Ultrafast Response Polyelectrolyte Humidity Sensor for Respiration Monitoring

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.

Spinning the Future: The Convergence of Nanofiber Technologies and Yarn Fabrication

The rapid advancement in nanofiber technologies has revolutionized the domain of yarn materials, marking a significant leap in textile technology. This review dissects the nexus between cutting-edge nanofiber technologies and yarn manufacturing, aiming to illuminate the pathway toward engineering advanced textiles with unparalleled functionality. It first discusses the fundamentals of nanofiber assemblies and spinning techniques, primarily focusing on electrospinning, centrifugal spinning, and blow spinning. Additionally, the study delves into integrating nanofiber spinning technologies with traditional and modern yarn fabrication principles, elucidating the design principles that underlie the creation of yarns incorporating nanofibers. Twisting technologies are explored to examine how they can be optimized and adapted for incorporating nanofibers, thus enabling the production of innovative nanofiber-based yarns. Special attention is given to scalable strategies like centrifugal and blow spinning, which are spotlighted for their efficiency and scalability in fabricating nanofiber yarns. This review further analyses recently developed nanofiber yarn applications, including wearable sensors, biomedical devices, moisture management textiles, and energy harvesting and storage devices. We finally present a forward-looking perspective to address unresolved issues in nanofiber-based yarn technologies.

Multi-Interface Engineering of MXenes for Self-Powered Wearable Devices

Self-powered wearable devices with integrated energy supply module and sensitive sensors have significantly blossomed for continuous monitoring of human activity and the surrounding environment in healthcare sectors. The emerging of MXene-based materials has brought research upsurge in the fields of energy and electronics, owing to their excellent electrochemical performance, large surface area, superior mechanical performance, and tunable interfacial properties, where their performance can be further boosted via multi-interface engineering. Herein, a comprehensive review of recent progress in MXenes for self-powered wearable devices is discussed from the aspects of multi-interface engineering. The fundamental properties of MXenes including electronic, mechanical, optical, and thermal characteristics are discussed in detail. Different from previous review works on MXenes, multi-interface engineering of MXenes from termination regulation to surface modification and their impact on the performance of materials and energy storage/conversion devices are summarized. Based on the interfacial manipulation strategies, potential applications of MXene-based self-powered wearable devices are outlined. Finally, proposals and perspectives are provided on the current challenges and future directions in MXene-based self-powered wearable devices.

Wearable Fiber SPR Respiration Sensor Based on a LiBr-Doped Silk Fibroin Film

Respiration is essential for supporting human body functions. However, a biocompatible fiber respiration sensor has rarely been discussed. In this study, we propose a wearable fiber surface plasmon resonance (SPR) respiration sensor using a LiBr-doped silk fibroin (SF) film. The SPR sensor monitors respiration by responding to airway humidity variation during inhalation and exhalation. We fabricated the SPR respiration sensor by depositing the core of a plastic-clad optical fiber with a gold film and an SF-LiBr composite film. The SF-LiBr composite film can absorb water through the interaction between water molecules and hydrogen bonds linking fibroin chains. Thus, humidity variation can change the SF-LiBr composite film's refractive index (RI), altering the phase-matching condition of the surface plasmon polaritons and shifting the SPR spectral dip. In experiments, we test the effect of the LiBr doping ratio on humidity response and confirm that the SF-22.1 wt % LiBr sensor has balanced performances. The SF-22.1 wt % LiBr sensor has a broad sensing range of 35-99% relative humidity (RH), a reasonable overall sensitivity of -6.5 nm/% RH, a fast response time of 135 ms, a quick recovery time of 150 ms, good reversibility, and good repeatability, which is capable of tracking different respiration states and patterns. Finally, we encapsulate this sensor in a conventional nasal oxygen cannula for wearable respiration monitoring, proving that the sensor is suitable for high-sensitivity, real-time, and accurate respiration monitoring.

Metal Microelectromechanical Resonator Exhibiting Fast Human Activity Detection

This work presents a MEMS resonator used as an ultra-high resolution water vapor sensor (humidity sensing) to detect human activity through finger movement as a demonstrator example. This microelectromechanical resonator is designed as a clamped-clamped beam fabricated using the top metal layer of a commercial CMOS technology (0.35 μm CMOS-AMS) and monolithically integrated with conditioning and readout circuitry. Sensing is performed through the resonance frequency change due to the addition of water onto the clamped-clamped beam coming from the moisture created by the evaporation of water in the human body. The sensitivity and high-speed response to the addition of water onto the metal bridge, as well as the quick dewetting of the surface, make it suitable for low-power human activity sensing.

