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Flexible strain sensors are emerging rapidly and overcoming the drawbacks of traditional strain sensors. However, many flexible sensors failed to balance the sensitivity, response time, and the desired sensing range. This work proposes a novel and cost-effective strain sensor which simultaneously achieved high sensitivity, fast response, and a good sensing range. It illustrates a prototype strain sensor realized with a nanocomposite constituting reduced graphene oxide and palladium as the primary sensing elements. These sensors were fabricated with manual screen-printing technology. The sensor exhibited an outstanding performance for the different strains ranging from 0.1% to 45%. As a result, a substantially high gauge factor around 1523 at a strain of as high as 45% and a rapid response time of 47 ms was obtained. This work demonstrated potential applications like real-time monitoring of pulse and respiration, and other physical movement detection, which become crucial parameters to be measured continuously during the COVID-19 pandemic.

We propose a microscale phloem-sap-dynamics sensor device to obtain the index of an internal plant condition regarding the transportation of primary photosynthates in phloem, which is an essential indicator of stable crop production under controlled-growth environments. In detail, we integrated a conventional Granier sensor with a thermal-flow sensor and devised an improved sensor device to quantify such index, including the information on velocity and direction of the phloem-sap flow using the microelectromechanical systems (MEMS) technology. The experimental results showed that although the proposed sensor device was approximately only 1/10 the size of the conventional Granier sensor, it could generate an output nearly equal to that of the conventional sensor. Furthermore, experiments using mimicked plants demonstrated that the proposed device could measure minute flow velocities in the range of 0–200 µm/s, which are generally known as the phloem-sap flow velocity, and simultaneously detect the flow direction.

In this paper, the result of developing a fully wireless pressure sensor based on endoscopy images for an endoscopic surgery is reported for the first time. The sensor device has structural color with a nm-scale narrow gap, and the gap is changed by air pressure. The structural color of the sensor is acquired from camera images. Pressure detection can be realized with existing endoscope configurations only. The inner air pressure of the human body should be measured under flexible-endoscope operation using the sensor. Air pressure monitoring, has two important purposes. The first is to quantitatively measure tumor size under a constant air pressure for treatment selection. The second purpose is to prevent the endangerment of a patient due to over transmission of air. The developed sensor was evaluated, and the detection principle based on only endoscopy images has been successfully demonstrated.

A wide dynamic range photogate-type active pixel image sensor based on a self-regulation principle is reported. A 5 ×5 element prototype sensor was fabricated using a 5 µm single-poly, single-metal negative channel metal oxide semiconductor (NMOS) silicon integrated circuit technology. The key techniques are based on the feedback of a floating diffusion (FD) potential to the photogate applied voltage and on the control of the sub-threshold characteristic of the photogate using an ion implantation process. The proposed sensor makes it possible to automatically adjust the sensitivity of each pixel using only three transistors. The conversion of the irradiated light to the output voltage is linear under low-light conditions, while it is nonlinear under bright light. The photo-electric conversion characteristics are similar to those of a linear-logarithmic image sensor. The dynamic range can be expanded without the need for an additional capacitor, an analog-to-digital (A/D) converter or any complex circuitry. Compared with a conventional photogate-type image sensor that was fabricated on the same chip, the dynamic range was expanded by 28 dB.

In this paper, we present a thermally measurable contact and noncontact-type thermoelectric sensor using a silicon-on-insulator (SOI) structure, because single-crystal silicon has a higher Seebeck coefficient than any other thermoelectric material. The sensor was designed to have 17 pairs of n- and p-type single-crystal silicon strips and a selective absorption area (SAA) that consists of aluminum and silicon dioxide. The thermoelectric sensor based on the SOI structure was fabricated and measured using a complementary metal–oxide–semiconductor (CMOS) process. The measured sensitivity of the sensor was 0.19 mV/°C, when human skin directly touched the sensor surface. In the noncontact measurement of the sensor while varying the distance between the lamp (EFD10EL/8, NEC) and the sensor, the maximum electromotive force (EMF) of the sensor was -1.55 mV at a 2 mm (6.09 mW) distance.

