Fabrication Of Humidity Sensors Research Articles

Laser Fabrication of Humidity Sensors on Ethanol-Soaked Polyimide for Fully Contactless Respiratory Monitoring.

Humidity-sensor-based fully contactless respiratory monitoring can eliminate the discomfort and infection risks associated with any wearable device. However, challenges in the facile fabrication of highly sensitive humidity sensors continue to hinder their widespread application for fully contactless respiratory monitoring. In this study, we introduce a simple method to fabricate highly sensitive humidity sensors. Our method employs laser-induced graphene (LIG) on an ethanol-soaked polyimide (PI) film as the electrode of the humidity sensor. The ethanol-soaked PI between adjacent LIG electrodes functions as the sensing material, enabling ion-conductive humidity sensing. Compared to the LIG humidity sensors fabricated on untreated PI films, LIG humidity sensors fabricated on ethanol-soaked PI films exhibit superior performance with higher linearity (R2 = 0.9936), reduced hysteresis (ΔH = 5.1% RH), and increased sensitivity (0.65%/RH). Notably, the LIG humidity sensor fabricated on the ethanol-soaked PI film can detect a person's breathing from a distance of 30 cm, a capability not achieved by sensors fabricated on untreated PI films. Moreover, incorporating these LIG humidity sensors into an array further enhances both the detection distance and the sensitivity for respiratory monitoring. Experimental results demonstrate that the LIG humidity sensor array can be employed for fully contactless on-bed respiration monitoring and for continuous, fully contactless monitoring of the respiratory rate during treadmill exercise. These results highlight the great potential of our LIG humidity sensors for various practical applications in medicine and sports.

High-sensitivity humidity sensor based on Poly(vinylidene fluoride)/Deep eutectic solvent composites

This study explores the incorporation of deep eutectic solvent (DES) into a poly (vinylidene fluoride) (PVDF) matrix to fabricate PVDF/DES composite thin films. Subsequently, electrodes were screen-printed onto the thin films to create PVDF/DES humidity sensors. The investigation delved into the crystalline structure of PVDF, the cross-sectional morphology and electrochemical property of PVDF/DES composite films, and the humidity sensing response of PVDF/DES humidity sensors. The study revealed that adding DES leads to the formation of porous composite films with electrical active phases and high electrical conductivity. The sensitivity of the PVDF/DES humidity sensor increases with the content of DES. This work provides a novel method for the fabrication of humidity sensors, holding significant potential in various applications such as respiratory monitoring.

MgAl-LDH nanoflowers as a novel sensing material for high-performance humidity sensing.

This work details a novel application of MgAl-LDH nanoflowers, applied in the fabrication of humidity sensors using quartz crystal microbalance (QCM). An oscillating circuit approach has been utilized to thoroughly investigate the humidity detection characteristics of QCM sensors that are fabricated using MgAl-LDH nanoflowers. The examination encompassed various parameters such as the sensors' response, humidity hysteresis, repeatability, and stability. Experimental results clearly indicate that these MgAl-LDH nanoflower-based QCM sensors exhibit a distinct logarithmic frequency response to varying moisture levels. Notably, the sensitivity of the sensors is intricately tied to the amount of MgAl-LDH nanoflowers utilized during the deposition process. Moreover, these sensors maintain remarkable stability across a wide humidity range spanning from 11% to 97% RH. Additionally, the MgAl-LDH nanoflower-based QCM sensors possess minimal humidity hysteresis and display swift dynamic response and recovery periods, further highlighting their potential for humidity detection applications.