Ultrathin hierarchical hydrogel-carbon nanocomposite for highly stretchable fast-response water-proof wearable humidity sensors

Wearable humidity sensors play an important role in human health monitoring. However, challenges persist in realizing high performance wearable humidity sensors with fast response and good stretchability and durability. Here we report wearable humidity sensors employing an ultrathin micro-nano hierarchical hydrogel-carbon nanocomposite. The nanocomposite is synthesized on polydimethylsiloxane (PDMS) films via a facile two-step solvent-free approach, which creates a hierarchical architecture consisting of periodic microscale wrinkles and vapor-deposited nanoporous hydrogel-candle-soot nanocoating. The hierarchical surface topography results in a significantly enlarged specific surface area (>107 times that of planar hydrogel), which along with the ultrathin hydrogel endow the sensor with high sensitivity and a fast response/recovery (13/0.48 s) over a wide humidity range (11-96%). Owing to the wrinkle structure and interpenetrating network between the hydrogel and PDMS, the sensor is stable and durable against repeated 180° bending, 100% strain, and even scratching. Furthermore, encapsulation of the sensor imparts excellent resistance to water, sweat, and bacteria without influencing its performance. The sensor is then successfully used to monitor different human respiratory behaviors and skin humidity in real time. The reported method is convenient and cost-effective, which could bring exciting new opportunities in the fabrication of next-generation wearable humidity sensors.

Chitosan-Based Highly Sensitive Viable Humidity Sensor for Human Health Monitoring

We report a sustainable resistive-type humidity sensor based on chitosan (CS) film deposited on an interdigitated Ti/Au electrode coated SiO2 substrate using a simple drop cast approach for human health monitoring. The sensor revealed remarkably high sensitivity (5.8 MΩ/%RH), fast response/recovery time (21 s/25 s), low hysteresis (∼9.3%), excellent reversibility, wide detecting range (11-95% RH), and high selectivity toward water vapor. The calculated associated uncertainty at different %RH indicates the excellent repeatability and stable performance of the sensor. The developed sensor is tested for different human breath patterns, and it is found that the sensor can clearly distinguish between the variations in rate and depth of respiration patterns during normal, fast, deep, and nasal breathing and can monitor for apnea-like situations. The sensor is also utilized to perform noncontact skin humidity sensing. Overall, the developed CS film humidity sensor provides a viable approach for the detection of respiratory disorders and human health issues, detected by skin moisture, in a noninvasive manner.

Recent Advancements in Physiological, Biochemical, and Multimodal Sensors Based on Flexible Substrates: Strategies, Technologies, and Integrations

Flexible wearable devices have been widely used in biomedical applications, the Internet of Things, and other fields, attracting the attention of many researchers. The physiological and biochemical information on the human body reflects various health states, providing essential data for human health examination and personalized medical treatment. Meanwhile, physiological and biochemical information reveals the moving state and position of the human body, and it is the data basis for realizing human-computer interactions. Flexible wearable physiological and biochemical sensors provide real-time, human-friendly monitoring because of their light weight, wearability, and high flexibility. This paper reviews the latest advancements, strategies, and technologies of flexibly wearable physiological and biochemical sensors (pressure, strain, humidity, saliva, sweat, and tears). Next, we systematically summarize the integration principles of flexible physiological and biochemical sensors with the current research progress. Finally, important directions and challenges of physiological, biochemical, and multimodal sensors are proposed to realize their potential applications for human movement, health monitoring, and personalized medicine.

Intelligent Multifunctional Sensing Systems based on Ordered Macro-Microporous Metal Organic Framework and Its Derivatives

Compared with nanomaterials-based sensors with single function, the development of multifunctional sensors shows high potential in comprehensive monitoring of personal health and environment, intelligent human-machine interfaces, and realistic imitation of human skin in prosthetics. Ordered macro-microporous metal-organic frameworks (MOFs)-enabled flexible and stretchable electronics are promising candidates for integrated multifunctional sensing systems. Herein, a three-dimensional ordered macro-microporous zeolite imidazolate framework-8 (3DOM ZIF-8) for humidity sensing and the derived ZnO within a hierarchically ordered macroporous-mesoporous-microporous carbon matrix (ZnO@HOMC) for gas sensor is constructed. Benefit from hierarchically ordered macroporous-mesoporous-microporous structure, the active site is fully exposed, and the charge transfer is accelerated. As a result, the multifunctional sensing systems show ultrafast response and recovery speed (10 s and 34 s), high sensitivity (R_air /R_gas = 38.6@50 ppm) to acetone, rapid humidity response speed (0.23 s) within changing humidity (RH 21%-99%), excellent stability and repeatability. Furthermore, in order to realize real-time monitoring of gas concentrations and humidity on mobile devices, an intelligent and portable sensor module is fabricated and wirelessly connected to a smartphone to effectively detect acetone concentration and humidity. This sensing technology shows fascinating applications in personal health, fitness tracking, electronic skins, artificial nervous systems, and human-machine interactions.

