Au Hierarchical Nanostructure-Based Surface Modification of Microelectrodes for Improved Neural Signal Recording.

Microelectrodes are widely used for neural signal analysis because they can record high-resolution signals. In general, the smaller the size of the microelectrode for obtaining a high-resolution signal, the higher the impedance and noise value of the electrodes. Therefore, to improve the signal-to-noise ratio (SNR) of neural signals, it is important to develop microelectrodes with low impedance and noise. In this research, an Au hierarchical nanostructure (AHN) was deposited to improve the electrochemical surface area (ECSA) of a microelectrode. Au nanostructures on different scales were deposited on the electrode surface in a hierarchical structure using an electrochemical deposition method. The AHN-modified microelectrode exhibited an average of 80% improvement in impedance compared to a bare microelectrode. Through electrochemical impedance spectroscopy analysis and impedance equivalent circuit modeling, the increase in the ECSA due to the AHN was confirmed. After evaluating the cell cytotoxicity of the AHN-modified microelectrode through an in vitro test, neural signals from rats were obtained in in vivo experiments. The AHN-modified microelectrode exhibited an approximate 9.79 dB improvement in SNR compared to the bare microelectrode. This surface modification technology is a post-treatment strategy used for existing fabricated electrodes, so it can be applied to microelectrode arrays and nerve electrodes made from various structures and materials.

Fabrication of Oblique Submicron-Scale Structures Using Synchrotron Hard X-ray Lithography.

Oblique submicron-scale structures are used in various aspects of research, such as the directional characteristics of dry adhesives and wettability. Although deposition, etching, and lithography techniques are applied to fabricate oblique submicron-scale structures, these approaches have the problem of the controllability or throughput of the structures. Here, we propose a simple X-ray-lithography method, which can control the oblique angle of submicron-scale structures with areas on the centimeter scale. An X-ray mask was fabricated by gold film deposition on slanted structures. Using this mask, oblique ZEP520A photoresist structures with slopes of 20° and 10° and widths of 510 nm and 345 nm were fabricated by oblique X-ray exposure, and the possibility of polydimethylsiloxane (PDMS) molding was also confirmed. In addition, through double exposure with submicron- and micron-scale X-ray masks, dotted-line patterns were produced as an example of multiscale patterning.

Fabrication of a Novel Nanofluidic Device Featuring ZnO Nanochannels.

We developed a novel fabrication method for nanochannels that are easily scaled up to mass production by selectively growing zinc oxide (ZnO) nanostructures and covering using a flat PDMS surface to make hollow nanochannels. Nanochannels are used in the biotechnological and environmental fields, being employed for DNA analysis and water purification, due to their unique features of capillary-induced negative pressure and an electrical double-layer overlap. However, existing nanochannel fabrication methods are complicated, costly, and not amenable to mass production. Here, we developed a novel nanochannel fabrication method. The pillar-like dense ZnO nanostructures were grown in a solution process, which is easily applicable to mass production. The size of the fabricated ZnO nanostructures has a thickness of 30-300 nm and a diameter on the order of 102 nm, which are easily adjusted by synthesis times. The ZnO nanostructures were covered by the flat polydimethylsiloxane (PDMS) surface, and then the cracks between ZnO nanostructures served as hollow nanochannels. Because the suggested fabrication process has no thermal shrinkage, the process has higher production efficiency than existing nanochannel mass production methods based on the thermal/pressure process. The mechanical strength of the fabricated ZnO nanostructures was tested with repetitive tape peeling tests. Finally, we briefly verified the nanochannel performance by applying the nanochannel to the micro/nanofluidic system, whose performance is easily evaluated and visualized by current-voltage relation.

Highly-robust, solution-processed flexible transparent electrodes with a junction-free electrospun nanofiber network.

Flexible transparent electrodes (FTEs) are widely used in a variety of applications, including flexible displays and wearable devices. Important factors in FTE design include active control of electrical sheet resistance, optical transparency and mechanical flexibility. Because these factors are inversely proportional to one another, it is essential to develop a technique that maintains flexibility while actively controlling the sheet resistance and transparency for a variety of applications. This research presents a new method for fabricating transparent electrodes on flexible polyimide films using electrospinning and copper electroless deposition methods. A flat metal network-based electrode without contact resistance was fabricated by heat treatment and electroless deposition onto the electrospun seed layer. The fabricated FTEs exhibited a transparency exceeding 80% over the entire visible light range and a sheet resistance of less than 10.0 Ω sq-1. Due to the heat treatment process, the adhesion between the metal network and the substrate was superior to other electrospinning-based transparent electrodes. Applicable to the large-area manufacturing process, the standard deviation of the network density of the fabricated large-area FTE was about 1%. This study does not require the polymer casting technique and has further advantages for mass production of electrodes and application to various fields.

One-directional flow of ionic solutions along fine electrodes under an alternating current electric field.

Electric fields are widely used for controlling liquids in various research fields. To control a liquid, an alternating current (AC) electric field can offer unique advantages over a direct current (DC) electric field, such as fast and programmable flows and reduced side effects, namely the generation of gas bubbles. Here, we demonstrate one-directional flow along carbon nanotube nanowires under an AC electric field, with no additional equipment or frequency matching. This phenomenon has the following characteristics: First, the flow rates of the transported liquid were changed by altering the frequency showing Gaussian behaviour. Second, a particular frequency generated maximum liquid flow. Third, flow rates with an AC electric field (approximately nanolitre per minute) were much faster than those of a DC electric field (approximately picolitre per minute). Fourth, the flow rates could be controlled by changing the applied voltage, frequency, ion concentration of the solution and offset voltage. Our finding of microfluidic control using an AC electric field could provide a new method for controlling liquids in various research fields.