Real-time Monitoring Of Humidity Research Articles

Eco-Friendly Textile-Based Wearable Humidity Sensor with Multinode Wireless Connectivity for Healthcare Applications.

Textile-based wearable humidity sensors are of great interest for human healthcare monitoring as they can provide critical human-physiology information. The demand for wearable and sustainable sensing technology has significantly promoted the development of eco-friendly sensing solutions for potential real-world applications. Herein, a biodegradable cotton (textile)-based wearable humidity sensor has been developed using fabsil-treated cotton fabric coated with a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) sensing layer. The structural, chemical composition, hygroscopicity, and morphological properties are examined using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), contact angle measurement, and scanning electron microscopy (SEM) analysis. The developed sensor exhibited a nearly linear response (Adj. R-square value observed as 0.95035) over a broad relative humidity (RH) range from 25 to 91.5%RH displaying high sensitivity (26.1%/%RH). The sensor shows excellent reproducibility (on replica sensors with a margin of error ±1.98%) and appreciable stability/aging with time (>4.5 months), high flexibility (studied at bending angles 30°, 70°, 120°, and 150°), substantial response/recovery durations (suitable for multiple applications), and highly repeatable (multicyclic analysis) sensing performance. The prospective relevance of the developed humidity sensor toward healthcare applications is demonstrated via breathing rate monitoring (via a sensor attached to a face mask), distinguishing different breathing patterns (normal, deep, and fast), skin moisture monitoring, and neonatal care (diaper wetting). The multinode wireless connectivity is demonstrated using a Raspberry Pi Pico-based system for demonstrating the potential applicability of the developed sensor as a real-time humidity monitoring system for the healthcare sector. Further, the biodegradability analysis of the used textile is evaluated using the soil burial degradation test. The work suggests the potential applicability of the developed flexible and eco-friendly humidity sensor in wearable healthcare devices and other humidity sensing applications.

High sensitivity Mach-Zehnder interferometric fiber-optic humidity sensor based on multimode interference enhancement

Variations in humidity can significantly disrupt the proper functioning of precision instruments and meters across various industrial, medical, and scientific applications, thereby necessitating real-time humidity monitoring. Here, a high sensitivity fiber-optic relative humidity (RH) sensor based on Mach-Zender interferometer (MZI) is proposed and demonstrated. The humidity sensor consists of single-mode fiber (SMF), peanut structure (PS), and multi-mode fiber (MMF). A multi-mode interference (MMI) enhanced fiber-optic MZI was constructed using a SMF-PS-SMF-MMF-SMF-PS-SMF cascade structure to detect small changes in the refractive index (RI) and expansion force of gelatin film caused by humidity variation. PS, as a beam splitter and combiner of MZI, provides the possibility for simple, economical, and efficient manufacturing of the fiber-based part of humidity sensors. The addition of MMF will excite higher-order core and cladding modes, laying the foundation for achieving high-sensitivity fiber-optic humidity sensors. The fiber-optic RH sensor prototype demonstrate a high sensitivity of about 0.633 nm/% RH in the RH range of 50 % RH to 80 % RH, and a low temperature cross-sensitivity of 0.165 % RH/°C. This sensor has the advantages of safe and easy manufacturing process, low manufacturing cost, and high sensitivity, providing continuous high accuracy humidity measurement results for humidity sensing in different fields.

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.

High-strength, antifogging and antibacterial ZnO/carboxymethyl starch/chitosan film with unique “Steel Wire Mesh” structure for strawberry preservation

