Humidity-sensitive Film Research Articles

Highly sensitive humidity sensor based on composite film of partially reduced graphene oxide and bacterial cellulose

Breathing is an important physiological activity of human body, which not only reflects the state of human movement, but also is one of the important health indicators. Breathing can change the concentration of water molecules, so monitoring humidity has gradually become a hot topic in modern research. In this study, a humidity sensing composite film with high sensitivity and short response time was made by using the mixture of graphene oxide (GO) and bacterial cellulose (BC) with simple dry film-forming method. L-ascorbic acid was used as reducing agent to reduce GO and improve the conductivity of GO/BC composite film (BG). The influence of different BC contents and the different reduction degree on the resistance change rate of composite film was investigated in details. The maximum resistance change rate of partially reduced BG humidity sensitive composite film reached up to 94%, and the response and recovery time were 13 s and 47 s respectively. Furthermore, the sensor shows obvious resistance change in noncontact sensing test and different breathing states. This kind of humidity sensitive film with fast response and high sensitivity has great potential in human health monitoring and noncontact sensing, and is of great significance in promoting health detection and intelligent life.

Multi-layer film surface functionalized PMF sensor for real-time humidity measurement

Real-time monitoring of environmental relative humidity (RH) plays a crucial role in ensuring both the quality of industrial production and the quality of life. In this work, polarization maintaining fiber (PMF) was used as carriers to creatively constructed multi-layer film surface functionalization on its cladding surface. The indium tin oxide (ITO) film and polyvinyl alcohol (PVA) doped graphite oxide (GO) composite polymer constitute multi-layer humidity sensitive film, which has the effect of dual absorption of water and improves the humidity sensitivity. Microporous composite polymer films created by laser processing increase the area for water evaporation and absorption, which improve response speed, and reduce recovery time. The surface functionalized PMF sensor exhibits high sensitivity and rapid response to humidity, achieving bidirectional repeated measurement of 0.28 nm/% within 10 s in the measurement range of 30%-90%, rendering it a reliable means for real-time monitoring of humidity.

Miniaturized Flexible Non-Contact Interface Based on Heat Shrinkage Technology.

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-Al2 O3 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 (0.5s). 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.

In-Fiber Breathing Monitoring Sensor by Micro-Tapered Long-Period Grating Structure

A simple in-fiber breathing monitoring sensor based on a micro-tapered long-period fiber grating (MT-LPFG) structure was proposed and demonstrated. The MT-LPFG structure was manufactured through the periodical tapering of an optical fiber utilizing a CO2 laser fusion splicer. Without the use of a humidity sensitive film, the MT-LPFG structure can directly and dynamically monitoring relative humidity with a fast response, based on which, the breathing process can be monitored in real time through the monitoring of its transmission affected by the humidity. Experiments show that breathing rate and breathing status during respiration were successfully monitored by placing the sensor structure nearby the nose in a breathing mask. A rise time of 0.237 s and a fall time of 0.299 s were realized in the breathing process monitoring. For its advantages of simple fabrication, miniature size, and fast response, the proposed sensor may found applications in visual observation of the patient’s breathing condition.

Nanodiamond/cellulose nanocrystals composite-based acoustic humidity sensor

This paper fabricated a high-performance nanodiamond (ND)/cellulose nanocrystal (CNC) composite-based acoustic humidity sensor. First, the effects of changes in both mass and viscosity of humidity-sensitive films on the resonance displacement, vibration modes, and impedance characteristics of the quartz crystal microbalance (QCM) were discussed using the finite element simulation software COMSOL Multiphysics. Then, the surface morphology and elemental composition of ND/CNC composites were characterized by TEM and EDS. Next, the humidity-sensing properties of ND/CNC composites, including response sensitivity, humidity hysteresis, dynamic characteristics, and stability, were tested at a relative humidity range from 11.3 % to 97.3 % RH combined with QCM transducers. In addition, the humidity-sensing performance of the QCM sensors with different ratios of composites was compared. The best ND/CNC-based QCM humidity sensors exhibited high sensitivity (54.1 Hz/%RH), low humidity hysteresis (3.2%RH), and fast response/recovery times. Then the adsorption process and sensitivity mechanism of water molecules on ND/CNC composites were analyzed using Langmuir isothermal adsorption model. Finally, the potential application of ND/CNC composite-based QCM humidity sensor was illustrated with respiration monitoring as an example.

