Showing papers in "Sensors and Actuators A-physical in 2020"

Flexible piezoelectric pressure sensor based on polydopamine-modified BaTiO3/PVDF composite film for human motion monitoring

Abstract: Flexible pressure sensors based on piezoelectric materials have been intensively investigated for their wide applications in wearable electronics. However, traditional films based on inorganic/organic composite piezoelectric materials face the bottleneck of defects and cracks or poor dispersion, which hinders the performance of pressure sensors. Herein, polydopamine (PDA) was introduced as a surface modification agent to modify barium titanate (BaTiO3, BTO), which was then blended with poly(vinylidene fluoride) (PVDF) matrix in different ratios to form uniform and homogeneous PDA@BTO/PVDF composites. Afterwards, the flexible piezoelectric pressure sensor was fabricated by a facial solution-casting method. This PDA-modification strategy can improve the dispersion of BTO into PVDF matrix, as well as reduce the interface hole defects and cracks between the two components. As a result, the 17 wt% PDA@BTO/PVDF sensor exhibited a fast response of 61 ms and a remarkable piezoelectric output voltage of 9.3 V, which showed obvious improvement as compared to the pristine PVDF and BTO/PVDF composite counterparts. In addition, as an energy supplier, the sensor could produce a maximum power of 0.122 μW/cm2 even with high load resistance of 70 MΩ. This pressure sensor was sensitive to various human motions, showing great potential in the applications of wearable electronics.

3D-printed sensors: Current progress and future challenges

Abstract: Due to the technological advances, sensors have found a significant role in different aspects of human life. The sensors have been fabricated via various manufacturing processes. Recently, additive manufacturing (AM) has become a common method for fabrication of a wide range of engineering components in many industries. This manufacturing method, commonly known as three-dimensional (3D) printing is based on melting and solidification that leads to production of a component with high dimensional accuracy and smooth surface finish. As precision and elegant techniques are needed in manufacturing of the sensors, AM has been utilized in fabrication of these parts in the last few years. In this study, we summarized and classified applications of different AM methods in manufacturing of sensors. In this context, we briefly reviewed and compared AM techniques and categorized 3D-printed sensors based on their applications. Moreover, fabrication of sensors via AM is explained in details, challenges and future prospect of this manufacturing process are discussed. Investigations on the performed studies proved that higher printing resolution, faster speed and higher efficiency are needed to reach a remarkable advance in the production of 3D-printed sensors. The presented data can be utilized not only for comparison, improvement and optimization of fabrication processes, but also is beneficial for next research in production of highly sensitive sensors.

Flexible temperature sensors: A review

Abstract: The research and development of wearable sensors have gained a growing interest due to the advantage of being flexible, thin, and light, which makes them highly desirable for robotic and healthcare applications. One of the essential devices in wearable technology is flexible temperature sensors for body temperature detection. Recent advances in flexible temperature sensors based on nanomaterials and conductive polymers are evaluated in this review. The temperature-responsive mechanisms, temperature-sensitive materials, and production methods of temperature sensors and recent outcomes of related papers are explained and classified in a detailed perspective. The flexible temperature sensors based on nanomaterial-filled polymer composites, printed/coated conductors, and textiles are explained in terms of structures and performances by giving examples from recent literature studies. The reliability and challenges of flexible temperature sensors are outlined from the point of view of structural design and implementation.

Microwave reflective biosensor for glucose level detection in aqueous solutions

Abstract: This article presents the design and analysis of a real-time non-invasive microwave microfluidic sensor for measuring glucose concentration in aqueous solutions. The sensor is made of an open-ended microstrip transmission line loaded with a complementary split-ring resonator (CSRR). The CSRR shows a very intense electric field concentration at resonance, which is highly sensitive to the dielectric sample loading. A microfluidic channel is designed to deliver the glucose solutions to the sensitive area of the device. By applying liquid samples to the channel, a resonance frequency shift is detectable in the reflection coefficient (S11) of the device. This in turn leads to a change in the |S11|. Both of the frequency shift and Δ|S11| can be used to measure the glucose level in the solution. Mathematical models are developed based on the measurement results of the glucose-water solutions using the resonance frequency shift and Δ|S11|. The developed sensing models are then used for detecting the glucose levels down to physiological values using the designed biosensor. The results prove the potential compatibility of the proposed biosensor for human glycaemia monitoring.

