Published papers

Direct wafer bonding of silicon wafers is a promising technology for manufacturing three-dimensional complex microelectromechanical systems as well as silicon-on-insulator substrates. Previous work has reported that the bond quality declines with increasing surface roughness, however, this relationship has not been quantified. This article explicitly correlates the bond quality, which is quantified by the apparent bonding energy, and the surface morphology via the bearing ratio, which describes the area of surface lying above a given depth. The apparent bonding energy is considered to be proportional to the real area of contact. The effective area of contact is defined as the area sufficiently close to contribute to the attractive force between the two bonding wafers. Experiments were conducted with silicon wafers whose surfaces were roughened by a buffered oxide etch solution (BOE, HF:NH4F=1:7) and/or a potassium hydroxide solution. The surface roughness was measured by atomic force microscopy. The wafers were direct bonded to polished "monitor" wafers following a standard RCA cleaning and the resulting bonding energy was measured by the crack-opening method. The experimental results revealed a clear correlation between the bonding energy and the bearing ratio. A bearing depth of similar to1.4 nm was found to be appropriate for the characterization of direct-bonded silicon at room temperature, which is consistent with the thickness of the water layer at the interface responsible for the hydrogen bonds that link the mating wafers.

High-precision fabrication is indispensable for high-speed silicon micro-rotors for power MEMS applications so as to minimize the rotor imbalance that deteriorates the rotor performance. Etch variation of deep reactive ion etch (DREE) process results in differences in rotor blade heights and thus rotor imbalance. A Fourier transform of the etch non-uniformity along the rotor circumference revealed the global etch variation across the wafer and local variations in etch rates depending on the concentration or proximity of the patterned geometry. Rotor imbalance arising from the global etch variation of DRIE process was estimated, which compared favorably to results obtained from spinning experiments. The global etch non-uniformity which culminates in rotor imbalance could be alleviated to 0.25% across a rotor of 4.2 mm diameter by optimizing the plasma chamber pressure. The developed DRIE recipe successfully reduced the rotor imbalance and thus enhanced the rotordynamic performance. The manufacturing processes presented herein are readily applicable to the constructions of other microstructures containing intricate geometries and large etched areas.

This paper presents microsized soft-magnetic rotational wings in an alternating magnetic field. The wireless power supply as well as control of magnetic microwings are readily applicable to microflight mechanisms. As a magnetic material, soft magnetic material, such as nickel-iron alloy was deposited via electroplating, which is a well-developed MEMS batch process. The magnetization of the soft-magnetic device, however, changes its direction and magnitude during rotation, where the shape magnetic anisotropy of the wings plays a vital role. The principle of the wing rotation in an alternating magnetic field was elucidated theoretically and showed favorable agreement with the experimental results with large-sized models. The aerodynamic performances of soft magnetic rotational wings were evaluated, including the microrotational wings 165 mug in weight consisting of 0.9-mm-long magnetic wings made of electroplated nickel-iron alloy and an axis of wing rotation made of a sharpened glass tube. The micromagnetic rotational wings presented here were utilized for microflight mechanisms, which achieved successful flights.

Multi-stack wafer bonding is one of the most promising fabrication techniques for creating three-dimensional (3D) microstructures. However, there are several bonding issues that have to be faced and overcome to build multilayered structures successfully. Among these are: (1) chemical residues on surfaces to be bonded originating from the fabrication processes prior to bonding; (2) increased stiffness due to multiple bonded wafers and/or thick wafers; (3) bonding tool effects; (4) defect propagation to other wafer-levels after high-temperature annealing cycles. The problems and the solutions presented here are readily applicable to any microelectromechanical. systems project involving the fabrication of multi-stack structures of two or more wafers containing intricate geometries and large etched areas.

This paper reports development of a hydrocarbon-fueled micro-combustion system for a micro-scale gas turbine engine for power generation and micro-propulsion applications. A three-wafer catalytic combustor was fabricated and tested. Efficiencies in excess of 40% were achieved for ethylene-air and propane-air combustion. A fabrication process for a six-wafer catalytic combustor was developed and this device was successfully constructed.