Theoretical and Simulation Analysis of Static and Dynamic Properties of MXene-Based Humidity Sensors

In this paper, the static and dynamic characteristics of the MXene-based IDE capacitive humidity sensor are investigated through theoretical modeling and simulation. It is found that the capacitance increases according to the thickness of the sensing film within a certain range while stopping increases along with the growth of the thickness when the thickness is over a threshold. When the thickness is at a tiny level, a larger thickness does not lead to a significant increase in the response time due to the diffusion mechanism of water molecules. When the thickness increases to certain extent, there is an evident relationship between the response time and the change of thickness. For the humidity-sensitive film, under the same relative humidity conditions, the capacitance has a positive correlation with temperature, and the response time shows the opposite trend. Subsequent studies on the sensitive mechanisms of MXene materials explain these phenomena and demonstrate the accuracy of the model. This provides a more accurate method for sensor design. The properties of the MXene capacitive humidity sensor can be optimized by changing its structure and adjusting material parameters.

Miniaturized High-Frequency Humidity Sensor Based on Quartz Crystal Microbalance

In this paper, a high-frequency quartz crystal microbalance (QCM) detection unit is fabricated by dry thinning or wet etching, whose resonant frequency is about 61 MHz and the package size is only 5.0 mm long and 3.2 mm wide. After coated a mixed solution of sodium alginate (SA) and γ-polyglutamic acid (PGA) as the humidity sensitive film over the detection unit, we obtain the proposed QCM sensor. Then, we analyze the variations of resonance frequency and relative humidity, and the response/recovery time. By practical tests, the proposed sensor exhibits a very high sensitivity of 2727.41 Hz/%RH and a decent detection limit of 0.011 %RH. The proposed QCM sensor could be widely applied to measure various environmental parameters for its high performance and portability. Due to its excellent performance and small size, we further prepared a 2× 4 sensor array which will be more widely tested and applied in the future.

Self-powered multifunctional monitoring and analysis system based on dual-triboelectric nanogenerator and chitosan/activated carbon film humidity sensor

A humidity sensor driven by a dual-TENG has been developed for humidity detection at room temperature. In this work, we report a polymer humidity sensor with high response linearity. The sensitive material is made of chitosan (CS) and activated carbon (AC). The sensor is driven by a triboelectric nanogenerator based on fluorinated ethylene propylene copolymer (FEP). The CS/AC humidity sensitive film is coated on a PET substrate with a nickel-chromium alloy interdigitated electrode by spray technique. The peak-to-peak voltage of TENG can be close to 700 V, and the maximum output power is 12.208 mW. The output voltage of TENG is rectified to 48 V to power the humidity sensor, and then we realize that the voltage change range of the humidity sensor is between full voltage of 0–48 V through adjusting the resistance ratio and designing operational amplifier circuit. The sensor has a wide detection range (0%~97% RH) and good stability. Its response/recovery time under 33% RH and 97% RH conditions are 12 s/20 s and 22 s/21 s, respectively. The sensor can realize obvious identification of breathing characteristics in practical application tests. Meanwhile, the sensor can also be used to monitor the distance of the finger from the sensor surface and the response when the water column passes over the sensor surface. In addition, we also sent the signal of the humidity sensor to the smart terminal via WIFI module to monitor the signal changes caused by breathing and finger movement, and matlab was used to analyze the breathing data.

The Effect of rGO-Doping on the Performance of SnO2/rGO Flexible Humidity Sensor.