A review of railway infrastructure monitoring using fiber optic sensors

Abstract: In recent years, railway infrastructures and systems have played a significant role as a highly efficient transportation mode to meet the growing demand in transporting both cargo and passengers. Application of these structures in extreme environmental situation under severe working and loading conditions, caused by the traffic growth, heavier axles and vehicles and increase in speed makes it extremely susceptible to degradation and failure. In the last two decades, a significant number of innovative sensing technologies based on fiber optic sensors (FOS) have been utilized for structural health monitoring (SHM) due to their inherent distinctive advantages, such as small size, light weight, immunity to electromagnetic interference (EMI) and corrosion, and embedding capability. Fiber optic-based monitoring systems use quasi-distributed and continuously distributed sensing techniques for real time measurement and long term assessment of structural properties. This allows for early stage damage detection and characterization, leading to timely remediation and prevention of catastrophic failures. In this scenario, FOS have been proved to be a powerful tool for meticulous assessment of railway systems including train and track behavior by enabling real-time data collection, inspection and detection of structural degradation. This article reviews the current state-of-the-art of fiber optic sensing/monitoring technologies, including the basic principles of various optical fiber sensors, novel sensing and computational methodologies, and practical applications for railway infrastructure monitoring. Additionally, application of these technologies to monitor temperature, stresses, displacements, strain measurements, train speed, mass and location, axle counting, wheel imperfections, rail settlements, wear and tear and health assessment of railway bridges and tunnels will be thoroughly discussed.

Progress and challenges in fabrication of wearable sensors for health monitoring

Abstract: Rapid development in sensor technology, has led to fabrication of high-performance wearable sensors with application in human health monitoring. As human health is an utmost importance issue, various high-tech equipment has been developed to use in continuous health monitoring. The favorable properties of wearable sensors, make them an appropriate choice in medical applications. In this context, wearable sensors designed, produced and utilized in human health care systems which can facilitate diseases diagnosis. Depend on the type of sensor, a specific parameter such as heart beat, pressure, and temperature can be monitored and measured. This review attempts to summarize the progress and challenges in fabrication of wearable sensors for human health monitoring. In this context, applications of wearable sensors are divided into (a) biophysical tracking, (b) biochemical monitoring, and (c) detection of real-time data. Moreover, details of medical wearable sensors are outlined. In this regard, types of sensors, their materials and fabrication processes are explained. Finally, performance and current challenges in this field are briefly discussed. The documented analysis and discussion in this paper show the advantages and limitations in this field, and also highlight needs for next researches. This review confirmed that the emergence of wearable sensors has opened new horizons for human health monitoring.

A review on piezoelectric ultrasonic motors for the past decade: Classification, operating principle, performance, and future work perspectives

Abstract: Hundreds of piezoelectric ultrasonic motors (PUSMs) have been proposed for scientific researches and developed for commercial applications in the past decade. They are surveyed and mainly classified into three types: standing wave motor (SWM), traveling wave motor (TWM) and hybrid modes motor (HMM), according to their operating principles. These different types of PUSMs are discussed in detail, in terms of their operating principles, structures, features and performances. The methods to realize the multi-degree-of-freedom (multi-DOF) motions of the PUSMs are also investigated based on the basic operating principles of the SWM, TWM and HMM. Some practical applications and representative designs of the PUSMs are introduced briefly. Finally, further efforts and research perspectives of the PUSM are summarized. This review contributes to a comprehensive understanding of the state of arts of the PUSM.

Metal-organic frameworks for QCM-based gas sensors: A review

Abstract: Gas safety has penetrated into every corner of life. As such, detection and control of hazardous gases or humidity will always be indispensable. Quartz crystal microbalance (QCM) platform modified by various sensing materials is a novel and high performance sensing device. As functional materials, metal-organic frameworks (MOFs) with unique physical and chemical properties show promise in gas sensing. In this review, we overviewed the application of MOFs and MOF-based materials in gas monitoring based on QCM. A variety of harmful gases (including formaldehyde, toluene, acetone, etc.) and humidity are mentioned. Future perspectives and challenges faced by MOFs in gas sensing will also be discussed. This review tries to summarize the development of MOF science and application in the research field of QCM-based gas sensor, and provide guidance to scientists who have just come into contact with this field.

