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A unidirectional microelectromechanical system (MEMS) microphone single module for handsfree and voice recognition systems in automobiles is presented. Because in-cabin noise and temperature variation affect the reduction in the handsfree and voice recognition system performance, noise suppression and increasing the signal-to-noise ratio (SNR) of the microphone are required. In this study, a capacitive MEMS microphone module with a high SNR and a unidirectional characteristic is achieved by designing the structure, package, and module of the MEMS microphone. To improve the SNR, the microphone is developed using a slit-edged membrane. The slit structure is designed to release the residual stress of the membrane to achieve improved sensitivity and SNR. The unidirectional characteristic of the microphone enables suppression of noise signals from undesired directions. The directional characteristic of the microphone is realized by attaching a porous SU-8 filter to delay the time to one of the two acoustic ports on the package. Tests on the proposed unidirectional MEMS microphone package and module show that an SNR of 62.4 dB and a front-back ratio of 27.1 dB are achieved.

We demonstrate a novel low power inductive wearable plethysmography system. This consists of ultra-sensitive 3D knitted helical coils integrated in a garment and an oscillator circuit with high quality factor. The low power oscillator is built using two cross coupled FET pairs with low capacitance drawing only 95 μA during operation and with a response time smaller than 10 μs . The sensor system is linear, with negligible hysteresis. The best compromise in sensitivity and power consumption is obtained with a 3D knitted helical coil using jersey knit with elastic yarn, a lower knitting needle size than recommended for the yarn and minimizing both the number of stitches per winding as well as the stitches containing metal. A sensitivity of 2.7 kHz per mm change in circumference with a power consumption of 6.85 mW per 30 ms measurement time is reported. This system can be used for long term breathing monitoring using a garment indistinguishable from everyday clothing.

A highly sensitive, large throughput and wide dynamic range (in the aspect of size and material type) broadband tunable microwave interferometer based rectangular waveguide wear debris detection system is presented. Passing of (non)ferrous and (non)conductive particles through the sensor, which causes inductance and/or capacitance changes, are detected as transmission signal variations of the system. We first demonstrate the sensing principle with the aluminum particle of 800 μm and epoxy resin particle of 1 mm3 at 6 GHz. Then, the effects of particle size and flow speed are tested with aluminum particles ranging from 200 μm to 1.8 mm. The measurement results show that the system is able to measure particle sizes with known material and differentiate nonconductive particles from conductive ones. With the proposed data processing techniques, this system is capable of detecting as small as 76 μm particles within an 8 mm outer diameter tube. And with future improvements on sensitivity and particle material characterization, the proposed system has the potential to differentiate (non)ferrous and (non)conductive particles and measure their sizes as a real-time wear debris sensing devices for rotating and reciprocating machines.

This work describes the development of an acoustic sensor based on piezoelectric ZnO thin film to be utilized for aero-acoustic measurements. The design of the device was carried out through CAD tool Coventorware. The backside of the device consists of the microtunnel and diaphragm structures. The Si-diaphragm thickness of the device was optimized for high sound pressure level (SPL) (upto 180 dB) using Coventorware. A microtunnel connects the cavity, created after diaphragm formation to the outside environment. The designed device has been fabricated using standard Si-fabrication technology. Microtunnel and diaphragm structures have been fabricated in a single step using wet etching technique which results in reduced cost as well as fewer number of process steps and increased yield of the device. The front side consists of a piezoelectric sensing layer of ZnO sandwiched between two sputter deposited electrode layers of Au covered with PECVD oxide on a thermally oxidized Si-diaphragm. The low cut-off frequency, bandwidth, resonance frequency and flat band sensitivity of the device have been found to be 35.2 Hz, 15 kHz, 78 kHz and 136.5 μV/Pa respectively. The fabricated device can be used for the SPL measurement in aircrafts, aerospace and similar applications.

This paper presents a relative pressure sensor fabricated by a standard industrial flexible printed circuit board (FPCB) technology. The passive wireless sensor consists of a variable sensing capacitor and a planar coil inductor. When the external pressure is applied to the sensing capacitor, it changes the air gap and shifts the LC resonance frequency which is detected by an antenna wirelessly. The sensing cavity of the variable capacitor is extended to the opening at the end of an embedded pressure channel in the FPCB strip to enable relative pressure measurement. Measurement results showed that the proposed sensor with such an open sensing cavity had much higher frequency sensitivity (up to 0.4 %/mmHg) compared with the sensor with a sealed cavity (< 0.01 %/mmHg in this work and ≤ 0.08 in literature). With the enhanced sensitivity, the measured pressure resolution could reach 0.2 mmHg. The potential application of the proposed sensor includes gauge pressure measurement with respect to local atmospheric pressure, such as intracranial pressure (ICP) monitoring, without complicated re-calibration procedures. The flat FPCB structure facilitates applications with limited space for sensor installation, such as in the subdural implantation procedures. Furthermore, the proposed technology enables future development of flexible microfluidic devices.

This paper presents a polyimide-based film bulk acoustic resonator (PI-FBAR) humidity sensor operating in resonant frequency and reflection coefficient S11 dual-mode. Models and experiments showed that those two modes are governed by different mechanisms, of which the former is based on the modulation of the humidity on the Young’s modulus of PI, while the latter is based on the modulation of the humidity on dynamic resistance. In the full-scale relative humidity (RH) testing range (20%-80%), the resonant mode achieved a sensitivity of 29.93 kHz/%RH (28.56 ppm/%RH) with ultra-high linearity (R2 = 0.9993), and the humidity hysteresis error is only about 1.32% RH. For the S11 response, an ultra-high average humidity sensitivity of 0.044dB/%RH (2095ppm/%RH) was achieved, and the sensitivity at 20% low relative humidity reached 0.075dB/%RH (3571ppm/%RH), which is more than 2 orders of magnitude higher than that of the resonant frequency response. And the limit of detection (LOD) for the humidity response of S11 is 0.035% RH, which is half lower than that of the resonant mode. Thus, we can choose a more appropriate sensing mode according to particular application or combine these two mechanisms in one test to expand the sensor competences by taking advantages of each method. In addition, as both of the two modes have high sensitivity to humidity and temperature, we deduced a method to calibrate the temperature for the humidity sensing. Based on this dual-mode mechanism, we can further achieve either temperature-calibrated humidity sensing or dual-variable sensing of humidity and temperature with more accurate sensing results.

