Humidity Sensitive Material Research Articles

A Flexible Wearable Sensor for In Situ Non-Destructive Detection of Plant Leaf Transpiration Information

This paper investigates an in situ, non-destructive detection sensor based on flexible wearable technology that can reflect the intensity of plant transpiration. The sensor integrates four components: a flexible substrate, a humidity-sensing element, a temperature-sensing element, and a self-adhesive film. It is capable of accurately and continuously measuring the temperature, humidity, and vapor pressure deficit (VPD) on the leaf surface, thus providing information on plant transpiration. We combined the humidity-sensitive material graphene oxide (GO) with a PDMS-GO-SDS flexible substrate as the humidity-sensing element of the sensor. This element exhibits high sensitivity, fast response, and excellent biocompatibility with plant interfaces. The humidity monitoring sensitivity of the sensor reaches 4456 pF/% RH, while the temperature sensing element has a sensitivity of approximately 3.93 Ω/°C. Additionally, tracking tests were conducted on tomato plants in a natural environment, and the experimental results were consistent with related research findings. This sensor can be used to monitor plant growth during agricultural production and facilitate precise crop management, helping to advance smart agriculture in the Internet of Things (IoT) for plants.

Humidity Sensing Using Polymers: A Critical Review of Current Technologies and Emerging Trends

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.

Design of low-cost humidity sensor based on chipless RFID

It is of great significance to design a low-cost chipless radio frequency identification(RFID) sensor for environmental humidity monitoring. To this end, polyvinyl alcohol (PVA) film is used as a humidity sensitive material, and the overall size of the rectangular substrate is 18 mm × 18 mm ×0.5 mm. Through the humidity sensing principle and simulation analysis, the change of environmental humidity causes the change of the dielectric constant of the humidity sensitive material PVA, which in turn affects the resonant frequency offset of the entire sensor. The simulation results show that the relative humidity working range of the designed humidity sensor is 21.9% ∼ 52.5%, the corresponding sensor resonant frequency range is 2.76 ∼ 2.51GHz, the total offset reaches 250 MHz, and the average sensitivity at the maximum relative humidity is 23.08MHz/% . The designed humidity sensor has the advantages of miniaturization, simple structure and low cost, and can be used for humidity monitoring in various target environments. Keywords.low cost; chipless RFID humidity sensor; polyvinyl alcohol; humidity monitoring; humidity sensitive material.

Respiratory rate monitoring based on all-fiber strain-induced humidity sensor

In this paper, an agarose coated-fiber Bragg grating (AG-FBG) based respiratory rate (RR) sensor for evaluating respiratory function and health status by all-fiber strain-induced humidity-sensitive material is proposed and experimentally demonstrated. The variation of the environment humidity leads to changes in the film morphology of AG, i.e., expansion and contraction, which impose strains on FBG to dynamically shift the central wavelength. In the experiment, the average sensitivity of the proposed AG-FBG-based RR sensor is 23.6 pm/%RH in the range of 30 %RH–90 %RH. Under the monitoring of a commercial FBG demodulator with a wavelength resolution of 1 pm, rich human breathing information is collected. The RR monitoring under different human postures and breathing patterns are respectively measured. In one breathing cycle, the response time and recovery time are 778 ms and 762 ms, respectively. The proposed sensor is simple and low-cost, shows easy signal demodulation, stable signal wavelength variation, and high repeatability, which has potential applications in vital signs monitoring and medical treatment.