Biosynthesis, structural characterization and humidity sensing properties of cellulose/ZnO nanocomposite

The present work demonstrates the applications of a nanocomposite of the cellulose polymer and ZnO nanoparticles with 1:1 weight ratio, prepared by a green assisted precipitation method for a high-performance resistive type humidity sensor. The morphology and nanostructure of prepared cellulose/ZnO composite were characterization by X-ray diffraction, Fourier transform infrared spectroscopy, field emission scanning electron microscopy and transmission electron microscopy are confirmed the decoration of ZnO nanoparticles on cellulose polymer matrix surface. The humidity sensing martials was coated onto the interdigital electrodes. The sensitivity, response/recover and stability studies performance of fabricated humidity sensors have been monitored in 5 to 98% relative humidity (%RH) at room temperature (37 °C). The response and recovery times of the fabricated sensors are observed as ≈ 26 s and ≈ 53 s respectively, and the sensitivity factor (Sf) is 2656 ± 103 Ω. Possible mechanisms of the humidity sensor based on water-induced conductivity increase are discussed. Also, the energy band of cellulose/ZnO nanocomposite was simulated by density functional theory (DFT) studies. The cellulose/ZnO nanocomposite humidity sensor has great potential for practical field applications.

Enhanced humidity sensing performance of LiF-doped MgTiO3 ceramics via spark plasma sintering

The fabrication of low-porosity ceramics for humidity sensing is crucial to prevent moisture entrapment, which poses significant challenges. Using Spark Plasma Sintering (SPS), we successfully densified fine-grained ceramic materials based on LiF-doped MgTiO3, comparing them with conventionally sintered counterparts. Structural and microstructural analyses, employing X-ray diffraction and Scanning Electron Microscopy (SEM), were conducted. Investigation into electrical and dielectric properties' variations concerning humidity levels was conducted and revealed that SPS sintered ceramics exhibit heightened sensitivity to moisture, as evidenced by resistivity, capacitance, and response time measurements. The implications of these results are discussed in depth, highlighting the potential of SPS as a promising method for fabricating humidity sensors with improved performance and reduced porosity-related issues.

A capacitive humidity sensor based on nanodiamond/silver nanoparticles (ND/Ag) nanocomposite with high stability and rapid response for respiratory monitoring

In this work, a high-performance capacitive humidity sensor based on nanodiamond/silver nanoparticles (ND/Ag) nanocomposite is proposed. The influence of silver nanoparticles with different content in nanodiamond is investigated, and the optimum ratio of 5 wt% Ag in the composite shows the maximum humidity sensing performance, which is three times than that of the pure nanodiamond based humidity sensor. Moreover, the proposed sensor exhibits a superior repeatability, ultrafast response time (<2 s) and recovery time (<1 s). More importantly, the sensor can maintain a good repeatability and rapid response speed when exposure to high humidity for a long time, demonstrating the excellent stability of the proposed sensor. Furthermore, complex impedance spectroscopy was applied to account for the underlying sensing mechanisms of the ND/Ag composite film humidity sensor. Finally, we design a circuit to realize non-contact breathing control by using the fabricated humidity sensor, which demonstrate that the fabricated humidity sensor could be applied to some auto-control systems by human-breath. This work demonstrated that ND/Ag composite film is an ideal candidate material for constructing high-performance humidity sensors for respiratory applications.

Design and Characterization of Printed Flexible Humidity Sensor

The development of humidity sensors is essential for applications in the environmental, agriculture, medical and semiconductor industries. This research focused on using advanced printed board circuit (PCB) printing technology to fabricate a humidity moisture sensor. The fabrication of humidity sensor was done using two different polymers: p-HEMA and polyimide, and the performance was compared. Humidity sensors with p-HEMA showed higher capacitance value and more sensitivity than the humidity sensors with polyimide. The variation in sensitivity was higher between the relative humidity from 60% to 90% than between 45% to 60%.

