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Humidity sensors are essential for monitoring humidity levels in various fields. This paper investigates the development and optimization of humidity sensitive SiC-Fe 3 O 4 polymer nanocomposites with varying Fe 3 O 4 concentrations fabricated using Direct Writing (DW) process. The inks were evaluated for flow behavior to ensure proper extrudability through the nozzle. The rheological behavior was then correlated with the printing parameters to optimize the printing accuracy of the nanocomposites. It was observed that all the inks exhibited shear-thinning behavior, enabling a smooth extrusion process. To ensure high-resolution features, a new image analysis method was introduced to quantify corner rounding in printed features, enabling the optimization of printing parameters. In terms of humidity sensing performance, all the inks displayed a change in electrical properties with humidity adsorption. The fabricated nanocomposites showed a decrease in electrical conductivity with increasing humidity, suggesting potential for humidity-sensing applications. Notably, the humidity sensitivity of the nanocomposites was highly dependent on the Fe 3 O 4 concentration in the inks. These findings provide valuable insights into the DW of SiC-Fe 3 O 4 polymer nanocomposites for humidity sensing applications. [ABSTRACT FROM AUTHOR]

In the post-pandemic era, human demand for a healthy lifestyle and a smart society has surged, leading to vibrant growth in the field of flexible electronic sensor technology for health monitoring. Flexible polymer humidity sensors are not only capable of the real-time monitoring of human respiration and skin moisture information but also serve as a non-contact human–machine interaction method. In addition, the development of moist-electric generation technology is expected to break free from the traditional reliance of flexible electronic devices on power equipment, which is of significant importance for the miniaturization, reliability, and environmentally friendly development of flexible devices. Currently, flexible polymer humidity sensors are playing a significant role in the field of wearable electronic devices and thus have attracted considerable attention. This review begins by introducing the structural types and working principles of various humidity sensors, including the types of capacitive, impedance/resistive, frequency-based, fiber optic, and voltage-based sensors. It mainly focuses on the latest research advancements in flexible polymer humidity sensors, particularly in the modification of humidity-sensitive materials, sensor fabrication, and hygrosensitivity mechanisms. Studies on material composites including different types of polymers, polymers combined with porous nanostructured materials, polymers combined with metal oxides, and two-dimensional materials are reviewed, along with a comparative summary of the fabrication and performance mechanisms of related devices. This paper concludes with a discussion on the current challenges and opportunities faced by flexible polymer humidity sensors, providing new research perspectives for their future development. [ABSTRACT FROM AUTHOR]

Biogel‐based ionic devices have emerged as versatile platforms for broad applications in wearable electronics, healthcare monitoring and bioelectronic interfaces. Electronic functions derived from fully edible natural soft materials are particularly attractive to allow low cost, green, biocompatible and biodegradable devices in biomedical areas as well as smart food package. This work presents a gummy‐inspired protein eutectogel by simply soaking gelatin hydrogels into solutions of natural deep eutectic solvent (NADES) consisting of sorbitol and citric acid to allow solvent exchange. Compared with the gelatin hydrogel, this natural eutectogel (NEG) exhibited anti‐drying and anti‐freezing performance, remarkably improved adhesiveness, room temperature degradability, as well as high mechanical toughness. The soaking conditions were investigated to tune the softness and ionic conductivity of the NEG, which revealed that the water content and acid ratio could significantly impact on the gel properties. Additionally, the eutectogel thin film exhibited good humidity sensing capability with a wide linear detection range (22%–98% RH), in which a soft patch was further demonstrated for breath test to detect various respiration frequencies. Thus, we believe the concept of NEG combining biopolymers and NADES can be explored in a broad range of soft devices for sensing, bio‐adhesives, energy supply or drug delivery. [ABSTRACT FROM AUTHOR]

The increasing environmental impact from electronic waste (e‐waste) has prompted research into sustainable materials for biodegradable and transient electronics. Although some progress has been achieved, further improvement in terms of performance and sustainability is needed. This study introduces a humidity sensor composed of biodegradable poly(D,L‐lactide‐co‐glycolide acid) (PDLG) in novel and multi‐structural morphologies. It highlights the role of the sensors’ microscopic structural features in their performance, particularly in humidity sensitivity, to maximize the retention and detection of water molecules. Techniques such as electrospinning and electrospray are used to achieve specific fiber and sphere morphologies. Oxygen plasma treatments tuned their surface hydrophilicity, enhancing moisture interaction. Physicochemical characterization revealed that plasma‐treated morphologies lost up to 93% of their weight after six weeks, demonstrating high sensor degradation. Functional tests showed that the sphere‐based sensor exhibited low hysteresis (0.19%), high sensitivity (3.9 × 10⁷ MΩ/% RH), excellent repeatability, and fast response time (0.43s) in the 60−95% RH range. Additionally, NaCl functionalization further improved detection sensitivity and extended the detection range down to 30% RH. The biodegradable nature of the PDLG sensors allows their natural decomposition into eco‐friendly by‐products, minimizing their environmental impact, and addressing the environmental challenges associated with e‐waste. [ABSTRACT FROM AUTHOR]

Exercise can strengthen the body, but due to the difference in exercise volume, which makes people's cardiorespiratory adaptive capacity different, unreasonable exercise load intensity is very easy to induce cardiovascular accidents. In this paper, an electronic skin sensor for monitoring human exercise status is proposed, which is a dual sensor for detecting changes in humidity and pressure by composites of poly-(vinyl alcohol) (PVA), citric acid (CA), and silver nanoparticles (AgNPs). The nanoporous sensor consisting of a PVA-CA-AgNP composite is flexible, lightweight, sensitive, and cost-effective. The sensor responds to unit humidity changes with a sensitivity as high as 0.34% RH–1 when the relative humidity is increased from 11 to 98%, and the pressure sensing/recovery time is 200/600 ms, respectively, when compressed by an external force. The sensor can be used not only for monitoring human movement and breathing patterns but also for further applications in information encryption. The sensor's continuous detection of human joint movements and breathing rhythms proves its effectiveness in health monitoring and confirms its stability and reliability in human movement and health applications. [ABSTRACT FROM AUTHOR]