Concurrent high force detection accuracy and extended battery lifetime are strongly expected in wearable gait monitoring systems, which are essential for many Internet of Health Things (IoHT) applications. In this article, a piezoelectric insole device and a rectifying circuitry-based technique is presented to achieve these two ultimate goals. Here, walking induced positive and negative charges are separated for plantar stress detecting and energy harvesting, respectively; these two functions are realized concurrently. Experimental results demonstrate that first, the high detection sensitivity of 55 mN and responsivity of 231 mV/N are achieved, satisfying the need for diagnosing various diseases; second, the electric energy of 9.6 pJ (2.67×10-15 W·hr) is stored during a walking cycle, showing the potential of extending the battery lifetime. The developed technique enhances the development of gait monitoring in IoHT.

In order to enhance the sensitivity, Pt coated ZIF-67 was proposed to detect humidity levels for human physiological signal monitoring. The proposed capacitive humidity sensors have superior sensitivity of 44.9 pF/%RH for ZIF-67 with 10 nm Pt, which is about 7.8 times of ZIF-67 with 2 nm Pt and 4490 times of pure ZIF-67. The specificity of water toward 7 interferential volatile organic compounds (VOCs) was analyzed for biomarker discrimination. The proposed sensor exhibits high stability and repeatability in five cycle tests. Normal, fast, deep, and slow breath were investigated to record reading, running, yoga, and sleeping activities, respectively. In addition, the low humidity level evaporated by human sweat on the skin surface can be detected with high sensitivity. This paper provides a highly sensitive humidity sensor for daily clinical monitoring and intelligent health applications.

Under the condition of low illumination, the traditional sky screen has low detection ability, which cannot meet the detection and test needs of dynamic projectile’s parameter in weapon field. In order to solve this problem, this paper proposes a design method of laser photoelectric detection target with array lens and high-power line laser, this method mainly uses the high-power line laser to form a fan-shaped laser light source with a certain thickness, and combines the fan-shaped laser light source plane and the receiving detection screen plane to form a laser detection screen. The array meso-lens is introduced to design the laser detection screen of receiving optical path with wide-angle. A high sensitivity array photodetector is used to capture the laser reflection information of projectile when it passes through the laser detection screen. According to the characteristics of the designed laser detection screen, this paper studies the calculation method of laser reflected energy of projectile, establishes the calculation model of laser reflection power of projectile and derives the signal-to-noise ratio calculation function of laser photoelectric detection target. Through the experimental test, the results show that compared with the traditional sky screen, the field of view of the designed laser photoelectric detection target is significantly increased, and the signal-to-noise ratio of the photoelectric detection is significantly higher than the traditional sky screen in low illumination environment.

A refractive index insensitive triple cladding quartz specialty fiber temperature sensor with D-type microcavity structure is proposed. It consists of a triple cladding quartz specialty fiber(TCQSF) with a D-type microcavity structure fused between two sections of single-mode fiber(SMF). Because the SMF and TCQSF mode fields do not match, the excited modes form a Mach-Zehnder(M-Z) interferometer, resulting in a mode interference phenomenon. Experimental results show that in the range of 30-90°C, the transmission spectrum has a red shift phenomenon, the temperature sensitivity is 138.03 pm/°C, and the wavelength shift has a good linear relationship with temperature change. When the external refractive index changes in the range of 1.3350-1.3466, the sensor has a low sensitivity to the refractive index, which can avoid the cross-sensitivity of simultaneous measurement of temperature and refractive index. The sensor is not only simple to fabricate and high in sensitivity, but also has good repeatability and stable response, and has wide application prospects in the field of sensing.

Microwave measurements of minute dielectric property variations have many advantages such as noninvasive test, rapid sensing capability, and being label-free. Several researches including concentration detection are promoted by measuring the minute dielectric property variations of samples. Highly sensitive and accurate device is required for measurement of electrolyte concentration in biological fluids, especially when measuring samples in nanoliter or less scale. In this paper, a miniaturized broadband coplanar waveguide (CPW) transmission-line (TL) sensor with enhanced sensitivity is proposed. The sensitive device is realized by introducing interdigital capacitance (IDC) at the central conduction band of the CPW TL. The sensor is fabricated using print circuit board (PCB) technology with a microfluidic channel designed on it. A calibration based on microwave network theory is used to obtain the characteristic impedance and propagation constant of the sensing part. And accordingly, variations in magnitude and phase of S-parameters are employed to express the changes in concentration of the sample. Furthermore, an extracted equation between S-parameters and electrolyte concentration of the sample is first proposed. Experiments show the proposed sensor can detect the minimum concentration of 0.05 g/l for the three electrolytes (NaCl, KCl, and GaCl2 in deionized (DI) water)). Compared with a corrugator coplanar transmission line and a split ring resonator senor, the proposed sensor has a much higher sensitivity. Moreover, the device is robust in front of cross sensitivity caused by external factors (such as temperature variations, moisture, etc.). Thus, the proposed sensor is well suited for low-cost, real-tine, and CMOS compatibles sensing technologies.

The capacitive sensors are widely used for liquid level measurement in different applications, but the contact type capacitive sensors suffer from frequent maintenance and drift of the sensor output. The present paper proposes a simple noncontact interdigital (IDT) fringing field capacitance sensor for liquid level measurement. The sensor can be made using flexible /non-flexible substrate, and it can be used to measure the liquid level of the vessel for both planar and non-planar surfaces. Theory of level measurement using the sensor has been discussed. Four different IDT sensor (1-n-1) configurations such as 1-1-1, 1-3-1, 1-5-1 and 1-11-1 have been designed, analysed, simulated (finite element method), and fabricated to measure water and insulating transformer oil levels from 0 to 40 mm. The analytical expressions are derived for 1-n-1 configuration to estimate the value of capacitance and the maximum field penetration depth of the sensors. The capacitance value of all four sensors varies almost linearly with the variation of liquid level. Experimental results show that the sensitivity of the 1-5-1 IDT sensor is 40 fF/mm. Performances of the sensors were compared with other capacitive sensors reported in the literature. The sensor is capable to detect the water level with a resolution of 0.1 mm.

