Published papers

© 2018 by the authors. The anatomy of a tooth was the inspiration for this tactile sensor study. The sensor consisted of a pole that was fixed in the middle of an acrylic base using a viscoelastic silicone elastomer. Four strain gauges were fixed three-dimensionally around the pole to detect its movement, which was formed in a single step in the assembly. When the load was applied to the side of the pole, the strain gauges were bent or released, depending on the direction of the applied load and the position of the strain gauges. The sensor device had the sensitivity of 0.016 mm -1 and 0.313 N -1 against the resistance change ratio. For the load detection experiment, a consistent pattern of full sine-curve, with a constant resistance change for the angles, was obtained for all of the four strain gauges, which confirmed the reliability of the sensor device to detect the direction of applied load. The amplitudes of the resistance change ratio remained to be consistent after loading-unloading processes at the frequency of 0.05-0.25 Hz.

© 2018 by the authors. Polymer-based flexible micro electro mechanical systems (MEMS) tactile sensors have been widely studied for a variety of applications, such as medical and robot fields. The small size and flexibility are of great advantage in terms of accurate measurement and safety. Polydimethylsiloxane (PDMS) is often used as the flexible structural material. However, the sensors are likely subject to large parasitic capacitance noise. The smaller dielectric constant leads to smaller influences of parasitic capacitance and a larger signal-to-noise ratio. In this study, the sensor underwent ultraviolet (UV) exposure, which changes Si-CH3 bonds in PDMS to Si-O, makes PDMS nanoporous, and leads to a low dielectric constant. In addition, we achieved further reduction of the dielectric constant of PDMS by washing it with an ethanol-toluene buffer solution after UV exposure. This simple but effective method can be readily applicable to improve the signal-to-noise ratio of PDMS-based flexible capacitive sensors. In this study, we propose reduction techniques for the dielectric constant of PDMS and applications for flexible capacitive force sensors.

Copyright © 2018 American Chemical Society. Room-temperature liquid metals such as GaInSn or EGaIn present the most attractive properties for soft and highly stretchable electronics. Recently, several methods have been investigated to functionalize the surface of the liquid metal via coatings and encapsulation. However, most can hardly be extended to other samples than droplets. In this study, we focus on the tunability of the process of galvanic replacement of Ga alloys with gold to form thin-film encapsulation. We characterized in-depth the obtainable composition and structure of a noble metal shell formed on the liquid metal via scanning electron microscopy, energy-dispersive X-ray, and topographic laser microscopy and highlighted the change in mechanism of galvanic replacement in different pH ranges. We showed the tunability of the surface morphology selection of different pH ranges, the solutions concentrations, and the reaction time. The adjustment of the pH of KAuBr4 solution to the preferential Ga2O3-free domain led to the successful formation of a sub-micrometer thin uniform coating with more than 60% of Au and reduced level of oxygen from 30% down to 10%. We finally demonstrated the effect of the coating composition on the electrical properties of the liquid metal using a simple and fast phase-drop measurement setup on the droplet and microchannels. A high correlation between the amount of noble metal deposited and the electrical properties of the droplets was demonstrated.

© 2018 by the authors. This paper demonstrates fatigue assessment based on eye blinks that are detected by dye-sensitized photovoltaic cells. In particular, the sensors were attached to the temple of eyeglasses and positioned at the lateral side of the eye. They are wearable, did not majorly disturb the user's eyesight, and detected the position of the eyelid or the eye state. The optimal location of the sensor was experimentally investigated by evaluating the detection accuracy of blinks. We conducted fatigue assessment experiments using the developed wearable system, or smart glasses. Several parameters, including the frequency, duration, and velocity of eye blinks, were extracted as fatigue indices. Successful fatigue assessment by the proposed system will be of great benefit for maximizing performance and maintenance of physical/mental health.

© 2018 by the author. Tactile displays have been extensively studied for several decades. However, owing to their bulkiness and stiffness, it has been difficult to integrate these displays with information devices to enable tactile communication between the devices and their users. This paper proposes a novel sheet-type electrovibration tactile display that consists of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate conductive layers and an insulation layer of polydimethylsiloxane. The tactile display is sufficiently thin and flexible for attaching onto various surfaces. In this study, the tactile display was micro-fabricated and characterized through experiments. The experimental results indicated that the tactile display exhibited good durability under bending and that it could present various tactile sensations depending on the type of voltage waveform. In addition, the effect of using a combination of electrovibration and thermal stimuli was also demonstrated. The sheet-type display was attached onto a Peltier element; the thinness of the structure enabled the display to conform to the element and ensure good heat transfer. In the experiment, subjects were asked to scan the display with their fingertips. The results showed that multiple tactile stimuli were also successfully perceived by the subjects.

