Ultrafast Response Polyelectrolyte Humidity Sensor for Respiration Monitoring

Synthesis and characterization of tetrathiol-substituted double-decker or ladder silsesquioxane nano-cores

Tetra(3-mercaptopropyl)-silsesquioxanes with double-decker (DDSQ) or ladder nano-cores were easily prepared from the corresponding tetraallyl derivatives through fast and convenient thiol-ene reactions. An additional tetrathiol-DDSQ with more flexible arms was also synthesized in high yield from the corresponding tetrachloro-DDSQ derivative. The three novel tetrathiol silsesquioxanes described represent versatile building blocks for the preparation of hybrid organic-inorganic materials.

Unique Noncontact Monitoring of Human Respiration and Sweat Evaporation Using a CsPb2Br5-Based Sensor

Respiration monitoring and human sweat sensing have promising application prospects in personal healthcare data collection, disease diagnostics, and the effective prevention of human-to-human transmission of fatal viruses. Here, we have introduced a unique respiration monitoring and touchless sensing system based on a CsPb₂Br₅/BaTiO₃ humidity-sensing layer operated by water-induced interfacial polarization and prepared using a facile aerosol deposition process. Based on the relationship between sensing ability and layer thickness, the sensing device with a 1.0 μm thick layer was found to exhibit optimal sensing performance, a result of its ideal microstructure. This sensor also exhibits the highest electrical signal variation at 0.5 kHz due to a substantial polarizability difference between high and low humidity. As a result, the CsPb₂Br₅/BaTiO₃ sensing device shows the best signal variation of all types of breath-monitoring devices reported to date when used to monitor sudden changes in respiratory rates in diverse situations. Furthermore, the sensor can effectively detect sweat evaporation when placed 1 cm from the skin, including subtle changes in capacitance caused by finger area and motion, skin moisture, and contact time. This ultrasensitive sensor, with its fast response, provides a potential new sensing platform for the long-term daily monitoring of respiration and sweat evaporation.

Wearable Sensing and Telehealth Technology with Potential Applications in the Coronavirus Pandemic

Coronavirus disease 2019 (COVID-19) has emerged as a pandemic with serious clinical manifestations including death. A pandemic at the large-scale like COVID-19 places extraordinary demands on the world's health systems, dramatically devastates vulnerable populations, and critically threatens the global communities in an unprecedented way. While tremendous efforts at the frontline are placed on detecting the virus, providing treatments and developing vaccines, it is also critically important to examine the technologies and systems for tackling disease emergence, arresting its spread and especially the strategy for diseases prevention. The objective of this article is to review enabling technologies and systems with various application scenarios for handling the COVID-19 crisis. The article will focus specifically on 1) wearable devices suitable for monitoring the populations at risk and those in quarantine, both for evaluating the health status of caregivers and management personnel, and for facilitating triage processes for admission to hospitals; 2) unobtrusive sensing systems for detecting the disease and for monitoring patients with relatively mild symptoms whose clinical situation could suddenly worsen in improvised hospitals; and 3) telehealth technologies for the remote monitoring and diagnosis of COVID-19 and related diseases. Finally, further challenges and opportunities for future directions of development are highlighted.

High-Performance Humidity Sensor Based on the Graphene Flower/Zinc Oxide Composite

Performance of an electronic device relies heavily on the availability of a suitable functional material. One of the simple, easy, and cost-effective ways to obtain novel functional materials with improved properties for desired applications is to make composites of selected materials. In this work, a novel composite of transparent n-type zinc oxide (ZnO) with a wide bandgap and a unique structure of graphene in the form of a graphene flower (GrF) is synthesized and used as the functional layer of a humidity sensor. The (GrF/ZnO) composite was synthesized by a simple sol–gel method. Morphological, elemental, and structural characterizations of GrF/ZnO composite were performed by a field emission scanning electron microscope (FESEM), energy-dispersive spectroscopy (EDS), and an x-ray diffractometer (XRD), respectively, to fully understand the properties of this newly synthesized functional material. The proposed humidity sensor was tested in the relative humidity (RH) range of 15% RH% to 86% RH%. The demonstrated sensor illustrated a highly sensitive response to humidity with an average current change of 7.77 μA/RH%. Other prominent characteristics shown by this device include but were not limited to high stability, repeatable results, fast response, and quick recovery time. The proposed humidity sensor was highly sensitive to human breathing, thus making it a promising candidate for various applications related to health monitoring.

