担当区分：筆頭著者,　責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）

Gel electrolytes are a promising material in the field of electronics, energy, and wearable devices due to their preferrable physical and electrochemical characteristics such as non-volatility, relatively large potential window, and high ionic conductivity. Ionic gels, however, possess toxicity that would potentially pollute environment and hinder application for environmental sensing. Decomposability and moderate toxicity are therefore required for disposable use. In this work, we develop a biodegradable ionic gel (BDiG) using a poly(ethylene glycol) diacrylate (PEGDA) combined with a biodegradable ionic liquid, 1-Butyl-3-methyl-Imidazolium bis(trifluoromethanesulfonyl)imide ([BMIM][TFSI]). The developed BDiG exhibites mechanical flexibility of fracture strain upwards of 300% and Young's modulus of 14 kPa. Electrical characteristics of the BDiG were a potential window of 4.8 V, ionic conductivity of 3.1 mS cm−1, and electrical double layer capacitance of 34 µF cm−2 when 90% weight ratio of ionic liquid was used. A tactile sensor is demonstrated as a proof of concept of ionic skin, whose electrical conductivity increases linearly with the pressure.

DOI： 10.1016/j.sna.2023.114574

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担当区分：筆頭著者,　最終著者,　責任著者   記述言語：英語   掲載種別：記事・総説・解説・論説等（学術雑誌）

With the development of wireless sensor networks, electrical waste that remains in the environment is an inevitable issue in achieving sustainability and progress in electronics. Transient electronics that disappear after a prescribed time are of interest in electronics and material sciences. Such devices comprise naturally sourced materials that degrade without harmful or toxic substances during biodegradation. Although there are reports on transient electronic devices, including transistors, sensors, and radio frequency circuits, insufficient research has been conducted on the energy storage essential for operating transient devices. This review highlights the recent progress in developing transient energy storage. First, materials for transient energy storage, including conductors, electrolytes, and gels, are introduced. Second, transient supercapacitors, pseudocapacitors, primary batteries, and secondary batteries, are described and summarized. Finally, this review concludes and discusses the prospect of transient energy storage. The continuous progress of transient batteries and integration with transient devices are promising for sustainable electronics in the future.

DOI： 10.1002/aesr.202300083

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担当区分：筆頭著者,　最終著者,　責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）

Choline lactate, an ionic liquid composed of bioderived materials, offers an opportunity to develop biodegradable electrochemical devices. Although ionic liquids possess large potential windows, high conductivity, and are nonvolatile, they do not exhibit electrochemical characteristics such as intercalation pseudocapacitance, redox pseudocapacitance, and electrochromism. Herein, bioderived ionic liquids are developed, including metal ions, Li, Na, and Ca, to yield ionic liquid with electrochemical behavior. Differential scanning calorimetry results reveal that the ionic liquids remained in liquid state from 230.42 to 373.15 K. The conductivities of the ionic liquids with metal are lower than those of the pristine ionic liquid, whereas the capacitance change negligibly. A protocol of the Organization for Economic Co-operation and Development 301C modified MITI test (I) confirms that the pristine ionic liquid and ionic liquids with metal are readily biodegradable. Additionally, an ionic gel comprising the ionic liquid and poly(vinyl alcohol) is biodegradable. An electrochromic device is developed using an ionic liquid containing Li ions. The device successfully changes color at −2.5 V, demonstrating the intercalation of Li ions into the WO3 crystal. The results suggest that the electrochemically active ionic liquids have potential for the development of environmentally benign devices, sustainable electronics, and bioresorbable/implantable devices.

DOI： 10.1002/smll.202302385

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担当区分：筆頭著者,　最終著者,　責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）

A facile, benign, and rapid dealloying method was employed to synthesize nanoporous Cu using an ammonium chloride (NH4Cl) saline solution and Mg as an etchant and a sacrificial metal, respectively. The large difference between the standard electrode potentials of Mg and Cu (−2.36 and +0.34 V, respectively) resulted in rapid etching at 10 nm/min and 303 K. Dealloying for 15 min yielded a nanoporous Cu foil, exhibiting a specific surface area of 24.1 m2/g. The raw materials Mg and NH4Cl are naturally abundant; Mg is classified as a biodegradable metal, while NH4Cl is recognized as a safe material by the United States Food and Drug Administration. No harmful byproducts were produced during dealloying. Therefore, the proposed dealloying method is environmentally benign and addresses the issue of the hazardous chemical waste produced via conventional electrochemical dealloying methods.

