Humidity Sensitive Material Research Articles

Biodegradable ion-conductive polyvinyl alcohol/okra polysaccharide composite films for fast-response respiratory monitoring sensors

Polyvinyl alcohol (PVA) and okra polysaccharide (OP) are biodegradable polymers with high hydrophilicity and good biocompatibility with potential for use as flexible humidity-sensitive materials. Herein, biodegradable flexible composite films (named POP films) were prepared from PVA, OP, and phytic acid using a solution-casting method based on. POP films exhibited excellent mechanical strength, flexibility, flame retardancy, water resistance, humidity response, and humidity-sensing characteristics. Notably, the POP humidity sensors exhibited a hysteresis value of 1.88 % relative humidity for the adsorption and desorption processes and good sensitivity over a wide humidity range of 35–95 %. In addition, the humidity sensor distinguished the frequency of nose breathing, and its response and recovery times were 0.9 and 1.98 s, respectively. The excellent performance of POP sensors in monitoring humidity and human respiratory rates demonstrates the sensor's potential for wearable smart devices.

Nafion/polyimide based programmable moisture-driven actuators for functional structures and robots

Sustainable and environmentally friendly actuators powered by humidity, light and magnetic field are of great significance to facilitate application of microrobots. Among them, moisture-driven actuators have attracted a growing interest as a result of the widespread presence and ease of use of humidity. Nevertheless, the rapid acquisition and large-scale application of the moisture-driven actuators is still difficult since synthesis process of humidity-sensitive materials is time-consuming and complex. In this paper, we proposed a facile and rapid method to prepare double-layer moisture-driven actuators by integrating commercial humidity-sensitive Nafion ™ membrane and polyimide (PI) tape with good stability. Compared with other double-layer moisture-driven actuators, the so-obtained actuator has excellent performance in both the bending curvature (from −0.98 cm−1 to 5.34 cm−1) and the response speed (0.29 cm−1/s). Through programmable structure design, a series of functional structures with complex deformation modes, i.e. torsion and bending, were realized. By imitating organisms like birds, vines, inchworms and ants, a series of soft robots have been developed based on the programmable structures to achieve behaviors including grasping, crawling and weight-bearing. The proposed programmable moisture-driven actuators have shed light on the fast development of environmental-friendly functional microrobots making use of ambient humidity.

Self-Powered Wearable Breath Sensor Cum Nanogenerator Using AuNR-rGO-PVDF Nanocomposite

Breathing is the most important function of living being. It is well known that breath contains 90% humidity along with various gases and volatile organic compounds (VOCs). These gases are released during various metabolic activities in the body. These gases present in the breath can act as biomarkers to showcase healthiness or abnormalities in the body. Therefore, breath sensors are the need of time to detect diseases at early stages and in noninvasive way. Herein, we report, simple breath sensor using Au nanorods (AuNRs) and its nanocomposite with reduced graphene oxide (rGO) and polyvinylidene fluoride (PVDF). The nanocomposites system is designed in such a way that each component has its advantages for breath sensing. Here, AuNR being a good humidity sensitive material detects the breath efficiently. The <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\beta $ </tex-math></inline-formula> phased PVDF being a very good ferroelectric material, contributes for voltage generation during sensing and enables the device to be self powered. The highly conducting rGO increases the sensitivity and voltage generation by facilitating the electron transport in the nanocomposite system. The nanocomposite was optimized with respect to PVDF, rGO and Au concentration. The as-synthesized materials were characterized by physiochemical characterization techniques such as field emission scanning electron microscopy (FESEM), X-ray diffraction analysis (XRD), UV-Visible spectroscopy, Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\beta $ </tex-math></inline-formula> phase formation of the PVDF was confirmed from the XRD peak at 20.41°. SEM results revealed that AuNR have length 15–20 nm and aspect ratio <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim $ </tex-math></inline-formula> 3–4. Breath sensing tests were carried out for as synthesized nanocomposite material. The nanocomposite exhibited high sensitivity towards breath and generated the voltage of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim $ </tex-math></inline-formula> 0.7 V within 0.8 s.

