Resistive-type Sensors Research Articles

Fingerprint-inspired triboelectric nanogenerator with a geometrically asymmetric electrode design for a self-powered dynamic pressure sensor

Effective microstructured active layers in capacitive or resistive-type pressure sensors have been adapted in triboelectric nanogenerator (TENG)-based self-powered pressure sensors to realize battery-free and highly sensitive operations. However, the maximum charge transfer cannot be achieved due to the geometry-driven partial contact status in a microstructured TENG, restricting its application to highly sensitive pressure sensors. In this report, inspired by fingerprint anatomy, a geometrically asymmetric paired-electrode TENG is designed to enhance the pressure-induced electrical output through a microelectrode (μ-electrode) on a microstructured TENG (μ-TENG). The μ-electrode is conformally formed along a 3D dielectric geometry; the shape and density of the μ-electrodes are customized by the finite element method (FEM) and validated experimentally. The optimized μ-electrode design endows higher pressure sensitivity with maximized charge transfer efficiency and exhibits reliable and robust pressure sensing performance. The artery pulse and acoustic pressure (with frequency ranges of 0.1–10 kHz) monitoring capabilities of the μ-TENG can be used for precise healthcare monitoring and discerning a sound signal.

Proposal of a Unified Sensing Circuit for Different Sensor Output Types

In recent years, the IoT of electronic devices has been rapidly progressing, and the demand for sensors used in these devices has been increasing. There are five output types of electric sensors: voltage type, current type, resistance type, capacitance type, and inductance type. Different types of output require different detection circuits. Therefore, it is necessary to use a different circuit for each type of sensor, which is a bottleneck for the price reduction. In addition, to prepare a detection circuit suitable for a particular application, it is sometimes necessary to purchase the circuit made by a specific manufacturer. In this case, the order-made circuit takes a long time to produce and needs more learning costs to train the user. This study developed a sensing system that can be used uniformly with a single circuit regardless of the type of sensor to solve these problems. In this study, sensors are divided into voltage type, current type, resistance type, capacitance type, and inductance type according to the type of electrical characteristic. The five types of sensors are automatically identified as energy conversion type or energy control type using a detection circuit. The difference between the energy conversion type and the energy control type circuit is whether the power source is AC voltage or 0[V]. In this research, the voltage type sensor and the current type sensor can be detected as the energy conversion type circuit, and resistance, capacitance, and inductance type sensors as energy control type circuits by using the same electric circuit. Thus, regardless of the sensor type, the detection circuit automatically classifies the sensor as either energy conversion type or energy control type and automatically measures all five types when a sensor is connected.

Performance of Flexible Strain Sensors With Different Transition Mechanisms: A Review

To promote the application of flexible strain sensors in the fields of athletics feedback, health monitoring, human–machine interface, and robotics, the design strategies of various strain sensors were reviewed according to their transition mechanisms from the input stimulus to the output signal. The transition mechanisms were categorized into four major types: resistance, capacitance, voltage (piezoelectric and triboelectric), and inductance/magnetism. The sensing performance of flexible strain sensors was summarized based on this categorization and compared according to the sensing mechanism. The study observed that the performance of resistance-type sensors depends on the sensor shape and structure of the conductive network. These sensors can detect various forms of strains and have a broad sensing range and high sensitivity. However, these require an external power source. Capacitance-type sensors display rapid response and high sensitivity to force variations. Thus, they can accurately detect subtle deformations with low energy consumption. However, these strain sensors also have certain disadvantages such as a marginal strain range and low repeatability. Voltage-type sensors are energy-saving devices that can directly convert mechanical energy into an electrical signal. However, they exhibit low sensitivity and cannot accurately detect subtle strains. Inductance/magnetism-type sensors can detect strains wirelessly. However, the signal-to-noise ratio is low, and the sensing range is narrow. Finally, the application and development prospects of flexible strain sensors were presented by describing their fabrication techniques and transition mechanisms.