A smart microfluidic DNA sensor with an integrated signal-processing circuit for electrochemical analysis has been successfully fabricated. The sensor comprises an integrated electrochemical sensing electrode, a microfluidic channel-type reactor, and operational amplifiers for electrochemical measurement. The microfluidic reactor employs a laminar flow principle. Generally, a relatively large and expensive system is necessary for electrochemical measurement. In the fabricated smart chip, signal-processing circuits for measuring cyclic-voltammogram characteristics are integrated, permitting cyclic-voltammograms to be successively measured, using only two simple sources of electrical power.

In this study, the characteristic improvement of Si (111) metal–oxide–semiconductor field-effect transistor (MOSFET) by decreasing the interface states has been investigated. As a process step for this characteristic improvement, long-time annealing in hydrogen ambient was performed after complementary MOS (CMOS) fabrication. It is found from the evaluated results that the interface state density of the Si (111) MOS structure decreases markedly upon increasing the annealing time. Furthermore, the S-factor and mobility of (111) MOSFETs were improved markedly. In particular, the field effect mobility of a (111) p-MOSFET was higher than that of a (100) p-MOSFET in spite of its possessing large interface states. It has been experimentally confirmed for the first time that long-time hydrogen annealing is an essential and effective process for the fabrication of (111) MOSFETs with a standard Si–SiO2 system.

A new electron-emission-type infrared light sensor, which uses the principle of electron emission from a Pb(Zr,Ti)O3 (PZT) thin plate, has been proposed. However, the electron supply route to the emission plane is not yet well understood. As pulsed infrared light was irradiated onto the sensor, the emission current increased rapidly and decreased slowly. The emission current peak value did not change during the continuous detection operation. When the infrared light is shielded, electron emission stops, the temperature of the PZT thin plate returns to the initial state, and an electric field appears due to spontaneous polarization. It is assumed that electrons are supplied through the PZT thin plate from the back-surface electrode to the emission plane via this electric field. In the pulsed infrared light irradiation experiment, we could recognize that the emission current peak value does not depend on the light shielding time. An electron supply model from the back-surface electrode through the PZT thin plate during a detection operation is proposed. In a quasi steady or thermal equilibrium state, the electron supply to the emission plane of the PZT thin plate is sufficient for a detection operation under ordinary irradiation conditions. Therefore, the proposed sensor is viable for use as an infrared sensor or imager under ordinary infrared light irradiation.

Multichannel 5 ×5-site Si microprobe electrode array has been developed for neural activity recording. Si microprobes were fabricated successfully at predetermined sites on a chip using Au dots and Si2H6 gas source molecular beam epitaxy (GS-MBE), a method based on vapor-liquid-solid (VLS) growth. Selective VLS Si growth allowed the design of three-dimensional (3D) microprobes with 40 µm spacing in a 5 ×5 array. The diameter and the length of the Si probe can be widely changed by changing the Au dot size and the Si growth time, respectively. In addition, the circular-cone-shaped Si probe has a shape suitable for penetration into neural tissues, and can be realized by increasing growth pressure. The mechanical strength of the Si probe was evaluated with observation of its bending and penetration into a gelatin membrane, which indicated that the Si probes are strong enough to withstand the application. Signal recording with the same amplitude as neural activity was also performed using the fabricated Si probe array chip. These results confirm that high-density neural signals from neural tissues can be obtained with the multichannel 3D VLS Si microprobe array chip.

The a-Si:H photodiode film is suitable for photoconversion in stacked-type image sensors. If a photocurrent in an a-Si:H p-i-n photodiode film is multiplied by the avalanche phenomenon, a highly sensitive image sensor can be realized. A photocurrent multiplication phenomenon was observed in a p-type hydrogenated amorphous silicon carbide (a-SiC:H)/i-type hydrogenated amorphous silicon (a-Si:H)/n-type crystalline silicon (c-Si) heterojunction p-i-n photodiode film. In this photodiode film, a maximum multiplication gain of photocurrent of 40 times was confirmed. To clarify the multiplication mechanism, the shot noise of the photocurrent in the a-Si:H p-i-n photodiode film was investigated. The measured shot noise in the multiplication region was larger than an ideal shot noise which was assumed to be free of excess noise. This indicates that the photocurrent was multiplied by avalanche process. The excess noise factor of our p-i-n photodiode film was also estimated.