DOI： 10.1021/acsanm.3c00320

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担当区分：筆頭著者,　責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）

This paper presents a novel time-logging system using a real time clock (RTC) powered by a vibrational energy harvester, triggered by a piezoelectric polyvinylidene fluoride (PVDF) film as an event-driven sensor. The power-management circuit normally remains in sleep mode to cut off the power supply to the micro-processor and the ferroelectric random access memory (FRAM), while the RTC alone is continuously powered by a battery to keep the time for seamless operation, thereby lowering the standby power consumption to as small as 231 nW. The power management circuit for a PVDF power generator can supply a load current upwards of 8 mA at 3.3 Vdc for the external circuit. Time-of-event recording is performed with energy consumption of 568 µJ per event.

DOI： 10.1016/j.sna.2023.114306

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担当区分：筆頭著者,　最終著者,　責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）

A pseudo-capacitor with transient behavior is applied in implantable, disposable, and bioresorbable devices, incorporating an Na ion-doped bioderived ionic liquid, molybdenum trioxide (MoO3)-covered molybdenum foil, and silk sheet as the electrolyte, electrode, and separator, respectively. Sodium lactate is dissolved in choline lactate as a source of Na ions. The Experimental results reveal that the Na ions are intercalated into the van der Waals gaps in MoO3, and the pseudo-capacitor shows an areal capacitance (1.5 mF cm−2) that is three times larger than that without the Na ion. The fast ion diffusion of the electrolyte and the low resistance of the MoO3 and Mo interface result in an equivalent series resistance of 96 Ω. A cycle test indicates that the pseudo-capacitor exhibited a high capacitance retention of 82.8% after 10 000 cycles. The transient behavior is confirmed by the dissolution of the pseudo-capacitor into phosphate-buffered saline solution after 101 days. Potential applications of transient pseudo-capacitors include electronics without the need for device retrieval after use, including smart agriculture, implantable, and wearable devices.

DOI： 10.1002/smll.202205598

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記述言語：英語   掲載種別：研究論文（学術雑誌）

In this paper, we report a moving magnet type electromagnetic actuator with three-degree-of-freedom, namely the displacement in XY axes and rotation around Z axis, by utilizing rare earth magnetic powder Sm-Fe-N. Parylene was utilized to fabricate springs to suspend four drive units and a center moving stage located in the four corners and the center of the actuator, respectively. The rare earth magnetic powder Sm-Fe-N with wax were embedded in the silicon frames of the drive units with a parylene layer, followed by magnetization by an 8 T pulse magnetic field to obtain permanent micromagnets. The motion of the center moving stage can be controlled by the drive units. To obtain an in-plane displacement in the XY axes, compensation coils were fabricated in a print circuit board (PCB) to eliminate the Lorentz force in the Z axis. The application of a direct current (DC) allowed the center moving stage to move in the XY axes by 55 μm and rotate almost 3 degrees around the Z axis, respectively. The resonance frequencies of the motion in the XY axes and the rotation around the Z axis were 630 Hz and 675 Hz, respectively.

DOI： 10.1109/JMEMS.2022.3183611

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記述言語：英語   掲載種別：研究論文（学術雑誌）

In this paper, we report a three-degree-of-freedom electromagnetic microactuator with pure in-plane bi-directional displacement in X and Y and rotation about Z. Moving coil scheme was adopted because pure in-plane actuation without vertical displacement was possible under a uniform magnetic field. The actuator mainly consists of four driving units and a center moving stage within a footprint of 6 mm × 6 mm. Each of the driving unit is formed by four driving springs made of silicon and gold and connected by a moving shuttle. Parylene was employed to make spring beams to connect the center moving stage and the shuttles of four driving units. In-plane Lorentz force was generated by the coupling between current in the driving springs and a uniform outer magnetic field. By controlling the strength of current and outer magnetic field, an in-plane bi-directional displacement as large as ± 40 µm in X and Y and rotation angle of ± 2 degrees about Z were measured, the corresponding resonant frequencies were 650 Hz and 780 Hz, respectively.

DOI： 10.1016/j.sna.2022.113606

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担当区分：筆頭著者,　最終著者,　責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）

We develop an environmentally benign supercapacitor, which decomposes by contact with water, incorporating an ionic liquid, carbon powder, a cellulose separator, and a molybdenum electrode. The ionic liquid is dispersed into a water-dissolvable polymer, poly(vinyl alcohol), to produce a solid electrolyte, so-called ionic gel. A carbon composite mixed with the ionic liquid maintains a gel form. The ionic gel and the carbon composite enable an all-solid-state supercapacitor, which can be charged at a voltage of 1.5 V. The supercapacitor shows areal and volumetric capacitances of 65 mF/cm2and 2.2 F/cm3, respectively. A cycle test reveals that capacitance retention and Coulombic efficiency are 77 and 90%, respectively. As for the dissolution test, the ionic gel and carbon composite dissolves in phosphate buffer solution in 18 days, and the Mo electrode is able to fully dissolve in 500-588 days. Potential applications of the environmentally benign supercapacitor include smart agriculture by monitoring of soil and disaster prevention by a wireless sensor network without the need for retrieval of devices after use.