Developing long-lasting humidity sensors is essential for sustainable advancements in nanotechnology. Prolonged exposure to high humidity can cause sensors to drift from their calibration points, leading to long-term accuracy issues. Our research aims to develop a fabrication method that produces stable sensors capable of withstanding the environmental challenges faced by humidity sensors. Traditional iron-based nanoparticles often require complex treatments, such as chemical modification or thermal annealing, to maintain their properties. This study introduces a novel, one-step synthesis method for iron-based thin films with exceptional stability. The synthesized films were thoroughly characterized using X-ray photoelectron spectroscopy (XPS) to evaluate their phase stability and nitride formation. The method proposed in this study employs an electrical sparking discharge process within a pure nitrogen atmosphere under a 0.2 T magnetic field, producing thin films composed of nanoparticles approximately 20 nm in size. The resulting films demonstrate superior performance in humidity sensing applications compared to conventional methods. This straightforward and efficient approach offers a promising path toward robust and sustainable humidity sensors. [ABSTRACT FROM AUTHOR]

Traditional photonic crystal films are becoming increasingly popular in the fields of smart sensing and optical devices due to their high brightness level, but at the same time there is a lack of color saturation and angle dependence, which seriously affects the application of structural color in visual response and display fields. Here, the optical performance of the photonic crystal films was improved by doping a certain amount of carbon black and carbon quantum dots during the film preparation process. Doping carbon quantum dots can effectively compensate for the optical brightness loss caused by the introduction of black substances by utilizing their self-sustained photoluminescence behavior that matches the photonic bandgap. In addition, the introduction of black nanoparticles can effectively enhance surface-resonant scattering, resulting in low angle-dependent structural colors, further expanding the application of structural color in optical display fields. [ABSTRACT FROM AUTHOR]

Carbon nanotubes (CNTs) are one‐dimensional nanomaterials with a hollow structure, high specific surface area and excellent chemical conductivity, which show great potential for application in the field of humidity‐sensitive responsive materials and humidity sensors. In this paper, CNTs were composited with a hybrid copolymer (HAC) of octavinyl POSS/acrylate to prepare the hybrid copolymer composites HAC/CNTs and their humidity sensors, which demonstrated exceptional thermal stability and fast response speed. Among them, acrylic resin serves as a humidity‐sensitive material; POSS introduce a stable three‐dimensional cage skeleton structure to the material; and CNTs are used as conductive fillers, which further enhance the stability of the material, as well as improve the sensitivity and humidity‐sensitive response speed of the material. The experimental findings indicate that the HAC/8.0% CNTs composite material exhibits a remarkable sensitivity of 119.94 KΩ/%RH within the humidity range of 11%–95%RH. Compared with HAC, HAC/8.0% CNTs demonstrate a significant reduction in response/recovery time from 16.0 s/70.0 s to 0.8 s/80.0 s, representing a considerable improvement in response speed; the wet hysteresis is reduced from 0.88 %RH to 0.77 %RH; and the temperature change coefficient is reduced from 0.035 %RH/°C to 0.008 %RH/°C over the temperature range of 20–60°C, indicating superior stability against temperature variation. Meanwhile, the humidity response mechanism of HAC/CNTs was discussed by using complex impedance spectrum. The hybrid copolymer composites, HAC/CNTs, that were prepared in this study have tremendous potential in the development of advanced, high‐performance, and miniaturizable humidity sensing technologies. Highlights: A hybrid copolymer composite humidity‐sensitive material HAC/CNTs is prepared.HAC/CNTs are compounds consisting of CNTs and POSS/acrylate hybrid copolymer.POSS improve the skeleton structural stability of acrylic resins.CNTs enhance the sensitivity and response speed of humidity sensor.The HAC/8.0% CNTs have higher response stability and faster response speed. [ABSTRACT FROM AUTHOR]

In the work, a high-performance Eu 2 Sn 2 O 7 –SnO 2 (ES-2) humidity sensors were prepared through a one-step hydrothermal method. Eu 2 Sn 2 O 7 is a bimetallic material can increase the adsorption capacity to water molecules and promote electron transfer, thus improving the sensitivity of the sensor. The introduction of Eu 2 Sn 2 O 7 promotes the increase of the content of oxygen vacancies in the ES-2 composite material, which promotes the accelerated decomposition of water molecules, and facilitates the formation of the chemisorbed layer to improve the sensitivity of the sensor and the response time. The increase in specific surface area of ES-2 provided more adsorption sites for H 2 O and also provided a good transport channel for water molecules to diffuse into the interior of the material. ES-2 humidity sensor exhibits high sensitivity (22,405 ± 363), small humidity hysteresis (1 ± 0.3 %), short response/recovery time (7/6 ± 0.5 s), good repeatability and stability (11%–95 % RH). This study not only promotes the development of SnO 2 -based humidity sensors, but provides new ideas for the preparation of high-performance humidity sensors. [ABSTRACT FROM AUTHOR]

Herein, this work presents the facile synthesis of Cerium (Ce3+) doped Cobalt-manganese-iron oxide ferrite as an effective humidity sensor operated at ambient temperature. This work reports, one step solution combustion technique to synthesize Co0.5Mn0.5Fe2−xCexO4 (x = 0.0, 0.050 and 0.1) ferrites. The structural and morphological features of the synthesized Co0.5Mn0.5Fe2−xCexO4 ferrites NPs were investigated through scanning electron microscope (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and ultraviolet visible (UV–visible) spectroscopy. The XRD results indicates the creation of single phase in pure Co0.5Mn0.5Fe2−xCexO4 ferrite that remains unchanged by the mechanism of Ce3+inclusion. The humidity sensing mechanism was systematically investigated using adsorption–desorption isotherms. Highest humidity sensing response was achieved for Co0.5Mn0.5Fe2−xCexO4 (x = 0.1) ferrite of the order of 98 % in comparison to the pure Co0.5Mn0.5Fe2−xCexO4 (x = 0) which is about 41 %. The sensors response and recovery times for the Co0.5Mn0.5Fe2−xCexO4 (x = 0.1) was recorded to be 36 s and 63 s respectively and the sensors long term stability was examined for a duration of 90 days. The results of the present investigation offer new dimensions to fabricate high performance, room temperature operable humidity sensors for next generation sensors applications.