Paper-Based Humidity Sensors as Promising Flexible Devices: State of the Art: Part 1. General Consideration.

In the first part of the review article "General considerations" we give information about conventional flexible platforms and consider the advantages and disadvantages of paper when used in humidity sensors, both as a substrate and as a humidity-sensitive material. This consideration shows that paper, especially nanopaper, is a very promising material for the development of low-cost flexible humidity sensors suitable for a wide range of applications. Various humidity-sensitive materials suitable for use in paper-based sensors are analyzed and the humidity-sensitive characteristics of paper and other humidity-sensitive materials are compared. Various configurations of humidity sensors that can be developed on the basis of paper are considered, and a description of the mechanisms of their operation is given. Next, we discuss the manufacturing features of paper-based humidity sensors. The main attention is paid to the consideration of such problems as patterning and electrode formation. It is shown that printing technologies are the most suitable for mass production of paper-based flexible humidity sensors. At the same time, these technologies are effective both in the formation of a humidity-sensitive layer and in the manufacture of electrodes.

Noncontact sensing for water area scanning identification based on Ho2O3/GO humidity sensor

With the advent of the era of intelligent perception and artificial intelligence, higher requirements on the response/recovery speed of humidity sensitive materials and the expansion of new applications of humidity sensors have been put forward. In this study, Ho2O3/GO composites were obtained by ultrasonic stirring of the GO solution with the synthesized Ho2O3 material. Benefiting from the presence of adsorbed hydroxyl oxygen on the surface and the regular arrangement of the nanosheets, the humidity response of Ho2O3/GO sensor has nearly doubled from 1102 to 2136, and the recovery time was reduced by more than half from 6 s to 1 s. Based on excellent humidity sensitive properties, the sensor was used for non-contact scanning to identify water zones of different shapes, thus presenting information on the distribution of water beneath the sensor. This research opens up further possibilities for the expansion of humidity sensors in emerging applications such as smart sensing.

Ultrafast and Sensitive Hydrophobic QCM Humidity Sensor by Sulfur Modified Ti3C2Tx MXene

MXenes, 2-D transition metal carbides, nitrides, and carbonitrides, have developed to be attractive nanomaterials for humidity-sensitive materials due to their abundant active sites, tunable surface chemistry, and outstanding stability. Nevertheless, metallic conductivity also makes it hard to extract the signal change from semiconductor resistive gas sensors. Here, we introduce quartz crystal microbalance (QCM) as a humidity sensor platform to collect the signal of MXene mass change directly replacing electronic signal detection. Surface modification was taken place for changing the surface functional groups during hydrofluoric acid (HF) etching for enhancing more humidity sensing performance of Ti3C2Tx MXene. Sulfurized Ti3C2Tx MXene QCM sensors show the highest humidity sensing performance than as-synthesized Ti3C2Tx MXene from 6 Hz/% up to 105 Hz/% with an ultrafast response and recovery time (13/6 s). Also, it was calculated up to 20 times more water absorption ratio compared to mass change of the as-synthesized Ti3C2Tx MXene, showing potential for high-performance and fast humidity sensors.