In this work, a multifunctional preservative film of ZnO/carboxymethyl starch/chitosan (ZnO/CMS/CS) with the unique “Steel Wire Mesh” structure is fabricated by the chemical crosslinked of ZnO NPs, CMS and CS. Unlike traditional nano-filled polymer film, the formation of the “Steel Wire Mesh” structure of ZnO/CMS/CS film is based on the synergistic effect of ZnO NPs filled CMS/CS and the coordination crosslinked between CMS/CS and Zn2+ derived from ZnO NPs. Thanks to the “Steel Wire Mesh” structure, the tensile strength and water vapor barrier of 2.5ZnO/10CMS/CS film are 2.47 and 1.73 times than that of CS film, respectively. Furthermore, the transmittance of 2.5ZnO/10CMS/CS film during antifogging test is close to 89 %, confirming its excellent antifogging effects. And the 2.5ZnO/10CMS/CS film also exhibits excellent long-acting antibacterial activity (up to 202 h), so it can maintain the freshness and appearance of strawberries at least 5 days. More importantly, the 2.5ZnO/10CMS/CS film is sensitive to humidity changes, which achieves real-time humidity monitoring of the fruit storage environment. Note that the preparation method of the film is safe, simple and environmentally friendly, and its excellent degradation performance will not bring any problems of food safety and environmental pollution.

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.

Ultra-thin CoAl layered double hydroxide nanosheets for the construction of highly sensitive and selective QCM humidity sensor

To achieve real-time monitoring of humidity in various applications, we prepared facile and ultra-thin CoAl layered double hydroxide (CoAl LDH) nanosheets to engineer quartz crystal microbalances (QCM). The characteristics of CoAl LDH were investigated by transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectric spectroscopy (XPS), Brunauer–Emmett–Telle (BET), atomic force microscopy (AFM) and zeta potential. Due to their large specific surface area and abundant hydroxyl groups, CoAl LDH nanosheets exhibit good humidity sensing performance. In a range of 11.3% and 97.6% relative humidity (RH), the sensor behaved an ultrahigh sensitivity (127.8 Hz/%RH), fast response (9.1 s) and recovery time (3.1 s), low hysteresis (3.1%RH), good linearity (R2 = 0.9993), stability and selectivity. Besides, the sensor can recover the initial response frequency after being wetted by deionized water, revealing superior self-recovery ability under high humidity. Based on in-situ Fourier transform infrared spectroscopy (FT-IR), the adsorption mechanism of CoAl LDH toward water molecules was explored. The QCM sensor can distinguish different respiratory states of people and wetting degree of fingers, as well as monitor the humidity in vegetable packaging, suggesting excellent properties and a promising application in humidity sensing.

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.

Real-time monitoring of humidity inside concrete structures utilizing embedded smart aggregates

The moisture level of hydraulic concrete structures strongly influences their strength and durability, so the real-time monitoring of the concrete humidity is crucial to ensure the regular operation of such structures. To this end, this study used an active monitoring method to observe the concrete humidity in real time by embedding pairs of piezoelectric smart aggregates (SAs) in concrete. The monitoring platform based on smart aggregates was constructed and measured five concrete specimens of two dimensions. The time-domain signals of the concrete specimens at different water content levels were obtained, and the wavelet packet energy analysis characterized the sensor signal energy quantitatively. On this basis, a humidity index was defined to describe the variation in the humidity of the concrete specimens. The experimental results demonstrate that the humidity index is practicable to monitor and evaluate the humidity inside concrete structures. Compared with traditional methods for monitoring the moisture content, the proposed technique does not need manual external excitation and can realize real-time, lossless, and long-time monitoring of concrete humidity.

Surface Engineering on Polyimide-Silver Films in Low-Cost, Flexible Humidity Sensors.

In this work, surface engineering is applied to polyimide (PI) films to fabricate low-cost Ag/PI wireless humidity sensors with a resonant frequency of 2.45 GHz. The sensors were obtained by in situ metallization technique coupled with inkjet printing, where PI plays triple roles as a flexible substrate, ion-exchange surface, and sensing material to moisture. Moreover, the humidity sensitivity can be enhanced by the improvement of hydrophilicity via loading with different ions on the PI surface, which has been demonstrated by Fourier transform infrared (FTIR), X-ray photoelectron spectroscopy (XPS), and contact angle measurements. The wireless humidity sensor loaded with K+ ions has the maximum sensitivity of 97.7 kHz/% RH at a low relative humidity range of 20-65% and 359.7 kHz/% RH at a high relative humidity of 65-90%, respectively. Accordingly, a sensing mechanism of the fabricated humidity sensor has been discussed in detail. On the other hand, the characteristics of the humidity sensor such as response and recovery speed and stability are analyzed. The mechanical performance tests show that the humidity sensor displays excellent flexibility and good mechanical stability. A strong adhesion between the Ag antenna and PI substrate can be found as well. The passive wireless humidity sensor described in this work has the advantages of having a simple structure, low cost, high sensitivity, long-term stability, and good mechanical properties, which has potential applications in automated industry and healthcare with real-time humidity monitoring.