Recent advances in sensor fault diagnosis: a review

Abstract: If this is to be REF-eligible it needs a full text file attached, this cannot be final published pdf but could be version after review but before publisher formatting applied researcher confirmed item not refabable so happy to process without file 13/5/20

Metal oxide semiconductors with highly concentrated oxygen vacancies for gas sensing materials: a review

Abstract: The introduction of oxygen vacancies into metal oxide semiconductors is an effective way to enhance their gas sensing performance. In this review paper, firstly, the roles of oxygen vacancies on band structure, electrical conductivity, optical absorption and gas adsorption are presented. The presence of highly concentrated oxygen vacancies narrows the bandgap width of semiconductors, thus reducing the energy required for electron transition. It also increases the active sites on the material surface and enhances the chemisorption, thus improving the adsorption performance of the material. In addition, it also improves the electrical conductivity and light absorption ability of the material. Then, this review paper briefly introduced the state of the art of metal oxide semiconductors with highly concentrated oxygen vacancies fabricated by various processes, which are mainly divided into direct and indirect methods. At last, the application of metal oxide semiconductors with highly concentrated oxygen vacancies in the field of gas sensors is reviewed.

One-step fabrication of SnO2 porous nanofiber gas sensors for sub-ppm H2S detection

Abstract: SnO2 porous nanofibers (NFs) were deposited on-chip by using a facile electrospinning method followed by heat treatment at 600 °C and used to detect H2S concentrations at sub-parts per million level. Morphological, compositional, crystal, and atomic structural properties of the as-spun and calcined SnO2 NFs were investigated by field emission electron microscopy, energy dispersive spectroscopy, X-ray diffraction, and high-resolution transmission electron microscopy, respectively. SnO2 porous NFs with an average diameter of 150 nm and consisting of many nanograins were successfully fabricated by on-chip electrospinning. The NFs were crystallized as the tetragonal structure of SnO2 with an average crystallite size and dislocation density of approximately 13.5 nm and 5.615 × 1015 lines/m2, respectively. The sensing characteristics of the SnO2 NF sensors were tested with 0.1–1 ppm H2S from 150 °C to 450 °C. The sensor achieved the optimal performance at 350 °C and exhibited gas response of 15.2 with fast response/recovery times of 15 s/230 s. The H2S gas sensing mechanisms of the SnO2 porous NF sensors were due to the modulation of the resistance along the surface depletion layer and the grain boundaries. The fabricated sensor also indicated a good selectivity to H2S, short-term stability, and the low detection limit of 1.6 ppb. The influence of humidity on the sensor’s performance in a low temperature range is also discussed.

Piezoelectric MEMS based acoustic sensors: A review

Abstract: This paper discusses piezoelectric acoustic devices based on widely used piezoelectric materials. Commonly used piezoelectric thin film deposition techniques and the influence of process parameters on the growth, texture and orientation of the films are discussed. Etching techniques are also outlined. A comparative study of different devices developed previously is given. Also, applications of developed devices in aero-acoustic and medical fields have been briefly discussed. Flow charts of various techniques of deposition along with a combined one for full acoustic device fabrication are given. Various techniques, frequently used for thin film characterization have been discussed. The testing and measurement techniques to determine the responses of acoustic devices such as sensitivity, resonance frequency, frequency response, piezoelectric co-efficient etc. have been briefly illustrated. This paper discusses common failure modes with respect to the field of use of acoustic devices. It also concisely discusses various reliability tests done in industries to assess the quality of the developed devices. This review has also suggested directions for future development of thin film acoustic sensors.