Asymmetrically Patterned Cellulose Nanofibers/Graphene Oxide Composite Film for Humidity Sensing and Moist-Induced Electricity Generation

The exploration of advanced functional materials from natural resources is significantly important to green and sustainable development. Herein, we design an ultrafast humidity-driven bending response system using asymmetrically patterned cellulose nanofiber (CNF)/graphene oxide (GO) composite films. The CNF/GO composite films are fabricated by vacuum-assisted filtration, followed by a surface imprinting technique. The results reveal that the composite films possess excellent linear response to humidity change and cycle stability in the relative humidity (RH) range from 25 to 85%. The curvature of the film varies from 0.012 to 0.260 cm^-1 as the RH changes from 25 to 85%, and the response time is only 3-5 s. The outstanding humidity response is attributed to the addition of GO that actively interacts with water, enhancing the flexibility and humidity sensitivity of the composite films. In addition, asymmetrical patterning improves the water transfer rate by confinement and renders an easy deformation of composite films under the same stress. Molecular dynamics simulation and finite element analysis are used to further elucidate the mechanism therein. Furthermore, this CNF/GO composite film is also an effective hygroelectric generator, with an output voltage as high as 286 mV. This smart CNF/GO film with responsive humidity-driven deformation shows potential applications as a biomimetic leaf, a proximity sensor, and a moisture-driven electricity generator. This work inspires a new approach of smart material design with nanocellulose and GO and promotes their further applications.

High-Performance Colorimetric Humidity Sensors Based on Konjac Glucomannan

High-humidity conditions (85-100% relative humidity (RH)) have very diverse effects on many aspects of people's daily lives. Despite remarkable progress in the development of structural coloration-based humidity sensors, how to significantly improve the sensitivity and visual humidity resolution of these humidity sensors under a high-humidity environment remains a great challenge. In this study, high-performance colorimetric humidity sensors based on environment-friendly konjac glucomannan (KGM) via thin-film interference are developed using a simple, affordable, and scalable preparation method. An effective strategy is demonstrated for substantially improving the sensor sensitivity and visual humidity resolution under a high-humidity environment via synergistic integration of multiorder interference peaks, sensor array technology, and superior water-absorbing polymer. The KGM full-range humidity sensors exhibit fast and dynamic response toward the humidity change without power consumption, and they also show high sensitivity and selectivity, little hysteresis, and excellent stability against high-humidity conditions. The KGM humidity sensors display extraordinary red shift of the reflection peak (e.g., 385 nm) and the visual humidity resolution as high as 1.5% RH in the visible range from 85 to 100% RH, which represent the largest spectra shift and highest visual humidity resolution, respectively, for structural coloration-based humidity sensors in high-humidity conditions.

Wearable Technology to Assist the Patients Infected with Novel Coronavirus (COVID-19)

Wearable technology plays a significant role in our daily life as well as in the healthcare industry. The recent coronavirus pandemic has taken the world’s healthcare systems by surprise. Although trials of possible vaccines are underway, it would take a long time before the vaccines are permitted for public use. Most of the government efforts are currently geared towards preventing the spread of the coronavirus and predicting probable hot zones. The essential and healthcare workers are the most vulnerable towards coronavirus infections due to their required proximity to potential coronavirus patients. Wearable technology can potentially assist in these regards by providing real-time remote monitoring, symptoms prediction, contact tracing, etc. The goal of this paper is to discuss the different existing wearable monitoring devices (respiration rate, heart rate, temperature, and oxygen saturation) and respiratory support systems (ventilators, CPAP devices, and oxygen therapy) which are frequently used to assist the coronavirus affected people. The devices are described based on the services they provide, their working procedures as well as comparative analysis of their merits and demerits with cost. A comparative discussion with probable future trends is also drawn to select the best technology for COVID-19 infected patients. It is envisaged that wearable technology is only capable of providing initial treatment that can reduce the spread of this pandemic.

Humidity-Sensing Properties of a BiOCl-Coated Quartz Crystal Microbalance

The performance of a bismuth oxychloride (BiOCl)-based quartz crystal microbalance (QCM) humidity sensor was studied using an oscillating circuit method. The BiOCl powder was prepared by a hydrolysis method. Scanning electron microscopy, X-ray diffraction, and Fourier transform infrared spectroscopy were used to characterize the BiOCl sample. Its humidity-sensing property was analyzed by combining it with a QCM at room temperature (25 °C). Experimental results indicated that the BiOCl-based QCM sensor showed good humidity characteristics from 11.3 to 97.3%, such as good logarithmic frequency response to humidity levels (R ² = 0.994), fast response time (5.2 s)/recovery time (4.5 s), good reversibility, stability, repeatability, and low humidity hysteresis. In addition, the response to human nose breaths showed excellent practicability. Finally, the humidity sensing mechanism of the BiOCl-based QCM humidity sensor was discussed in detail. This work demonstrates that BiOCl is a promising candidate material for humidity detection.