Preparation of graphene oxide composited zinc oxide films by electrostatic spray deposition for humidity sensors

This work aimed to study the preparation of graphene oxide (GO) and zinc oxide (ZnO) composited films by electrostatic spray deposition for use in fabricated humidity sensors. The physical properties of prepared films were examined with X-ray diffraction, Raman spectroscopy, and scanning electron microscope. Subsequently, a humidity sensor was fabricated using a low-cost interdigitated electrode pattern with the print circuit board. The humidity sensing behaviors were assessed with fixed humidity levels in a saturated aqueous salt solution in the humidity range of 11 – 93%RH. The impedance value of the device was obtained with a precision LCR meter. It was found that the composited film of 0.50 %wt. GO demonstrated the highest humidity response with optimized humidity sensitivity, hysteresis error, and response/recovery times of 2.68 ´ 103, 4.89%, and 228/19 s, respectively. Moreover, the equivalent circuits for the prepared device at various humidity levels were acquired with impedance spectroscopy to propose the mechanism for the humidity sensing of the device.

Highly Sensitive Self-Powered Humidity Sensor Based on a TaS2/Cu2S Heterostructure Driven by a Triboelectric Nanogenerator.

Self-powered humidity sensors with high response and good stability have attracted extensive interest in environmental monitoring, medical and health care, and sentiment detection. Because of its high specific surface area and good conductivity, two-dimensional material has wide application in the field of humidity sensing. In this work, we proposed a novel self-powered high-performance TaS2/Cu2S heterostructure-based humidity sensor driven by a triboelectric nanogenerator (TENG) made with the same structure. The TaS2/Cu2S heterostructure was prepared via the chemical vapor deposition method, and then, electrolytic and ultrasound treatments were introduced to further increase the surface area. The fabricated humidity sensor showed ultrahigh sensitivity (S = 3.08 × 104), fast response (2 s), low hysteresis (3.5%), and great stability. First-principles calculation results demonstrated the existence of an electron transport channel with a low energy barrier (-0.156 eV) from the Cu2S to TaS2 layer in the heterostructure, which improves the surface charge transfer of the material. The TaS2/Cu2S heterojunction-based TENG can generate an output voltage of 30 V and an output current of 2.9 μA. Furthermore, the proposed self-powered humidity sensor verified the potential ability of detecting human respiratory frequency, skin humidity, and environmental humidity. This work provides a new and feasible path for research in the field of humidity sensors and promotes the application development of self-powered electronic devices.

An enhanced MEMS-based polyimide capacitive-type relative-humidity sensor with halloysite nanotube as a modifier

Polymers have been widely used in the fabrication of humidity sensing materials, although, issues including sensitivity and hysteresis still exist, limiting its potential. Herein, we aim to enhance the humidity sensing performances of a polyimide (PI) capacitive-type relative-humidity sensor via a simple and efficient approach. Specifically, modified halloysite nanotubes (HNTs) were doped in the polyimide sensing layer to optimize the humidity sensing capabilities of the MEMS (Micro-electromechanical Systems)-based sensor. The configurational and morphological characteristic of the sensing membrane with different content of the nanofiller were characterized. The humidity sensing capabilities of the enhanced sensors were investigated under the relative humidity (RH) range of 10–90%. Owing to the rational structure design of the sensing layer, the modified sensor exhibited enhanced sensitivity, low humidity hysteresis and good linearity. The highest sensitivity of the device with the best overall performance reached 0.87 pF/%RH, which is 1.81 times that of the pure PI based humidity sensor. Meanwhile, the sensor maintained moderate humidity hysteresis (2.18%) as well as good linearity (R2 = 0.9813) within the humidity boundary from 10 to 90% RH. The aforementioned advantages of the PI-HNTs-NH2 based humidity sensor demonstrate its promising application in humidity measurement. As a low-cost and effective additive, HNTs own great application potential in the preparation of polymer sensing devices. This work provides valuable guidance for fabricating humidity sensors with outstanding sensing performances.

Flexible Humidity Sensor with High Sensitivity and Durability for Respiratory Monitoring Using Near-Field Electrohydrodynamic Direct-Writing Method.