DOI： 10.1021/acsami.2c00915

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記述言語：英語   掲載種別：研究論文（学術雑誌）

We propose the use of a laminated wafer with a conductive diamond layer for forming cavities as an alternative silicon-on-insulator wafer for micro-electro mechanical system (MEMS) sensors. Since this wafer has no insulator such as a buried oxide (BOX) layer but a conductive layer, it is not charged during plasma treatment in MEMS sensor fabrication processes. The conductive diamond layer was formed on a base wafer doped with boron of more than 2 × 1021 atoms cm-3 by microwave-plasma-enhanced chemical vapor deposition. The resistivity of this layer was 0.025 ω cm, and this layer can be selectively etched to a base wafer made of silicon crystal, such as a BOX layer. In addition, a silicon wafer can be bonded to its layer without voids with gaps of more than 2 nm by surface-Activated bonding. Therefore, we believe that the laminated wafer studied here is useful for the fabrication processes for MEMS sensors that may otherwise be damaged by plasma treatment.

DOI： 10.35848/1347-4065/ac6056

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記述言語：英語   掲載種別：研究論文（国際会議プロシーディングス）

In this paper, we report a moving coil type electromagnetic MEMS actuator with three-axis actuation which can be used in the application of optical image stabilizer (OIS). Compared with the conventional OIS actuators which are based on conventional machining and assembling technique, the actuator we developed is based on MEMS technique and can be used to actuate the CIS directly. The actuator has four driving units and one center moving stage. Connecting with driving units by parylene beams, the center moving stage is actuated by the application of current and outer magnetic field. The experimental results show that the in-plane displacement in X and Y and rotation in Z are nearly 50 m and more than 2 degrees, respectively. Both the displacement and rotation angle can be improved by increase the current or magnetic field. The resonant frequencies of single direction displacement and rotation are 652 Hz and 780 Hz, respectively, which are sufficiently high for practical applications.

DOI： 10.1109/MEMS51670.2022.9699555

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記述言語：英語   掲載種別：研究論文（学術雑誌）

In this paper, we report an actuator with a magnet made of novel rare earth magnetic powder Sm-Fe-N with diameter of 2 µm to 5 µm. Micro magnets were fabricated by mixing the magnetic powder with wax at temperature of 90 C. Alignment of the easy axis process was employed to enhance the remanence of the micro magnets. A 8 T strong pulse magnetic field was utilized to magnetize the micro magnets. The properties of magnetic powder were revealed with vibrating sample magnetometer by measuring micro magnets with various magnetic powder weight loading percentages. In the best condition, the remanence reaches 0.9 T and the energy product reaches 70 kJ m-3. After that, the electromagnetic actuator was fabricated by embedding the magnetic powder in deep-reactive-ion-etched Si cavity with wax binder and parylene capping. The completed microelectromechanical system structure was mounted on a four-layer printed circuit board with embedded coils. Upon the application of a direct current for actuation, a displacement of 180 μm was obtained from a device volume of only 1.5 mm3 with a low actuation power consumption of 11.5 mW.

DOI： 10.1088/1361-6439/abfeb3

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担当区分：筆頭著者,　責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）

In the era of a trillion sensors, a tremendous number of sensors will be consumed to collect information for big data analysis. Once they are installed in a harsh environment or implanted in a human/animal body, we cannot easily retrieve the sensors; the sensors for these applications are left unattended but expected to decay after use. In this paper, a disposable temperature sensor that disappears with contact with water is reported. The gel electrolyte based on an ionic liquid and a water-soluble polymer, so-called ionic gel, exhibits a Young's modulus of 96 kPa, which is compatible with human muscle, skin, and organs, and can be a wearable device or in soft robotics. A study on electrical characteristics of the sensor with various temperatures reveals that the ionic conductivity and capacitance increased by 12 times and 4.8 times, respectively, when the temperature varies from 30 to 80 °C. The temperature sensor exhibits a short response time of 1.4 s, allowing real-time monitoring of temperature change. Furthermore, sensors in an array format can obtain the spatial distribution of temperature. The developed sensor was found to fully dissolve in water in 16 h. The water-dissolvability enables practical applications including healthcare, artificial intelligence, and environmental sensing.

