Laser direct writing of Ga_(2)O_(3)/liquid metal-based flexible humidity sensors

Flexible and wearable humidity sensors play a vital role in daily point-of-care diagnosis and noncontact human-machine interactions.However,achieving a facile and high-speed fabrication approach to realizing flexible humidity sensors remains a challenge.In this work,a wearable capacitive-type Ga_(2)O_(3)/liquid metal-based humidity sensor is demonstrated by a one-step laser direct writing technique.Owing to the photothermal effect of laser,the Ga_(2)O_(3)-wrapped liquid metal particles can be selectively sintered and converted from insulative to conductive traces with a resistivity of 0.19Ω·cm,while the untreated regions serve as active sensing layers in response to moisture changes.Under 95%relative humidity,the humidity sensor displays a highly stable performance along with rapid response and recover time.Utilizing these superior properties,the Ga_(2)O_(3)/liquid metal-based humidity sensor is able to monitor human respiration rate,as well as skin moisture of the palm under different physiological states for healthcare monitoring.

Surface Acoustic Wave Humidity Sensors Based on(1120) ZnO Piezoelectric Films Sputtered on R-Sapphire Substrates

ZnO films on R-sapphire substrates are prepared and characterized by x-ray diffraction and scanning electron microscopy, which indicate that the thin films are well crystallized with （1120） texture. Love wave and Rayleigh wave are used for fabrications of humidity sensors, which are excited in [1100] and [0001] directions of the （1120） ZnO piezoelectric films, respectively. The experimental results show that both kinds of sensors have good humidity response and repeatability, and the performances of the Love wave sensors are better than those of the Rayleigh wave sensors at room temperature. Moreover, the theoretical calculations of the mass sensitivity of the sensors are a/so carried out and the calculated results are in good agreement with the experimental measurements.

Functionalized Hydrogel-Based Wearable Gas and Humidity Sensors

Breathing is an inherent human activity;however,the composition of the air we inhale and gas exhale remains unknown to us.To address this,wearable vapor sensors can help people monitor air composition in real time to avoid underlying risks,and for the early detection and treatment of diseases for home healthcare.Hydrogels with three-dimensional polymer networks and large amounts of water molecules are naturally flexible and stretchable.Functionalized hydrogels are intrinsically conductive,self-healing,self-adhesive,biocompatible,and room-temperature sensitive.Compared with traditional rigid vapor sensors,hydrogel-based gas and humidity sensors can directly fit human skin or clothing,and are more suitable for real-time monitoring of personal health and safety.In this review,current studies on hydrogel-based vapor sensors are investigated.The required properties and optimization methods of wearable hydrogel-based sensors are introduced.Subsequently,existing reports on the response mechanisms of hydrogel-based gas and humidity sensors are summarized.Related works on hydrogel-based vapor sensors for their application in personal health and safety monitoring are presented.Moreover,the potential of hydrogels in the field of vapor sensing is elucidated.Finally,the current research status,challenges,and future trends of hydrogel gas/humidity sensing are discussed.

Orange dye polyaniline composite based impedance humidity sensors

This study presents the fabrication and investigation （PANI） composite films. A blend of 3 wt.% OD with 1 of humidity sensors based on orange dye （OD） and polyaniline wt.% PANI was prepared in 1 ml water. The composite films were deposited on glass substrates between pre-deposited silver electrodes. The gap between the electrodes was 45 um. The sensing mechanism was based on the impedance and capacitance variations due to the absorption/desorption of water vapor. It was observed that with the increase in relative humidity （RH） from 30% to 90%, the impedance decreases by 5.2 × 10^4 and 8.8 × 10^3 times for the frequencies of 120 Hz and 1 kHz, respectively. The impedance-humidity relationship showed a more uniform change compared to the capacitance-humidity relationship in the RH range of 30% to 90%. The consequence of annealing, measuring frequency, response and recovery time, and absorption-desorption behavior of the humidity sensor were also discussed in detail. The annealing resulted in an increase in sensitivity of up to 2.5 times, while the measured response time and recovery time were 34 s and 450 s, respectively. The impedance-humidity relationship was simulated.