FLEXIBLE HUMIDITY SENSORS BASED ON NANOCELLULOSE FOR WEARABLE ELECTRONICS

Problem. Humidity control is necessary for many areas of modern human life, including agriculture, food and automotive industries, electronics manufacturing, medicine and everyday life. A new trend in medical sensors is the use of flexible wearable sensors that can be attached to a person's body or the clothing adjacent to it. Such sensors are able to repeat the shape of the body and deform as needed, without breaking or causing discomfort to the patient. Today, nanocellulose (NC) is a rather promising humidity-sensitive material, which is characterized by high hydrophilicity and sufficient flexibility, and at the same time it is a biodegradable material.&#x0D; The purpose of the work. Development and research of flexible humidity sensors based on nanocellulose as a moisture-sensitive film, depending on the nature of the raw material and the method of NC manufacturing, as well as the configuration of the device electrodes.&#x0D; Research results. Three groups of humidity sensors were manufactured: capacitive sensors with electrodes in the form of expanded capacitor; resistive sensors made on the base of interdigital electrodes; resistive sensors made on the base of planar-parallel electrodes. In each configuration, 4 different nanocellulose hydrogels were used, made from reed or wheat straw by hydrolysis or TEMPO methods. The static and dynamic characteristics of humidity sensors were investigated, and conclusions were drawn regarding the optimal type of electrode configuration, raw materials and the method of NC synthesis. The largest sensitivity and the smallest hysteresis are demonstrated by the sensor made on the basis of an interdigital electrode and nanocellulose, synthesized by the TEMPO method - 0.164 (%RH)-1 and 1.5%, respectively. However, the high speed (response time - 6 s, recovery time - 10 s), short-term stability (deviation during device measuring under constant humidity for 1 h - 1.4%) and repeatability of results (deviation during cycling between different humidity levels - 1.6%) are observed for sensors, manufactured by the hydrolysis method, and for both types of sensors (resistive and capacitive device).&#x0D; Conclusions. It was established that the static parameters (response, sensitivity and reversibility) of humidity sensors depend mainly on the type of electrode configuration and the source material for the NC synthesis, while dynamic parameters (repeatability during cycling, short-term stability, response time and recovery time) depend on NC synthesis method. At the same time, it was shown that resistive sensors demonstrate significantly better parameters of sensitivity and hysteresis compared to capacitive devices. The obtained flexible humidity sensors can be used in wearable medical electronics.

Simultaneous humidity and temperature measurement sensor based on two cascaded long-period fiber gratings

An optical fiber sensor based on two cascaded long-period fiber gratings (LPFG) for simultaneous measurement of humidity and temperature is proposed. In the cascade structure, the resonant peak wavelengths of the two LPFGs (i.e. LPFG1 and LPFG2) are in different bands, in which the resonant peak wavelengths of LPFG1 are between 1325 nm and 1465 nm, and the resonant peak wavelengths of LPFG2 are between 1470 nm and 1575 nm. Then, the humidity sensitive material and thermal sensitive material are combined with LPFG1 and LPFG2 respectively to achieve high-sensitivity humidity and temperature measurement. In our sensor structure, the mixture of humidity sensitive material polyimide (PI) and graphene quantum dots (GQDs) was coated on the surface of LPFG1, and the humidity sensitivity of 78 pm/%RH was obtained in the range of 45%RH∼75%RH. Then, the surface of LPFG2 was encapsulated with dimethyl silicone oil (DSO) thermal sensitive material, and the temperature sensitivity of LPFG2 reached 445 pm/∘C. Finally, the sensitivity measurement matrix is constructed by using the temperature and humidity sensitivities obtained by two LPFG, which can realize the simultaneous measurement of humidity and temperature and eliminate the cross-sensitivity. The sensor has the advantages of simple structure, good stability, low cost, high sensitivity and double parameter measurement, and it has a certain practical application prospect.

A high-sensitive microwave humidity sensor based on one-step laser direct writing of dielectric silver nanoplates

Microwave technology has emerged as an appealing alternative for building humidity sensors with multiple detection parameters. This paper demonstrates the one-step laser direct writing fabrication of microwave humidity sensors. Silver nanoplates were used as both the humidity-sensitive material and a precursor to highly conductive Ag electrodes. Particularly, the unirradiated silver nanoplates film shows an unusual lossy dielectric nature, with the effective complex dielectric permittivity being dramatically tuned by humidity stimuli. Through a laser-induced sintering mechanism, laser irradiation bursts the conductivity by approximately 10 orders of magnitude with a maximum conductivity value of 9.7 × 106 S/m. A prototype microwave humidity sensor is designed and fabricated and its humidity sensing performances are comparatively studied and discussed. The measured results show that the sensor displays a high humidity sensitivity of up to 12.61 MHz/RH or 0.288 dB/RH. This work suggests a good candidate approach for the realization of high-performance microwave humidity sensors with a fast and simplified advantage.