© 2018 Elsevier B.V. Current methods of generating asymmetric lipid vesicles produce only single types of vesicles, which poses a challenge for investigation of vesicles with different lipid leaflet combinations using a single device. Here, we describe a device for sequentially generating asymmetric lipid giant vesicles (GVs) with various combinations of asymmetric lipid leaflets. Various combinations of planar asymmetric lipid bilayers are formed by sliding and contacting the water in oil (phospholipid) (W/O) droplets in the collecting and jetting wells of our device. Next, we generate asymmetric lipid vesicles using a pulsed-jetting method. We sequentially generate three types of GVs: two asymmetric GVs containing fluorescent-conjugated phospholipids (either (i) rhodamine or (ii) BODIPY) on the outer leaflet, and (iii) a symmetric GV with phosphatidylcholine (PC) on the inner and outer leaflets. The use of asymmetric GVs with various combinations of asymmetric lipid leaflets reveals that increase in membrane phosphatidylethanolamine (PE) concentration influences cinnamycin activity and promotes the phospholipid flip-flop dynamics. This system will be useful for investigating activities of proteins or peptides on GV membranes with various combinations of lipid leaflets.

© 2018 The Royal Society of Chemistry. Two-dimensional (2D) microdroplet arrays with indexed sample concentration gradients have been receiving considerable attention for high-throughput biological and medical analyses. However, the preparation of such an array by conventional methods mandates precise pipetting and/or pumping. In this paper, we introduce a method to spontaneously generate 2D-arrayed aqueous droplets using a well array, for which coarse pipetting is sufficient. The wells are connected in rows and columns via narrow channels. Aqueous solutions impregnated in the well array are split into droplets in every single well as a subsequently introduced immiscible solvent self-propagates and divides the solution at the channels. A concentration gradient of the samples can be formed across the connected solution in the well array; once droplets are generated, each droplet possesses a different sample concentration depending on its position in the array. We experimentally determined the optimal well dimensions and solvent species to obtain a high yield of droplet generation. We next demonstrated a 2D droplet array with a two-sample concentration gradient. Finally, the applicability of the system was demonstrated through a cell viability assay using a sample that induced apoptosis. We believe the proposed method contributes to simplification and miniaturization of the system to generate droplet arrays and thus is applicable to biological and medical analysis.

© 2018 by the authors. This paper demonstrates a triple-coaxial flow device to continuously produce a hydrogel microtube using a microfluidic technique. The hydrogel microtube can encapsulate a microbial suspension, while allowing the diffusion of oxygen and nutrients into the microtube and preventing microbes from passing into or out of the microtube. The microtubes also enable the collection of the microbes after task completion without contaminating the environment. In our previous study, we used a double-coaxial flow device to produce the microtubes, but continuous production was a challenge. In the present study, we developed a microfluidic device that fabricates a triple-coaxial flow to enable continuous production of the microtubes. Here, we characterize the production capacity of the microtubes along with their properties and demonstrate bioremediation using microtubes encapsulating a microbial suspension.

© 2018 by the authors. Forming microspheres or microbeads from nanofibrous materials has recently attracted research interest for their applications in various fields, because these structures greatly impact cellular behaviors and functions. However, conventional methods of preparing microspheres or microbeads have limitations, such as limited variety of material. Here, we propose a new fabrication process for forming a nanofibrous microsphere composed of bacterial cellulose (BC), which is synthesized through fermentation by specific bacteria. The process uses a hydrogel fiber containing spherical cavities. The bacteria encapsulated into the cavities produce BC, resulting in the formation of BC microspheres. Because of its simplicity, robustness, and cost-effectiveness, this process is promising for applications, such as in biochemical engineering and cell delivery systems.

© 2017 Elsevier B.V. Sloshing is the movement of liquid in a container induced by an external force. The phenomenon has been extensively studied at the macroscopic scale because of the importance of liquid control in tanks. Sloshing at the microliter scale should be explored for microdroplets in Lab-on-a-chip applications. We therefore verified the sloshing characteristics of droplets in microliter-sized wells. A water-in-oil droplet in the well was vibrated horizontally at between 20 and 100 Hz. Circular and elliptic wells between 2 and 5 mm sizes were applied for the well geometry. The surface chemistry of the well was additionally changed by using amphiphobic coatings to examine the effect of the interfacial tension between the well surface and the liquid, because we considered it a dominant factor at the microliter scale. Decreasing the well diameter caused the natural frequency to increase and the sloshing to suppress. This agreed with Housner's equation, which is conventionally used for macroscopic scale. The experiments also revealed that decreasing the interfacial tension caused the natural frequency to increase and resulted in drastic suppression of the sloshing, which was characterized by Bond number, similarly with the cases under microgravity. We experimentally verified that at the microliter scale, the interfacial tension is dominant to control the sloshing phenomenon.