Recent progress of diversiform humidity sensors based on versatile nanomaterials and their prospective applications

Humidity sensors are of significance in various fields,such as environmental and food quality monitoring,industrial processing,wearable and flexible electronics,and human health care.High-performance humidity sensors with high sensitivity,rapid response time,and good stability are of paramount importance in humidity sensing.In this paper,diversiform humidity sensors with different sensing mechanisms are summarized,including resistive,impedance,capacitive,quartz crystal microbalance(QCM),surface acoustic wave(SAW),field-effect transistor(FET),and optical fiber humidity sensors.Versatile nanomaterials such as graphene,transition-metal chalcogenide,MXenes,black phosphorus(BP),boron nitride(BN),polymers,and nanofibers were promising building-blocks for constructing humidity sensors.The latest progress in the wearable and flexible humidity sensors,and self-powered humidity sensors was summarized.The diversiform applications of the humidity sensors with great prospects were demonstrated in various fields in terms of human respiratory monitoring,skin dryness diagnosing,fingertip approaching,and non-contact switch.Moreover,the challenges and prospects of nanomaterials-based humidity sensors were discussed.

Self-assembled graphene oxide/polyethyleneimine films as high-performance quartz crystal microbalance humidity sensors

As humidity is one of the most widely demanded environmental parameters,the precision of its detection is significant.An advanced humidity sensor will improve the validity of the humidity monitoring system.In this study,a facile chemical layer-by-layer self-assembly(CLS)method was developed for fabricating graphene oxide(GO)/polyethyleneimine(PEI)multilayer films.Owing to the chemical bonding between the PEI and GO,and the intrinsic stickiness of the PEI,layered films with different numbers of layers were successfully prepared using the CLS method and confirmed through ultraviolet-visible(UV-Vis)spectroscopy and the mass loading of quartz crystal microbalance(QCM).Morphological measurements revealed that the roughness and thickness of the films increased exponentially with the number of bilayers.The GO/PEI films were deposited on QCM electrodes using the CLS method to produce the humidity sensors.The humidity measurement results showed a high sensitivity(37.84 Hz/%RH)and rapid response/recovery(<5 s/8 s)of the optimal sensor,which was superior to that of recently developed QCM sensors.

Capacitive-Resistive Measurements of Cobalt-Phthalocyanine Organic Humidity Sensors

In this study, the fabrication and characterization of capacitive humidity sensors using cobalt-phthalocyanine （CoPc） as the active material were presented. Thin films of CoPc were deposited by drop casting on glass substrates with pre-deposited aluminum electrodes to form A1/CoPc/A1 surface-type humidity sensors. The effect of humidity on the electrical properties of the CoPc film was investigated by measuring capacitance and resistance of the samples at four different frequencies of the applied voltage. It was observed that the capacitance of the sensor increased while the resistance decreased with raising the relative humidity. It was also found that the values of capacitance and resistance decreased with increasing frequency. The optical absorption spectra and optical band gap energy of CoPc films were measured. The structure of CoPc powder and thin films has been studied by X-ray diffraction （XRD）, scanning electron microscopy （SEM）, and atomic force microscopy （AFM）. Results of XRD studies show that the film structure is polycrystalline with the monoclinic structure while thin films have a peak for annealing temperatures with （100） orientation. Also, the surface morphology （grain size and roughness） for CoPc films have been studied by AFM.

Highly sensitive humidity sensors based on hexagonal boron nitride nanosheets for contactless sensing

Humidity sensors with high sensitivity,rapid response,and facile fabrication process for contactless sensing applications have received considerable attention in recent years.Herein,humidity sensors based on hexagonal boron nitride(h-BN)nanosheets that are synthesized by a facile ultrasonic process have been fabricated,which display an ultrahigh sensitivity of 28,384%at 85%relative humidity(RH),rapid response/recovery time(3.0/5.5 s),and long-term stability in a wide humidity detection range(11%-85%RH),superior to most of the reported humidity sensors.The high sensitivity can be ascribed to the massive hydrophilic functional groups absorbed on the h-BN nanosheet surface.Benefiting from the high humidity sensing performances,contactless Morse code messaging and human writing and speech recognition have been demonstrated.This work demonstrates the great potential of the high-performance h-BN nanosheet-based humidity sensors for future contactless sensing devices.

