresis in the microchannel built on a substrate which has a huge roughness and a joint.

Fabrication method of moth-eye using UV-curable polydimethylsiloxane with vitrification by vacuum ultraviolet light

Moth-eye, which is the nanoscale tapered pillars array and has a remarkable anti-reflection property, has been paid more attention to be coated on optical materials for high performance anti-reflection surface. However, there are some problems in conventional fabrication method of moth-eye structure, such as labor-intensive mold master for nanoimprint fabrication. In order to overcome these problems, we demonstrated a novel fabrication method of moth-eye structure, which is possible to apply on the large area of non-flat surfaces at low cost by printing (stamping). The process was realized with an UV-curable silicone elastomer and glass transformation from silicone elastomer to SiO2 by vacuum-ultraviolet light (VUV) exposure. As a result, the fabricated PDMS-made moth-eye showed 94.3 % transparency and the vitrified SiO2 one showed 95.5 %, compared with plain surface of 92.0 % in the visible range from 470nm to 700nm.

Conformation dependent non-linear impedance response of DNA in nanofluidic device

Applying high intensity electric field (>MV/m), a non-linear measurement of DNA was conducted by impedance spectroscopy. Low alternate voltage was enough to generate high field strength (0.5 MV/m) in the nanometer gap to several micrometer gap and providing non-faradaic condition. With high field strength, DNA was subjected to electrostatic forces, which induced the conformational change and motion. DNA interaction with alternated field would reflect to the non-liner responses of the impedance signal. We could differentiate the DNA characteristic based on length and conformation of 100, 500, 1000, 5000, 10000bp and λ DNA (48kbps). With this format, we obtained 4 times higher sensitivity than the conventional impedance measurement. Furthermore, the mechanism was qualitatively proofed by the visualized behavior of DNA by fluorescent microscopy.

Release agent-free low-cost double transfer nanoimprint lithography for moth-eye structure

Glass is one of the essential materials for optical devices due to its transparency. It usually loses about 4% of energy of an incident light at its interface. In order to suppress its reflection, moth-eye structure, which is the nanoscale tapered pillar array, has a remarkable anti-reflection performance. In its fabrication method, nanoimprint lithography has been paid attention due to the availability for large area. However, there are some technological problems such as labor-intensive process and deterioration of the molds shape happened in surface treatment of release agent to fabricate moth-eye. In order to overcome these problems, we try to use polydimethylsiloxane (PDMS) as the mold due to ease of replication by its low surface energy, and apply to NIL method to fabricate moth-eye structure. As a result, we succeeded to fabricate the moth-eye structure on a glass by this process and raise its transparency from 92% to 96% at visible light range.

Trapping and isolation of single prokaryotic cells in a micro-chamber array using dielectrophoresis

The vast majority of prokaryotic species are difficult or impossible to culture in laboratories, which makes it difficult to study these organisms using conventional biochemical techniques. Methods that enable the physical isolation of single prokaryotic cells would thus facilitate the characterization of previously unstudied organisms by eliminating or reducing the need for cultivation. Most current methods for single-cell isolation were designed for eukaryotic cells mainly of mammals, which are non-motile and much larger than prokaryotic cells. We therefore developed a micro-chamber array-based method for the isolation of single prokaryotic cells using dielectrophoresis. Here, we demonstrated the applicability of the method using two prokaryotic species, Escherichia coli (bacteria) and Haloferax volcanii (archaea), which differ both in size and biochemical composition. Our results showed that cells of either organism are trapped with an applied electric field of 5 to 20 MV m−1 and 50 kHz to 3 MHz, and that the optimum combination of dielectrophoresis voltage and frequency depends on the cell type. We suggest that this technique is useful for trapping single cells of diverse prokaryotic species.

Three-dimensional visualizing method at nanoscale resolution for printing behavior

Printed electronics, which patterns electrical components for large area electronic devices by using printing technology, is paid more attention in electric industry to fabricate, such as flat panel displays, solar cells, flexible electronics, etc. In this field, the larger area and finer resolution is strongly required from the application-side. For this purpose, we proposed a soft nano-printing technology using elastomer-made printing-plate, which has a potential to realize nanoscale printing. However, the deformation of such a soft printing plate will induce another problem, which is the deformation of the printing plate during the printing. In this study, we developed an observation tools to visualize the deformation of printing plate and ink inside during printing process, in order to study both the influence of deformation of printing plate and the mechanism of printing. As a result, we successfully realized and demonstrated the 3D visualization of the printing plate and ink at the same time at nanometer resolution.

Nano-pattern molding technique using photocurable silicone elastomer

Heat-curable silicone elastomer was widely used as a material for micro/nano structure replication. However, deformation under the thermal residual stress imposed by heat curing substantially reduces their replication accuracy, which is a critical challenge for nanostructures. In this study, a photo-curable silicone elastomer was evaluated as a suitable material for nanostructure replication. The usability of the material was demonstrated by verification of optical and mechanical characteristics, and nanostructure replication accuracy. As the result, the cured elastomer was shown to be superior to existing heat-curable silicone elastomers to have mechanical strength, and was shown as same as in optical property. Most importantly, the photo-curable elastomer provided extremely high replication accuracy due to its thermal shrinkage less than 0.02%, compared to 2.91% in the heat-curable elastomer.