High-Sensitivity and Environmentally Friendly Humidity Sensors Deposited with Recyclable Green Microspheres for Wireless Monitoring.

The reliable, high-sensitive, wireless, and affordable requirements for humidity sensors are needed in high-precision measurement fields. Quartz crystal microbalance (QCM) based on the piezoelectric effect can accurately detect the mass changes at the nanogram level. However, water-capture materials deposited on the surface of QCM generally show disadvantages in either cost, sensitivity, or recyclability. Herein, novel QCM-based humidity sensors (NQHSs) are developed by uniformly depositing green microspheres (GMs) of natural polymers prepared by the chemical synthesis of the emulsification/inner gel method on QCM as humidity-sensitive materials. The NQHSs demonstrate high accuracy and sensitivity (27.1 Hz/% RH) owing to the various hydrophilic groups and porous nano-3D deposition structure. Compared with the devices deposited with a smooth film, the frequency of the NQHSs shows almost no changes during the cyclic test and exhibits long-term stability. The NQHSs have been successfully applied to non-contact sensing human activities and remote real-time humidity monitoring via Bluetooth transmission. In addition, the deposited humidity-sensitive GMs and QCM substrate are fully recycled and reused (72% of the original value). This work has provided an innovative idea to construct environmental-friendly, high-sensitivity, and wireless humidity sensors.

PI-SiO<sub>2</sub>/NiI<sub>2</sub> colorimetric humidity sensor with fast response and recovery

The organic-inorganic hybrid colorimetric humidity sensor which can obtain environmental humidity by electrical signals and color changes has broad application prospects in the field of humidity monitoring because of its high feature color discrimination, excellent stability, and simple preparation process. However, its long response-recovery time is generally not conducive to real-time humidity monitoring. In this paper, nanometer silica particles are doped into polyimide(PI)-nickel(II) iodide(NiI<sub>2</sub>) organic-inorganic hybrid materials to fabricate PI-SiO<sub>2</sub>/NiI<sub>2</sub> composite films and colorimetric humidity sensors. Then their surface morphologies and humidity sensing properties are studied. It is found that PI-SiO<sub>2</sub>/NiI<sub>2</sub> film possesses a honeycomb-like surface morphology, the humidity sensitivity of PI-SiO<sub>2</sub>/NiI<sub>2</sub> colorimetric humidity sensor is better than that of other NiI<sub>2</sub> based humidity sensors, its characteristic color is distinct in a range of 11%–97% RH humidity, and the humidity response time of the PI-SiO<sub>2</sub>/NiI<sub>2</sub> colorimetric humidity sensor is less than 1.5 s, and the recovery time is less than 18 s. The research result indicates that the doping of nanometer silica particles can effectively improve response-recovery properties of the organic-inorganic hybrid colorimetric humidity sensor, which is helpful in improving the performance of the sensor.

Wearable humidity sensors based on bar-printed poly(ionic liquid) for real-time humidity monitoring systems