The development of a flexible and high-performance humidity sensor is essential to expand its new applications, such as personal health monitoring and early diagnosis. In this work, SnO2/rGO nanocomposites were prepared by one-step hydrothermal method. The effect of rGO-doping on humidity sensing performance was investigated. Scanning electron microscopy, transmission electron microscopy, X-ray diffraction and Raman spectroscopy were used to characterize the nanostructure, morphology and chemical composition of SnO2/rGO nanocomposites. The SnO2/rGO humidity sensitive film was prepared by electrospinning on a polyimide film modified with gold electrodes. The humidity test results show that different doping ratios of rGO have different effects on humidity sensing properties. Among them, the sensor with 2 wt% rGO-doping has a high sensitivity (37,491.2%) within the humidity range as well as the fast response time (80 s) and recover time (4 s). Furthermore, the sensor with 2 wt% rGO-doping remains good flexibility and stability in the case of bending (1000 times). The sensitivity of the 2 wt% rGO-doping sensor at the bending radius (8 mm and 4 mm) is 48,219% and 91,898%, respectively. More importantly, the sensor could reflect different breathing states clearly and track breathing intervals as short as 3 s. The SnO2/rGO flexible humidity sensor with accuracy, flexibility and instantaneity as well as the facile fabrication strategy is conceivable to be applied in the potential application for human health real-time monitoring.

Numerical simulation and optimal design of photonic crystal fiber hygrometer coated with PVA/GO composite film

In this paper, a new hygrometer sensor based on a taper-photonic crystal fiber (PCF)-dislocation structure coated with polyvinyl alcohol/graphene oxide (PVA/GO) composite film is proposed and analyzed by the beam propagation method (BPM) method and experimental measurement. Due to the moisture absorption characteristic, the refractive index (RI) of the PVA/GO composite film changes linearly, so the PCF RI interferometer can realize high sensitivity humidity detection. In order to further promote its sensing performance, we use CO2 laser lattice to optimize the sensor with a narrower linewidth and an enhanced RI sensitivity. By optimizing the sensing structure and testing the humidity sensitive film, the research results show that the humidity sensitivity can reach 0.0824 nm/%RH in the range of 20%RH-90%RH.

Carbon Nanotubes/Graphene Oxide Composite Applications in Fiber RH Sensor

We propose a high-sensitivity optical fiber relative humidity (RH) sensor based on multiwalled carbon nanotubes (MWCNTs)/graphene oxide (GO) composite, which functions as the humidity-sensitive film. The optical component of this demonstrated sensor is a single-mode fiber-solid core fiber-single-mode fiber-based multimode interference structure. For the composite sensitive film, GO is used as a dispersant for two types of MWCNTs with different lengths. Subsequently, GO-dispersed MWCNT hybrid films are formed on fiber surfaces by the dip-coating method. Herein, the hydroxylated MWCNTs can function as supporting structures to avoid the formation of layer-by-layer stacked GO, and this destacking humidity-sensitive film is beneficial to enhance water molecules' absorptive capacity. To clarify the principle, the interaction between composite humidity-sensitive film and water molecules is investigated. The experimental results reveal that the interference fringe shows a redshift with the increase in RH. When RH changing from 40% to 90%, the sensor probe with long-range MWCNT-based film has higher sensitivity than that with short-range MWCNT-based film, which demonstrates the effect of the supporting action of MWCNTs. Besides, a larger specific surface area and pore size of long-range GO-MWCNTs benefit sensitivity enhancement. In addition, this proposed sensor has good repeatability, stability, and a large linear measurement range (40% RH-90% RH).

High-performance flexible self-powered tin disulfide nanoflowers/reduced graphene oxide nanohybrid-based humidity sensor driven by triboelectric nanogenerator

Flexible self-powered humidity sensors with high response and good flexibility have attracted extensive interest in wearable electronics, motion monitoring and personal healthcare. In this work, we reported a high-performance flexible tin disulfide nanoflowers/reduced graphene oxide (SnS2/RGO) nanohybrid-based humidity sensor driven by a poly(tetrafluoroethylene) triboelectric nanogenerator (TENG). The humidity sensitive film of SnS2/RGO was screen-printed on a flexible PET substrate with Au interdigital electrodes. The TENG has a peak-to-peak voltage of 500 V and maximum output power of 378 μW. The tribotronic constant voltage source is achieved by integrating TENG with voltage rectifier and stabilizer circuits to steadily power the humidity sensor. The performance of the self-powered triboelectric humidity sensor (TEHS) at different humidity levels was systematically investigated, which exhibits high steady output voltage (0–24 V), fast response/recovery time (4 s/3 s for 33%RH, 6 s/15 s for 97%RH), wide sensing range (0–97%RH), great stability, and ultralow power consumption (29.78 μW). Furthermore, the TEHS demonstrated the potential ability of monitoring human breath with differ frequency, human cough, and finger approach in various applications for humidity sensing. This work provides a novel thought to design the self-powered humidity sensor, and also proposes the promising applications of TENG for the self-powered electronic devices.