A facile spray pyrolysis fabrication of Sm:CdS thin films for high-performance photodetector applications

Abstract: Achievement of high-performance photodetectors based on CdS is a key field of research and challenge in the current scenario. Here, facile fabrication and characterization of novel samarium (1, 3 and 5 wt.% Sm)-doped CdS thin films for the photodetector applications have been demonstrated. The fabricated films show good crystallinity with crystallites size ranging 18–30 nm. The morphology and homogeneity of Sm-doping ingrown films were confirmed through scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDX). Field emission SEM study reveals the low dimension nanograins formation and the films are free from voids and cracks. The effects of Sm-doping on linear and nonlinear optical properties of the fabricated thin films have been elucidated. The optical parameters such as refractive index, energy gap, susceptibilities were noticed to be reduced by Sm-doping in CdS thin films. An emission peak around 536 ± 14 nm was observed in PL spectra of pure CdS which was found to be shifted and quenched by Sm-doping. Finally, the photodetector performance of the fabricated thin films has been investigated for 532 nm laser light. The photodetector based on the 1 wt.% Sm:CdS shows an improved performance (higher responsivity of 1.01 AW−1, higher detectivity of 2.21 × 1012 Jones, excellent photosensitivity of ∼4.9 × 103, and very high external quantum efficiency (EQE) of 257 %) compared to pure CdS (responsivity of 0.213 AW−1, detectivity of 7.43 × 1011 Jones, photosensitivity of ∼2.0 × 103, and EQE of 249.70 %). These results propose a much simpler route to achieve high-quality CdS films for photodetector applications.

A novel, co-located EMG-FMG-sensing wearable armband for hand gesture recognition

Abstract: Gestures play an important role in human-computer interaction, providing a potentially intuitive way to bridge the gap between human intention and the control of smart devices. Electromyography (EMG) and force myography (FMG) are two commonly-adopted wearable sensing modalities for gesture recognition. Previous research approaches utilize only a single modality (EMG or FMG) at any given muscle location, thus limiting the amount of potentially-useful biological hand gesture information. We thus propose a novel co-located approach (EMG and FMG) for capturing both sensing modalities, simultaneously, at the same location. We developed a novel hand gesture recognition armband consisting of 8 co-located EMG-FMG sensing units (size and weight of each EMG-FMG sensing unit was 11 × 13 × 6 mm and 0.90 g, respectively). Five subjects performed a hand gesture recognition experiment for American Sign Language digits 0–9 while wearing the co-located EMG-FMG armband on the forearm. Hand gesture classification accuracy was 81.5 % for EMG only, 80.6 % FMG only, and 91.6 % for co-located EMG-FMG. These results suggest that co-located EMG-FMG may lead to higher hand gesture classification accuracy than sensing approaches using either EMG or FMG in isolation. To the best of our knowledge, this is the first prototype that measures EMG and FMG simultaneously at the same muscle location for hand gesture recognition. Implications of this work could positively impact a variety of muscle activity monitoring research applications including biomechanics modeling, prosthesis control, and gesture recognition.

Transition metal dichalcogenides-based flexible gas sensors

Abstract: In recent years, two-dimensional (2D) layered materials, such as MoS2, MoSe2, WS2, WSe2, SnS2, etc., have gained enormous interest in sensing applications with low-power consumption owing to their unique electrical, chemical, and mechanical properties. Among a broad range of sensors, rapid advances in flexible and wearable sensors have paved the way for smart sensing applications, including electronic skin (e-skin), home security, air- and health-monitoring, etc. This review aims at summarizing recent progress in energy-efficient flexible gas sensors by utilizing 2D transition metal dichalcogenides (TMDCs) materials. Main concepts and different approaches are overviewed for optimizing the gas sensing characteristics on flexible sensing platforms. The different strategies and challenges for incorporating 2D TMDCs materials into e-skin-oriented and e-textiles gas sensors are also highlighted. In addition, this review also includes the challenges and future perspectives for TMDCs materials in emerging flexible and wearable gas sensing field.

Flexible Piezoelectric Nanogenerators Using Metal-doped ZnO-PVDF Films.

Abstract: Piezoelectric nanomaterial-polymer composites represent a unique paradigm for making flexible energy harvesting and sensing devices with enhanced devices' performance. In this work, we studied various metal doped ZnO nanostructures, fabricated and characterized ZnO nanoparticle-PVDF composite thin film, and demonstrated both enhanced energy generation and motion sensing capabilities. Specifically, a series of flexible piezoelectric nanogenerators (PENGs) were designed based on these piezoelectric composite thin films. The voltage output from cobalt (Co), sodium (Na), silver (Ag), and lithium (Li) doped ZnO-PVDF composite as well as pure ZnO-PVDF samples were individually studied and compared. Under the same experimental conditions, the Li-ZnO based device produces the largest peak-to-peak voltage (3.43 Vpp) which is about 9 times of that of the pure ZnO based device, where Co-ZnO, Na-ZnO and Ag-ZnO are 1.2, 4.9 and 5.4 times, respectively. In addition, the effect of doping ratio of Li-ZnO is studied, and we found that 5% is the best doping ratio in terms of output voltage. Finally, we demonstrated that the energy harvested by the device from finger tapping at ~2 Hz can charge a capacitor with a large output power density of 0.45 W/cm3 and light up an ultraviolet (UV) light-emitting diode (LED). We also showed the device as a flexible wearable motion sensor, where different hand gestures were detected by the device with distinctive output voltage amplitudes and patterns.