Piezotronic effect enhanced Schottky-contact ZnO micro/nanowire humidity sensors

A ZnO micro/nanowire has been utilized to fabricate Schottky-contacted humidity sensors based on a metal-semiconductor-metal （M-S-M） structure. By means of the piezotronic effect, the signal level, sensitivity and sensing resolution of the humidity sensor were significantly enhanced when applying an external strain. Since a higher Schottky barrier markedly reduces the signal level, while a lower Schottky barrier decreases the sensor sensitivity due to increased ohmic transport, a 0.22% compressive strain was found to optimize the performance of the humidity sensor, with the largest responsivity being 1,240%. The physical mechanism behind the observed mechanical-electrical behavior was carefully studied by using band structure diagrams. This work provides a promising way to significantly enhance the overall performance of a Schottky-contact structured micro/nanowire sensor.

A review on Ti_(3)C_(2)T_(x)-based nanomaterials:synthesis and applications in gas and humidity sensors

Ti_(3)C_(2)T_(x),which is a novel two-dimensional(2 D)material,has received enormous interest in the field of sensor technology due to its large surface area,excellent electrical conductivity,and abundant active surface sites.In recent years,several Ti_(3)C_(2)T_(x)-based gases and humidity sensors have been developed and reported.In this review,we focus on the latest applications of Ti_(3)C_(2)T_(x)-based nanomaterials in gas and humidity sensors.First,the synthesis of Ti_(3)C_(2)T_(x) from the dangerous fluorine-containing etching process to the safe fluorine-free preparation method was discussed,and the structures of the Ti_(3)C_(2)T_(x) controlled by different delamination methods were also outlined.Subsequently,the functionalization of pristine Ti_(3)C_(2)T_(x) and composite strategies for enhancing its gas and humidity sensing performance were reviewed.In addition,the gas and humidity sensing mechanisms of sensors based on Ti_(3)C_(2)T_(x) were also summarized.Finally,the challenges and opportunities for the use of Ti_(3)C_(2)T_(x) gas and humidity sensors were discussed to provide guidance on the promising potential of Ti_(3)C_(2)T_(x) in this field.

Characterization of vanadyl phthalocyanine based surface-type capacitive humidity sensors

This study presents the fabrication and characterization of novel surface-type capacitive humidity sensors using vanadyl phthalocyanine（VOPc） as the active material.The devices,which comprise three different thicknesses, have been fabricated using the thermal evaporation technique.A thin film of VOPc is deposited on thoroughly cleaned glass substrates with pre-patterned Ag electrodes.The capacitive effect of the samples under humidity has been investigated. Comparison of the samples with different thicknesses shows that the thinnest device seems more sensitive towards humidity.The humidity dependent capacitance properties of the sensor make it beneficial for use in commercial hygrometers.

Fabrication of Humidity Sensors Based on Laser Scribed Graphene Oxide/SnO Composite Layers

Humidity sensors have been widely applied to detect environment humidity in various fields. However, most of humidity sensors cannot provide performance needed for high sensitivity and fast response. We report one type of capacitive-type humidity sensors composed of laser-scribed graphene(LSG) as sensing electrodes and graphene oxide/tin dioxide(GO/SnO2) as a sensing layer. The LSG is reduced graphene oxide(rGO) electrodes resulted from selective reducing of GO within a GO/SnO2 composite layer by laser scribing method, and the sensing layer is the un-scribed GO/SnO2 composite. The sensor fabrication is a one-step process which is facile and cost-efficient. When a mass ratio of GO:SnO2 in the composite reaches 1:1, the humidity sensor(named as LSG-GS1) has the best properties than other ratios, which exhibits high sensitivity in the range of 11%~97% relative humidity(RH). In addition, the LSG-GS1 also has very quick response/recovery time(20 s for adsorption and 18 s for desorption) when RH changes from 23% to 84%, and very good stability after monitoring for 41 days. Such excellent performances of the humidity sensor can be attributed to synergistic effect of SnO2 and GO within the composite layer.