Piezoelectric nanofibers sensors cannot be mass-produced due to the limitation of electrode technology. Herein, we proposed a parallel-stripes electrode pattern for the first time, which enables using the screen-printing technology to print electrodes on piezoelectric nanofibers mats to prepare piezoelectric nanofibers sensors. Firstly, combining PVDF-TrFE nanofibers with different electrodes to prepare piezoelectric sensors and comparing their sensitivity, we prove that PVDF-TrFE nanofibers sensor based on parallel-stripes electrode has piezoelectric output. What is more, the sensor has the highest sensitivity when the parallel-stripes electrode is parallel to the oriented piezoelectric nanofibers. And then, we directly printed parallel-stripes electrode on oriented PVDF-TrFE nanofibers mat, and the vibration test of 50,000 cycles verifies that the PVDF-TrFE nanofibers sensor prepared by this way possess satisfactory stability. Our research provides a new electrode pattern for preparing stable and reliable electrodes for piezoelectric nanofibers sensors via screen printing. Moreover, this electrode further increases the sensitivity of the piezoelectric nanofibers sensor.

A quartz resonant pressure sensor for ultra-high pressure measurement has been designed and verified. The pressure conversion unit of the sensor is composed of a pressure cylinder and a flexible lever, which can accurately and efficiently meet the role of converting and passing the ultra-high pressure to the resonant unit. The resonant unit of the sensor uses quartz DETF resonators and forms a differential output structure to meets the requirements of high precision and stability. The theory of the overall sensor design scheme is consistent with the simulation results, which verifies the feasibility of the design scheme. The experimental results show that in the pressure range of 120 Mpa, the sensitivity of the ultra-high pressure sensor is 27.3 Hz/Mpa, the linearity reaches 0.0112%, and the total accuracy is 0.0365%. The sensor can be used for high-precision and high-stability detection of ultra-high pressure, which is necessary in special fields such as deep-sea exploration.

A soil yeast count monitor that includes a soil yeast sensor and converter was developed for plant growing applications. The monitor can linearly transform soil yeast count into a frequency output. Moreover, the monitor’s adjustable sensitivity control circuits can increase sensitivity when low-nutrient soil is used. During measurements, the input voltage of the proposed converter was 0 to 2.2 V, and the sensitivity was controllably adjusted by 2.217, 2.952, 3.651, 4.388, and 5.118 MHz/V. The range of soil yeast count was 0 to $7\times10$ 8 colony forming units (CFU), and the adjustable sensitivities were 0.654, 0.871, 1.091, 1.311, and 1.529 MHz/108 CFU. The maximum linear error of the complete system was −0.66%, and the chip size was $1.49\times1.49$ mm2. The system was experimentally tested on two types of soil: fluvisol and paddy soil. Through measurements and experiments, the soil yeast count monitor was proven functionally.

As 2D materials are aggressively penetrating the arena of conventional metal oxides-based gas sensors, tungsten disulfide (WS2) as a ‘beyond graphene and beyond MoS2 layered material’ is appearing with shining visibility owing to its exquisite electronic, surface, optical, physicochemical and thermal properties. Micromechanically exfoliated or homogeneously/heterogeneously synthesized mono/few-layer(s) of this transition metal di-chalcogenide (TMDCs), with excellent tunability of almost all the properties, have intrigued worldwide gas sensor researchers to demonstrate various device configurations using WS2 leading to ultrahigh sensitivity with ppb level detection capacity even at room temperature. This review article presents a holistic overview of the WS2 based gas sensors encompassing the impact(s) of the different device architectures (viz. planar, Schottky, heterojunctions, field-effect, and heterojunction bipolar transistors) on the gas sensing performance indices (like sensitivity, selectivity, response/recovery time, humidity effect and baseline drift). Starting with the brief overview of the sensing mechanism of WS2, followed by elaborate discussions on the unique features/advantages and bottlenecks of each of the innovative device architecture investigated, this review concludes by focusing the possible direction to circumvent the most common challenges (like slow and incomplete recovery, vulnerability to humidity and oxidation), that slacken the commercial viability of such devices.

Nitrogen-vacancy (NV) centers have potential applications in medical imaging and distributed sensing, for which the integrated NV magnetometer array is needed. However, the microwave (MW) drive module limits the development of the compact NV magnetometer system and array. In this paper, wireless MW is applied to two NV magnetometer systems (Sensor I and Sensor II) innovatively. We verified that the magnetic-field sensitivity was approximately equal to 77.1 nT•Hz−1/2 and 44.6 nT•Hz−1/2 for Sensor I and Sensor II, respectively, when the distance between the transmitting and receiving antennas was 1 m. This method can be extended to the NV magnetometer array. The receiving antenna and power amplifier are integrated into the NV magnetometer array system for miniaturization. This expands the utility of NV magnetometers in sensing and medical imaging. Furthermore, we achieved remote heart sound records with a signal-to-noise ratio (SNR) of 5.93 dB and 4.68 dB for Sensor I and Sensor II, respectively, by applying the principle of NV-based demodulator to the system. This would further extend the advantages of the wireless MW in the NV magnetometer. The NV magnetometer could receive the frequency point carrying heart sound signal to reach up to 16, which can avoid the interference of the 2.4 GHz frequency band of Bluetooth and Wi-Fi in complex environments.

When ethanol and water are mixed into gasoline, the dielectric properties of gasoline will change. Based on this characteristic, we propose an antenna sensor and study its radiation and sensing performance in detection of ethanol gasoline in 6-7GHz band. In order to avoid the corrosion of the oil sample on the antenna, the antenna adopts a multilayer dielectric substrate structure. A $2\times 2$ artificial magnetic conductor (AMC) array is loaded on the bottom of the antenna to increase the gain of the sensor and reduce the return loss. The antenna sensor has good radiation pattern with a peak gain of 4 dB and high quality factor(Q) up to 69. Furthermore, the antenna sensor shows the advantages of high sensitivity and wide detection range on the detection of oil-water mixture and ethanol gasoline concentration. In order to verify the simulated results, the radiation and sensing performance of the antenna were measured, and the measured results were basically consistent with the simulated ones. Therefore, the proposed antenna sensor can be used for real-time wireless detection in ethanol gasoline transportation.