A significant enhancement in visible-light photodetection properties of chemical spray pyrolysis fabricated CdS thin films by novel Eu doping concentrations

Abstract: This work deals with the preparation of undoped and rare earth europium-doped cadmium sulfide (Eu:CdS) films by spray pyrolysis procedure. X-ray diffraction (XRD), field emission-scanning electron microscopy, energy dispersive X-ray spectroscopy, ultraviolet-visible spectroscopy and photo-electrical studies were used to analyze the properties of the films. XRD analysis of CdS films showed polycrystalline nature with most preferred orientation along (002) and (101) for Eu:CdS. Eu doping improved the optical properties, the optical band gap of Eu:CdS films is noted to be larger compare to pure CdS thin films. Optical studies reveal the direct band gap variation from 2.43 eV to 2.48 eV with rising Eu doping from 1 wt.% to 5 wt.%. The increase in the nonlinear optical parameter was observed on Eu doping in CdS. The photo-electrical studies done for Eu:CdS films indicates that Eu doping has noticeable influence on electrical properties. The responsitivity for pure was ∼0.0629 AW−1 which increased to 0.614 AW−1, when 5 wt.% Eu was doped. The external quantum efficiency was enriched ∼ 8 times for 5 wt.% Eu:CdS compared to pure. The rise and decay time of Eu:CdS photodetector and its ON/OFF ratio was studied which is noticed ∼10 times larger compared to pure CdS. The observed enhancement in photodetector properties, morphology, crystalline quality and optical properties are caused by Eu doping concentration and hence will be more applicable in optoelectronics.

Self-sensing capabilities of cement-based sensor with layer-distributed conductive rubber fibres

Abstract: Piezoresistive-based cementitious composites have unique advantages compared with other sensors for structural health monitoring. A novel cement-based sensor incorporating conductive rubber was explored in this study, including its raw materials, manufacturing process, electrical conductivity, self-sensing efficiency, repeatability and compressive strength. Different water to binder (w/b) ratios of composites and loading stress magnitudes were also investigated. The results show that the conductive rubber fibres had capacity to improve the electrical conductivity of cementitious composite, with percolation threshold approximately from 60–100 rubber fibres (0.96–1.6 vol.% to specimen). The higher w/b ratio, the lower resistivity and the more inconspicuous percolation the composite was. Furthermore, the composites at w/b ratio of 0.38 were provided with best piezoresistivity, whose fractional changes of resistivity reached 25% under stress magnitude of 10 MPa when reinforced by 80 rubber fibres (1.28 vol.%). The sensitive coefficient proposed could describe piezoresistive sensitivity in different stress magnitudes, because of the non-linear resistivity changes with increased stress amplitude. As for the mechanical properties, satisfactory compressive strength of 36 MPa was obtained for the composite filled with 80 rubber fibres (1.28 vol.%) at w/b ratio of 0.38. Therefore, the conductive rubber embedded cementitious composites have great application potential as new cement-based sensors for concrete structural health and pavement traffic monitoring.