Dynamics of a three-dimensionally movable microflight mechanism were analyzed both theoretically and experimentally. The microflight mechanism is composed of magnetic rotational wings that rotate and generate thrust in an alternating magnetic field and a body with magnetic anisotropy that contributes to attitude control. The device consisted of 2.5-mm-long wings weighing 3.5 mg which were fabricated with MEMS technology. A wing rotational frequency of 500 Hz provided enough thrust for liftoff. Experimental data obtained through high-speed camera images show good agreement with theory and also quantify the magnetic anisotropy of the microflight mechanism, which cannot be estimated theoretically. Simultaneous actuation and attitude control by an external magnetic field presented herein, which culminated in simplification and small weight of the device and thus the successful flight, is applicable to other MEMS devices.

マイクロロボット移動用の回転翼に使用される磁性体は軟磁性体であり，回転中に磁化の大きさ，方向ともに変化する．本論文において，回転中の磁化および磁気トルクの変化を，計算および数ミリサイズのラージスケール磁性体翼による実験から解析した．磁性体の磁気特性とともに，形状磁気異方性が重要な役割を担っていることを明らかにした．また翼長0.9ミリメートルの磁性体回転翼をMEMS (Microelectromechanical Systems) 製作技術を用いて実現した．

マイクロロボットの移動には，地面との摩擦がなく，かつ３次元の移動が可能な飛行が有効である．本論文では，推力を得る方法として，交流磁場中で回転する磁性体回転翼を提唱し，数ミリサイズのラージサイズモデルによる実験を通じ，実現可能性を示した．

In this paper, we present a three-layered device for preparation of heterologous core layers linearly aligned in a hydrogel sheet by a simple four-step process without using pumps. Recently, cell-based sensors have been developed for detection of odorant molecules owing to the sensitivity and selectivity of olfactory receptors to chemical substances. To detect various odorant molecules with these sensors, it is necessary to array multiple types of sensor cells expressing different olfactory receptors. We therefore propose a sheet-shaped sensor chip for a sensor-cell array. Previously, hydrogel sheets with heterogeneous cell patterns were produced using microfluidic devices with syringe pump systems, causing loss of both sample volumes and time for settings. Here, we develop a device that enables preparation of hydrogel sheets with a small amount of samples and with a single manual pipette.

This paper reports successful re-formation of a lipid bilayer using an air bubble. Lipid bilayers have been used for membrane protein studies and biosensor applications. Droplet-contact-method is a simple and effective method to form the bilayers. A major challenge in using the bilayers originates from their vulnerability against physical and electrical disturbance, which leads the bilayer rapture and the droplet fusion. We previously proposed a concept to re-form a bilayer by introducing an air bubble to cut-and-split the merged droplet. In this work, we investigated the key parameters to guide the bubble and drastically enhanced the yield up to 97%.

Titanium is widely used in biomedical components. As a promising advanced manufacturing process, electropolishing (EP) has advantages in polishing the machined surfaces of material that is hard and difficult to cut. This paper presents the fabrication of a titanium microchannel using the EP process. The Taguchi method was adopted to determine the optimal process parameters by which to obtain high surface quality using an L9 orthogonal array. The Pareto analysis of variance was utilized to analyze the three machining process parameters: applied voltage, concentration of ethanol in an electrolyte solution, and machining gap. In vitro experiments were conducted to investigate the fouling effect of blood on the microchannel. The result shows that an applied voltage of 20 V, an ethanol concentration of 20 vol.%, and a machining gap of 10 mm are the optimum machining parameters by which to enhance the surface quality of a titanium microchannel. Under the optimized machining parameters, the surface quality improved from 1.46 to 0.22 µm. Moreover, the adhesion of blood on the surface during the fouling experiment was significantly decreased, thus confirming the effectiveness of the proposed method.

This paper reports a cartridge-type device to produce sensor cells with spontaneous solution exchange. Our group has developed a cell-based odorant sensor expressed olfactory receptors. These sensor cells presented excellent sensitivity and specificity, which would be useful for a wide range of sensor applications. However, production of such sensor cells requires multiple times of aseptic solution exchanges. In this work, we develop a cartridge-type cell culture device that utilizes capillary force for spontaneous solution exchange without pumps or tubes. When the cartridge is set to the exchange stage, the specified volume of solution is exchanged immediately. We experimentally examine the solution exchange, and demonstrate the response of a sensor cell to a chemical stimulus in the exchanged solution. We consider that the device would be useful to produce multiple types of sensor cells.

In order to improve the quality of life of dialysis patients, our group have been developing an implantable hemofiltration device (IHFD) composed of multiple layers of dialysis membranes and microfluidic channels. To improve the hemodialysis performance of IHFD, preventing the negative filtration, which is caused by the oncotic pressure of blood, is mandatory. In this study, we fabricated IHFDs with five different microchannel designs and experimentally investigated the performance of each device in in vitro experiment. In addition, the successful IHFD was further evaluated by ex vivo experiments with a beagle dog. The experiments verified the effectiveness of the microchannel design, which will be used for the IHFD for in vivo experiments with pigs in the future.