Humidity sensors based on octaphenylcyclotetrasiloxane hypercrosslinked porous polymers for non-contact sensing and respiratory monitoring

Polyhedral oligomeric silsesquioxane (POSS) is renowned for its facile modification and excellent stability, rendering it a promising candidate for humidity-sensitive materials. Achieving humidity sensors based on POSS with broad response ranges and fast response times remains a considerable hurdle. In this investigation, four organic-inorganic hybrid porous polymers, denoted as HCP-X, were synthesized through Friedel-Crafts reaction utilizing octaphenylcyclotetrasiloxane (OPCTS). The incorporation of hydrophilic functional groups onto HCP-X polymers increases their specific surface area and enhances their adsorption capacity for water molecules, thereby accelerating the humidity sensor's response to humidity. The resulting humidity sensors derived from HCP-X demonstrated rapid response times ranging from 0.6 s to 7 s. Notably, the HCP-B-SO3 humidity sensor exhibited outstanding performance due to the synergistic effects of -OH and -SO3- groups, with a 6 orders of magnitude impedance change across the entire measured humidity range (11 % - 97 % RH), a rapid response time of 0.2 s, and excellent repeatability. It also showcased consistent long-term stability over a 30-day duration, maintaining its performance integrity at both 11 % and 97 % RH levels. The resulting humidity sensor was further utilized into non-contact switches and integrated into masks for health monitoring applications, underscoring its substantial potential in non-contact sensing and healthcare domains.

Enhancing the sensitivity of a humidity sensor by using PVA and GQDs hybrid diaphragms and the harmonic Vernier effect.

What we believe to be a novel fiber optic Fabry-Perot interferometer (FPI) humidity sensor based on polyvinyl alcohol (PVA) doped graphene quantum dots (GQDs) was proposed and experimentally studied. This sensor consisted of a sensing FPI and a reference FPI in parallel. The two sensing cavities FPI were composed of humidity sensitive materials PVA and PVA-GQDs, respectively. Experimental comparative studies had found that doping GQDs in PVA increases humidity sensitivity by 2.1 times. Four reference cavity FPIs were prepared by splicing single-mode fiber and quartz capillary, and then they were combined with two sensing cavity FPIs to form two Vernier effect (VE) sensors and two harmonic Vernier effect (HVE) sensors. Experimental research had found that the sensitivities of PVA as a sensing material for the VE sensor and HVE sensor were -1.0804 nm/%RH and-1.6566 nm/%RH, respectively. The sensitivities of PVA-GQDs as sensing materials for the VE sensor and HVE sensor were-3.1527 nm/%RH and 7.3343 nm/%RH, respectively. Moreover, both HVE sensors had minimal temperature crosstalk. PVA-GQDs were excellent humidity sensitive materials that significantly improve the sensitivity of humidity sensors, making it promising candidate for humidity sensing in various applications.

Wireless chipless RFID temperature and humidity sensor based on Fe2O3-Co3O4/SnO2/rGO composites

Due to its convenience and affordability, chipless RFID technology has become increasingly popular for detecting environmental temperature and humidity. This paper presents a wireless chipless RFID sensor for measuring temperature and humidity. The sensor utilizes a composite material consisting of Fe2O3-Co3O4、SnO2、and rGO to achieve accurate detection of both temperature and humidity. We investigated the change of RFID resonance frequency under different temperature and humidity variations. The temperature and humidity sensitive material is coated on the surface of the antenna by spinning coating machine. The combined structure of nanoparticle-attached rods of Fe2O3-Co3O4, granular clusters of SnO2 and flakes of rGO enhances the sensor’s sensitivity to temperature and humidity measurements. The sensitivity of the temperature and humidity sensor is approximately-0.293 MHz/°C and −2.12 dB/°C during a change in ambient temperature from 10 °C to 85 °C, and approximately −2.11 MHz/°C and 0.19 dB/%RH during a change in humidity from 10 %RH to 65 %RH at normal ambient temperature. The response time of the temperature and humidity sensor to temperature and humidity changes is 15.3 s and 10.1 s, and the recovery time is 21.7 s and 17.5 s, respectively. The wireless chipless RFID sensor has good sensitivity and responsiveness for temperature and humidity measurements.

Preparation and Mechanism Analysis of High-Performance Humidity Sensor Based on Eu-Doped TiO2.