Ferrofluid droplet manipulation using an adjustable alternating magnetic field

Abstract: Magnetically actuated droplet manipulation offers a promising tool for biomedical and engineering applications, such as drug delivery, biochemistry, sample handling in lab-on-chip devices and tissue engineering. In this study, characteristics of an adjustable alternating magnetic field generated by a magnetic coil for droplet manipulation was investigated which enables more control on droplet transport, and it can be considered as a suitable alternative for moving magnets or an array of micro-coils. By adjusting the magnetic flux density, the duty cycle and applied magnetic frequency, the manipulation of water-based ferrofluid droplets with a bio-compatible surfactant for different volumes was comprehensively examined. Also, the platform was able to manipulate the ferrofluid droplets completely immersed in oil. This solves the problem with droplet evaporation which has previously been reported for droplet manipulation on the surface. Furthermore, an analytical model is proposed for the movement of the ferrofluid droplet on the hydrophobic surface. The model predictions are in good agreement with experimental results. Also, the effects of magnetic flux density, duty cycle, frequency and the distance between the coils on the mixing process were studied. Results showed that the droplet movement on the hydrophobic surface was fully synchronized with the generated signal while the droplet moved backward in the oil after the magnetic field was turned off. By decreasing magnetic flux density, droplet volume, duty cycle, as well as increasing applied magnetic frequency, the step lengths become more uniform, the resolution of droplet displacement increases, but the average-velocity decreases.

A review on fabrication, characterization and implementation of wearable strain sensors

Abstract: The paper presents a substantial review on the different kinds of strain sensors that have been employed as wearable sensing prototypes. The importance of strain sensors lies in their low cost, high sensitivity and multifunctional applications. The flexible strain sensors have been developed with printing techniques that have generated prototypes with varied electrical, mechanical and thermal characteristics. These types of devices have been primarily used for biomedical applications, where a small amount of deflection holds a crucial worth to monitor acute and chronic anomalies in human beings. Among the major areas in healthcare applications where strain sensors have been utilized, wearable sensing holds a pivotal role due to their capability of ubiquitous monitoring. The wearable sensors have been designed and fabricated with a range of processing materials, based on their respective applications. Along with the significant research related to the fabrication and implementation of wearable strain sensors, explanation related to the challenges of the current sensors and their futuristic possibilities have also been showcased in the paper.

Design of graphene metasurface based sensitive infrared biosensor

Abstract: The article investigates graphene metasurface based infrared biosensor. Graphene metasurface perturbations are added in the Si3N4 waveguide to create leaky wave structure. Biomolecules in blood plasma form are placed over the Si3N4 waveguide to observe the sensing characteristics. Numerical results of absorption, reflectance, electric field and sensitivity are presented in this paper. The sensitivity is calculated from the shift in the absorption peak of biosensor. The sensitivity of the proposed biosensor is also compared with the previously published biosensor designs. In addition, the design results in the form of absorption and reflectance is observed for the different physical parameters like waveguide height (S), biomolecules/air layer height(B), SiO2 layer height(C) and graphene perturbations period(G). The corresponding electric field response is also presented for the proposed design at different frequencies to show the leakage of light in the biomolecules layer.

Flexible and lead-free piezoelectric nanogenerator as self-powered sensor based on electrospinning BZT-BCT/P(VDF-TrFE) nanofibers

Abstract: Recently, the flexible and environmental-friendly piezoelectric generators have drawn much attentions due to the power-supplying apply applications of powering implantable and wearable devices. In this work, an environmental-friendly and flexible piezoelectric nanogenerator is proposed based on electrospinning nanofiber which is composed of 0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 (BZT-BCT) and polyvinylidene fluoride–trifluoroethylene P(VDF-TrFE). The as-prepared nanofiber mats with different amounts of doping of BZT-BCT nanoparticles varied from 0 wt% to 50 wt% are characterized by XRD and SEM. Based on the testing results, the nanofiber generator with 40 % content of BZT-BCT exhibits the excellent output performance, which produces the output voltage as high as 13.01 V under cyclic tapping under 6 N and 10 Hz, which is mostly attributed from the doping of the BZT-BCT with high piezoelectric coefficient. The generator can be deployed as the self-powered sensor, which can measure the tensile and compressive deformation, the movement of different parts of body. Due to the advantages of flexibility and environmental kindness, this developed nanogenerator has great potential for wearable and implantable devices.