Advanced flexible humidity sensors:structures,techniques,mechanisms and performances

Flexible humidity sensors are widely used in many fields,such as environmental monitoring,agricultural soil moisture content determination,food quality monitoring and healthcare services.Therefore,it is essential to measure humidity accurately and reliably in different conditions.Flexible materials have been the focusing substrates of humidity sensors because of their rich surface chemical properties and structural designability.In addition,flexible materials have superior ductility for different conditions.In this review,we have summarized several sensing mechanisms,processing techniques,sensing layers and substrates for specific humidity sensing requirements.Aadditionally,we have sorted out some cases of flexible humidity sensors based on different functional materials.We hope this paper can contribute to the development of flexible humidity sensors in the future.

Multifunctional Flexible Humidity Sensor Systems Towards Noncontact Wearable Electronics

In the past decade,the global industry and research attentions on intelligent skin-like electronics have boosted their applications in diverse fields including human healthcare,Internet of Things,human–machine interfaces,artificial intelligence and soft robotics.Among them,flexible humidity sensors play a vital role in noncontact measurements relying on the unique property of rapid response to humidity change.This work presents an overview of recent advances in flexible humidity sensors using various active functional materials for contactless monitoring.Four categories of humidity sensors are highlighted based on resistive,capacitive,impedance-type and voltage-type working mechanisms.Furthermore,typical strategies including chemical doping,structural design and Joule heating are introduced to enhance the performance of humidity sensors.Drawing on the noncontact perception capability,human/plant healthcare management,human-machine interactions as well as integrated humidity sensor-based feedback systems are presented.The burgeoning innovations in this research field will benefit human society,especially during the COVID-19 epidemic,where cross-infection should be averted and contactless sensation is highly desired.

Humidity Sensing of Stretchable and Transparent Hydrogel Films for Wireless Respiration Monitoring

Respiratory monitoring plays a pivotal role in health assessment and provides an important application prospect for flexible humidity sensors.However,traditional humidity sensors suffer from a trade-off between deformability,sensitivity,and transparency,and thus the development of high-performance,stretchable,and low-cost humidity sensors is urgently needed as wearable electronics.Here,ultrasensitive,highly deformable,and transparent humidity sensors are fabricated based on cost-effective polyacrylamide-based double network hydrogels.Concomitantly,a general method for preparing hydrogel films with controllable thickness is proposed to boost the sensitivity of hydrogel-based sensors due to the extensively increased specific surface area,which can be applied to different polymer networks and facilitate the development of flexible integrated electronics.In addition,sustainable tapioca rich in hydrophilic polar groups is introduced for the first time as a second cross-linked network,exhibiting excellent water adsorption capacity.Through the synergistic optimization of structure and composition,the obtained hydrogel film exhibits an ultrahigh sensitivity of 13,462.1%/%RH,which is unprecedented.Moreover,the hydrogel film-based sensor exhibits excellent repeatability and the ability to work normally under stretching with even enhanced sensitivity.As a proof of concept,we integrate the stretchable sensor with a specially designed wireless circuit and mask to fabricate a wireless respiratory interruption detection system with Bluetooth transmission,enabling real-time monitoring of human health status.This work provides a general strategy to construct high-performance,stretchable,and miniaturized hydrogel-based sensors as next-generation wearable devices for real-time monitoring of various physiological signals.