This manuscript presents the use of three novel technologies for the implementation of wireless green battery-less sensors that can be used in agriculture. The three technologies, namely, additive manufacturing, energy harvesting, and wireless power transfer from airborne transmitters carried from UAVs, are considered for smart agriculture applications, and their combined use is demonstrated in a case study experiment. Additive manufacturing is exploited for the implementation of both RFID-based sensors and passive sensors based on humidity-sensitive materials. A number of energy-harvesting systems at UHF and ISM frequencies are presented, which are in the position to power platforms of wireless sensors, including humidity and temperature IC sensors used as agriculture sensors. Finally, in order to provide wireless energy to the soil-based sensors with energy harvesting features, wireless power transfer (WPT) from UAV carried transmitters is utilized. The use of these technologies can facilitate the extensive use and exploitation of battery-less wireless sensors, which are environmentally friendly and, thus, "green". Additionally, it can potentially drive precision agriculture in the next era through the implementation of a vast network of wireless green sensors which can collect and communicate data to airborne readers so as to support, the Artificial Intelligence and Machine Learning-based decision-making with data. [ABSTRACT FROM AUTHOR]

Humidity sensors are used in various applications to provide suitable environmental conditions. High-performance humidity sensors require highly sensitive active sites to detect water molecules. In this study, a nanopillar-array-based electrode (NAE) was developed, which has a large specific surface area and is applicable to various humidity-sensing materials. The NAE, which was fabricated via photo-lithography and soft lithography, exhibited superior electrochemical capacitance and diffusion behavior compared to flat electrodes. The NAE-based humidity sensor exhibited a high sensitivity and linearity, low hysteresis error, and long-term stability for a duration of 25 days. Moreover, the humidity sensor maintained a consistent impedance signal in a mechanically bent state. Furthermore, the real-time monitoring performance of the humidity sensor was demonstrated by measuring humidity changes during plant transpiration. [ABSTRACT FROM AUTHOR]

We report the structural properties and humidity sensing abilities of polypyrrole (PPy) composites containing ZnO nanoparticles (NPs) and Li2O. The PPy was synthesized following chemical oxidation method. The PPy50–ZnO(NPs)50–Li2Ox (where x = 0,2,4,6,8 & 10) composites were prepared by physical mixing using ball-milling method. The as-prepared composites were subjected to XRD, FTIR and EDAX. The structural analysis using these tools hinted at the presence of Li2O in the composites and the effect of variation of Li2O in the composites is emphasized. The humidity sensing ability of pristine PPy and the composites has been investigated, at room temperature, by measuring the change in electrical resistance of the sample due to the variation in relative humidity from 11 to 97% RH. The response to humidity and recovery times were determined. The mechanism of humidity sensing in these composites was understood by Grotthus theory. Among all the composites, the composite x = 10 (PZL10) has exhibited very good response and recovery time and has the potential to be used as a low-cost and highly effective humidity sensor. [ABSTRACT FROM AUTHOR]

Abstract Because there is more demand for freshwater around the world and the world’s population is growing at the same time, there is a severe lack of freshwater resources in the central part of the planet. The world’s current population of 7.2 billion people is expected to grow to over 9 billion by the year 2050. The vast majority of freshwater is used for things like cooking, cleaning, and farming. Most industrialised countries are in desperate need of smart irrigation systems, which are now a must-have because of how quickly technology is improving. In article presents IoT based Sensor integrated intelligent irrigation system for agriculture industry. IoT based humidity and soil sensors are used to collect soil related data. This data is stored in a centralized cloud. Features are selected by CFS algorithm. This will help in discarding irrelevant data. Clustering of data is performed by K means algorithm. This will help in keeping similar data together. Then classification model is build using the SVM, Random Forest and Naïve Bayes algorithm. Model is trained, validated and tested using the acquired data. Historical soil and humidity related data is also used in training the model. K-means SVM hybrid classifier is achieving better results for classification, prediction of water demand and saving fresh water by intelligent irrigation. K-means SVM hybrid classifier has achieved accuracy rate of 98.5 percent. Specificity, recall and precision of K-means SVM hybrid classifier is also higher than random forest and naïve bayes classifier.

In the post-pandemic era, human demand for a healthy lifestyle and a smart society has surged, leading to vibrant growth in the field of flexible electronic sensor technology for health monitoring. Flexible polymer humidity sensors are not only capable of the real-time monitoring of human respiration and skin moisture information but also serve as a non-contact human–machine interaction method. In addition, the development of moist-electric generation technology is expected to break free from the traditional reliance of flexible electronic devices on power equipment, which is of significant importance for the miniaturization, reliability, and environmentally friendly development of flexible devices. Currently, flexible polymer humidity sensors are playing a significant role in the field of wearable electronic devices and thus have attracted considerable attention. This review begins by introducing the structural types and working principles of various humidity sensors, including the types of capacitive, impedance/resistive, frequency-based, fiber optic, and voltage-based sensors. It mainly focuses on the latest research advancements in flexible polymer humidity sensors, particularly in the modification of humidity-sensitive materials, sensor fabrication, and hygrosensitivity mechanisms. Studies on material composites including different types of polymers, polymers combined with porous nanostructured materials, polymers combined with metal oxides, and two-dimensional materials are reviewed, along with a comparative summary of the fabrication and performance mechanisms of related devices. This paper concludes with a discussion on the current challenges and opportunities faced by flexible polymer humidity sensors, providing new research perspectives for their future development.

MXenes have received worldwide attention across various scientific and technological fields since the first report of the synthesis of Ti3C2 nanostructures in 2011. The unique characteristics of MXenes, such as superior mechanical strength and flexibility, liquid-phase processability, tunable surface functionality, high electrical conductivity, and the ability to customize their properties, have led to the widespread development and exploration of their applications in energy storage, electronics, biomedicine, catalysis, and environmental technologies. The significant growth in publications related to MXenes over the past decade highlights the extensive research interest in this material. One area that has a great potential for improvement through the integration of MXenes is sensor design. Strain sensors, temperature sensors, pressure sensors, biosensors (both optical and electrochemical), gas sensors, and environmental pollution sensors targeted at volatile organic compounds (VOCs) could all gain numerous improvements from the inclusion of MXenes. This report delves into the current research landscape, exploring the advancements in MXene-based chemo-sensor technologies and examining potential future applications across diverse sensor types. [ABSTRACT FROM AUTHOR]