Fast-Response Low Humidity Sensor Based on Ionic Liquid Functionalized MOF-801 by Solvent-Assisted Linker Exchange

In this work, strongly hydrophilic ionic liquid (IL) was introduced into the metal organic frameworks (MOFs) by the solvent-assisted linker exchange (SALE) method. Low humidity sensors based on IL functionalized MOF-801 materials were prepared. The as-prepared sensor shows a high response of 20.3 (in the humidity range of 0% RH to 35% RH, 0 ppmv to 8138 ppmv), small hysteresis (~1% RH) and good long-term stability (>100 days). In particular, the optimized sensor shows fast response (~400 ms) and high resolution (1% RH). This work provides a potential humidity sensitive material for preparing high performance low humidity sensors.

Performance analysis of humidity sensor prepared by CeO2/molecular sieves type 4A composite

In this paper, CeO2/molecular sieves type 4A composite (CeO2/4A) was prepared through a simple method and was then used as a humidity-sensitive material. The humidity sensor prepared of CeO2/4A exhibits more than five orders of magnitude of impedance variation at 200 Hz in the relative humidity (RH) range from 11 % to 95 %. The CeO2/4A sensor shows a good humidity-sensitive response value of S = 1.04 × 105, appropriate response/recovery time (9/80 s within 5 min), and high long-term stability. In addition, the complex impedance plots of the CeO2/4A sensor have been measured to understand the humidity sensing mechanism. The present investigation demonstrates that CeO2/4A is of great potential humidity-sensitive material.

Recent Studies on the Humidity Sensor: A Mini Review

For the time being, a number of humidity measurement instruments made of a variety of different materials and operating on a number of different principles are accessible. There are numerous ways that may be utilized to categorize them. I begin by discussing the most commonly used designs for flexible humidity sensors. Following that, the section on humidity-sensitive materials, the sensing mechanism, the major properties of humidity sensors, and the influence of temperature on humidity sensors are discussed. The study concludes with a review of current research in this area, as well as the challenges that exist. The majority of the studies included in this review were published between 2020 and 2022. These remarks are expected to serve as an overview to humidity measurement for researchers and graduate students who are new to the field or who are already working in it and would like to learn more about the issue.

Fabry-Pérot cavity based on polyimide cold-spliced and Vernier effect for relative humidity sensing application

Utilizing humidity sensitive material polyimide (PI) and optical Vernier effect, a humidity sensor with high sensitivity is fabricated and verified by experiments. The sensor consists of two Fabry-Perot interferometers (FPIs) in parallel. The Fabry-Perot (F-P) cavity of FPI1 is composed of two single mode fibers (SMF) cold spliced through PI, which is a PI cavity. When the ambient humidity rises, the expansion of PI after absorbing water molecules directly changes the F-P cavity length of FPI1, so FPI1 is particularly sensitive to humidity. The relative humidity (RH) sensitivity of a single FPI1 is measured to be 0.5361 nm/%RH. The F-P cavity of FPI2 is composed of "SMF - quartz capillary - SMF" in turn hot splicing, which is an air cavity. FPI2 with air cavity is insensitive to RH. The FSRs of FPI1 and FPI2 are controlled to approach, and the Vernier effect is generated after they are connected in parallel, which further amplifies the RH sensitivity of FPI1. When using Vernier effect to measure RH, the RH sensitivity of the sensor is as high as − 7.6221 nm/%RH, which increases the RH sensitivity of a single FPI1 by nearly 14.2 times. Moreover, the temperature cross sensitivity of the sensor is about 0.067%RH/°C, which is very small. This sensor has the advantages of easy fabrication, low cost, high sensitivity, good repeatability and stability, and fast response.