In this paper, we report new photoacoustic sensing (PAS) probe design based on silicon acoustic delay lines (SADLs). It consists of an optical fiber for light delivery and two SADLs for detecting/relaying the generated PA signals to an outside receiving transducer. Ultrasound testing shows that acoustic signals can be transmitted along the SADLs with low loss up to 17~20 MHz. To improve the PA excitation efficiency, a beveled tip is formed at the distal end of the optical fiber to align the optical illumination region with the acoustic detection zone of the SADLs. New polymer micro linkers fabricated with high-resolution 3-D printing are used to maintain the mechanical stability and acoustic isolation within the probe assembly. For demonstration, a prototype SADL-PAS probe was designed and fabricated. Its PA sensing performance (e.g., sensitivity, linearity, and depth resolution) is characterized. Testing results show that the SADL-PAS probe provides superior sensitivity with considerable high-frequency improvement. Ex-vivo tests in the chicken breast are conducted to demonstrate the PA target detection in biological tissues.

Target flowmeter is commonly used for measuring flow rates, but there are some shortcomings of the conventional target flowmeter, such as complex structure, low sensitivity, non-linearity. Here, a Surface Acoustic Wave (SAW) sensor-based target flowmeter has been proposed where a SAW sensor is placed on the target rod of the flowmeter. When liquid flows through the pipe in contact with the target rod, the force is produced. This force then acts on the target rod and deflects the target rod from its original position, which results strain in it. SAW sensor senses the strain level at different flow rates, and the output is in terms of delayed time response of the order of a nanosecond. The output is non-linear with respect to the flow rate and it is linearized with the help of a neural network technique. This minimizes the non-linearity error up to 0.27% of full-scale output (FSO). The modification in the target type flowmeter is done with the help of the SAW sensor in such a way that the flow transmitter can transmit sensed information through the wireless channel. Simulation and mathematical approaches are reported in this paper for the range of 0 to 1200 LPH.

There is a need to develop sensors that can measure vital signs such as pulse rate and blood pressure easily; such vital signs can be calculated by measuring arterial strain. To this end, a new method that uses a plastic fiber Bragg grating (FBG) sensor to measure pulsation strain at human fingertips is proposed herein. Plastic fiber has a smaller Young’s modulus than that of the glass fiber. Under the same applied pressure, the plastic FBG sensor deforms more than the silica glass FBG sensor, and the Bragg wavelength shift length becomes longer, which results in higher detection sensitivity. The pulsation strain of the fingertip was measured by the proposed method based on this characteristic. The most sensitive signal detection point in the fingertip was the measurement point 10 or 15 mm from the distal interphalangeal joint in the fingertip direction. Strain signals with periodic peaks were measured from five participants. From the strain signal measured with the cuff attached to the arm, the peak disappeared when the cuff moved, and it reappeared when the cuff was opened. Therefore, the measured signal waveform changes based on the blood flow condition. The pulse rate calculated from the peak interval of the measured signal showed high correlation with the reference pulse rate, and the measurement accuracy was ±5.5 bpm. It may be possible to calculate the blood pressure and pulse rate from the measurement signal of the pulsation strain of the fingertip obtained using the plastic FBG sensor.

A highly-sensitive temperature sensor with controllable sensitivity based on Vernier effect by cascading a tunable extrinsic Fabry-Perot interferometer (FPI) and a fixed reflective Lyot filter (RLF) is theoretically investigated and experimentally demonstrated. The temperature sensitivity can be tuned by modulating the cavity length of the extrinsic FPI and online monitoring the envelope of superimposed spectrum with optical spectrum analyzer (OSA). The FPI works as the reference arm to tune the temperature sensitivity of the sensing system, while the RLF with 1-meter polarization maintaining fiber (PMF) acts as the sensing probe. Experimental results prove that by changing the cavity length of the FPI, the sensitivities of −3.82 nm/°C, −8.33 nm/°C and −14.63 nm/°C can be achieved. Compared with the single sensing element, the sensitivities are magnified by 3.78, 8.25 and 14.49 times. The proposed temperature sensor is feasible to be applied practically in scenarios which require different temperature sensitivities in demanded temperature detection ranges.

Monitoring the cable terminal’s partial discharge (PD) is of great significance to ensure the cable line’s safe and stable operation. To improve the anti-interference ability and real-time performance of partial discharge detection, this paper develops a partial discharge Mach-Zehnder interferometer (MZI) detection system for cross-linked polyethylene (XLPE) cable terminals. The optical fiber is implanted into the 10 kV XLPE cable terminal, and airgap defects of different volumes are set in the main insulation. The optical fiber inside the cable terminal serves as the sensing head of the MZI signal arm. PD test was carried out on the samples, and the test results of fiber optic MZI and high-frequency current transformer (HFCT) were compared and analyzed. The results showed that the 10 kV XLPE cable terminal does not produce PD due to optical fiber implantation. The partial discharge of 196 pC can be recognized by the fiber optic MZI for the cylindrical airgap defect sample with a base radius of 1.5 mm and a height of 3 mm. For the cylindrical airgap defect sample with a base radius of 1.5 mm and a height of 1 mm, the sensitivity of partial discharge detection by fiber optic MZI is higher than that by HFCT. The sensitivity of partial discharge inception voltage (PDIV) detection is increased by 27.5%.

A temperature sensor based on erbium-doped fiber cascaded peanut taper (EDFCPT) structure in fiber ring laser (FRL) cavity is proposed and studied. The cascaded peanut structure was fabricated by erbium-doped fiber. The cladding mode and core mode interference and the strong thermo-optical effect of erbium-doped fiber were used to realize the high sensitivity measurement of temperature. The mode interference and thermal sensitivity effects of EDFCPT in a broadband super-continuous light source and a fiber ring laser cavity were studied and compared experimentally. Besides, Erbium-doped fiber cascaded peanut (EDFCP) structure was designed to compare performance with EDFCPT. The experimental results show that EDFCPT has a higher signal to noise ratio (SNR) (~50dB) and a narrower 3-dB bandwidth (~0.12nm) than the broadband light source. Among them, the temperature sensitivity of EDFCPT is 571pm/°C which is two times higher than the EDFCP (~249nm/°C) structure at range from 5°C-55°C. The proposed temperature sensor has the advantages of high sensitivity, good repeatability, simple fabrication, compact structure, etc. It has a good application prospect in the field of aerospace and life health monitoring.