Piezoelectric pressure sensor based on flexible gallium nitride thin film for harsh-environment and high-temperature applications

Abstract: Piezoelectric materials are promising for pressure sensors in a variety of industrial applications such as automotive and petroleum fields. Typical piezoelectric sensors rely heavily on lead zirconate titanate (Pb[ZrxTi1-x]O3, PZT) transducers. However, for broader applications of piezoelectric sensors, the PZT is a suboptimal candidate due to its unstable output at temperatures above 200 °C and potential environmental hazard. A recent research objective is to produce a more effective, safer, and eco-friendly material than PZT for piezoelectric pressure sensors by incorporating lead-free materials, such as other ceramics or polymeric composites. Among lead-free materials, gallium nitride (GaN) has a notable piezoelectric coefficient and demonstrates the greatest potential to replace the PZT as a result of its performance in high temperature and pressure operating conditions. In this study, GaN thin film is used as a sensing material, where the sensor is made by a simple layer transferring process after the removal of silicon substrate. The output potential values of GaN with respect to gas pressure levels are 42.3, 76.8, 98.7, and 122.1 mV for 50, 100, 150, and 200 psi, respectively, well matched to simulated results. Additionally, the potentials measured at elevated temperatures produce reliable outputs at high temperatures up to 350 °C. Furthermore, the stability of sensor outputs at room temperature and elevated temperatures with various pressure levels was confirmed.

A flexible strain sensor based on CNTs/PDMS microspheres for human motion detection

Abstract: In this paper, a novel method for the preparation of flexible strain sensors based on multi-walled carbon nanotubes/polydimethylsiloxane (MWCNTs/PDMS) microspheres (MSs) is proposed. The conductive and elastic MWCNTs/PDMS MSs were prepared by the oil-in-water Pickering emulsion mechanism at room temperature which served as the sensing materials. After being covered with the PDMS matrix, the flexible strain sensor was obtained and it exhibited high conductivity of 157.64 Ω ∙ c m , high strain range of more than 40 %, high sensitivity (GF = 7.22), and high stability (>500 stretch and release cycles). Besides, the as-prepared strain sensor successfully recognized different finger gestures, and the movements of wrists, elbows and knees, which proved the sensor’s application prospects in the fields of human motion detection, electronic skins of bionic robots and human-machine interfaces, etc.

VOC gas sensor based on hollow cubic assembled nanocrystal Zn2SnO4 for breath analysis

Abstract: Hollow cubic assembled nanocrystal Zn2SnO4 was synthesized via one-step hydrothermal method for VOC gas sensing applications. The obtained Zn2SnO4 materials had a uniformly hollow cubic structure with an average size of approximately 1 μm and a wall thickness of about 150 nm formed from nanocrystals of around 14 nm. The X-ray absorption near-edge structure results showed no structural disorder and/or lattice damage around the Zn-absorbing atoms and the Zn oxidation state of 2+ in the host lattice of the hollow cubic Zn2SnO4. The gas sensing characteristics of the prepared Zn2SnO4 material were tested to C3H6O, C2H5OH, CH3OH, NH3, H2, and CO at 350 °C–450 °C and results showed that the sensors exhibited a good response to acetone and ethanol gases. The highest response values were 47.80 for 125 ppm of acetone and 7.52 for 10 ppm of ethanol at 450 °C. The Zn2SnO4 hollow cubic sensor demonstrated high sensitivity and selectivity to acetone with good stability and a detection limit of 175 ppb. The VOC sensing mechanism of the hollow cubic Zn2SnO4-based sensor was also discussed. The findings indicated that hollow cubic Zn2SnO4 is a promising material for use in excellent VOC gas sensing application towards breath analysis.

Review of Non-invasive Continuous Glucose Monitoring Based on Impedance Spectroscopy

Abstract: The incidence of diabetes is increasing at an alarming rate worldwide, and glucose monitoring is critical to diabetes management. Non-invasive continuous blood glucose monitoring is highly sought after for diabetes management. While enzyme based approaches already have commercial products, they are invasive, inconvenient for patients to use, and cannot be continuous. Non-invasive glucose detection by spectroscopy methods have garnered much interest in recent years, however, still facing many challenges to be adopted for clinical applications. Among spectroscopy methods, electrical impedance methods have the advantages of being low cost and user-friendly. This review summarizes recent advances in non-invasive continuous glucose monitoring based on skin impedance measurement by electrical impedance spectroscopy (EIS), with emphasis on the devices that can potentially be integrated into wearable platforms. The discussion includes physical designs of bioimpedance sensors, signal measurement strategies, modeling and parameter estimation methods to extract blood glucose levels, and portable system designs. At the end, the challenges of EIS based glucose estimation devices and the trend of non-invasive glucose monitoring techniques are discussed.