Cu(OH)2 nanowires/graphene oxide composites based QCM humidity sensor with fast-response for real-time respiration monitoring

Respiration is an essential physiological process of human body and acts as a vital sign for personal health. Humidity sensing is a promising way to monitor respiration based on the humidity change when people breathing. Here we demonstrate Cu(OH)2 nanowires and graphene oxide (GO) based quartz crystal microbalance (QCM) humidity sensor for real-time respiration monitoring. The Cu(OH)2 nanowires/GO composites humidity-sensitive film was fabricated by the method of in-situ growth and drop-casting. The morphology and structure of the Cu(OH)2 nanowires/GO composites were characterized by energy dispersive spectrometer (EDS), Raman, Fourier transform infrared (FT-IR), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopic (TEM), high resolution TEM (HRTEM), and energy dispersive X-ray spectroscopy (EDXS). The nanocomposites with large surface area and abundant oxygen-rich functional groups lead to good humidity sensing performance. When changing relative humidity (RH) between 0 and 80%, our device exhibits high sensitivity (52.1 Hz/%RH), fast response and recovery (within 1.9 s and 7.6 s, respectively) and high linearity (R2 = 0.9968). Different breathing patterns can be distinguished, including normal breathing, apnea, speaking, nose and mouth respiration, and humidity variations before and after water intake, showing potential applications in analyzing human health-status.

A fully inkjet-printed transparent humidity sensor based on a Ti3C2/Ag hybrid for touchless sensing of finger motion.

Inkjet-printing was used to prepare a flexible and transparent humidity sensor with a Ti3C2/Ag hybrid as the humidity-sensitive film and poly(diallyldimethylammonium chloride) (PDDA) as the adhesive layer. The sensor demonstrates an ultrahigh sensitivity (106 800%), a rapid response (80 ms), and excellent bending resistance. We demonstrate that an array of sensors can track moving fingers in a non-contact way and map the distance of the fingers away from the sensor surface. Therefore, our humidity sensors have great potential for novel human-machine interfacing such as touchless control of electronics and collision control between robots and humans in a cobot setting.

Humidity Sensor Preparation by <italic>In Situ</italic> Click Polymerization

In situ methods to prepare sensitive films can improve material film-forming properties, the reproducibility and stability of the resultant films. Mercaptopropyl polyhedral oligomeric silsesquioxane, 1,4-divinylbenzene, and sodium p-styrene sulfonate were in situ crosslinked via thiol-ene click polymerization on an interdigital electrode surface to form humidity-sensitive films. The humidity-sensing properties of the sensors were systematically researched, and the optimal sensor shows a high sensitivity and minimal humidity hysteresis. The reproducibility, water resistance, and stability of the sensors prepared by this method were studied, and the sensing mechanism was investigated by complex impedance spectroscopy, equivalent circuits, and phase diagrams.

Humidity Sensor Based on Hetero-Core Structured Fiber Optic Covered with Layer-by-Layer Thin Film

Humidity sensors based on hetero-core structured fiber optic has been fabricated. In the hetero-core structured fiber optic, a short piece of singlemode fiber optic is inserted between multi-mode fibers. Layer-by-layer (LBL) technique is applied to prepare humidity sensitive film. The humidity sensitive LBL film, prepared using polyacrylate as polyanion and polyallylamine as polycation, was deposited on the singlemode fiber part of the hetero-core structured fiber optic. According to increasing of deposition cycle, the propagating loss increased because of increasing the refractive index at the sensing part by the formation of the film. After deposition procedure, the sensor part was immersed in calcium chloride solution to dope the deliquescence material in the thin film. The humidity sensors ware characterized as a function of relative humidity (RH). The sensor shows that the propagating light power increased according to increasing of RH. This is because the dry film having a higher refractive index absorbed moisture and swollen, and then the refractive index decreased resulting in the decreasing of the leakage of the propagating light at the singlemode fiber part. In addition, the increasing of calcium chloride concentration caused the enhancement of the sensitivity. This is because calcium chloride is deliquescence material and absorbed water from atmosphere, resulting in the swelling, decreasing of the refractive index at low humidity.