TiO2 is a typical semiconductor material, and it has attracted much attention in the field of humidity sensors. Doping is an efficient way to enhance the humidity response of TiO2. Eu-doped TiO2 material was investigated in both theoretical simulations and experiments. In a simulation based on density functional theory, a doped Eu atom can increase the performance of humidity sensors by producing more oxygen vacancies than undoped TiO2. In these experiments, Eu-doped TiO2 nanorods were prepared by hydrothermal synthesis, and the results also confirm the theoretical prediction. When the doping mole ratio is 5 mol%, the response of the humidity sensor reaches 23,997.0, the wet hysteresis is 2.3% and the response/recovery time is 3/13.1 s. This study not only improves the basis for preparation of high-performance TiO2 humidity sensors, but also fills the research gap on rare earth Eu-doped TiO2 as a humidity-sensitive material.

Novel Technologies towards the Implementation and Exploitation of "Green" Wireless Agriculture Sensors.

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.

Humidity sensor based on tapered no-core fiber coated with Ti3C2Tx MXene

An optical fiber relative humidity (RH) sensor based on tapered no-core fiber (TNCF) coated with Ti3C2Tx MXene is proposed. The fluoride etching method is used to prepare Ti3C2Tx, which is used as the humidity sensitive material with a multi-layer structure, large specific surface area and rich hydrophilic groups, and optical deposition method is used to integrate Ti3C2Tx with TNCF. The diameter of TNCF between two sections of SMF is reduced to 11.6 μm to enhance RH sensitivity. The refractive index (RI) of Ti3C2Tx changes as a result of variations in RH, and this ultimately causes a shift in the transmission spectrum. The sensitivity of the device is −77 pm/%RH in the range of 33 %RH to 75 %RH. In the region of 75 %RH to 91 %RH, the sensitivity increases to 685 pm/%RH, the linear correlation coefficients of the two regions are 0.9521 and 0.9940, respectively. On average, the response time is approximately 3.5 s and the recovery time is about 12.7 s. The sensor may be utilized in possible domains, including biological, chemical, and food processing. Its benefits include a simple construction, cheap cost, high reversibility, and stability.

High-sensitivity, fast-response quartz crystal microbalance humidity sensors coated with nanodiamond/halloysite nanotube composites

This paper develops a composite mixture of halloysite nanotubes (HNTs) and nanodiamond (ND) as a humidity-sensitive material for a quartz crystal microbalance (QCM) to achieve a balance between rapid response/recovery times and high sensitivity. The ND-HNTs composite-integrated QCM sensor exhibited rapid response/recovery times of 2/1 and 4/2 s when transitioning from an environmental humidity of 55 % RH to both low (11.3 % RH) and high (97.3 % RH) humidity levels. Throughout the humidity range of 11.3 %∼97.3 % RH, the sensor demonstrated a sensitivity of up to 98.14 Hz/%RH and low humidity hysteresis of around 1.95 % RH, good repeatability and stability. Furthermore, a detailed analysis of the response mechanism for the ND-HNTs composite-integrated QCM humidity sensor was conducted by integrating a bimodal exponential kinetic adsorption model. This paper introduces a new approach for fabricating QCM humidity sensors that possess both heightened sensitivity and rapid response/recovery times.