Multifunctional flexible sensors based on ionogel composed entirely of ionic liquid with long alkyl chains for enhancing mechanical properties

Ionogel, with high transparency, excellent mechanical properties, and notable recoverability is an up-and-coming material for making skin-like iontronic devices. Herein, a pure ionic liquid design strategy is proposed for preparing tough ionogel material by one-pot copolymerization of two polymerizable ionic liquids, 1-vinyl-3-butylimidazolium tetrafluoroborate ([VBIm][BF4]) and 1-dodecyl-3-vinylimidazolium tetrafluoroborate ([C12VIm][BF4]), in a non-polymerizable ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm][BF4]). Experiments and theoretical simulations confirm the important role of an ionic liquid-phobic “microdomain” made-up of the long alkyl chains of [C12VIm]+ in the improvement of mechanical properties. Due to a synergistic effect of the ionic liquid-phobic microdomains, ionic bond interactions, and molecular entanglement, the resultant ionic-liquid-based ionogels (IL-PILs-IG) show good mechanical properties (tensile stress ca. 4.5 MPa, tensile modulus ca. 3.2 MPa), excellent transparency (91%), high thermal stability (>300 °C), and good recoverability. Several IL-PILs-IG-based sensors are further developed. IL-PILs-IG-based resistive-type sensors show diverse sensory capabilities towards temperature, strain, and humidity. Capacitive-type sensors based on IL-PILs-IG exhibited a low detection limit as low as 25 Pa, and showed no deviation over 1,000 loading/unloading cycles. This study might provide a new path for designing and fabricating ionogels with good mechanical properties, transparency, and excellent fatigue resilience for skin-inspired ionic conductive sensors in detecting complex human motions.

Studying of a sensitive material based on Ecoflex and CNTs for flexible strain sensors

Nowadays there is a great need for the development of flexible strain sensors that can register human body’s movements. In the field of wearable and smart electronics such sensors are actively being developed. Resistive-type flexible sensors are the easiest to manufacture. Their mechanism of sensitivity to deformations is based on a change in electrical resistance during deformations. In this work, we have developed the functional material for strain sensor with high tensile properties, strength and electrical conductivity. This material based on a matrix of silicone elastomer and a multi-walled carbon nanotubes (MCNTs) filler. The material showed a high elongation of 950 % with a tensile strength of 1.437 MPa. The manufacturing process included laser structuring of MCNTs to form an electrically conductive network. The linear gauge factor was 3.4, and the angular gauge factor was 0.26.

Response Comparison of Resistor- and Si FET-Type Gas Sensors on the Same Substrate

The response characteristics of a field-effect transistor (FET)-type gas sensor are compared with those of the resistor-type gas sensor fabricated on the same Si substrate. Both types of gas sensors have the same sensing material prepared by the same process. Indium oxide (In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ) film is adopted as a sensing material to sense NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> gas. The two types of gas sensors have different response characteristics depending on the operating condition. The resistor-type gas sensor shows relatively constant response at all operating currents, and the Si FET-type gas sensor shows high response in the subthreshold region. Both types of sensors have the highest response at 105 °C, and the current of the Si FET-type gas sensor can change up to ~ 3.8×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> times at 500 ppb of NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> gas, which is ~8.15 times higher than that of the resistor-type gas sensor. In addition, when the pre-bias method ( V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">pre</sub> = -3 V) is applied to the Si FET-type gas sensor, the response of the Si FET-type sensor overwhelms that of the resistor-type sensor over the entire measurement temperature and concentration range.