In this study, the authors present a dielectric-resonator-based sensor to characterize liquids. The sensor is based on a dielectric resonator (DR) fed by a slot-coupling mechanism in the ground plane of a microstrip transmission line (TL). The obtained device is a fully passive sensor that a radiofrequency (RF) signal can interrogate. A small hole was drilled on the DR surface to allocate a drop of the liquid under test (LUT); as a consequence, the resonant frequency of the sensor will depend on the electromagnetic characteristics of the LUT. The device is modeled both with an equivalent circuit model and through an electromagnetic modal analysis. The cavity perturbation technique is used to study the impact of the LUT on the resonant frequency and modal distribution; full-wave simulations corroborate the theoretical results. Finally, a prototype of the proposed sensor tuned to work in the 2.45 GHz band has been designed, manufactured, and measured. The device is cost-effective given the small size and practical to use, thanks to the minimal volume of the pool, which calls for quantities of LUT in the order of $0.13~\mu l$ . The prototype has been tested with an ethanol-water solution set. The experimental results match well with simulations and show good repeatability. The sensitivity of the prototype resulted in being 718 kHz per percentage of ethanol in water.

LC resonant wireless passive pressure sensor has been receiving increased attention nowadays. In this paper, an innovative LC-C resonant wireless passive pressure sensor based on dual capacitors was designed and manufactured to replace the traditional LC device. In addition to the inductor and capacitor structure required by the traditional LC device, this paper innovatively designs the fixed capacitor as the inter-layer interconnection structure. The key structural dimensions of the sensor were determined through the numerical analysis of the planar inductor theory and the electromechanical coupling simulation of the dual capacitors model in the previous stage, and the processing of the sensor was carried out through MEMS technology. The final test results showed that within the pressure range of 5kPa to 100kPa, the sensor’s average sensitivity was greater than 69.0 kHz/kPa, while the maximum linear fitting error was smaller than 4.18%. The temperature drift in atmospheric environment was 2.7kHz/°C, and the sensitivity temperature drift was 0.0062kHz/(kPa·°C).

A fiber-optic sensor based on a microfiber coupler (MFC) for dual parameter detection of acetaminophen and temperature is proposed. The microfiber structure is manufactured by hydrogen-oxygen flame-heating technology. The characteristics of the transmission spectrum were studied theoretically and experimentally under conditions of different acetaminophen concentrations and temperatures. Acetaminophen concentration detection sensitivities of 102.4 pm/( $\mu $ mol/mL) and −0.01 dB/( $\mu $ mol/mL) in the range of 10 - $50~\mu $ mol/mL, as well as temperature sensitivities of −0.32 nm/°C and −0.001 dB/°C, were obtained for the proposed sensor. These results indicate that the photonic sensor has potential as a tool for pharmaceutical quality control and hospital wastewater treatment.

Volatile organic compounds (VOCs) are pervasive in the environment and their real-time continuous monitoring can facilitate better understanding of their effects on human health by combining environmental factors with physiological conditions. The scope of wearable sensors for detection of VOCs is evident as the accuracy of the sensor prediction depends on its proximity to the VOC source along with the sensitivity and selectivity of the sensor itself. In this paper, we present a low-power wearable e-nose system based on a capacitive micromachined ultrasonic transducer (CMUT) array. CMUTs offer inherent benefits of excellent mass resolution, easy array fabrication, and integration with electronics, which make them an appropriate choice as a transducer element for gravimetric e-nose systems. A 5-channel CMUT sensor array was chemically functionalized and used for the detection of four volatiles, ethanol, toluene, p-xylene, and styrene. All the channels of the sensor array achieved a resolution below 10 ppm within 0.2–3% of OSHA-PEL time-weighted average (TWA) for each volatile. For each test cycle, the maximum frequency shift, the rate of adsorption, and the rate of desorption were extracted as features. Linear discriminant analysis (LDA) was applied to visualize the discrimination performance of the sensor array. The system performance was characterized using an automated testing system. The presented sensor system can be used for identification of volatiles with suitable pattern-recognition techniques.

Sensitivity enhancement was demonstrated in a nonreciprocal fiber cavity operating near the exceptional point (EP). A fiber Bragg grating (FBG) was used to couple a pair of modes propagating in the opposite direction in a fiber ring cavity and give rise to mode splitting in the spectrum. The different roundtrip losses between the two counter-propagating modes showing nonreciprocity is introduced by coupling an extra fiber ring with an isolator. It could enter the EP in the nonreciprocity cavity due to the pair of modes coalesced in both the real and imaginary parts. In order to resolve the weak mode splitting near the EP, the mode linewidth was narrowed by compensating the roundtrip loss by a section of the pumped active fiber. The experimental results showed that a larger change in the mode splitting strength was observed in the nonreciprocity cavity when the temperature in the cavity was controlled by the incident power, indicating an obvious improvement of sensitivity in the EP-based fiber sensor.

During underwater acoustic detection, the positioning accuracy of the bionic cilia MEMS vector hydrophone is affected by its own sensitivity. MEMS vector hydrophone operate in different temperature environments, so studies on the environmental temperature and the sensitivity of the MEMS vector hydrophone are of great significance. In this paper, based on the sensitivity of the MEMS vector hydrophone at room temperature (22°C), its sensitivity variation is studied within the temperature range of 0–40°C. The relationship between the temperature and the piezoresistor in the MEMS vector hydrophone is theoretically analyzed, and it is concluded that the sensitivity change is within ±0.6dB. The sensitivity change of MEMS vector hydrophone is proved to be within ±0.3 dB via COMSOL 5.4 simulation software, and the MEMS vector hydrophone is verified experimentally by the hydrophone temperature characteristic testing system. The results show that the variation of the sensitivity is within ±0.5 dB, which is basically consistent with the theoretical analysis results and simulation results. It can be said that under normal non-extreme ambient temperature, the operating temperature has little effect on the sensitivity of the MEMS vector hydrophone.

B-dot sensor, as an important device, is extensively employed to measure the current flowing in the magnetically insulated transmission line (MITL). To shield the output signal of the B-dot sensor from electron flow and high electrical field in the MITL anode-cathode (A-K) gap and also avoid the electrical breakdown in the A-K gap, the sensor is often covered with the metallic film and located in a conductor cavity penetrating the MITL electrode hole simultaneously. The frequency response properties of such kind of sensor are investigated. The experiment results indicate that the upper limited response frequencies of the sensors with and without the attached $5-\mu $ m-thick Ni-Chrome foil are 50 MHz and 148 MHz, respectively. Moreover, as a result of penetration of magnetic flux into the cavity’s wall and the wire loops wound to form the sensor, the low-frequency components of the B-dot sensor output signal are amplified, consequently the falling edge of the waveform given by the B-dot probe will be distorted. We have proposed a method to make a first-order correction for such effect by using a passive resistor-capacitor integrator instead of the numerical process suggested previously. The integrator combines the behaviors of integration and compensation of the B-dot sensor output. The actual measured current waveform can be directly obtained by multiplying the output signal of the integrator by the sensitivity of the B-dot monitor. The B-dot sensors and this post-processing method have been successfully used on a pulsed power accelerator.