Development of craquelure patterns in paintings on panels

Panel paintings are multi-layer structures composed of humidity-sensitive materials. Preventing or limiting stresses in these structures, generated by the loss or gain of moisture, requires an understanding of the relevant processes and risks. A three-dimensional elastic model of a panel painting was used to analyse surface stresses and understand how crack patterns are developed in the two-layer structure of the pictorial layer—the gesso and the paints. Two historically important paint types were considered—egg tempera and oil paints, laid on a gesso produced following historical procedures. Two scenarios of stress development were analysed: permanent cumulative drying shrinkage of paints or gesso, owing to gradual loss of water or evolution of the molecular composition of the binders, and moisture-induced cyclic swelling of the wood substrate. Ratios of distances between cracks in the tangential and longitudinal directions of a wood panel to the layer thickness were estimated for increasing magnitudes of materials’ dimensional change in the two scenarios. The critical values of the ratios for which stress in the midpoint between the cracks dropped below the value inducing strain at break in the materials and saturation of the crack patterns occurred, was approximately 3–4 or 5–6 for the paints and the gesso, respectively. The critical distance normalized to the gesso thickness between cracks parallel to the wood grain induced by cyclic swelling of the wood substrate due to relative humidity variation in the range of 50–70% was 6. The study demonstrated that crack spacings in the fully developed crack systems remain sensitive only to the thicknesses of paint or gesso layers which, therefore, can be derived from the crack pattern geometry. Existing flaws in gesso were found not to increase the risk of new crack development.

A novel photonic skin sensor based on tapered micro nano fiber structure coated with sodium polyacrylate film

Biomimetic skin sensing devices play an important role in next-generation healthcare, robotics, and bioelectronics. In recent years, biomimetic skin-sensing technologies have gained significant attention due to their broad-ranging utility in the fields of healthcare and automation, and future wearable devices designed for health monitoring, robotic applications, and ultra-precise industrial positioning will necessitate the incorporation of flexible optical systems. A novel photonic skin flexible sensor (PSFS) based on partially neutralized sodium polyacrylate (PAS-50) is reported for the first time in this paper. In this study, a novel wearable sensor was designed for real-time detection of temperature and humidity. A single-mode tapered fiber (SMTF) structure based on the principle of evanescent wave is proposed for fast response and real-time monitoring of humidity and temperature. The sensor incorporates an optical micro/nanofiber (MNF) as the core sensing node, while a hydrophilic and ductile Sodium Polyacrylate (PAS) is chosen as the humidity-sensitive material that overcomes the limitation of commonly reported electrical, optical, and material-based flexible devices. Due to the strain amplification effect of the MNF, the sensitivity of the material to relative humidity (RH) is significantly improved, reaching 0.1411 dB/%. In addition, the temperature response exhibits outstanding sensitivity (0.4 dB/°C) in the human skin surface temperature range (30 ∼ 48 °C). Notably, the sensor exhibits a rapid response with a remarkable response time of 170 ms. The proposed PSFS presents novel opportunities for the advancement of future biomimetic skin devices, thus playing a pivotal role in the advancement of next-generation robotics and medical monitoring technologies.

Oxygen plasma treatment-enhanced humidity sensing performance of MoS2 nanoparticles-anchored nitrogen-doped laser-induced graphene

The monitoring of humidity has elicited considerable interest in diverse fields. Nevertheless, the intricate nature of preparatory procedures employed for humidity sensors poses a formidable challenge in the pursuit of a streamlined, expeditious, and highly efficient technique for fabricating humidity-sensitive materials. Herein, the fundamental formation principle of polyimide (PI) film serves as the underpinning for the application of a swift and effective laser direct writing technique in creating a composite integrating MoS2 nanoparticles anchored within nitrogen-doped laser-induced graphene (MoS2@NLIG). Subsequently, a rapid low-temperature oxygen plasma treatment is administered to enhance humidity sensing capabilities of the MoS2@NLIG composite. The oxygen plasma treatment-enhanced MoS2@NLIG humidity sensor exhibits exceptional capability in detecting relative humidity (RH) with a significant sensitivity of 61,219.97 Ω/%RH across an extensive range from 11% to 95%RH, while maintaining a hysteresis as minimal as 3.72%. Moreover, the sensor exhibits outstanding response and recovery times with respective values of 9 s and 15.5 s. Additionally, the humidity sensor demonstrates the capacity to identify human exhalation, potentially serving as a valuable electronic device for monitoring clinical respiration.