Investigation of relative humidity-sensing performance of capacitive and resistive type sensor based on TDTBPPNi metalloporphyrin dielectric layer

Investigation of relative humidity-sensing performance of capacitive and resistive type sensor based on TDTBPPNi metalloporphyrin dielectric layer

Application of a Bio-Nanocomposite Tissue as an NIR Optical Receiver and a Temperature Sensor

The application of a type of bio-nano tissue, consisting of tobacco cells and multiwalled carbon nanotubes, as a sensitive near-infrared region (NIR) bolometer operating at room temperature was investigated. An electrical resistor-type sensor was fabricated by the evaporation of lateral gold contacts. A very low dark conductivity was achieved by a high-voltage treatment of a strongly conducting nanotube network. Although before the treatment the electrical transport was dominated by the percolative transport in the nanotube network, after the voltage stress the electronic transport in the low-conductivity tissue was controlled only by the intrinsic electrical properties of the biological matrix. In this latter case, the conductivity was extremely temperature and humidity dependent. However, on operating the sensor tissue in a small temperature window around room temperature, the change in humidity could be neglected. Under these conditions, the multifunctional device functioned as a very high sensitivity temperature sensor with a temperature coefficient of resistance of more than −20%/K. Sensitive bolometer operation with a very good signal-to-noise ratio was demonstrated by irradiation of the tissue with low-power LEDs in the near-infrared range between 780 and 1720 nm.

Sub-10 parts per billion detection of hydrogen with floating gate transistors built on semiconducting carbon nanotube film

Semiconducting carbon nanotubes (CNTs) based gas sensors have shown great promise owing to the high sensitivity, low cost and compatible to integrated circuits (ICs), but are not able to detect H2 with sub-10 ppb concentration, which is necessary to leakage monitoring in nuclear power plants. Here, we significantly enhance the limit-of-detection (LOD) and comprehensive performance of CNT based hydrogen sensors mainly through adopting a floating gate (FG) field-effect-transistor (FET) structure. The sensors are constructed by using high-purity semiconducting CNT network film as channel, Pd nanoparticles as H2-sensitive material, and Y2O3 dielectric layer between them as gate insulator. The introduction of Y2O3 film brings a co-optimization of sensitivity, selectivity and stability to CNT film gas sensors through the synergistic amplifying effect and protection effect. Specifically, the fabricated FG FET hydrogen sensors with the optimized Y2O3 thickness show a record LOD of 90 ppb at room temperature and even down to 5 ppb at ∼100 °C. Compared with conventional resistance-type sensors, the FG FET sensors exhibit more than one magnitude enhancement on response. Furthermore, the FG FET sensors exhibit excellent stability and well compatibility to CNT ICs, and are potential used as an excellent platform to develop smart sensors.

Determining sensor-based field capacity for irrigation scheduling

Irrigated agriculture is a major consumer of freshwater and acreage of irrigated land has increased as producers have become increasingly reliant on irrigation to ensure adequate yields and reduce production risks. Information about the soil’s field capacity, or the water content after a saturated soil has drained for at least 24 h, is needed to determine the soil’s readily available water, or the amount of water that can be safely removed by plants. To address the challenges associated with using soil water sensors in highly variable soils, soil water measurements were included in ongoing cotton irrigation studies during the 2017 through 2019 growing seasons at the University of Missouri Fisher Delta Research Center near Portageville. Sensor-based field capacity values were compared among multiple commercially available sensors. Time-domain reflectometry sensors (Acclima TDR 315) were installed at four depths and five locations each of the three years. In addition, resistance-type sensors (Irrometer Watermark 200SS) were installed at five depths and five locations for two years. The goal was to provide guidance on how best to use soil moisture sensor data for irrigation management. Although water continued to move in the profile for more than 24 h after saturation, the average change between 24 and 48 h at each observed depth was < 0.015 m3 m−3. Observed values for coarser textured soils were much wetter after 24 h than expected based on published field capacity values and in one case, even a small difference in sensor location had a large effect on the observed field capacity. Furthermore, soil apparent electrical conductivity was highly correlated to measured field capacity, which could result in a much faster and less expensive way to estimate field capacity compared to collecting and analyzing soil cores. These findings demonstrate that the application of soil moisture sensors for irrigation management is site specific, and differences can be observed over short distances within a field. The research is continuing to better meet the needs of agricultural producers, consultants, research and extension personnel, and others for information to improve irrigation management.