This paper describes a method capable of individual and selective re-formation of ruptured BLMs (bilayer lipid membranes) by using air bubbles in arrayed devices. BLMs play an important role in membrane protein studies, biosensor applications, and drug screening. In such applications, data throughput is a key issue, but BLMs are easily ruptured by physical/electrical disturbance, which makes it difficult to operate a large BLM array. In this work, we propose a bubble-assisted BLM re-formation method: An aqueous phase is divided by a small air bubble covered with a lipid monolayer. After the bubble departed, the divided aqueous phases are brought into contact with the lipid monolayer, resulting in formation of BLM. This method enables selective BLM re-formation in a multichannel device. To verify individual and selective BLM re-formation using this method, we developed a 4ch bubble-assisted device and demonstrated repetitive BLM re-formation.

This study describes a gas-flow device that enhances the solubility of gas-phase odorants into aqueous solution for cell-based sensors. Cell-based sensors using olfactory receptors have attracted attention because of their high sensitivity and selectivity. However, most of previous studies used liquid-phase odorants as a target sample due to their low solubility of gas-phase odorants even though the applications expect the gas-phase sensing. Therefore, we focus on gas-phase odorant detection using the gas-flow device. Here, we clarify the impact of the velocity of convection inside the solution to enhancement of the solubility, based on computational simulation. Moreover, we experimentally verify efficiency of odorant dissolution in two types of devices, compared with the negative control using a sample odorant.

Cell-based sensors using olfactory receptors have attracted attention because of their high sensitivity and selectivity. A challenge is efficient partition of gaseous samples to the aqueous solution where the cell-based sensors reside. In this paper, we developed a device with a gas-flow channel, which allowed inflow of gaseous odorants over a well filled with aqueous solution. We considered that shear stress induced by the gas-flow at the gas-liquid interface initiate convectional mixing inside liquid and in turn, promotes partitioning of gaseous odorants into the solution. Solubility of a gaseous odorant was experimentally found to increase with the mixing strength. We further demonstrated efficient detection of a gaseous odorant using the sensor cells with the developed gas-flow device.

Tactile displays, which present virtual surfaces and tactile information to the users, can potentially augment quality and quantity of information in human-computer interaction. In this presentation, we discuss how we solve the inverse problems of the tactile displays, i.e., how to derive the control parameters of the displays when the target tactile sensations are given, in which we propose the microfabricated tactile samples to deduce quantitative correlation between the surface properties and tactile perception.

In the present study, we propose a largely deformable and miniaturized soft actuator that is made by an elastic rubber bladder (called a pouch) with a low-boiling-point liquid. When the temperature of the low-boiling-point liquid reaches 34 - C, the liquid inside the pouch evaporates, and the whole structure inflates. Thanks to the proposed fabrication method, we can make a miniaturized pouch of approximately 5 mm in diameter with a thin rubber membrane, and the pouch can inflate to a volume of 86 times or more compared to its initial volume and can generate approximately 20 N at maximum. We calculated the deformation model and developed the fabrication process through investigation of the thickness and the inflation volume of the pouch with respect to the process parameters. We then experimentally characterized the actuator with respect to the generated force, time response, and repeatability of the inflation. We believe that micro elastic pouch motors will contribute to soft robotic systems as a new component as a result of having unique characteristics, such as millimeter size and large deformability.

An artificial cell membrane is applied to study the pore formation mechanisms of bacterial pore-forming toxins for therapeutic applications. Electrical monitoring of ionic current across the membrane provides information on the pore formation process of toxins at the single pore level, as well as the pore characteristics such as dimensions and ionic selectivity. However, the efficiency of pore formation detection largely depends on the encounter probability of toxin to the membrane and the fragility of the membrane. This study presents a bilayer lipid membrane array that parallelizes 4 or 16 sets of sensing elements composed of pairs of a membrane and a series electrical resistor. The series resistor prevents current overflow attributed to membrane rupture, and enables current monitoring of the parallelized membranes with a single detector. The array system shortens detection time of a pore-forming protein and improves temporal stability. The current signature represents the states of pore formation and rupture at respective membranes. The developed system will help in understanding the toxic activity of pore-forming toxins.