Stretchable respiration sensors: Advanced designs and multifunctional platforms for wearable physiological monitoring

Respiration signals are a vital sign of life. Monitoring human breath provides critical information for health assessment, diagnosis, and treatment for respiratory diseases such as asthma, chronic bronchitis, and emphysema. Stretchable and wearable respiration sensors have recently attracted considerable interest toward monitoring physiological signals in the era of real time and portable healthcare systems. This review provides a snapshot on the recent development of stretchable sensors and wearable technologies for respiration monitoring. The article offers the fundamental guideline on the sensing mechanisms and design concepts of stretchable sensors for detecting vital breath signals such as temperature, humidity, airflow, stress and strain. A highlight on the recent progress in the integration of variable sensing components outlines feasible pathways towards multifunctional and multimodal sensor platforms. Structural designs of nanomaterials and platforms for stretchable respiration sensors are reviewed.

Highly Selective Metal-Organic Framework Textile Humidity Sensor

The increase in demand and popularity of smart textiles brings new and innovative ideas to develop a diverse range of textile-based devices for our daily life applications. Smart textile-based sensors (TEX sensors) become attractive due to the potential to replace current solid-state sensor devices with flexible and wearable devices. We have developed a smart textile sensor for humidity detection using a metal-organic framework (MOF) as an active thin-film layer. We show for the first time the use of the Langmuir-Blodgett (LB) technique for the deposition of a MIL-96(Al) MOF thin film directly onto the fabrics containing interdigitated textile electrodes for the fabrication of a highly selective humidity sensor. The humidity sensors were made from two different types of textiles, namely, linen and cotton, with the linen-based sensor giving the best response due to better coverage of MOF. The TEX sensor showed a reproducible response after multiple cycles of measurements. After 3 weeks of storage, the sensor showed a moderate decrease in response. Moreover, TEX sensors showed a high level of selectivity for the detection of water vapors in the presence of several volatile organic compounds (VOCs). Interestingly, the selectivity is superior to some of the previously reported MOF-coated solid-state interdigitated electrode devices and textile sensors. The method herein described is generic and can be extended to other textiles and coating materials for the detection of toxic gases and vapors.

High-Performance Magnesium-Carbon Nanofiber Hygroelectric Generator Based on Interface-Mediation-Enhanced Capacitive Discharging Effect

This study reports the concept of a water/moisture-induced hygroelectric generator based on the direct contact between magnesium (Mg) alloy and oxidized carbon nanofibers (CNFs). This device generates an open-circuit voltage up to 2.65 V within only 10 ms when the unit is placed in contact with liquid water, which is higher than the reduction potential of magnesium. The average peak short-circuit current density is ∼6 mA/cm², which is among the highest values yet reported for water-induced electricity generators. Our results indicate that galvanic corrosion occurs at the interface between the CNF and Mg electrode, but the device can still generate electricity because of the high contact resistance caused by the work function difference between Mg and CNF and the surface oxidation. The oxidized CNF is shown to absorb water/moisture and get reduced, leading to a capacitive discharging effect to provide enhanced signal amplitude and sensitivity. These devices are found to be highly sensitive to small quantities of water, and their high output voltage and current make them useful for the detection of water vapor in the human breath as well as changes in ambient humidity. The Mg/CNF systems thus provide a new technology for use in the fabrication of self-powered water/moisture sensors and the development of portable electric power generators.

Highly Flexible Multilayered e-Skins for Thermal-Magnetic-Mechanical Triple Sensors and Intelligent Grippers

This work reports a novel triple-functional electronic skin (e-skin) which shows both wonderful thermal-magnetic-mechanical sensing performance and interesting magnetic actuation behavior. The flexible e-skin comprises thermo-sensitive, magnetic, and conductive tri-components, and their sensitive characteristics under 5-70 °C, 0-1200 mT, and 0.1-5.1 MΩ are studied, respectively. Owing to the unique piezoresistive characteristic and magnetorheological effect, the e-skin exhibits a rapid response time (38 ms) to the external stimuli. The assembled e-skin with the triple-layer structure can act as a functional sensor to monitor various human motions, magnetic fields, and environmental temperatures. Based on this e-skin, an intelligent magneto-active gripper is further developed, and it can be used to grasp and transport targets by the actuated force of magnetic field under various working conditions. Importantly, the multi-functional sensing capability endows the gripper with real-time deformation and ambient temperature perception characteristics. As a result, because of the ideal multi-field coupling sensing and magnetic active features, this e-skin shows a wide prospect in wearable electronics, man-machine interactions, and intelligent transport systems.

Rapid response flexible humidity sensor for respiration monitoring using nano-confined strategy

Development of wearable devices for continuous respiration monitoring is of great importance for evaluating human health. Here, we propose a new strategy to achieve rapid respiration response by confining conductive polymers into 1D nanowires which facilitates the water molecules absorption/desorption and maximizes the sensor response to moisture. The nanowires arrays were fabricated through a low-cost nanoscale printing approach on flexible substrate. The nanoscale humidity sensor shows a high sensitivity (5.46%) and ultrafast response (0.63 s) when changing humidity between 0% and 13% and can tolerate 1000 repetitions of bending to a curvature radius of 10 mm without influencing its performance. Benefited by its fast response and low power assumption, the humidity sensor was demonstrated to monitor human respiration in real time. Different respiration patterns including normal, fast and deep respiration can be distinguished accurately.