In this study, an integrated pressure and temperature sensor device for a flexible endoscope with long-term stability in in vivo environments was developed and demonstrated. The sensor, which is embedded in the thin wall of the disposable endoscope hood, is intended for use in endoscopic surgery. The device surface is coated with a Cr layer to prevent photoelectronic generation induced by the strong light of the endoscope. The integrated temperature sensor allows compensation for the effect of the temperature drift on a pressure signal. The fabricated device pressure resolution is 0.4 mmHg; the corresponding pressure error is 3.2 mmHg. The packaged device was used in a surgical simulation in an animal experiment. Pressure and temperature monitoring was achieved even in a pH 1 acid solution. The device enables intraluminal pressure and temperature measurements of the stomach, which facilitate the maintenance of internal stomach conditions. The applicability of the sensor was successfully demonstrated in animal experiments.

In this study, we propose a magnetically driven micro-mirror, constructed using negative photoresist SU-8 containing magnetic particles, as a magnetic actuator and torsion bar structure. Because the magnetic polymer composite uses thick negative photoresist SU-8 as the main material, the micro-mirror is simply fabricated in just a few steps by conventional photolithography and deep reactive ion etching. A fabricated prototype of the micro-mirror, which is magnetically driven by using an external magnetic field, is shown to deflect with two-dimensional optical deflection angles of 6.5 and 12.5°. Moreover, Lissajous scanning motion of the fabricated mirror is achieved.

In this study, the stable detection principle of a MEMS hardness sensor with “reference plane” structure is theoretically analyzed and demonstrated with experimental results. Hardness measurement independent of contact force instability is realized by the optimum design of the reference plane. The fabricated devices were evaluated, and a “shore A” hardness scale (JIS K 6301 A) was obtained as the reference in the range from A1 to A54 under a stable contact force. The contact force dependence on hardness sensor signals was effectively reduced by 96.6% using our reference plane design. Below 5 N contact force, the maximal signal error of hardness is suppressed to A8. This result corresponds to the detection capability for fat hardness, even when the contact force is unstable. Through experiments, stable detection of human body hardness has been demonstrated without any control of contact force.

In this study, a post CMOS reliable formation process for high-aspect-ratio SU-8 structures on integrated circuits is newly proposed. Enhancement of SU-8 adhesiveness is realized by forming a thin SU-8 layer (called an adhesive layer) over the surface of the circuit before the SU-8 structures are formed. Improvement of adhesion of thick SU-8 structures is very important to guaranty the reliability of MEMS microsensors. The negative effect of the adhesive SU-8 layer on the mechanical properties of silicon movable structures has been estimated and discussed with simple analytical formulae. Also, the effect of the adhesive layer on the silicon substrate has been demonstrated with test structure patterns. On the basis of the developed technique, a tactile sensor device has been successfully fabricated as an example of the application of this technique.

An electrochemical sensor has been developed on a single chip in which potentiostat and sensor electrodes are integrated. Sensor chips were fabricated using 5.0 µm complementary metal–oxide–semiconductor (CMOS) technology. All processes including the CMOS process, postprocessing for fabricating sensor electrodes and passivation layers, and packaging were performed at Toyohashi University of Technology. The integration makes it possible to measure electrochemical signals without having to use a bulky external electrochemical system. The potential between the working electrode and the reference electrode was controlled using an on-chip potentiostat composed of CMOS transistors. The chip characteristics were verified by electrochemical measurement, namely, by cyclic voltammetry. Potassium ferricyanide solution was measured to obtain results that fit well to the theoretical formula. A clear proportional relationship between peak height and the concentration of the sample solution was obtained using the proposed sensor chip, and the dynamic range obtained was 0.10 to 8.0 mM.