Flexible wearable humidity sensors have seen significant development in healthcare fields. However, most sensors have only one electrical response mode with low visualization, thus limiting applications. Here, a wearable color-electric dual-mode linked-response humidity sensor was provided for intimate humidity monitoring. Specifically, nickel (II) iodide (NiI2) and polyacrylonitrile (PAN) were chemically treated and sodium hydroxide (NaOH)-modified, respectively. Subsequently, the modified NiI2 and PAN were configured into a spinning solution to fabricate humidity-sensitive NiI2/PAN membranes utilizing the solution blow-spinning method. Polyimide-based fork-finger electrodes were then laminated onto the humidity-sensitive NiI2/PAN membrane to assemble a wearable color-electric bimodal NiI2/PAN humidity sensor. The sensor was characterized by a fast response time (6.19 s) and recovery time (9.58 s), wide humidity-monitoring range (11–98% RH), relatively small hysteresis (1.9% RH), and excellent stability (> 30 d). As the relative humidity (RH) increased from 11 to 98%, the color of the Ni2/PAN membrane changed drastically from black to light yellow, demonstrating significant potential in enhancing the visualization of humidity sensors and optimizing their monitoring effects. [ABSTRACT FROM AUTHOR]

Developing long-lasting humidity sensors is essential for sustainable advancements in nanotechnology. Prolonged exposure to high humidity can cause sensors to drift from their calibration points, leading to long-term accuracy issues. Our research aims to develop a fabrication method that produces stable sensors capable of withstanding the environmental challenges faced by humidity sensors. Traditional iron-based nanoparticles often require complex treatments, such as chemical modification or thermal annealing, to maintain their properties. This study introduces a novel, one-step synthesis method for iron-based thin films with exceptional stability. The synthesized films were thoroughly characterized using X-ray photoelectron spectroscopy (XPS) to evaluate their phase stability and nitride formation. The method proposed in this study employs an electrical sparking discharge process within a pure nitrogen atmosphere under a 0.2 T magnetic field, producing thin films composed of nanoparticles approximately 20 nm in size. The resulting films demonstrate superior performance in humidity sensing applications compared to conventional methods. This straightforward and efficient approach offers a promising path toward robust and sustainable humidity sensors.

Flexible and wearable humidity sensors play a vital role in daily point-of-care diagnosis and noncontact human-machine interactions. However, achieving a facile and high-speed fabrication approach to realizing flexible humidity sensors remains a challenge. In this work, a wearable capacitive-type Ga2O3/liquid metal-based humidity sensor is demonstrated by a one-step laser direct writing technique. Owing to the photothermal effect of laser, the Ga2O3-wrapped liquid metal particles can be selectively sintered and converted from insulative to conductive traces with a resistivity of 0.19 Ω·cm, while the untreated regions serve as active sensing layers in response to moisture changes. Under 95% relative humidity, the humidity sensor displays a highly stable performance along with rapid response and recover time. Utilizing these superior properties, the Ga2O3/liquid metal-based humidity sensor is able to monitor human respiration rate, as well as skin moisture of the palm under different physiological states for healthcare monitoring.

Abstract Human respiration reflects abundant physiological information and could enable non‐invasive monitoring, providing information about various biological parameters such as respiration rate and depth. The rapidly growing field of humidity sensors bring forward the requirement for good performance. In this work, by virtue of good hydrophilicity and conductivity, a MXene/thermoplastic polyurethane (TPU) composite film is prepared by coating MXene nanosheets on chitosan‐modified TPU electrospun nanofibers via electrostatic interactions, for fabricating a humidity sensor. Based on the principle that the tunnel resistance changing with water molecules influences the distance of MXene nanosheets, the MXene/TPU humidity sensor exhibits fast response (12 s), wide humidity response range (11–94% RH), low hysteresis (

Wearable sensors have garnered considerable attention due to their flexibility and lightweight characteristics in the realm of healthcare applications. However, developing robust wearable sensors with facile fabrication and good conformity remains a challenge. In this study, a conductive graphene nanoplate‐carbon nanotube (GC) ink is synthesized for multi jet fusion (MJF) printing. The layer‐by‐layer fabrication process of MJF not only improves the mechanical and flame‐retardant properties of the printed GC sensor but also bolsters its robustness and sensitivity. The direction of sensor bending significantly impacts the relative resistance changes, allowing for precise investigations of joint motions in the human body, such as those of the fingers, wrists, elbows, necks, and knees. Furthermore, the data of resistance changes collected by the GC sensor are utilized to train a support vector machine with a 95.83% accuracy rate for predicting human motions. Due to its stable humidity sensitivity, the sensor also demonstrates excellent performance in monitoring human breath and predicting breath modes (normal, fast, and deep breath), thereby expanding its potential applications in healthcare. This work opens up new avenues for using MJF‐printed wearable sensors for a variety of healthcare applications. [ABSTRACT FROM AUTHOR]

In this paper, flexible substrate integrated waveguide (SIW) resonators have been designed and fabricated on polyimide substrates for humidity sensing applications. The proposed SIW resonant cavity allows the resonator to obtain the maximum humidity sensitivity and meet the demand for flexible microwave sensing detection. Meanwhile, the humidity response performance can be further significantly enhanced by introducing nanodiamond (ND) sensing material. Three prototypes of ND-coated SIW sensors with different bending radii are measured to analyze their humidity sensing performance. The experimental results demonstrate that the proposed ND-coated SIW sensor with the minimum bending radius can achieve a maximum humidity sensitivity of 1.09 MHz/% relative humidity (RH) in the high RH region (>75.3% RH) and a low humidity hysteresis of 1.8% in the range of 11.3–97.3% RH. This study provides a promising candidate to realize flexible microwave sensors with excellent sensing performance. [ABSTRACT FROM AUTHOR]

Wearable strain–humidity sensors are increasingly recognized for their role in healthcare and detecting human motion signals. However, many strain–humidity sensors require complex manufacturing processes and have limited applicability. In this study, based on the good miscibility between natural rubber latex (NRL) and poly (3,4‐ethylenedioxythiophene):poly (4‐styrenesulfonate) (PEDOT:PSS), PEDOT:PSS/NRL composite conductors are transferred onto fabric substrate through screen printing and prepared high‐performance strain–humidity sensor. The PEDOT:PSS/NRL blends strain–humidity sensor exhibited good stability and durability (500 cycles) at 10% strain, achieves a gauge factor (GF) of 123.8 with excellent linearity, making it operational in detecting various human motions, such as finger bending, elbow bending, wrist bending, and so on. In addition, the PEDOT:PSS and water molecules interaction results in a humidity response time as low as 0.72 s and recovery time as low as 0.85 s for the PEDOT:PSS/NRL composite strain–humidity sensor, and a wide relative humidity detection range (6–83%), allowing it to be utilized for precise detection of human breathing. Furthermore, it can monitor the use's joint movement, facial muscle movement, and respiratory rate, which show its potential application prospect in health monitoring. [ABSTRACT FROM AUTHOR]