The humidity of breath can serve as an important health indicator, providing crucial clinical information about human physiology. Significant progress had been made in the development of flexible humidity sensors. However, improving its humidity sensing performance (sensitivity and durability) is still facing many challenges. In this work, near-field electrohydrodynamic direct writing (NFEDW) was proposed to fabricate humidity sensors with high sensitivity and durability for respiration monitoring. Due to the applied electric field, dense carbon nanotube/cellulose nanofiber (CNT/CNF) networks formed during the printing process that enhance the sensitivity of the sensor. The prepared sensor showed excellent humidity responses, with a maximum response value of 61.5% (ΔR/R0) at 95% relative humidity (RH). Additionally, the sensitivity film prepared by the NFEDW method closely fits the poly(ethylene terephthalate) (PET) substrate, endowing the sensor with outstanding bending (with a maximum curvature of 4.7 cm-1) and folding durability (up to 50 times). The sensitivity of the prepared sensor under different simulated conditions, namely, nose breathing, mouth breathing, coughing, yawning, breath holding, and speaking, was excellent, demonstrating the potential of the sensor for the real-time monitoring of human breath humidity. Thus, the high-performance flexible humidity sensor is suitable for human respiration and health monitoring.

Humidity Sensing Mechanism and Respiration Monitoring of WO3 Nanoplates with a Monoclinic Crystal Phase

Functioning of hydrothermally synthesized WO3 nanoplates was investigated for humidity sensing and respiration monitoring under different breathing conditions. The monoclinic phase was identified by the X-ray diffraction (XRD) technique. The average crystallite size was calculated by Williamson-Hall (W–H) plot (27 nm) and modified Scherrer equation (24 nm). The optical band gap was calculated as 2.7 eV using UV–visible spectroscopy. The field emission electron microscopy (FE-SEM) and high resolution transmission electron microscopy (HR-TEM) micrographs of readied WO3 have confirmed the formation of microstructured nanoplates having an average diameter of 216 nm. Fluorine-doped tin oxide (FTO) substrate was used for the deposition of film and also used as an electrode. The investigation of humidity was carried out at different relative humidity (RH)−11%, 33%, 44%, 54%, 74%, and 95%. The fabricated humidity sensor has shown excellent reversibility, stability and very small humidity hysteresis (<2%) at room temperature. The maximum response was observed as 41.95% at 95% RH with response and recovery time as 2 s and 134 s respectively. During the 30 d of observation, only a 0.4% decrease in response was observed. The fabricated WO3-based humidity sensor was investigated for human respiration having respiration rates of 2.51 min−1, 3.09 min−1 and 3.74 min−1.

CNT-PDMS foams as self-powered humidity sensors based on triboelectric nanogenerators driven by finger tapping

An increasing number of frequently applied portable electronics has raised the significance of self-powered systems. In this regard, triboelectric nanogenerators (TENGs) have drawn considerable attention due to their diversity of design and high power output. As a widely used material in TENG electrodes, polydimethylsiloxane (PDMS) shows attractive characteristics, such as electron affinity, flexibility, and facile fabrication. To achieve active TENG-based humidity sensing, we proposed a straightforward method to enhance the hydrophilicity of PDMS by two parallel approaches: 1. Porosity induction, 2. Carbon nanotube (CNT) compositing. Both of the mentioned processes have been performed by water addition during the synthesis procedure, which is not only totally safe (in contrast with the similar foaming/compositing routes), but also applicable for a wide range of nanomaterials. Applying the modified electrode as a single-electrode TENG-based humidity sensor, demonstrated an impressive enhancement of sensing response from 56% up to 108%, compared to the bare electrodes. Moreover, the detecting range of ambient humidity was broadened to higher values of 80% in a linear behavior. The fabricated humidity sensor based on a CNT-PDMS foam not only provides superior sensing characteristics but also is satisfactory for portable applications, due to being lightweight and desirably self-powered.

Fully integrated wearable humidity sensor for respiration monitoring.