In this paper, a high sensitive fiber-optic Fabry-Perot interferometer sensor based on gold diaphragm for acoustic detection has been proposed and experimentally demonstrated. The extrinsic Fabry-Perot interferometer (EFPI) comprises a 140-nm-thick gold diaphragm and the end-face of a fiber-optic collimator, both of which are enclosed into a structure made of glass tubes. A fiber-optic collimator is first adopted in acoustic sensing to reduce the light loss and improve the sensitivity. The experimental results show that the proposed sensor has a wide flat response range from 400 Hz to 12 kHz, almost covering the primary frequency component of audible sound. The pressure sensitivity and the minimum detectable acoustic pressure level (MDP) are measured to be −175.7 dB re 1rad/ $\mu $ Pa@150Hz and $95.3~\mu $ Pa/Hz1/2@2kHz, respectively. The proposed sensor has advantages of high sensitivity, wide flat response, low cost, and simple fabrication, which shows its potential as a fiber optical microphone with high sensitivity and high acoustic quality in practical applications.

Weak acoustic signals detection from heavy background noise plays an important role in different research fields. However, the detection limit, namely the detection capability of minimal detectable pressure, of sensitivity still hinders the performance of ordinary acoustic sensors. We propose an acoustic metamaterials-based sensor for weak signals detection. We show that the trapezoidal structure with gradient refractive index (GRIN) can effectively realize the pressure field enhancement through acoustic wave compression effect. The acoustic pressure amplitudes can be amplified more than 20 times around the maximum pressure gain frequencies even with different thicknesses of acrylic plates and measured positions, which makes it possible to achieve a broadband acoustic enhancement. Moreover, we numerically demonstrate that both harmonic and periodic impulse signals can be detected much easier based on the GRIN metamaterial due to the acoustic enhancement. In addition, we also experimentally demonstrate that harmonic and periodic impulse signals can be effectively recovered from the background noise due to the improved signal to noise ratios (SNRs). All the results indicate that acoustic metamaterials-based sensor can be well used for weak acoustic signals detection.

The coustic emission (AE) b value plays an important role in studying structural damage evolution and disaster precursors. The b value can be obtained from the least square fit between AE event number and magnitude (the Gutenberg-Richter relationship), where the magnitude is generally calculated through the maximum amplitude of an AE signal. However, the magnitudes obtained from amplitudes of different AE signals may be quite different, due to the influence of coupling state between the sensor and specimen (expressed as the sensor sensitivity coefficient, ${Sc}$ ). Thus, the b values obtained from a single sensor and without magnitude correction may not be reliable. To address this, this study used pencil-lead breaks and far-field theory to obtain ${Sc}$ values. One ${Sc}$ value may be as much as double another, demonstrating that signal maximum magnitudes should be corrected before b value estimation. The ${Sc}$ -corrected median magnitude is then treated as the AE event magnitude and used to estimate the b value. Application to uniaxial compression AE data from a red sandstone indicates that the improved magnitude reduces the instability compared to magnitude based on a single sensor. The b value time series among sensors exhibit large differences, while the improved b values agree well with AE event locations and rock specimen failure mode. This research demonstrates for the first time the necessity of amplitude correction in AE magnitude and b value computation.

A novel high sensitivity all-fiber Fabry-Perot interferometer (FPI) gas refractive index (RI) sensor based on hole-assisted one-core fiber (HAOCF) and Vernier effect was proposed and demonstrated. The sensor is fabricated by splicing a section of HAOCF to a section of silica tube (ST) fused with the lead-in single-mode fiber (SMF), ensuring that the sensor is composed of an air-cavity FPI and a silica-cavity FPI. The cascaded dual-cavities generate the Vernier effect due to a similar free spectrum range (FSR), which significantly improve the sensitivity of the sensor. Owing to the air hole in the cladding of the HAOCF, gases with different RI can enter or leave the in-fiber air cavity, which makes the device usable as a gas RI sensor. The sensor was used to monitor the RI changes of air with different pressures and provided a high sensitivity of −9462.4 nm/RIU. The proposed sensor has a potential to find further applications in the fields of gas composition detection, medical diagnostics and environmental monitoring.

This paper reports on a fiber-optic directional microphone with a bionic MEMS diaphragm for low-frequency sound source localization. The diaphragm consists of two 3 mm $\times3$ mm wings connected by a 1 mm $\times1$ mm bridge, and two 0.2 mm $\times0.1$ mm torsion beams anchor the diaphragm to the Silicon-On-Insulator (SOI) frame along the bridge’s central axis. The bridge causes the mechanical coupling between the two wings, resulting in rocking and bending vibration modes of the diaphragm. The designed microphone contains a closed back cavity and works by measuring vibration at one of the diaphragm’s distal edges by a fiber-optic Fabry-Perrot interferometer (FPI). Simulation and experimental results demonstrated that the microphone can make a significant directional response in a wide low-frequency band centered on the rocking-mode eigenfrequency, giving a typical bi-directional polar pattern, but its directionality disappears at the bending-mode eigenfrequency. The directional sensitivity of the microphone was determined to be 1.86 mV/° at 300 Hz based on the measured linear response to the incident angle in an angular range from $\theta =0$ to 60°. The minimum detectable pressure (MDP) of the microphone relies on both the incident angle and sound frequency, and MDP was measured to be $12.68\mu $ Pa/ $\surd $ Hz at 300 Hz and $\theta = 100^{\circ }$ . The fiber-optic FPI serving as the acousto-optic transducer offers the directional microphone high sensitivity and strong immunity to electromagnetic interference, rendering it suitable for low-frequency sound source localization in harsh environments, especially for tracing the environmental noise sources.