Simultaneous Measurement of Humidity and Temperature Based on ZnO-Coated Hollow Core Bragg Fiber

An optical fiber sensor for simultaneous measurement of relative humidity (RH) and temperature is proposed, which is constructed by sandwiching one section of hollow core Bragg fiber (HCBF) between two sections of single mode fiber (SMF). The ultrathin ZnO film is coated on the outer surface of HCBF as humidity-sensitive material. The dips in the transmission spectrum caused by different anti-resonant (AR) modes of HCBF have different sensitivities to the RH and temperature, and then the simultaneous measurement of above two parameters can be realized by solving a cross matrix. For the two dips monitored in the experiments, the sensitivities of the proposed sensor to RH are 0.7 pm/%RH and 16.95 pm/%RH in the range of 46–73 %RH, and the sensitivities to temperature are 25 pm/°C and 26.8 pm/°C in the range of 20-60 °C, respectively. Taking advantage of compact structure, good stability, easy fabrication and the proposed sensor exhibits great potential application in fields of chemical sensors and biochemical detection.

Compact breath monitoring based on helical intermediate-period fiber grating

Tiny and soft breath sensors offer unique capabilities in a continuous and long-term recording of vital physiological parameters for human health monitoring. Here, we propose an all fiber-optic breath sensor based on helical intermediate-period fiber grating (E-HIPFG) written in an elliptic core polarization-maintaining fiber. This E-HIPFG-based breath sensor shows ultrafast response time and recovery time, which are 34.46 ms and 41.34 ms, respectively. The response time and recovery time are three orders faster than the corresponding values of conventional optical fiber-based breath sensors with humidity-sensitive material. In addition, this sensor does not combine with any additional coating materials or fiber architectures, increasing the stability in a continuous and long-term detection for breath monitoring. The E-HIPFG sensor is insensitive to external influences, such as temperature, torsion and strain, endowing the possibility of stable detection in the moving body. Particularly, the temperature sensitivity of the sensor is 2.75 pm/°C, which is two orders lower than the corresponding value of previous reports. Due to its considerable stability, quick response, and insensitivity to external influences, the E-HIPFG-based breath sensor offers significant potential for long-term and compact breathing monitoring devices.

Guar Gum/Ethyl Cellulose-Polyvinyl Pyrrolidone Composite-Based Quartz Crystal Microbalance Humidity Sensor for Human Respiration Monitoring.

In this work, the guar gum (GG) and the electrospinned ethyl cellulose-polyvinyl pyrrolidone (EC-PVP) nanofibers were used as humidity-sensitive materials for fabricating a quartz crystal microbalance (QCM) sensor. Fourier transform infrared spectroscopy, scanning electron microscopy, water contact angle test, and X-ray photoelectron spectra were used to characterize the synthesized GG/EC-PVP composite material, confirming its successful preparation and good hydrophilicity. The humidity sensitivity experiments were performed at room temperature. The GG/EC-PVP-coated QCM sensor has high sensitivity (55.72 Hz/%RH) and low hysteresis (2.8% RH) in a wide relative humidity range (0-97% RH), short response/recovery time (26/2 s), excellent selectivity, good repeatability, and stability. The combined action of hydrophilic groups and porous structure enhances the humidity sensitivity. The GG/EC-PVP sensor can be used to capture and measure typical breathing patterns in different human basic emotions due to its good performance. Furthermore, a lie-detector system was also designed for judging the lying through detecting the emotional breathing pattern of the subjects.