A new electrical conductivity (EC) sensor with Pt electrodes on a Si substrate was proposed for use in agricultural applications. The complementary metal oxide semiconductor (CMOS) logic technology-compatible sensor was successfully fabricated with a small chip (area, 25 mm2). This sensor was used for an alternating voltage, owing to the polarization of the Pt electrode and its best operation at 10 kHz frequency. Measurements were conducted on water with electrical conductivities ranging from 10-2 to 101 S/m, which shows sufficient linearity for use as a sensor for agriculture applications. It was also possible to be miniaturized. In an agricultural application, measurements were carried out with the sensor inserted in the rock wools, which was confirmed by the linearity of the values obtained using the sensor. In addition, successful real-time measurements using the sensor were conducted by inserting the sensor into the compost in the vicinity of the roots of a tomato seedling. This sensor provides a promising solution for an EC sensor for broad fields of agricultural applications.

Three-dimensional microtubes and microprobes with MOSFET circuits are integrated for use in chemical and electrical neural interface applications with microelectronics. We propose a vapor–liquid–solid (VLS) method to realize both the microtubes and microprobes, which are 2 µm and 3.6 µm in diameter, respectively, and can be fabricated after the on-chip MOSFET processes. The on-chip NMOSFET shows electrical characteristics with a threshold voltage of 1.2 V and a subthreshold swing of 145 mV decade−1, confirming that subsequent fabrications of the tube and probe are compatible with the inclusion of the on-chip circuits. The prototype oxide tube, which has 2.7 µm inner diameter and 29 µm height, has a flow rate of 550 nl min−1 with an external pump pressure of 33.5 kPa. The impedance of the on-chip Si probe, which has 2 µm diameter and 30 µm height, measured at 1 kHz in a saline environment is 2 MΩ. Insertion into a gelatin membrane confirms that both the tube and the probe show sufficient penetration capabilities.

A charge-transfer-type pH sensor operated by charge accumulation was demonstrated, and it was confirmed that its pH sensitivity was 1130 mV/pH with five signal integration cycles. The most familiar pH sensor using a semiconductor is the ion-sensitive field effect transistor (ISFET). The sensitivity of our pH sensor was 20 times higher than that of the ISFET. The sensitivity of the ISFET, which is similar in structure to a metal–insulator–semiconductor (MIS) structure, is restricted by the Nernst equation (59 mV/pH at room temperature). The output signal from an ISFET disappears into its noise as pH variation becomes small. The charge-transfer-type pH sensor increases the pH output signal to integrate signals in the time domain that are noise-free. It is expected that with this sensor it is possible to measure small pH signal variations.

In this paper we report the development of n-type and p-type Si-microprobe arrays fabricated by using in situ doping in vapor-liquid-solid (VLS) growth employing a gas-source molecular beam epitaxy (GS-MBE) system as the growth environment. VLS growth using Si2H6 only gives intrinsic Si microprobes with the resistivity of ∼104 Ω-cm, which decreases to ∼10-2 Ω-cm after phosphorous diffusion at 1100°C. However, by incorporating in situ doping into the VLS growth method, more conductive probes (resistivity ∼10-3 Ω-cm) can be realized at a temperature less than 700°C. The site and diameter of the VLS-grown probe can be controlled and the growth rate is higher than that of a poly-Si or epitaxial Si crystal grown by the vapor-solid (VS) method. Due to the low processing temperature, in situ doping is effective for realizing highly conductive probe arrays with smart sensor devices by a standard IC process followed by VLS growth. The wide range of doping leads to the possibility of using these probes for the fabrication of vertical active devices such as diodes and transistors.

In this study, a microchip for measurement of hemoglobin in human blood has been proposed, fabricated and evaluated. The measurement principle of hemoglobin is based on the “cyanmethemoglobin method” that calculates the cyanmethemoglobin concentration by absorption photometry. A glass/silicon/silicon structure was used for the microchip. The middle silicon layer includes flow channels, and 45° mirrors formed at each end of the flow channels. Photodiodes and metal oxide semiconductor (MOS) integrated circuits were fabricated on the bottom silicon layer. The performance of the microchip for hemoglobin measurement was evaluated using a solution of red food color instead of a real blood sample. The fabricated microchip exhibited a similar performance to a nonminiaturized absorption cell which has the same optical path length. Signal processing output varied with solution concentration from 5.32 V to 5.55 V with very high stability due to differential signal processing.