Ultra-sensitive relative humidity sensor formed by two parallel Fabry-Perot interferometers and Vernier effect

In this paper, a relative humidity (RH) Vernier sensor based on gelatin diaphragm is proposed, in which the gelatin diaphragm is coated on the end surface of the quartz capillary by the dot-dip method, so that the prepared diaphragm is only 3 μm thick. The sensing cavity FPI1 is an air-cavity made of single-mode-fiber (SMF), quartz capillary (QC) and gelatin diaphragm. Gelatin diaphragm is an excellent humidity sensitive material, therefore FPI1 has high sensitivity to humidity. The reference cavity FPI2 is also an air-cavity spliced in the order of SMF-QC-SMF. When FPI1 and FPI2 have similar free spectral range, they form a Vernier effect in parallel, achieving maximum humidity sensitivity. The experimental results show that in the range of 1300 nm to 1650 nm, the envelope formed by two parallel FPIs is particularly regular and has a free spectral range of 95.2 nm. By tracking the variation of the envelope dip with humidity, the sensor obtained a highly fitting polynomial relationship in the range of 57 %RH − 84 %RH. However, in the range of 75 %RH − 84 %RH, the sensor achieved an excellent linear fitting relationship, with a linearity of up to 0.9997 and a sensitivity of up to 27.9726 nm/%RH. This is the most sensitive gelatin Vernier humidity sensor ever reported. And the crosstalk of temperature of the Vernier humidity sensor is only 0.0186 %RH/℃. The sensor is easy to manufacture, ultra-high sensitivity, small temperature crosstalk, good performance and low cost, and has good reversibility and stability. It is a competitive humidity candidate sensor.

Ti3C2-MXene used to enhance response/recovery speed and thermal stability of acrylate copolymer humidity sensor under abnormal temperature changes

MXene is a typical kind of 2D material with high specific surface, electrial conductivity, self-lubrication and abundant surface groups, which has received wide attention in the fields of electrochemistry, lubrication, sensors and electromagnetic shielding. In this work, MXene-containing polymer matrix composites (MHMA@ Ti3C2) are prepared to enhance the humidity response properties of conventional polyacrylate humidity sensitive materials. The result shows that the humidity response curve of MHMA@ Ti3C2 humidityfilm shows good linearity (R2 = 0.9992) in the relative humidity range of 30–90 %RH, with a high sensitivity of 63.7 kΩ/%RH. Meanwhile, the humidity response/recovery time of the MHMA@ Ti3C2 humidity film shortened from 310 s/312 s to 30 s/35 s, resulting in a much faster response speed. The wet hysteresis was reduced from 2.1 %RH to 0.34 %RH, and the lowest temperature change coefficient was reduced from 0.60 %RH/℃ to 0.44 %RH/℃ compared with MHMA. In the composites, MXene component significantly enhances the electronic migration capability during the humidity response, which improved the humidity-sensitive properties of the composites, and at the same time enhanced the response speed and temperature change resistance of the composites.

A controllable humidity sensitivity measurement solution based on a pure FPI fiber tip and the harmonic Vernier effect

Hygroscopic films used to enhance the humidity sensitivity of fiber-optic sensors often experience material aging and nonlinear response issues. To address this issue, this study proposes a controllable humidity sensitivity measurement solution that offers a simple and cost-effective fabrication process without requiring humidity-sensitive material sensitization. The solution is based on a Fabry-Perot interferometer (FPI) fiber tip and the harmonic Vernier effect. By employing a cascade structure of single-mode fiber (SMF)-hollow core fiber (HCF)-no core fiber (NCF) with fiber grinding techniques, the FPI-based fiber sensing probe constructed a resonance cavity to detect small variations in air refractive index (RI) induced by humidity. A reference spectrum, which can flexibly control parameters, such as free spectral range (FSR), contrast, phase, and intensity, is superimposed onto the sensing spectrum acquired by the FPI fiber sensing tip. By extracting features from the internal envelope generated by the harmonic Vernier effect, the mapping relationship between the center wavelength of the inner envelope and the humidity is determined. The harmonic Vernier effect amplifies the humidity sensitivity of the fiber-optic FPI sensing tip, with the magnification factor determined by the optical path length (OPL) detuning of the created reference and sensing spectra. For the harmonic order j = 4, a humidity sensitivity of approximately 76.01 pm/% RH@ 24 ℃ was achieved without using hygroscopic material sensitization. The proposed solution offers numerous advantages and enables high-precision and high environmental reliability humidity monitoring.