DOI： 10.1021/acsami.0c10229

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記述言語：英語   掲載種別：研究論文（学術雑誌）

We investigated the influence of the shear force on a roll-type transfer technique developed to realize high transfer ratio and high positioning accuracy. Roll-type stamping transfer is a promising method for integrating solid-state devices on flexible substrates. However, the accuracy of the roll-type stamping transfer method is insufficient for tiny devices, such as Micro-Electro-Mechanical-Systems (MEMS) sensors, integrated circuits, and surface-mounting chips, because of the shear stress that occurs at the contact of the two rolls. In this work, the shear force was measured at various roll radii and rotational speeds. The results suggest that adjustments of the roll radius can reduce the shear stress to less than 10 kPa. The low shear stress improves the misalignment angle by 2 degrees, with a high roll rotational speed, which enables the development of a hybrid sensing system for flexible and rigid devices at a large-scale.

DOI： 10.1541/ieejsmas.140.79

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担当区分：筆頭著者,　責任著者   記述言語：英語   掲載種別：研究論文（国際会議プロシーディングス）

This paper presents a novel time logging system using a real time clock (RTC) powered by a vibrational energy harvester to record the time of events when piezoelectric PVDF film as an event-driven sensor is activated. The power-management circuit normally remains in the sleep mode to cut power supply to the micro-processor and the ferroelectric random access memory. On the other hand, the RTC alone is powered by a battery to keep the time by seamless operation, which lowers the standby power consumption as small as 231 nW. As the PVDF film generates power, the developed power management circuit can supply load current upwards of 8 mA at 3.3 Vdc for external circuit to record the time of events with the energy consumption of 467 μJ.

DOI： 10.1088/1742-6596/1407/1/012045

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担当区分：筆頭著者,　責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）

A gelatinized ionic liquid of [TMPA] + [TFSI] − suspended in a polymer is proposed for the first time as a dielectric material for micro-electromechanical systems (MEMS) vibrational energy harvesters. Owing to the high-density charges in the electrical double layer (EDL) formed at the gel/electrode interface, a high contrast of capacitance modulation is realized, thereby enhancing the electrostatic induction current. A typical current of 4.6 µA rms cm -2 is experimentally confirmed for a 10 kΩ load resistance connected to a DC voltage of 1.5 V, which is thought to be 86-fold greater than that produced by an energy harvester with a simple air-gap.

DOI： 10.1541/ieejsmas.139.7

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担当区分：筆頭著者,　責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）

A water-dissolvable electrolyte is developed by combining an ionic liquid (IL) with poly(vinyl alcohol) (PVA), which decays over time by contact with water. An IL generally consists of two species of ions (anion and cation), and forms an electrical double layer (EDL) of a large electrostatic capacitance due to the ions accumulated in the vicinity of a conductive electrode when voltage is applied. In a similar manner, the ionic gel developed in this work forms an EDL due to the ions suspended in the conjugated polymer network while maintaining the gel form. Test measurements show a large capacitance of 13 µF cm−2 within the potential window of the IL. The ionic gel shows an electrical conductance of 20 µS cm−1 due to the ionic conduction, which depends on the weight ratio of the IL with respect to the polymer. The developed ionic gel dissolves into water in 16 h. Potential application includes the electrolyte in disposable electronics such as distributed sensors and energy harvesters that are supposed to be harmless to environment.

DOI： 10.1002/smll.201800937

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担当区分：筆頭著者,　責任著者   記述言語：英語   掲載種別：研究論文（国際会議プロシーディングス）

This paper presents a new use of piezoelectric PVDF film as an event-driven sensor switch that activates the power-management circuit for a subsequent wireless sensor ZigBee such that it would not consume extra power during the sleep mode. The circuit is designed to pick up a mechanical vibration with the PVDF film and control the self-holding circuit to keep flowing the drive current to the load until a reset pulse signal is received. The developed power management circuit can supply load current upwards of 33 mA at 3.3 Vdc for load resistance ranging between 100 Ω and 1 MΩ.

DOI： 10.1088/1742-6596/1052/1/012070

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担当区分：筆頭著者,　最終著者,　責任著者   記述言語：日本語   掲載種別：記事・総説・解説・論説等（学術雑誌）

DOI： 10.1299/jsmemag.127.1269_36

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担当区分：筆頭著者,　責任著者   記述言語：英語   掲載種別：研究論文（国際会議プロシーディングス）

Combined with a coupled torsional structure at the anchor, a temperature compensated cantilever resonator was developed to reduce frequency dependence. The device shows the frequency dependence of 856 ppm with the temperature ranging from 25°C to 80°C. FEM simulation indicates that the coupled torsional structure at the anchor allows us to reduce temperature dependence of resonators to be 170 ppm with temperature ranging from -40°C to 80°C, optimizing wafer dopant, density, and mode.

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