Flexible and Sensitivity-Adjustable Pressure Sensors Based on Carbonized Bacterial Nanocellulose/Wood-Derived Cellulose Nanofibril Composite Aerogels.

For sustainability and environmental friendliness, the renewable biomaterials including cellulose have been widely used in flexible electronics, such as pressure sensors. Herein, the carbonized bacterial nanocellulose with excellent conductivity and wood-derived cellulose nanofibrils are combined to prepare the aerogel through directional ice-templating and freeze-drying. The obtained composite aerogel, which has a porous structure and aligned channels, is further employed as an active layer to prepare the resistive-type pressure sensor on a paper substrate. This pressure sensor exhibits remarkable flexibility, fast response, reliability, and especially adjustable sensitivity in a wide pressure range (0-100 kPa). In addition, the sensor's working mechanism and potential applications, such as motion detection, footstep recognition, and communication with smartphones via Bluetooth, are also well demonstrated. Moreover, this work provides novel insights into the development of green pressure sensors and the utilization of sustainable natural biomaterials in high-tech fields.

Haptic Display Responsive to Touch Driven by Soft Actuator and Soft Sensor

Haptic display transfers the sense of touch by applying forces, vibrations, or motions to the user. This article presents a novel bidirectional haptic display responsive to touch. The device is composed of a flexible tactile sensor and tactile actuator, and the actuator is actuated according to the touch signals from the tactile sensor. The tactile sensor is a thin array of resistive type sensors that covers the top of the soft dielectric elastomer actuator array. The tactile sensor measures the normal forces (0 $-$ 6 $N$ ) and positions of touches and then sends these signals to control the corresponding tactile actuator cells. The tactile actuator works within the frequency range of 0 $-$ 300 Hz, provides up to 520 $\mu \text{m}$ of displacement and, 600 mN of normal force, much greater than the human hand threshold (50 mN). And thus, the device can transfer different feelings to users. In this research, we develop and evaluate a stand-alone bidirectional haptic display. Then, we took the psychophysical test and introduced the master—slave system whereby the master device controls the slave device depending on the master input.

A High-Performance Pressure Sensor Based on Discrete Structure and Multiple-Contact Mechanism for Detecting Human Motions and Image Description

A flexible pressure sensor is proposed based on discrete structure and multiple-contact mechanism (DSMM). It has capability to optimize performances of those resistive-type pressure sensors, which have disadvantage of small measuring range to trade for super high sensitivity. The proposed DSMM pressure sensor has tri-layer structure consisting of top electrode, graphite nanosheets/polyimide (GNS/PI) resistor matrix, and bottom electrode. As the contact area between sensing elements enlarges, more and more pixels of resistor matrix contact with top electrode to realize parallel connection for expanding measuring range up to 410 kPa. The pressure sensor still exhibits the high sensitivity (0.38 /kPa for low pressure, 0.79 /kPa for high pressure) and ultralow detection limit (15 mg), and there is no response delay or hysteresis in contact process. Human motions and tactile image are measured, which demonstrates that the pressure sensors have applicability for biological signals monitoring and have potential applications combining machine vision and tactile.

Intrinsically adhesive, highly sensitive and temperature tolerant flexible sensors based on double network organohydrogels