Water-durability and high-performance all-fiber humidity sensor using methyldiethanolamine-photopolymer-PDMS structure

In the context of optical fiber humidity sensing, the long-term stability of sensors in high humidity and dew environments such as bathrooms or marine climates remains a challenge, especially since many humidity sensitive materials are water soluble. In this study, we use methyldiethanolamine, pentaerythritol triacrylate and Eosin Y to form a liquid-solid structure humidity sensitive component, the outermost layer is coated with PDMS passivating layer to ensure the stability and durability of the humidity sensor under the conditions of dew and high humidity. The liquid microcavity of the sensor consists of methyldiethanolamine-pentaerythritol triacrylate composite solution, and the sensitivity is several times higher than that of the liquid-free cavity sensor. The sensitivity of the sensor to temperature is verified (0.43 nm/°C and 0.30 nm/°C, respectively) and temperature crosstalk is compensated using a matrix. The compact structure allows for ultra-fast response (602 ms) and recovery time (349 ms). Our work provides a promising platform for efficient and practical humidity and other gas monitoring systems.

Sulfonated hypercross-linked porous organic polymer based humidity sensor

Humidity sensors are of great significance in industry, agriculture and human activities, and many nanomaterials are already used for humidity sensing. Linear ionic polymers with hydrophilic groups have exhibited good humidity detection performance because they can adsorb water and then dissociate charged carriers. However, it suffers from the instability under high relative humidity that caused by polymer chain swelling and disengagement. To overcome this shortage, in this work, a new sensor based on hypercross-linked porous organic polymer (POP) with sulfonic acid group was fabricated to humidity detection in wide humidity range. The POP with strong covalent bonds exhibits remarkable chemical and thermal stability, and sulfonic acid group in the pore can act as fully accessible active site to enhance its ability to adsorb water. The obtained sensor shows high response (∼77) in a wide relative humidity range (11%−95% RH), short response/recovery time (9 s/21 s) and good stability. Complex impedance curves and density functional theory have been carefully investigated to understand its sensing mechanism. This work provides a new direction for the development of the impedance-type humidity sensitive materials in subsequent research and practical applications.

Smart mask based on lead-free perovskite humidity sensor for labor intensity grading by breath monitoring

Unaware fatigue causes inefficiencies and unsafe behaviors. Therefore, an easily-used technique to monitor labor intensity is critical. In this work, the humidity-sensitive material Cs3Cu2I5 was used to monitor breath and grade labor intensity without contact. The Cs3Cu2I5 showed reversible conversion to the CsCu2I3 phase by being exposed to water. EMI-TFSI was added to improve the material’s conductivity and electrochemical stability, helping to passivate surface defects and adjust the response sensitivity. Further mechanism explanation was explored by density functional theory calculation and electrochemical impedance spectroscopy. Switching from 40 % RH to 90 % RH, the sensor resistance changed by 161.3 megohms with a relative change ratio of 0.967. Besides, the sensor has a short response/recovery time (e.g., 2.0/0.5 s switching from 40 % RH to 60 % RH), with good repeatability and stability in high humidity. Then, the developed high-performance lead-free sensor was integrated into a smart mask with a neural network algorithm to monitor and forecast breath, and grade labor intensity. The model was built with 240 sample data and 19 constructed features. The accuracy of identifying three intensity levels in external validation was 0.9. Based on the sensing data and the grading model, a real-time labor intensity monitoring and visualization platform was constructed.