Humidity sensors are of significance in various fields, such as environmental and food quality monitoring, industrial processing, wearable and flexible electronics, and human health care. High-performance humidity sensors with high sensitivity, rapid response time, and good stability are of paramount importance in humidity sensing. In this paper, diversiform humidity sensors with different sensing mechanisms are summarized, including resistive, impedance, capacitive, quartz crystal microbalance (QCM), surface acoustic wave (SAW), field-effect transistor (FET), and optical fiber humidity sensors. Versatile nanomaterials such as graphene, transition-metal chalcogenide, MXenes, black phosphorus (BP), boron nitride (BN), polymers, and nanofibers were promising building-blocks for constructing humidity sensors. The latest progress in the wearable and flexible humidity sensors, and self-powered humidity sensors was summarized. The diversiform applications of the humidity sensors with great prospects were demonstrated in various fields in terms of human respiratory monitoring, skin dryness diagnosing, fingertip approaching, and non-contact switch. Moreover, the challenges and prospects of nanomaterials-based humidity sensors were discussed. [ABSTRACT FROM AUTHOR]

Humidity sensors are fabricated on base of nanocellulose (NC) using different initial raw materials (reed stalks or wheat straw) and by means of different extraction methods (TEMPO-oxidation or acid hydrolysis). In addition, nanocomposites from NC with addition of polyvinyl alcohol (PVA) are used to improve the mechanical characteristics of nanocellulose films obtained by hydrolysis method. The static and dynamic characteristics of humidity sensors are measured, and their sensitivity, response, hysteresis, response and recovery times, as well as short- and long-term stability are determined. The influence of initial materials and extraction methods for NC, as well as amount of humidity-sensitive material, on device parameters is established. The dependence of sensor sensitivity on the NC mass is determined. In particular, it is shown that NC sensors made of reed have higher sensitivity, but worse stability and dynamic parameters compared to sensors made of wheat. The maximum value of sensitivity (0.204 (%RH) -1 ) is observed for sensor based on the NC film obtained of reed by TEMPO-oxidation method. Minimal signal fluctuations (10%) during continuous operation for 1 h are observed for NC sensors obtained of wheat by the hydrolysis technique. Improved response time and recovery time (7 s and 6 s) are available for NC sensors obtained of wheat by the TEMPO-oxidation method. As shown, the NC film mass of 0.3 mg is favourable for all sensors. The effect of test signal frequency is as follows: improving of sensitivity occurs at 100 Hz, and of all other parameters—at 1000 Hz. [ABSTRACT FROM AUTHOR]

Properties porous silica glasses are systemized in this review. Also the methods of formation of the nanoparticles’ ensembles of the substances, which are useful for the microelectronics, are described. Porous silica glasses are perspective due to their chemically resistance, mechanical strength and development of their inner surface. Own electrical resistance is too big for the porous glasses’ specimens with standard sizes, that’s why ones use them as matrixes for luminescence type of sensors mainly. The luminescent properties’ dependence on the molecular structure of investigate substances (such as dyes and metal oxides), as well as on the nanoparticle ensemble technological formation condition is studied. It is revealed to which certain gases dyes of the specified type are sensitive and why it occurs in such manners. It is shown due to which of the described nano-size systems’ properties sensitivity is appeared. Besides, one can embed a conductive phase into the pores of glass due to features of its structure. After such treatment one can use the porous glasses as matrix for formation of resistance type sensors.

Highlights Imine groups in covalent organic framework (COF) films act as dual-active sites for humidity sensing, inducing an intrinsic enhanced mechanism of reversible protonated tautomerism via water molecule-induced hydrogen bonding. The cis-ketoimine reciprocal isomerization induces a stretching vibration effect for the ordered conjugated conductive frame of COF films, realizing fast response, wide range, and high sensitivity characteristics for humidity detection. Resistance changes of COF film-based sensors keep a strong linear relationship with low-range relative humidity, reflecting the quantitative sensing mechanism at the molecular level.

This manuscript presents the use of three novel technologies for the implementation of wireless green battery-less sensors that can be used in agriculture. The three technologies, namely, additive manufacturing, energy harvesting, and wireless power transfer from airborne transmitters carried from UAVs, are considered for smart agriculture applications, and their combined use is demonstrated in a case study experiment. Additive manufacturing is exploited for the implementation of both RFID-based sensors and passive sensors based on humidity-sensitive materials. A number of energy-harvesting systems at UHF and ISM frequencies are presented, which are in the position to power platforms of wireless sensors, including humidity and temperature IC sensors used as agriculture sensors. Finally, in order to provide wireless energy to the soil-based sensors with energy harvesting features, wireless power transfer (WPT) from UAV carried transmitters is utilized. The use of these technologies can facilitate the extensive use and exploitation of battery-less wireless sensors, which are environmentally friendly and, thus, “green”. Additionally, it can potentially drive precision agriculture in the next era through the implementation of a vast network of wireless green sensors which can collect and communicate data to airborne readers so as to support, the Artificial Intelligence and Machine Learning-based decision-making with data.

Abstract Wearable sensors have garnered considerable attention due to their flexibility and lightweight characteristics in the realm of healthcare applications. However, developing robust wearable sensors with facile fabrication and good conformity remains a challenge. In this study, a conductive graphene nanoplate‐carbon nanotube (GC) ink is synthesized for multi jet fusion (MJF) printing. The layer‐by‐layer fabrication process of MJF not only improves the mechanical and flame‐retardant properties of the printed GC sensor but also bolsters its robustness and sensitivity. The direction of sensor bending significantly impacts the relative resistance changes, allowing for precise investigations of joint motions in the human body, such as those of the fingers, wrists, elbows, necks, and knees. Furthermore, the data of resistance changes collected by the GC sensor are utilized to train a support vector machine with a 95.83% accuracy rate for predicting human motions. Due to its stable humidity sensitivity, the sensor also demonstrates excellent performance in monitoring human breath and predicting breath modes (normal, fast, and deep breath), thereby expanding its potential applications in healthcare. This work opens up new avenues for using MJF‐printed wearable sensors for a variety of healthcare applications.