Respiration monitoring is a promising alternative to medical diagnosis of several diseases. However, current techniques of respiration monitoring often require expensive and cumbersome devices which greatly limit their medical applications. Here, we present a fully integrated wearable device consisting of a flexible LCP-copper interdigital electrode, a sensing layer and a wireless electrochemical analysis system. The developed humidity sensor exhibits a high sensitivity, a good repeatability and a rapid response/recover time. The long-term stability is over 30days at different relative humidity. By integrating the flexible humidity sensor with miniaturized electrochemical analysis system (0.8cm × 1.8cm), response current concerning respiration can be wirelessly transmitted to App-assisted smartphone in real time. Furthermore, the fabricated humidity sensor can realize skin moisture monitoring in a touch-less way. The large-scale production of miniaturized flexible sensor (4mm × 6mm) has significantly contributed to commercial deployment.

Facile Fabrication of a Bio-Inspired Leaf Vein-Based Ultra-Sensitive Humidity Sensor with a Hygroscopic Polymer.

Bio-inspired materials have received significant interest in the development of flexible electronics due to their natural grid structures, especially natural leaf vein networks. In this work, a bio-inspired leaf vein-based flexible humidity sensor is demonstrated. The proposed sensor is composed of a leaf/Al/glycerin/Ag paste. The Al-deposited leaf vein networks are used as a bottom electrode with a resistance of around 100 Ω. The humidity sensor responds well to relative humidity (RH) levels ranging from 15% to 70% at room temperature. The fabricated humidity sensor exhibits an ultra-sensitive response to different humidity conditions due to the biodegradable insulating hygroscopic polymer (glycerin), specifically the ionic conductivity reaction. To further verify the presence of ionic conduction, the device performance is tested by doping NaCl salt into the hygroscopic polymer sensing layer. In addition, both the repeatability and flexibility of the sensor are tested under different bending angles (0°, 90°, 180°, and 360°). The bioinspired ultrasensitive humidity sensor with a biocompatible and biodegradable sensing layer holds great potential, especially for health care applications (e.g., respiratory monitoring) without causing any body harm.

Low-Power Wearable Breath Monitoring Device With Humidity- and Ammonia-Sensing Functions (May 2022)

This article presents a wearable breath-monitoring device integrated with humidity- and ammonia-sensing functions. The breath-monitoring device was constructed using a piezoelectric cantilever array and hollow flexible polydimethylsiloxane (PDMS) membrane. Important parameters of breath, such as the respiratory rate and tidal volume, can be detected without actuated energy. The Nafion-based capacitive humidity sensor and ZnO nanoparticle-based resistance ammonia sensor were compactly integrated into the housing ring of a breath monitoring device. The fabricated humidity sensor demonstrated a faster response in the sensing range below 75% compared with a commercial product. The sensitivity of the fabricated ammonia sensor reached 0.16 ppm/mV and the sensor exhibited an approximately fourfold shorter response time compared with the commercial product. Two applications were tested for the developed device prototype. One was monitoring the humidity and ammonia of the environmental air along with breath parameters while breathing. The other was to examine the amount of breath air exhaled from the body of the subject. The total power consumed by the developed prototype device was approximately 46 mW, which is significantly lower than that of existing commercial products. This compact and lightweight prototype device demonstrates significant potential for wearable applications.

Screen-printed graphene-carbon ink based disposable humidity sensor with wireless communication

Humidity sensing is crucial for several industrial, environmental, and healthcare applications, many of which require sensors in flexible form factors and with features such as disposability and facile fabrication processes. Herein, we present a flexible, cost-effective, and disposable humidity sensor developed on paper substrate. The screen-printed graphene-carbon (G-C) ink-based humidity sensor demonstrates good sensing performance in terms of change in resistance (∼12.4 Ω/%RH) in humidity ranging from 25%RH to 91.7%RH. The sensor displays high flexibility (studied at bending radiuses 40 mm, 30 mm, 25 mm, and 20 mm), appreciable stability (> 4 months), high repeatability (> 100 cycles), short response/recovery time (∼4 s/∼6 s towards respiration rate monitoring) and good reproducibility (minor variations ∼ ± 1 Ω/%RH). The efficacy of fabricated humidity sensor is evaluated for spatial humidity monitoring, respiration rate monitoring, and soil moisture monitoring. Finally, the real time monitoring of humidity is also demonstrated via wireless transmission of data to a smartphone to display the potential of the fabricated sensor for environmental, agricultural, and healthcare applications.