Hydrogel-based flexible sensors are of promising applications in various fields, but fabrication of such sensors with integrated high performances remains a challenge. In this work, flexible sensors (both strain sensors and pressure sensors) with integrated high performances are fabricated utilizing double network (DN) organohydrogels. Because of the unique structure of DN organohydrogels, the flexible sensors exhibit intrinsic adhesion without introducing components that are often used to obtain adhesive hydrogels, such as polydopamine, nucleobases or proteins. In addition, outstanding temperature tolerance (−18 to 80 °C), high stretchability (>2000%), tensile strength (>300 kPa), self-healing ability (96.5%) and transparency (90%) are also achieved. Resistive-type strain sensors of DN organohydrogels achieve high gauge factor (GF = 2.58), low response time (0.18 s), large sensing range (0–1000%) and reversible sensing ability (>1000 cycles). Sandwich-shaped capacitive-type pressure sensors comprising DN organohydrogel electrodes with reliefs exhibit a high sensitivity of 2.14 kPa−1. Such flexible sensors can be applied in monitoring various human motions and subtle physiological activities and further promoted as wireless sensors on the basis of a Bluetooth protocol.

Effect of Capping-Agent Concentration on Size and Size Dispersity of Palladium Nanoparticles for Resistive-Type Hydrogen Sensors

The size attributes in palladium nanoparticle yield were found to influence the hydrogen sensor response in resistive devices, and the sensing mechanism was correlated with the variation of particle size from large to small in a particular synthesis product. The quantity of polyvinylpyrrolidone (PVP), the stabilizer used in this study, was varied during synthesis, and the resulting sizes were determined by high-resolution transmission electron microscopy (HRTEM). The size tuning by the capping agent PVP was also confirmed by UV–Vis spectroscopy via a detailed analysis of the experimental spectra, which revealed an interesting shift of the major absorption peaks with the increase in PVP. Glancing-angle x-ray diffraction (GAXRD) studies were undertaken to highlight the face-centred cubic crystallinity of the drop-cast nanofilms on thin glass substrates, as well as to evaluate the variation in crystallite cluster size by analyzing the major diffraction peaks. The size variation from HRTEM and GAXRD studies was found to match within the limits of experimental accuracy. The hydrogen sensor studies showed good room temperature response with typical size-dependent characteristics. The mechanistic control of the hydrogen activity over the mixed sized nano-films at room temperature (RT) and beyond RT is elaborately discussed.

Fabrication of Channel Circuit Electrodes and Flexible Graphene Resistive Sensors for Detecting Dinitrotoluene 2,4 (DNT)

This paper presents flexible explosive sensors possibly manufacturable using high volume techniques. The flexible sensors were created using channel circuit electrodes (CCE) coated with hydrazinium graphene (HG) by a sprayer. The channel circuits produced using paste fill of fine traces is a simple yet largely unreported technique for creating electrodes. The CCE were prepared by molding electrode patterns into resin and filling the channels with conductive paste, producing trace widths of 100 µm and thickness of 4 µm. Its resistance was 5–10 kΩ/each electrode of 60 cm long in serpentine shape. Sensors created using HG as the resistive layer on CCE showed their ability to detect 18.1 ppb of Dinitrotoluene 2,4 (DNT), a precursor to trinitrotoluene (TNT). To form a complete package, resistive type sensors could be made into a portable detection system using simple electronics; we demonstrated the system with battery powered portable electronics.

Robust and sensitive pressure/strain sensors from solution processable composite hydrogels enhanced by hollow-structured conducting polymers

Flexible sensors based on conductive hydrogels have found potential applications in many emerging electronic devices, but they often suffer from poor mechanical properties, single response mode and low sensitivity. Herein, robust and sensitive pressure/strain sensors are designed and fabricated utilizing novel composite hydrogels composed of hollow polyaniline spheres (HPS), poly(vinyl alcohol) (PVA) and phytic acid (PA). By taking advantages of structure-derived elasticity of hollow spheres, conductivity of doped conducting polymers, flexibility of polymer matrix and physically cross-linked structure, the composite hydrogels exhibit outstanding mechanical properties, strain sensitivity, piezoresistivity and solution processability. The optimized composite hydrogel possesses high tensile strength (9.3 MPa), stretchability (>493%) and toughness (2.6 MJ/m3). Resistive-type strain sensors assembled from the composite hydrogel can achieve a gauge factor (GF) of 2.9 in the strain range of 0%~300%, a GF of 7.4 in the strain range of 300%–450%, a response time of 0.22 s and high reliability (1000 cycles). Capacitive-type pressure sensor with ultrahigh sensitivity of GF = 3.6 kPa−1 is fabricated by sandwiching a dielectric layer between two composite hydrogel films. Interestingly, piezoresistivity of such composite hydrogels makes them promising materials for piezoresistive-type pressure sensors and visualization of pressure. On the basis of their high performances, flexible sensors of the composite hydrogels are applied in monitoring various human motions, physiological activities and bending/vibration deformations in daily life.