Abstract Humidity sensors are used in many applications. The design of fast sensors that can operate in explosive environments is a difficult task. Therefore, current research efforts aim at combining reliability, sensitivity and high sensing speed. The use of structured ultrathin hydrogels perfectly meets these requirements. Nanostructures are directly fabricated with a two‐photon‐polymerisation (2PP) 3D printer to use them as templates for hydrogels. After the 3D printing multiple templates are coated with ultrathin films of poly(2‐hydroxyethyl‐methacrylate) (pHEMA) using initiated chemical vapor deposition (iCVD). p(HEMA) is a humidity responsive hydrogel which changes its thickness by orders of magnitude depending on the ambient conditions. The 3D printed structures are optimized to give both a fast response time, and an optical read‐out method for visible wavelengths. Upon hydrogel swelling, the height of the nanostructure pillars increases, keeping their periodicity constant. This induces a change in intensity of the first ‐order refraction peak, which can be easily measured also at low humidity levels. The humidity response of the nanostructures is measured and an influence for different hydrogel thicknesses and humidity flow rates is observed. The ultrathin film with the lowest thickness of 50 nm shows the fastest response to relative humidity, which is much faster than commercial sensors with 8 s response time.

Agriculture is one of the drivers for societies and economies worldwide. There are several challenges to be addressed in this sector, such as food security and climate change. Agriculture 4.0 and the Internet of Things (IoT) technology paradigms aim to address these challenges, allowing for sustainable usage of natural resources and application of novel farming practices. This paper introduces an innovative smart IoT-based prototype system that supports precision irrigation of crops and enables real-time exploitation of data for minimization of errors, as well as forecasting. It is based on microprocessors and a Single-Board Computer (SBC) for data logging using various sensors that monitor soil moisture, air humidity, air temperature and UV light. Farmers’ Intelligent Actuation systems have full access to all relevant raw data, in order to choose when, where and how much to irrigate. The respective precision irrigation prototype system has been evaluated over series of experiments and the respective findings are presented in this paper.

High‐performance miniaturized flexible sensors are becoming increasingly important in wearable electronics. However, miniaturization of devices often requires high‐precision manufacturing processes and equipment, which limits the commercialization of flexible sensors. Therefore, revolutionary technologies for manufacturing miniaturized flexible sensors are highly desired. In this work, a new method for manufacturing miniaturized flexible humidity sensor by utilizing heat shrinkage technology is presented. This method successfully achieves much smaller sensor and denser interdigital electrode. Utilizing this method, a miniaturized flexible humidity sensor and array are presented, fabricated by anchoring nano‐Al2O3 into carbon nano‐tube as the humidity sensitive film. This heat shrinkage technology, forming wrinkle structure on the humidity sensitive film, endows the sensor with a high sensitivity over 200% (ΔR/R0) at humidity levels ranging from 0 to 90%RH and a fast recovery time (0.5 s). The sensor allows non‐contact monitoring human respiration and alerting in case of an asthma attack and the sensor array can be adaptively attached to the wrist as a non‐contact human–machine interface to control the mechanical hand or computer. This work provides a general and effective heat shrinkage technology for the development of smaller and more efficient flexible circuits and sensor devices. [ABSTRACT FROM AUTHOR]

Respiratory rate and skin humidity are important physiological signals and have become an important basis for disease diagnosis, and they can be monitored by humidity sensors. However, it is difficult to employ high-quality humidity sensors on a broad scale due to their high cost and complex fabrication. Here, we propose a reliable, convenient, and efficient method to mass-produce humidity sensors. A capacitive humidity sensor is obtained by ablating a polyimide (PI) film with a picosecond laser to produce an interdigital electrode (IDE), followed by drop-casting graphene oxide (GO) as a moisture-sensitive material on the electrode. The sensor has long-time stability, a wide relative humidity (RH) detection range from 10% to 90%, and high sensitivity (3862 pF/%RH). In comparison to previous methods, the technology avoids the complex procedures and expensive costs of conventional interdigital electrode preparation. Furthermore, we discuss the effects of the electrode gap size and the amount of graphene oxide on humidity sensor performance, analyze the humidity sensing mechanism by impedance spectrum, and finally perform the monitoring of human respiratory rate and skin humidity change in a non-contact manner. [ABSTRACT FROM AUTHOR]

Transition metal carbides and nitrides (MXenes) are an emerging class of 2D materials, which are attracting ever‐growing attention due to their remarkable physicochemical properties. The presence of various surface functional groups on MXenes' surface, e.g., F, O, OH, Cl, opens the possibility to tune their properties through chemical functionalization approaches. However, only a few methods have been explored for the covalent functionalization of MXenes and include diazonium salt grafting and silylation reactions. Here, an unprecedented two‐step functionalization of Ti3C2Tx MXenes is reported, where (3‐aminopropyl)triethoxysilane is covalently tethered to Ti3C2Tx and serves as an anchoring unit for subsequent attachment of various organic bromides via the formation of CN bonds. Thin films of Ti3C2Tx functionalized with linear chains possessing increased hydrophilicity are employed for the fabrication of chemiresistive humidity sensors. The devices exhibit a broad operation range (0–100% relative humidity), high sensitivity (0.777 or 3.035), a fast response/recovery time (0.24/0.40 s ΔH−1, respectively), and high selectivity to water in the presence of saturated vapors of organic compounds. Importantly, our Ti3C2Tx‐based sensors display the largest operating range and a sensitivity beyond the state of the art of MXenes‐based humidity sensors. Such outstanding performance makes the sensors suitable for real‐time monitoring applications. [ABSTRACT FROM AUTHOR]