Compact plasmonic fiber tip for sensitive and fast humidity and human breath monitoring

We demonstrate a plasmonic fiber tip for relative humidity (RH) detection by integrating a gold nanomembrane onto the end-face of a multimode optical fiber via a flexible and high-efficiency transfer method. Fast water condensation/evaporation is responsible for the high performance of the fiber tip in response to RH. A high sensitivity of 279 pm/%RH is obtained in the range of $ 11\% \sim 92\% {\rm RH} $11%∼92%RH. Taking advantage of the fast dynamics (response and recovery times of 156 ms and 277 ms), the plasmonic fiber tip offers an excellent detection capability to human breaths at varied frequencies and depths. The compact, easy-fabrication, and fast-dynamics plasmonic platform has versatile potential for practical applications, including environmental and healthcare monitoring, as well as biochemical sensing.

Breathable Nanomesh Humidity Sensor for Real-Time Skin Humidity Monitoring

The importance of monitoring the condition of skin is increasing as its relevance to health is becoming more well understood. Inappropriate humidity levels can cause atopic dermatitis or hair loss. However, conventional film substrates used in electronic skin monitoring devices cause accumulation of sweat or gas between the device and biological tissue, leading to negative effects in long-term humidity measurements. Thus, real-time measurements of skin humidity over long periods are difficult using conventional film devices. Here, a breathable nanomesh humidity sensor that can monitor skin humidity for a long time is developed by using biocompatible materials such as gold, poly(vinyl alcohol), and Parylene C. The sensor presents excellent gas and sweat permeability and precisely detects the humidity level of an object for a long time. This study demonstrates the successful real-time detection of the humidity level from human skin and also detects the relative humidity of a plant surface over a prolonged period. This sensor is expected to have wide applicability for cultivating delicate plants as well as to reveal correlations between skin humidity and disease for biomedical applications.

A Flexible Multimodal Sensor That Detects Strain, Humidity, Temperature, and Pressure with Carbon Black and Reduced Graphene Oxide Hierarchical Composite on Paper

Flexible sensors (FSs) are the key components of intelligent equipment and wearable devices, thus attracting increasing research interests in recent years. However, the preparation of multifunctional FS with good degradability in a natural environment is still challenging. In this work, we fabricated a flexible multimodal sensor that can detect multiple stimuli with only one device by spraying the mixture of carbon black (CB) and reduced graphene oxide (rGO) on a paper substrate. Scanning electron microscopy visualization indicated the CB particles absorbed on the surface of rGO, which then overlayered together, constructing a hierarchical structure. Benefiting from this unique structure, the obtained FS was demonstrated to have good sensitivity for strain, humidity, temperature, and pressure as well as multiple stimuli and was used to monitor human respirations as well as body motions, such as finger and elbow bending and head nodding. Besides, the sensor can be easily degraded in water being free of electronic pollution, but it also can be reused after the soaking-drying process, implying its reliability. This degradable and multimodal FS may find great potential in flexible electronics.

Freestanding Laser-Assisted Reduced Graphene Oxide Microribbon Textile Electrode Fabricated on a Liquid Surface for Supercapacitors and Breath Sensors

Graphene microribbons (rGO-MRs) are highly desired for their high electrical conductivities and specific surface areas, which contribute to multiple applications in thin, flexible, textile supercapacitors, sensors, and actuators. Herein, we demonstrate a facile method for creating reduced graphene oxide microribbons with microscale architecture utilizing a simple blue-violet diode laser under ambient conditions. This method takes advantage of the photochemical reduction mechanism of self-assembled graphene oxide liquid crystals (GO-LC), allowing rGO-MR patterns to be directly printed on the solution surface. The rGO-MR films demonstrated tunable diameters and can be tailored into any geometries. A maximum intrinsic electrical conductivity for rGO-MR reaching 325.8 S/m was observed. The rGO-MR textile electrodes can be assembled into microsupercapacitors with a high areal specific capacitance of 14.4 mF/cm², a low charge-transfer impedance, and an exceptional cycling performance with a retained 96.8% capacitance after 10 000 cycles. The rGO-MR films also experience changes in resistance in response to the moisture adsorption from human breaths and therefore can also be employed as a breathing sensor for health monitoring. The presented facile method for creating multilayered rGO-MR films directly on liquid surfaces can further expand the potential for three-dimensional printing graphitic materials for various multifunctional applications.