A new open-access online database for resistive-type gas sensor properties and performance

A new database has been established which will allow the gas sensor researcher to quickly and effectively compare published data to evaluate the state-of-the-art, identify deficiencies in the field, and discover trends in properties and performance that would otherwise be unidentifiable from the efforts of any one research group. This Open-access Database Of Resistive-type Sensors (ODORS) will remain openly accessible through a web browser at (www.odorsdatabase.com). The interface allows the user to search for, filter, and sort results based on materials composition, gas analyte, response magnitude, morphology, synthesis method, and many other relevant properties. The database has been established with data from over 150 recently published papers, but its further growth will rely on researchers around the world submitting their own published data via an easy-to-use form also available on the website. It is to the benefit of all researchers that they submit their own published data for inclusion in the database to increase discoverability and citations of their papers and exposure to their work.

Eco-friendly, self-healing, and stretchable graphene hydrogels functionalized with diol oligomer for wearable sensing applications

Graphene-based materials are of emerging interest; however, low stretchability limits their potential for wearable and sustainable electronics. Thus, this study presents the development of deformable and self-healing graphene hydrogels functionalized with polyurethane diol oligomer for wearable sensing applications. Controlling the loading of the oligomer, acting as a non-toxic plasticizer and providing abundant hydrogen bonds, is crucial to turn the functionalized hydrogels into a quasi-solid state with good stretchability and rapid healing ability (at room temperature). In sensing measurements, resistive-type sensors employing the functionalized hydrogels as the active channel show high responsiveness towards either small temperature changes (ΔT ∼ 0.2 °C near 25 °C) or the existence of ammonia and nitrogen dioxide gases at very low concentrations (0.7–20 ppm and 0.8–3.5 ppm, respectively). Notably, the ammonia detectability of our material is excellent in a wide relative humidity range (0–65 %). Besides, the sensors can be elongated to 30 %, and the healing ability of the hydrogels can recover the sensing capability of damaged sensors (about 90 % in 30 s). Overall, the obtained results suggest that our functionalized graphene hydrogels are promising for developing wearable sensing applications like e-skin and e-nose.

Development and fabrication of highly flexible, stretchable, and sensitive strain sensor for long durability based on silver nanoparticles–polydimethylsiloxane composite

In this study, silver nanoparticles (AgNPs) conductive filler was attached on polydimethylsiloxane (PDMS) substrate to produce resistive-type strain sensors. The sandwich-structured-like PDMS covered AgNPs strain sensors (Ag-PDMS) and AgNPs modified PDMS composite strain sensors (Ag@PDMS) were fabricated by a drop-casting method. The effect of AgNPs loading such as 0.20 wt%, 0.25 wt%, and 0.30 wt% with PDMS was investigated and electromechanical performance were compared between Ag-PDMS and Ag@PDMS composite strain sensors. Based on the strain performance, the gauge factor of strain sensor fabricated with AgNPs loading with 0.25 wt% in PDMS (Ag0.25@PDMS) composite revealed the gauge factor of 10.08 with a strain range of 70%, which demonstrated fast time response, good stretchability, durability, and repeatability. Ag0.25@PDMS composite strain sensor fabricated in the present study is able to be used in finger motion detection, which indicates the capability of the sensor to be used for human body motion capture and other highly sensitive strain-sensing applications.