Background: Agriculture is a vital part of the economy that feeds a part of the world expanding population. However, the sector faces several difficulties, including water constraints, soil degradation, and climate change. The demand for increased security and safety in agricultural activities worsens these problems. This paper suggests an IoT-based agriculture environment and security monitoring system address these issues. Aims: The main objective of this work is to develop an intelligent monitoring and automated irrigation system that minimizes water use based on individual conditions and enables real-time environmental Methods: The Arduino Uno R3 microcontroller based on Microchip ATmega328P is used to gather environmental data such as humidity, temperature, soil moisture levels. The Arduino board is integrated with a relay and RTC module to water plants at precise times. It also has a passive infrared sensor to detect intruders. ESP32 camera is also used to record the current condition of the agricultural ground automatically. A sample of reading in an interval of 1 hour is recorded. This method involves wirelessly transmitting the data to cloud-based platforms. Results: The value of the sensor at the start of a wet condition is 0. When this occurs, the motor pump turns off, and the measured value is communicated to the microcontroller through NodeMCU. The motor pump automatically turns ON and OFF when plants receive enough amount of water. Discussion: It has been observed the water pump starts as soon as the temperature threshold of 19.5 °C is achieved. It has been observed that as humidity increases, it is unnecessary to water the plants as the plants take up water vapor from the surface of the leaves under humid conditions. Conclusions: This research offers a convenient prototype model that can simultaneously measure temperature, moisture, and humidity, identify intrusion, and remotely monitor it over the internet. It was concluded that with the decrease in moisture of the soil, the plants need to be watered. [ABSTRACT FROM AUTHOR]

This paper describes the synthesis and characterization of seven different copper(II) coordination compounds, as well as the formation of a protonated ligand involving all compounds from the same reaction. Their synthesis required hydrothermal conditions, causing the partial in situ transformation of 5-fluoro uracil-1-acetic acid (5-FUA) into an oxalate ion (ox), as well as the protonation of the 4,4′-bipyridine (bipy) ligand through a catalytic process resulting from the presence of Cu(II) within the reaction. These initial conditions allowed obtaining the new coordination compounds [Cu2(5-FUA)2(ox)(bipy)]n·2n H2O (CP2), [Cu(5-FUA)2(H2O)(bipy)]n·2n H2O (CP3), as well as the ionic pair [(H2bipy)+2 2NO3−] (1). The mother liquor evolved rapidly at room temperature and atmospheric pressure, due to the change in concentration of the initial reagents and the presence of the new chemical species generated in the reaction process, yielding CPs [Cu(5-FUA)2(bipy)]n·3.5n H2O, [Cu3(ox)3(bipy)4]n and [Cu(ox)(bipy)]n. The molecular compound [Cu(5-FUA)2(H2O)4]·4H2O (more thermodynamically stable) ended up in the mother liquor after filtration at longer reaction times at 25 °C and 1 atm., cohabiting in the medium with the other crystalline solids in different proportions. In addition, the evaporation of H2O caused the single-crystal to single-crystal transformation (SCSC) of [Cu(5-FUA)2(H2O)(bipy)]n·2n H2O (CP3) into [Cu(5-FUA)2(bipy)]n·2n H2O (CP4). A theoretical study was performed to analyze the thermodynamic stability of the phases. The observed SCSC transformation also involved a perceptible color change, highlighting this compound as a possible water sensor. [ABSTRACT FROM AUTHOR]

Humidity sensors with high sensitivity, rapid response, and facile fabrication process for contactless sensing applications have received considerable attention in recent years. Herein, humidity sensors based on hexagonal boron nitride (h-BN) nanosheets that are synthesized by a facile ultrasonic process have been fabricated, which display an ultrahigh sensitivity of 28,384% at 85% relative humidity (RH), rapid response/recovery time (3.0/5.5 s), and long-term stability in a wide humidity detection range (11%–85% RH), superior to most of the reported humidity sensors. The high sensitivity can be ascribed to the massive hydrophilic functional groups absorbed on the h-BN nanosheet surface. Benefiting from the high humidity sensing performances, contactless Morse code messaging and human writing and speech recognition have been demonstrated. This work demonstrates the great potential of the high-performance h-BN nanosheet-based humidity sensors for future contactless sensing devices. [ABSTRACT FROM AUTHOR]

Highlights: Imine groups in covalent organic framework (COF) films act as dual-active sites for humidity sensing, inducing an intrinsic enhanced mechanism of reversible protonated tautomerism via water molecule-induced hydrogen bonding. The cis-ketoimine reciprocal isomerization induces a stretching vibration effect for the ordered conjugated conductive frame of COF films, realizing fast response, wide range, and high sensitivity characteristics for humidity detection. Resistance changes of COF film-based sensors keep a strong linear relationship with low-range relative humidity, reflecting the quantitative sensing mechanism at the molecular level. Human metabolite moisture detection is important in health monitoring and non-invasive diagnosis. However, ultra-sensitive quantitative extraction of respiration information in real-time remains a great challenge. Herein, chemiresistors based on imine-linked covalent organic framework (COF) films with dual-active sites are fabricated to address this issue, which demonstrates an amplified humidity-sensing signal performance. By regulation of monomers and functional groups, these COF films can be pre-engineered to achieve high response, wide detection range, fast response, and recovery time. Under the condition of relative humidity ranging from 13 to 98%, the COFTAPB-DHTA film-based humidity sensor exhibits outstanding humidity sensing performance with an expanded response value of 390 times. Furthermore, the response values of the COF film-based sensor are highly linear to the relative humidity in the range below 60%, reflecting a quantitative sensing mechanism at the molecular level. Based on the dual-site adsorption of the (–C=N–) and (C–N) stretching vibrations, the reversible tautomerism induced by hydrogen bonding with water molecules is demonstrated to be the main intrinsic mechanism for this effective humidity detection. In addition, the synthesized COF films can be further exploited to effectively detect human nasal and oral breathing as well as fabric permeability, which will inspire novel designs for effective humidity-detection devices. [ABSTRACT FROM AUTHOR]

In this work, Er3+-doped Cobalt nanoferrite particles (CoErxFe2−xO4; x = 0.0, 0.01, 0.03, 0.05) were synthesised using an optimised citrate-gel auto-combustion method and analysed for structural, magnetic, and electrical properties. Preliminary X-ray diffraction and Raman spectroscopy studies confirm the formation of a single-phase spinel structure. Average crystallite sizes from the Williamson-Hall method are in the range of 28 to 32 nm, which are in close agreement with TEM data. Temperature dependent magnetization The ZFC-FC curves show blocking temperatures that are higher than room temperature. The hysteresis loop clearly demonstrates an increase in saturation magnetization of cobalt ferrite with Er doping concentration. Electrical resistance measurements in different relative humidity conditions (RH 10–95%) and frequencies were done using a high-frequency LCR meter. When compared to pure Cobalt Ferrite, Er doped Cobalt Ferrite samples have lower electrical resistance and higher sensitivity. The variation of resistance with respect to humidity conditions suggested that Er cobalt ferrite nanoparticles are potentially suitable for resistive humidity sensor applications. [ABSTRACT FROM AUTHOR]