Optical Humidity Sensor Based on CdSe/ZnS Quantum Dots Modified by Porous Silica

AbstractUltra‐sensitive optical humidity sensors based on nanomaterials play important roles in the fields of science and industry. The related research has been reported, however, the underlying physical mechanism is still unclear. In order to clarify this point, and further improve the sensing performance, herein, optical humidity sensor based on porous silica/quantum dots (PS/QDs) hybrid nanoarchitecture has been developed. PS is selected to enhance emission of QDs due to its large surface roughness, which can improve light utilization. Under optimized conditions, the emission from the hybrid PS/QDs can be enhanced 6.5 times compared with the as‐grown QDs. With the increase of relative humidity from 20% to 80%, the hybrid PS/QDs demonstrate excellent response, wide sensing range, fast response time (22–40 s), good selectivity, and repeatability. Based on laser spectroscopy, the sensing mechanism is ascribed to the passivation of the surface defects by water molecules, which results in enhanced emission. This work provides a simple and low‐cost method to fabricate humidity sensors and shows promising applications in environmental detection.

Integrated Capacitive- and Resistive-Type Bimodal Relative Humidity Sensor Based on 5,10,15,20-Tetraphenylporphyrinatonickel(II) (TPPNi) and Zinc Oxide (ZnO) Nanocomposite.

The development of high-performance humidity sensors to cater for a plethora of applications, ranging from agriculture to intelligent medical monitoring systems, calls for the selection of a reliable and ultrasensitive sensing material. A simplistic device architecture, robust quantification of ambient relative humidity (% RH), and compatibility with the contemporary integrated circuit technology make a bimodal (capacitive and resistive) surface-type sensor to be a prominent choice for device fabrication. Herein, we have proposed and demonstrated a facile realization of a 5,10,15,20-tetraphenylporphyrinatonickel (II)–zinc oxide (TPPNi-ZnO) nanocomposite-based bimodal surface-type % RH sensor. The TPPNi macromolecule and ZnO nanoparticles have been synthesized by an eco-benign microwave-assisted technique and a thermal-budget chemical precipitation method, respectively. It is speculated from the morpohological study that specific surface area improvement, via the provision of ZnO nanoparticles on micro-pyramidal structures of TPPNi, may reinforce the sensing properties of the fabricated humidity sensor. The relative humidity sensing capacitive and resistive characteristics of the sensor have been monitored in 40–85% relative humidity (% RH) bandwidth. The fabricated sensor under the biasing conditions of 1 V of applied bias (Vrms) and 500 Hz AC test frequency exhibits a significantly higher sensitivity of 387.03 pF/% RH and 95.79 kΩ/% RH in bimodal operation. The average values of both the response and recovery times of the capacitive sensor have been estimated to be ∼30 s. It has also been debated why this high degree of sensitivity and considerable reduction in response/recovery time has been obtained. In addition, the intense and wide bandwidth spectral response of the TPPNi-ZnO nanocomposite indicates that it may also be utilized as a potential light-harvesting heterostructured nanohybrid in future studies.

Cellulose Fibers Extraction from Cotton Stalk via One-Step Acid Bleachable Pretreatment for Fiber-Optic Humidity Sensor Fabrication

Cellulose Fibers Extraction from Cotton Stalk via One-Step Acid Bleachable Pretreatment for Fiber-Optic Humidity Sensor Fabrication