Wearable chemical sensors can provide crucial human-physiology information by monitoring humidity in various environments. Capacitive-type humidity sensors based on poly(ionic liquid) (PIL) as the sensing material, which was synthesized via the polymerization of an ionic liquid (IL) were developed. The PIL films fabricated through a facile bar-printing process in ambient air, exhibited outstanding capacitive characteristics over a broad relative humidity (RH) range of 10%−80% at frequencies between 100 Hz and 1 MHz. Notably, high sensitivity (1.8 nF/%RH), good linearity (R2 = 0.980), low hysteresis, and fast response (~20 ms) of the developed humidity sensors were achieved. The corresponding sensing mechanism of the PIL for capacitive-type humidity sensors was studied by investigating the chemisorption of water molecules on the PIL surface and proton hopping through multiple layers of water molecules physisorbed on chemisorbed water molecules through single hydrogen bonding. In addition, the operational stability of the wearable humidity sensors developed in this study was investigated by analyzing the capacitive characteristics of the PIL sensing material at various temperatures (20–60 °C), even under a bending state and after 20,000 bending-stress cycles with a strain of 30%. Finally, a real-time humidity monitoring system integrated with signal processing and wireless communication modules was demonstrated by analyzing real-time capacitive signals acquired from a wearable humidity sensor attached to a human wrist and by visualizing processed humidity information on a user’s smartphone.

High sensitive and fast response humidity sensor based on polymer composite nanofibers for breath monitoring and non-contact sensing

A rapid and stable polymeric humidity sensor is demonstrated using electrospun sulfonated poly (ether ether ketone)/polyvinyl butyral (SPEEK/PVB) composite nanofiber and applied in breath monitoring and non-contact sensing for the first time. First, SPEEK was fabricated into thin film sensor by drip casting and nanofiber sensor by electrospinning, respectively. By comparing the humidity sensing characteristics of the two sensors, it is found that the hysteresis and response/recovery time of the nanofiber humidity sensor have been significantly improved. To further improve its sensing performance, we incorporated different contents of polyvinyl butyral (PVB) to the electrospinning solution to prepare SPEEK/PVB composite nanofiber humidity sensors. The morphology and diameter of nanofibers with different PVB contents were observed by Scanning Electron Microscope. The composite nanofiber humidity sensor exhibited high sensitivity, wide working range of relative humidity, small hysteresis (2.68 %), fast response time (<1 s), rapid recovery time (5 s) and stable sensing performance when the ratio of SPEEK to PVB is 1:3. In addition, the breath monitoring and non-contact sensing performance of SPEEK/PVB = 1:3 composite nanofiber humidity sensor was also studied. This work indicates that electrospun SPEEK/PVB nanofibers have good application prospects as humidity sensors for real-time humidity monitoring and non-contact sensing devices.

Self-Powered Humidity Sensor Based on Polypyrrole/Melamine Aerogel for Real-Time Humidity Monitoring

Measuring or monitoring air humidity is an increasing demands in the various fields. In this work, a simple and convenient method was utilized to prepare a melamine aerogel (MA) based sensor for humidity measurement. The sensing material was made from polymerization of polypyrrole (PPy) in the matrix of MA. The surface morphology was characterized by scanning electron microscopy (SEM) to examine the inner structure of the sensing material. MA was chosen as the substrate owing to the porous structure, which facilities the adsorption of water molecules from the air. The self-powered humidity sensor was manufactured by adhering conductive tapes on both side of PPy modified MA (PPy@MA). The sensor has a flexible, ultralight and ultraporous structure, and could generate humidity-induced open-circuit voltage from the concentration gradient of the H <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> ions. The PPy@MA sensor exhibited the response and recovery time of 1.1 s and 4.5 s, respectively, when it is used for sensing the flowing wet air (RH 75 %). Furthermore, the sensitivity of the self-powered humidity sensor was evaluated by assessing the moisture changes of the human skin, and the moisture from the human breath.

Simultaneous in situ temperature and relative humidity monitoring in mechanical ventilators using an array of functionalised optical fibre long period grating sensors

An array of optical fibre long period gratings (LPGs) has been demonstrated for biomedical application to monitor temperature and relative humidity (RH) changes in the air delivered by a mechanical ventilator operating in different modes. The LPG array consists of two gratings, where one was kept bare to monitor the temperature change and the second was modified with 10 layers of silica nanoparticles to measure relative humidity.A mesoporous film was deposited on the surface of an optical fibre LPG using the layer-by-layer method. The sensor was calibrated in a bench model against a commercially available temperature and relative humidity sensor and sensitivity of the sensor was 0.46 ± 0.01 nm/ oC and 0.53 nm/RH%, respectively.The tip of the sensor array was modified with reflection mirror and placed inside an endotracheal tube (ETT) and tested in typical clinical equipment to enable the in-situ real-time monitoring of humidity and temperature. Temperature and RH changes associated with the breathing frequency of 8 and 15 breaths per minute were successfully monitored using developed sensor array.