Designing new medical devices with advanced humidity sensors is of great significance for patients with incontinence-associated dermatitis (IAD). The primary goal of this study is to test the humidity-sensing mattress system for patients with IAD in clinical settings. The design of the mattress is set at 203 cm, with 10 × 3 sensors, dimensions of 19 × 32 cm, and a weighted bearing of 200 kg. The main sensors consist of a humidity-sensing film, a thin-film electrode (6 × 0.1 mm), and a glass substrate (500 nm). The sensitivity of the test mattress system showed that the resistance-humidity sensor was at a temperature of 35 °C ( V 0    = 30 V,  V 0   = 350 mV), with slope at 1.13 V/fF,  f  = 1 MHz, 20–90% RH, and a response time of 20 s at 2  μ m. In addition, the humidity sensor reached 90% RH, with a response time of less than 10 s, a magnitude of 1 0 7 –1 0 4   Ω , 1 mol%,  CrO 1.5 , and F O 1.5 , respectively. This design is not only a simple, low-cost medical sensing device, but also opens a new pathway for developing humidity-sensing mattresses in the field of flexible sensors, wearable medical diagnostic devices, and health detection. [ABSTRACT FROM AUTHOR]

Breathable, flexible, and highly sensitive pressure sensors have drawn increasing attention due to their potential in wearable electronics for body‐motion monitoring, human‐machine interfaces, etc. However, current pressure sensors are usually assembled with polymer substrates or encapsulation layers, thus causing discomfort during wearing (i.e., low air/vapor permeability, mechanical mismatch) and restricting their applications. A breathable and flexible pressure sensor is reported with nonwoven fabrics as both the electrode (printed with MXene interdigitated electrode) and sensing (coated with MXene/silver nanowires) layers via a scalable screen‐printing approach. Benefiting from the multi‐layered porous structure, the sensor demonstrates good air permeability with high sensitivity (770.86–1434.89 kPa−1), a wide sensing range (0–100 kPa), fast response/recovery time (70/81 ms), and low detection limit (≈1 Pa). Particularly, this sensor can detect full‐scale human motion (i.e., small‐scale pulse beating and large‐scale walking/running) with high sensitivity, excellent cycling stability, and puncture resistance. Additionally, the sensing layer of the pressure sensor also displays superior sensitivity to humidity changes, which is verified by successfully monitoring human breathing and spoken words while wearing a sensor‐embedded mask. Given the outstanding features, this breathable sensor shows promise in the wearable electronic field for body health monitoring, sports activity detection, and disease diagnosis. [ABSTRACT FROM AUTHOR]

Respiration is an important physiological process of the human body and can assess an individual's health status. Humidity detection is an effective method for real‐time breath monitoring. Herein, the α‐Co(OH)2‐based quartz crystal microbalance humidity sensor (Co‐QCM sensor) is prepared via the hydrothermal method. The Co‐QCM sensor is characterized by scanning electron microscopy, Fourier‐transform infrared, and X‐ray diffraction. The α‐Co(OH)2 with hierarchical nanostructure and hydroxy functional groups leads to good humidity‐sensing performance. The Co‐QCM sensor exhibits high sensitivity (23.1 Hz/% relative humidity), fast response and recovery times (within 3 and 4 s, respectively), high linearity (R2 = 0.994), and long‐term stability (≈30 days) in the humidity range of 4–73.3%. In addition, different human breathing signals, including free breathing, shallow breathing, and deep breathing, can be detected and distinguished by this humidity sensor, showing the potential application in human respiratory detection. [ABSTRACT FROM AUTHOR]

Wood is an inherently hygroscopic material which tends to absorb moisture from its surrounding. Moisture in wood is a determining factor for the quality of wood being employed in construction, since it causes weakening, deformation, rotting, and ultimately leading to failure of the structures resulting in costs to the economy, the environment, and to the safety of residents. Therefore, monitoring moisture in wood during the construction phase and after construction is vital for the future of smart and sustainable buildings. Employing bio‐based materials for the construction of electronics is one way to mitigate the environmental impact of such electronics. Herein, a bio‐graphene sensor for monitoring the moisture inside and around wooden surfaces is fabricated using laser‐induced graphitization of a lignin‐based ink precursor. The bio‐graphene sensors are used to measure humidity in the range of 10% up to 90% at 25 °C. Using laser induced graphitization, conductor resistivity of 18.6 Ω sq−1 is obtained for spruce wood and 57.1 Ω sq−1 for pine wood. The sensitivity of sensors fabricated on spruce and pine wood is 2.6 and 0.74 MΩ per % RH. Surface morphology and degree of graphitization are investigated using scanning electron microscopy, Raman spectroscopy, and thermogravimetric analysis methods. [ABSTRACT FROM AUTHOR]

Abstract Mesoporous silica is commonly used as matrix for humidity sensors which operate on the principle of relative humidity (RH)‐dependent water uptake and read‐out by resistance (R) monitoring. Although numerous studies have been dedicated to improving sensitivity with conductive additives, the role of the pore architecture on the sensing behavior has not been systematically investigated so far. Herein, the effects of pore size and porosity on resistive sensing performance in the 0.5–85% RH range are showcased. Across various sensors, a clear correlation is identified between mesopore size and linear RH sensing range. Sensors with larger pores (≈15 nm) exhibit linear response in the 65 to 85% RH range with larger slope (ΔlogR/ΔRH) than sensors with smaller pores (

MXenes have received worldwide attention across various scientific and technological fields since the first report of the synthesis of Ti3C2 nanostructures in 2011. The unique characteristics of MXenes, such as superior mechanical strength and flexibility, liquid-phase processability, tunable surface functionality, high electrical conductivity, and the ability to customize their properties, have led to the widespread development and exploration of their applications in energy storage, electronics, biomedicine, catalysis, and environmental technologies. The significant growth in publications related to MXenes over the past decade highlights the extensive research interest in this material. One area that has a great potential for improvement through the integration of MXenes is sensor design. Strain sensors, temperature sensors, pressure sensors, biosensors (both optical and electrochemical), gas sensors, and environmental pollution sensors targeted at volatile organic compounds (VOCs) could all gain numerous improvements from the inclusion of MXenes. This report delves into the current research landscape, exploring the advancements in MXene-based chemo-sensor technologies and examining potential future applications across diverse sensor types.