In this study, a novel capsaicin concentration detector was proposed for individuals with capsaicin sensitivity. Capsaicin damages these individuals’ chromatids and organs such as the eyes, gastrointestinal organs, and skin. The proposed capsaicin concentration detector comprises a capsaicin sensor and a capsaicin concentration converter, which can linearly convert capsaicin concentration into a duty cycle output. In addition, the innovative automatic sensitivity control circuits automatically increase the detector sensitivity when low capsaicin concentration is sensed. Through measurement, all functions of the capsaicin sensor and capsaicin concentration converter were proven accurately. The measured differential voltage of the proposed converter was 0.1–1.4 V, and the corresponding sensitivities of 55.35, 45.4, 34.5, 25.17, and 13.3 %/V were changed automatically. The capsaicin concentration ranged from 57.134 to 1347.71 Scoville heat units (SHU), and the corresponding sensitivities of 0.066, 0.043, 0.031, 0.021, and 0.008 %/SHU were obtained automatically. The maximum linear error of the proposed detector was 0.633%, and the chip size was $1.49\times1.49$ mm2. Finally, the whole system was tested on two chili peppers—paprika and poblano peppers. The proposed detector could successfully detect capsaicin concentration.

This paper presents an analytical model of AlGaN/GaN MOS-HEMT based pH sensor for the first time to determine pH of different electrolyte solutions. Gouy-chapman stern model has been used to calculate the surface charge density. The results obtained using analytical model have been verified and show good agreement with the simulated results. Cavity length, thickness of AlGaN barrier layer and Al composition have been optimized to improve the sensitivity of the device. It has been observed that the drain current and threshold voltage decreases with increase in pH of the electrolytic solutions. Proposed AlGaN/GaN MOS-HEMT sensor demonstrate a quick response to the pH changes. The maximum drain-on-sensitivity of the device is 132mA/mm-pH. Surface potential and threshold voltage sensitivity of 0.95mV/pH and 950mV/pH respectively have been obtained which is much higher than the Nernstian limit(59.2mV/pH).

A mathematical model is necessary to analyze sensor performance and optimize its design. However, most of the existing models of piezoelectric micro-machined hydrophones are incomplete and too complex to understand. There has been no mathematical model for the sensitivity of piezoelectric hydrophones. To address this shortcoming, this paper proposes a mathematical model of a piezoelectric micro-machined hydrophone. The stresses in the piezoelectric layer and static displacement of the diaphragm under uniform sound pressure are derived. In addition, the resonant frequency and sensitivity of the hydrophone are analyzed. The finite element simulation is used to verify the validity of the proposed mathematical model. On the basis of the mathematical model and simulation, the diaphragm radius, top electrode area, and piezoelectric film thickness are optimized. Furthermore, the hydrophone is fabricated, and its morphology is observed. The impedance-frequency spectrum of the hydrophone is measured by HIOKI IM3536 LCR impedance analyzer. The tested resonant frequency is 0.5 MHz, which is relatively close to the calculated result of 0.537 MHz, so the validity of the proposed mathematical model is further validated. Also, the reason why the test result is less than the calculated result is explained. The effective electromechanical coupling coefficient derived from the impedance curve is 7.5%. The developed mathematical model can provide a useful guide for designing high performance piezoelectric micromachined hydrophone and make preliminary predictions of its characteristics.

Compared with apodization, the piston-mode technique is a preferable solution to suppress the spurious transverse modes in surface acoustic wave (SAW) devices. This paper extends the technique to the SAW resonator (SAWR) strain sensors to achieve a high performance, that is, conquer some negative influence in the wireless strain sensing system caused by the spurious modes, such as low signal to noise ratio (SNR), high misreading rate and demodulation difficulty. The principle of piston-mode operation was presented and a modified expression was given for primary design of the piston-mode waveguide, which takes into account the anisotropy difference in different electrode regions. SAWRs based on the Al/ST-quartz with the conventional and two common piston-mode configurations were designed, simulated, fabricated and measured. The experiments to evaluate the wireless performance and strain sensitivity of the SAWR strain sensors with different electrode configurations were performed.

Relatively simple structures are necessitated for achieving high-performance refractive index sensors. Herein, we propose a less sophisticated sensing platform based on a thin metal film resonance cavity. A super-strong electric field in an analyte medium is formed via multi-wave interference, and the effective refractive index of the cavity mode strongly depends on the refractive index of the tested medium. In a relatively ideal situation, the sensitivity and maximum figure of merit of our device can reach 1,456,700 nm/RIU and 1,234,500 /RIU, respectively, which are both much higher than those of most sensing methods. This sensor shows great potential for low-cost sensing with high performance in biological or chemical sensing. Moreover, the cavity resonance configuration with high quality and strong electric field is versatile and may facilitate conventional surface plasmon resonance sensors in enhancing the strength of the detection field. Moreover, it can be applied to integrated optics and high-resolution spectroscopy.

To improve the stability and sensitivity of pH sensors based on AlGaN/GaN high electron mobility transistors (HEMTs), we proposed a modified sensing structure by integrating a reference HEMT device. This structural pH sensor exhibits a typical Nernstian behavior with a sensitivity of 54.38 mV/pH, which is higher than the value of 49.43mV/pH derived from the AlGaN/GaN HEMT-based sensor without integrating the reference HEMTs device. Furthermore, the stability of the new structural sensor is enhanced by approximately 19.2% in comparison with that of its traditional counterpart. The improved performances are analyzed using an electrical double-layer model together with an equivalent circuit model, and we find that the new sensor structure has a larger capacitor and exhibits a better rectification effect, hence decreasing the electrical noise and enhancing the stability of the testing signal. Meanwhile, the new sensor structure displays a smaller equivalent resistance and results in a larger available output current, hence exhibiting a higher sensitivity.

A new method of SPR signal amplification based on dynamic field enhancement at the sensor surface to detect C-reactive protein (CRP) is proposed. The gold nanoparticles-detection antibody (AuNPs-dAb) conjugates, as the dynamical agent, were applied to enhance the biosensor sensitivity. The mechanism of SPR sensitivity enhancement is due to the strong evanescent field coupling between the localized SPR (LSPR) of AuNPs and SPR of gold-silver alloy film which proved by COMSOL Multiphysics and MATLAB simulation. The biosensor sensitivity got with sandwich immunoassay (cAb/CRP/AuNPs-dAb) is 5.56-fold of the structure without AuNPs (cAb/CRP/dAb). Moreover, concentration of AuAg-SPR biosensor in CRP detection was lower for an order of magnitude than that of the conventional gold film cladding SPR (Au-SPR) biosensor, according to its direct reaction with the immobilized cAb. The dynamic field enhancement based AuAg-SPR biosensors have high sensitivity in detecting trace biomarker of CRP, and the improvement can also be applied to the detection of other biomarkers in low concentration sensitively.