Outstanding Humidity Chemiresistors Based on Imine-Linked Covalent Organic Framework Films for Human Respiration Monitoring

Human metabolite moisture detection is important in health monitoring and non-invasive diagnosis.However,ultra-sensitive quantitative extraction of respiration information in real-time remains a great challenge.Herein,chemiresistors based on imine-linked covalent organic framework(COF)films with dual-active sites are fabricated to address this issue,which demonstrates an amplified humidity-sensing signal performance.By regulation of monomers and functional groups,these COF films can be pre-engineered to achieve high response,wide detection range,fast response,and recovery time.Under the condition of relative humidity ranging from 13 to 98%,the COFTAPB-DHTA film-based humidity sensor exhibits outstanding humidity sensing perfor-mance with an expanded response value of 390 times.Furthermore,the response values of the COF film-based sensor are highly linear to the relative humidity in the range below 60%,reflecting a quantitative sensing mechanism at the molecular level.Based on the dual-site adsorption of the(-C=N-)and(C-N)stretching vibrations,the revers-ible tautomerism induced by hydrogen bonding with water molecules is demonstrated to be the main intrinsic mechanism for this effective humidity detection.In addition,the synthesized COF films can be further exploited to effectively detect human nasal and oral breathing as well as fabric permeability,which will inspire novel designs for effective humidity-detection devices.

Recent Progress in Cellulose-Based Flexible Sensors

Flexible sensors are attractive due to potential applications in body exercise and ambient gas monitoring systems.Cellulose and its derivatives have combined superiorities such as intrinsic and structural flexibility,ease of chemical functionalization,moisture sensitivity,and mechanical stability,enabling them to be promising candidates as flexible supporting substrates and flexible sensitive materials.Significant progress consequently has been achieved to improve mechanical,electrical,and chemical performance.The latest advance in materials synthesis,structure design,fabrication control,and working mechanism of novel cellulose-based flexible sensors are reviewed and discussed,including strain sensors,humidity sensors,and harmful gas sensors.Various strategies were summarized to enhance sensor performance by surface group modifications,inorganic and organic conducting fillers optimization,multilayer structure design.Newly emerged processing techniques of self-assembly,vacuum filtration,and 3D printing were introduced as well to construct multiscale microstructures.The integration of multiple sensors toward smart and healthy exercise monitoring system is briefly reviewed.The facing challenges and future opportunities of cellulose-based flexible sensors were discussed and proposed at the end.This review provides inspiration and guidelines on how to design and fabricate cellulose-based flexible sensors.

Humidity Sensing Properties of Pre-irradiation Grafted HDPE Membranes

Grafted high density polyethylene (HDPE) films have been prepared by grafting sodium styrene sulfonate (SSS) and acrylic acid (AA) onto HDPE using preirradiation method.The structure and surface morphology of both grafted and ungrafted HDPE films were observed by FTIR spectroscopy and scanning electron microscope (SEM),respectively.The humidity sensing properties of grafted HDPE films were investigated.The results show that the impedance of the film decreases with increasing of relative humidity (RH).The films display high sensitivity with nearly three orders of magnitude change in the resistance during the relative humidity variation from 5% to 98%.Grafted HDPE films exhibit excellent properties as humidity sensors.

Design of ispPAC-based Humidity Sensor Signal Processing Circuits

The widely used sensitive elements of humidity sensors can be divided into 3 types,i.e.,resistor,capacitor,and electrolyte.Humidity sensors consisting of these sensitive elements have corresponding signal processing circuit unique to each type of sensitive elements.This paper presents an ispPAC (in-system programmable Programmable Analog Circuit) -based humidity sensor signal processing circuit designed with software method and implemented with in-system programmable simulators.Practical operation shows that humidity sensor signal processing circuits of this kind,exhibit stable and reliable performance.

Metal Oxide Heteronanostructures Prepared by Electrospinning for the Humidity Detection: Fundamentals and Perspectives

Humidity is a critical environmental parameter for several production processes and its control/monitoring is of great importance in maintaining the quality of goods and products. In this context, metallic oxide ceramic nanostructures are materials of great technological interest in the fabrication of moisture sensors because they have good chemical/structural stability and high surface area/volume ratio. The electrical response of these sensors relates to the chemisorbed and physisorbed layers of water molecules on the surface of the ceramic particles and to the capillary condensation of water in the microscopic pores between the particles. Based on these aspects, this work presents the fundamentals, electrical/electronic properties, influence of dopants, novel preparation procedure by electrospinning and perspectives of application of TiO2:WO3 metal oxide heteronanostructures as humidity sensors.