Effective use of biomass ash as an ultra-high humidity sensor

The paper reports a facile, green, cost-effective, and sustainable track to fabricate humidity sensors with superior performance. This frugal way make use of biomass ashes, left by combustion and annealing of natural wood pulp paper, that can emerge as an alternative source of sensing materials. In comparison to other humidity sensors, the optimized humidity sensor based on annealed tissue ash exhibited ultra-high sensitivity of six orders of magnitude (∼106), excellent selectivity (against H2, CO, CH4) and good stability (1–8 weeks) in the range of 15–90% RH at room temperature. Further characterization was carried out to elucidate the sensing mechanism, which includes chemical composition, functional group, crystal structure, surface morphology and elemental composition of the ashes by XRF, FT-IR, XRD, SEM and EDS analysis, respectively. The proposed strategy and fabricated sensors also enable the real-time humidity monitoring in human breath, which demonstrates the feasibility of its practical application ability as a flexible and wearable humidity sensor.

Enhanced Moisture-Reactive Hydrophilic-PTFE-Based Flexible Humidity Sensor for Real-Time Monitoring.

Flexible sensors connected to cell phones are a promising technology that can aid in continuously monitoring signals in our daily lives, such as an individual’s health status and information from buildings, farms, and industry. Among such signals, real-time humidity monitoring is crucial to a comfortable life, as human bodies, plants, and industrial environments require appropriate humidity to be maintained. We propose a hydrophilic polytetrafluoroethylene (H-PTFE)-based flexible humidity sensor integrated with readout circuitry, wireless communication, and a mobile battery. To enhance its sensitivity, linearity, and reliability, treatment with sodium hydroxide implements additional hydroxyl (OH) groups, which further enhance the sensitivity, create a strong linearity with respect to variations in relative humidity, and produce a relatively free hysteresis. Furthermore, to create robust mechanical stability, cyclic upward bending was performed for up to 3000 cycles. The overall electrical and mechanical results demonstrate that the flexible real-time H-PTFE humidity sensor system is suitable for applications such as wearable smart devices.

Fabrication of Porous Silicon Based Humidity Sensing Elements on Paper

A roll-to-roll compatible fabrication process of porous silicon (pSi) based sensing elements for a real-time humidity monitoring is described. The sensing elements, consisting of printed interdigitated silver electrodes and a spray-coated pSi layer, were fabricated on a coated paper substrate by a two-step process. Capacitive and resistive responses of the sensing elements were examined under different concentrations of humidity. More than a three orders of magnitude reproducible decrease in resistance was measured when the relative humidity (RH) was increased from 0% to 90%. A relatively fast recovery without the need of any refreshing methods was observed with a change in RH. Humidity background signal and hysteresis arising from the paper substrate were dependent on the thickness of sensing pSi layer. Hysteresis in most optimal sensing element setup (a thick pSi layer) was still noticeable but not detrimental for the sensing. In addition to electrical characterization of sensing elements, thermal degradation and moisture adsorption properties of the paper substrate were examined in connection to the fabrication process of the silver electrodes and the moisture sensitivity of the paper. The results pave the way towards the development of low-cost humidity sensors which could be utilized, for example, in smart packaging applications or in smart cities to monitor the environment.

A plastic optical fibre sensor for real-time humidity monitoring

Certain kinds of cellulose, such as a hydroxyethylcellulose, swell in a humidatmosphere and show a decrease in refractive index. Using this effect, a simpleoptical humidity sensor with fast response and high sensitivity, which is basedon the plastic optical fibre structure change from leaky to guided, waseasily fabricated and could be used for real-time humidity monitoring.