A ballistocardiogram (BCG) is a recording of the mechanical movement of the body caused by cardiac ejection of blood. We propose a method to obtain the BCG of a body by utilizing the pressure inside a sensor tube placed between a cushion and a bed frame. A theoretical model of the propagation of movement through the cushion that generates pressure is presented to investigate the cushion viscoelasticity effect. The stiffness constants and damping coefficient of an elastic cushion and a memory foam cushion were investigated. We also investigated the influence of the sensor tube diameter on sensitivity and the propagation of mechanical movement through these different cushions. Finally, the BCG of a subject was measured for various cushion conditions and the S/N ratios of all cases were evaluated to find the optimal sensor tube and placement conditions. The sensitivity is proportional to the sensor tube diameter and the S/N ratio is almost inversely proportional to the cushion thickness. However, for all cases, the S/N ratio ranges from 26.4 dB to 44.3 dB. The best sensor tube placement was under the neck. The experimental results revealed the same trends as the analysis of the model.

Underwater acoustic ranging and detection usually require hydrophones to have high sensitivity at narrowband operating frequencies. Different from traditional sensitization methods with the help of new materials and solid/liquid resonance structure, this paper proposes a low-cost, high-sensitivity hydrophone based on a resonant air cavity containing an affordable MEMS microphone inside to sense the focused acoustic waves transmitted from the external water across the solid shell into the air cavity. Both finite element simulations and experiments are carried out to test its acoustic sensing performances. Resonance frequency and directivity of the proposed hydrophone are determined by the air cavity size and the acoustic mode, but is independent of the solid shell, which facilitates the precise designability. The sensitivity of the developed hydrophone at the resonance frequency is up to −157 dB re 1V/ $\mu $ Pa calibrated by commercial standard hydrophone TC4013. Finally, several further considerations, improvements and applications on the proposed hydrophone are discussed.

In this study, a novel response acquisition method was proposed and verified in a capacitive tactile sensing array, which collected signals from shared capacitors between two adjacent sensing units. In the proposed design, the electrodes of a capacitor exhibited intentional misalignments in the xy -plane and four types of misalignments existed in the array. The tactile sensing array was then numerically evaluated, fabricated, and demonstrated. With the proposed method, the previously undetectable normal force was resolvable with noticeable (at least 225%) spatial resolution enhancement. Because the composition of the sensing array, number of capacitors in the array, and electrode dimensions in the capacitors were all unaltered from the conventional design, high detection sensitivities (on average 3.261 pF/ N for the normal force) were obtained. Additionally, although the four types of sensing unit required four corresponding algorithms to derive the shear angles, results proved that the supportiveness was remained and accuracy was unaltered compared to existing similar works.

In this research, the numerical analyzing procedure for a two-dimensional (2D) solid-fluid phononic crystal (PnC) demultiplexer is presented. The demultiplexer acts as a selective sensor that could separate three dangerous heavy metals (CuSO4, MgSO4 and MnSO4) with a very low concentration of 1% added in water. The platform consists of a T-model waveguide with three hollow cylinder branches. The PnC demultiplexer is designed with a square arrangement of infinite rods of Stainless steel embedded in the distilled water substrate. To determine the band structure and transmission spectrum, the finite element method has been evaluated. The main waveguides aremade by eliminating one row or one column of scatterers. For each branch, the radius of inclusions is changed to have three different defect branches. Hence, the intended demultiplexer can classify three frequencies in the range of kHz. The demultiplexer showed higher performance parameters for all mixtures. Where, the highest transmission intensity is about 95% for MgSO4-water mixture. The highest sensitivity (66000 Hz.cm3/g) was obtained for theMgSO4-watermixturewhile the highest quality factor (10694) and FOM(2437 cm3/g)were obtained for the CuSO4-water mixture. Themaximum crosstalk (The worst value) among the three channels equals the value of −25.22 dB. The proposed demultiplexer can act as an eco-sensor for detecting water pollutions with higher performance and selectivity.

International audience; In this work, a fiber optic-based Fabry-Perot interferometer (FFPI) developed for simultaneous heart rate (HR) and pulse pressure (PP) measurement was investigated. Particularly, the FFPI was designed with simplicity in configuration for highly precise and non-invasive arterial distension measurement whose output was then demodulated via fringe counting to simultaneously obtain both HR and PP information through fringe pattern analysis and Kirchhoff-Love’s plate theory, respectively. The sensitivity was then characterized by linear fitting of measured PP inducing a number of interference fringes, whose slope represented the FFPI sensitivity. Experiments were conducted both on a simulating device and healthy subjects, each measurement carried out at least 10 times. Obtained results demonstrated the FFPI sensitivity for PP measurement in both experiments to be ~1.916 mmHg/fringe. For HR measurements, an average difference of 1.24% was found when compared to a digital sphygmomanometer employed as reference. Analysis of the FFPI resolution revealed the impact of the fringe counting technique, interrogating wavelength, and sensing material properties on measurement accuracy. Consequently, the thickness of the transducing thin film was found to have the most impact on PP demodulation. Therefore, optimization of the aforementioned parameters could lead to the development of a more accurate FFPI for PP measurement.

A fully integrated regenerative CO2 MEMS sensor using a micro-resonator coated with Linear Polyethylenimine as an adsorbent layer is presented. The pyramidal cantilever micro-resonator used has a resonant frequency of 315.24 kHz with a Q factor of 1125 at atmospheric pressure. The micro-resonator was integrated with a sustaining amplifier that consumes 8 mW. The MEMS sensor sensitivity was characterized in a CO2 concentration range from 1200 to 10500 ppm, showing a frequency shift of −12 Hz at 1200 ppm with a slope of −3.50 mHz / ppm over the characterized range. The adsorption and recovery times are characterized to be as low as 170 seconds and 280 seconds, respectively. The impacts of pressure, temperature and humidity on the sensor were also characterized. The behavior of the MEMS sensor is compared to that of an optical CO2 sensor.