High-performance humidity sensor for multipurpose applications by recycling of potato peel bio-waste

This study suggests the development of a biocompatible and eco-friendly humidity sensor fabricated by using a naturally available potato peel bio-waste as the substrate and the humidity sensitive material. Entire fabrication process of this potato peel-based humidity sensor (PPHS) took less than two minutes owing to the facile and handmade patterning of silver interdigitated electrodes on the surface of dry potato peel. The fabricated humidity sensor yielded a high sensitivity (70 kΩ/ % RH) due to the presence of abundant polar hydrophilic functional groups. The achieved results were stable for more than seven days and repeatable data was obtained for three different samples. Furthermore, a quick response/recovery time of 0.96/1.65 s was recorded for nose breathing with a small hysteresis of only 0.21 % for adsorption and desorption process. The sensor exhibited a good linearity (R2 = 0.997) in the practical applications-based humidity sensing range of 40–85 % RH. Efficient use of PPHS was successfully demonstrated in multiple applications including medical diagnosis, skin care, real-time environment monitoring and touchless sensing owing to its biocompatibility and nonhazardous nature. In addition to the above listed multifunctional applications of PPHS, economic use of potato peel in commercial devices can help in recycling the worldwide wastage of this bio-waste.

Research Progress on Humidity-Sensing Properties of Cu-Based Humidity Sensors: A Review

Novel humidity sensors based on semiconducting metal oxides with good humidity-sensing properties have attracted extensive attention, which due to their high sensitivity at room temperature, high safety, low hysteresis, and long-term stability. As a typical p-type semiconductor metal oxide, CuO is considered to be a high-performance moisture-sensitive material; however, with the development of production, the complex working environment has put forward higher requirements for its humidity sensitivity, especially sensitivity and stability. In this regard, workers around the world are working to improve the moisture-sensing properties of sensing elements. In this review, the humidity-sensing properties of CuO-based moisture-sensitive materials are comprehensively summarized, focusing on effective measures to improve the moisture-sensing properties of CuO-based moisture-sensitive materials, including surface modification and nanocomposites. The future research of semiconducting metal oxide humidity-sensitive materials is also prospected.

Sensors for simultaneous measurement of temperature and humidity based on all-dielectric metamaterials.

In this study, a new type of sensors based on all-dielectric metamaterials that can measure temperature and relative humidity simultaneously was designed and theoretically analyzed in detail. The proposed metamaterial sensor consists of a quartz substrate in the bottom layer, polydimethylsiloxane (PDMS) in the middle layer, and a periodic silicon structure on the top layer. CST Studio Suite was used to determine the transmission spectrum of the metamaterials in the near-infrared band using finite integration, and two transmission dips were observed. Then, polyvinyl alcohol (PVA) was used as the humidity-sensitive material to be coated on the surface of this metamaterial sensor, and these two transmission dips were used to measure the temperature and relative humidity simultaneously. Simulation results showed that the sensitivities of the two dips to the temperature are -0.224 and -0.069 nm/°C, and the sensitivities to the relative humidity are -0.618 and -0.521 nm/%, respectively. Based on the sensitivity matrix, the temperature and the relative humidity can be measured simultaneously. The proposed sensor has the advantages of polarization insensitivity, small size and low loss, which makes it have many application potentials in various research fields, including physics, biology and chemical sensing.

High sensitivity chipless RFID humidity sensor tags are based on SnO2/G nanomaterials

In this study, a chipless RFID humidity sensor tag based on SnO2/G (Graphene) nanomaterial with recognition and sensing functions is proposed. Chip-less RFID sensors are usually composed of tag ID and sensing materials. In this paper, similar nano-spherical SnO2 powder was generated by hydrothermal method and doped with G to get SnO2/G mixture. For chip-free RFID tags, a microstrip coupled line resonator is proposed, which consists of a coupled line segment and three line segments of different shapes. Among them, u-shaped resonators coated with humidity sensitive materials for humidity sensing, and the other two resonators can code four different states. Through testing the sensitivity, hysteresis, repeatability and response characteristics of humidity sensor, it is proved that humidity sensitive material has excellent humidity sensing performance. At 11%-98%RH, the frequency offset is up to 257 MHz, achieving a sensitivity of 2.95MHz/%RH. HFSS simulation and experimental results verify that the proposed integration of humidity sensor and RFID technology has great application potential.