Polyelectrolyte humid membranes anchored to the gold surface on flexible polyimide substrate and their water durability

A novel flexible resistive-type humidity sensor was fabricated by printing a pair of comb-like gold electrodes on a polyimide (PI) film, followed by printing with copolymers made of [2-[(methacryloyloxy)ethyl] dimethyl]propylammonium bromide (MEPAB), methyl methacrylate (MMA), and 2-(cinnamyloxy)ethyl methacryate (CEMA) as a humidity-sensing film. The self-crosslinkable polyelelctrolyte copolymers contained different contents of MEPAB/MMA/CEMA = 70/27/3, 70/25/5, 70/23/7, and 70/20/10. A gold electrode was pretreated with 2-(mercaptoethyl) cinnamamide (MEC) containing a thiol-coupling agent for the purpose of anchoring the humidity-sensitive polyelectrolyte to the gold electrode on the flexible PI substrate. UV irradiation on the humidity-sensitive film stimulated a crosslinking reaction between the cinnamate and cinnamamide groups via [2π+2π] addition. After soaking in water for 60 min, the resulting sensor using MEC showed water durability and increased resistance of 42–65%, corresponding to a reduction in relative humidity of 2.25–2.98%RH. The flexible humidity sensor also showed good hysteresis (−1.1 to −0.8%RH), an acceptable response time (97 s), and good stability at high temperature and high humidity.

A novel serial high frequency surface acoustic wave humidity sensor

A novel topology of surface acoustic wave (SAW) humidity sensor was proposed. An interdigital electrode (IDE) was added in serial with a SAW resonator (SAWR) as the sensitive component. Silicon-containing polyelectrolyte humidity sensitive film was deposited on the surface of IDE instead of SAWR. By this way, the stringent technical requirements for film coating on SAW surface could be avoided, which was helpful to bring down the processing cost of the sensor. At the same time, vacuum packaged SAWR could provide a more stable and high operating frequency (433 MHz) for the sensor, which was profitable for both stability and sensitivity of the sensor. Sensitivity, stability, response-time, temperature coefficient of the novel sensor were tested and compared with the traditional SAWR sensor in our previous work. The results showed that both sensitivity (0.4 kHz/%RH at low humidity level; >1 kHz at high humidity level) and stability (noise level <10 Hz) of the sensor have got remarkable improvement for the new design.

The electrical responses to humidity of a composite of polyaniline and polyelectrolyte

Poly(dimethylaminoethyl methacrylate) and poly(glycidyl methacrylate) were simultaneously cross-linked to form a polyelectrolyte humidity sensitive film on an interdigitated gold electrode, which was further coated with a layer of soluble polyaniline (PANI). The composite so prepared was characterized by UV–vis spectroscopy and scanning electron microscopy. Its electrical response towards humidity was investigated at room temperature and compared with that of PANI and the polyelectrolyte separately. It was revealed that the composite showed an impedance as low as 1.8 × 10 5 Ω even at 0%RH. In contrast, the impedance of the polyelectrolyte at less than 20%RH was too high to be measured, and PANI showed very low humidity sensitivity. The low impedance, good sensitivity as characterized by a linear change in impedance from 1.8 × 10 5 to 9 × 10 3 Ω over 0–95%RH, fast response and small hysteresis, makes the composite a promising candidate for humidity sensors capable of detecting very low humidity.

Sensitivity Improvement of a Humidity Sensor Based on Silica Nanospheres on a Long-Period Fiber Grating

This work addresses a new configuration that improves the sensitivity of a humidity sensor based on a long-period fiber grating coated with a SiO2-nanospheres film. An intermediate higher refractive index overlay, deposited through Electrostatic Self-Assembly, is placed between the fiber cladding and the humidity sensitive film in order to increase the total effective refractive index of the coating. With this intermediate design, a three-fold improvement in the sensitivity was obtained. Wavelength shifts up to 15 nm against 5 nm were achieved in a humidity range from 20% to 80%.