High-force soft pneumatic actuators based on novel casting method for robotic applications

Abstract: Soft pneumatic actuators (SPAs) play an important role in leading the development of soft robotics, and enable diverse and complex hardware design for soft robots. However, due to the limitation of the inherent characteristics of SPAs’ constitutive materials as well as fabrication tools and techniques, they tend to exhibit a lower driving force. In this paper, we propose a class of high-force SPAs that are fabricated by a novel method, embedded core casting. This method is enabled by two main steps: First, a SPA is designed in two parts, an actuating core composed of a fiber-reinforced airbag and an elastic holder made of soft materials, and they are fabricated independently; second, both parts are combined together by assembly and recasting techniques, producing a finished actuator. The bending SPAs fabricated by this method can bear air pressure up to 400 kPa, which provides important hardware for the development of high-load soft robots. Further, we demonstrate the applications of these soft SPAs in soft grippers, continuum manipulators, and soft wearable robots. This study helps improve the fabrication process for fiber-reinforced actuators and propose new high-force SPAs for robotic applications.

A review of optical interferometry techniques for VOC detection

Abstract: Exposure to volatile organic compounds (VOCs) is widely associated with adverse health effects. Detection and monitoring of VOCs are important for maintaining safe and healthy industrial and domestic environments. Interferometry is a highly-sensitive optical measurement technique that has been widely applied to a vast range of physical parameters from the speed of light to temperature and has also been used to detect VOCs at the sub-ppm range. Owing to the vast range of interferometer arrangements and processing techniques, this review assesses the different approaches adopted in detecting VOCs. Different interferometry arrangements including the Fabry-Perot interferometry, Sagnac interferometry and Mach-Zehnder interferometry are reviewed for VOC detection, including the different sensing films and materials employed. We present the basis of each technique, applications and limitations. The different interferometry techniques are summarized by comparing the sensitivity, limit of detection, linearity, response time and the challenges of current interferometry techniques. Lastly, prospects to realize a miniaturized, high-sensitive and multiplex interferometric sensors based on the recent technology are suggested.

Synthesis of WO3 nanoflakes by hydrothermal route and its gas sensing application

Abstract: The controlled morphology and size of inorganic materials have attracted intense interest, as these parameters play an important role in determining sensing properties. Herein, WO3 thin film is hydrothermally grown on FTO substrate at 175 ℃ with the assistance of seed layer deposited by spray pyrolysis technique. The WO3 thin film was characterized by X-ray Diffraction (XRD), micro-Raman spectroscopy, Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and UV–vis Spectroscopy for determination of physico-chemical properties. Moreover, X-ray Photoelectron Spectroscopy (XPS) analysis is carried out to understand chemical states and boding. Systematic gas sensing studies were performed for NH3, H2S and CO gases under static condition. The sensing study reveals, WO3 nanoflake exhibits a superior sensor response to NH3 gas. Moreover, it exhibits higher sensitivity to NH3. Gas sensing properties indicate WO3 nanoflakes holds promise to become a potential candidate for NH3 gas detection at the expense of lower power consumption.

Highly elastic capacitive pressure sensor based on smart textiles for full-range human motion monitoring

Abstract: Human motion analysis has become an important clinical step for collecting data in the healthcare field. However, the typical movement monitorings have to be faced with many different challenges about tools, reliability, and working range of sensors for applications at different positions on the human body. To fulfill this aim, the design and fabrication of a multi-purpose capacitive pressure textile sensor for wearable electronics applications are presented. Because of the high elasticity of the dielectric layer of spacer fabric, the capacitive pressure sensor exhibits a very fast recovery time (7 ms) and high cycling stability (> 20,000). Besides, the stacking structure of the electrode layers (SWCNT/Silver paste) due to excellent durability even under large deformations (grasping, bending, stretching), and breathable for the skin in applications. As the practical demonstrations, the pressure sensors are embedded into a textile glove for grasp motion monitoring, and smart socks for walking gait analysis during activities of daily living. More importantly, an adaptive fuzzy-neuro network algorithm has been developed and adapted in order to increase the accuracy of the gait monitoring system under actual and realistic wearing test conditions.