Skin-Interfaced Superhydrophobic Insensible Sweat Sensors for Evaluating Body Thermoregulation and Skin Barrier Functions

Monitoring sweat rate is vital for estimating sweat loss and accurately measuring biomarkers of interest. Although various optical or electrical sensors have been developed to monitor the sensible sweat rate, the quantification of the insensible sweat rate that is directly related to body thermoregulation and skin barrier functions still remains a challenge. This work introduces a superhydrophobic sweat sensor based on a polyacrylate sodium/MXene composite sandwiched between two superhydrophobic textile layers to continuously measure sweat vapor from insensible sweat with high sensitivity and rapid response. The superhydrophobic textile on a holey thin substrate with reduced stiffness and excellent breathability allows the permeation of sweat vapor, while preventing the sensor from being affected by the external water droplets and internal sensible sweat. Integrating the insensible sweat sensor with a flexible wireless communication and powering module further yields a standalone sensing system to continuously monitor insensible sweat rates at different body locations for diverse application scenarios. Proof-of-concept demonstrations on human subjects showcase the feasibility to continuously evaluate the body's thermoregulation and skin barrier functions for the assessment of thermal comfort, disease conditions, and nervous system activity. The results presented in this work also provide a low-cost device platform to detect other health-relevant biomarkers in the sweat (vapor) as the next-generation sweat sensor for smart healthcare and personalized medicine.

Ultrasensitive Humidity Sensors with Synergy between Superhydrophilic Porous Carbon Electrodes and Phosphorus-Doped Dielectric Electrolyte

Capacitive humidity sensors have been used for human health monitoring, but their performance may be poor in terms of sensitivity and response time, because of limitations in sensing materials and insufficient knowledge about sensing mechanisms. Herein, a new combination of humidity sensing materials to assemble thin-film based capacitive-type sensors is proposed by using PA-doped polybenzimidazole (PA-PBI) as an electrolyte and laser-carbonized PA-PBI as a carbon electrode (PA-PBI-C). Based on PA involved laser scribing, the flexible sensor can reach excellent humidity-sensing performances with an ultrahigh sensitivity (1.16 × 106 pF RH%-1, where RH represents the relative humidity), a superior linearity (R2 = 0.9982), a fast response time (0.72 s), and a low hysteresis in a wide RH range from 1% to 95%. By further studying P-O decorated porous carbon electrode with superhydrophilicity and the solid-state dielectric electrolyte featured by a high dielectric constant, a synergistic sensing mechanism consisting of a "Water reservoir" and a "Bridge" is established to support advanced health-monitoring applications such as the respiration patterns and skin condition where both sensitivity and response time are critical.

Directly Growing Graphdiyne Nanoarray Cathode to Integrate an Intelligent Solid Mg-Moisture Battery

A continuous humidity and solar-light dual responsive intelligent solid Mg-moisture battery (SMB) with a graphdiyne nanosheets array was fabricated. The integrated battery works based on a new concept of chemical bond conversion on the surface of the graphdiyne nanosheets array that is grown in situ on a 3D melamine sponge (GDY/MS). The unique structure, excellent catalytic, and semiconductor performance of GDY endows the GDY/MS with some outstanding characteristics on trapping and transferring water molecules, catalyzing HER, and utilizing solar energy, making the GDY/MS a new generation cathode for a high-performance intelligent SMB. The performance of the GDY/MS-based smart SMB (GSMB) can be continuously tuned by humidity and solar-light. The GSMB shows a significant positive correlation between open circuit potential (OCP) and humidity, while the natural band gap of GDY makes it further act as a photoelectrode to capture light and generate photoelectrons. The GSMB can be applied as a self-power humidity monitor with an ultrafast response time of <0.24 s, a recovery time of <0.16 s, and a sensitive (36,600%) respiratory sensing performance. This simple and efficient battery-made strategy represents the future development direction of self-power supply equipment, intelligent electronic devices, and intelligent battery integration.

Fast-speed, Highly Sensitive, Flexible Humidity Sensors Based on a Printable Composite of Carbon Nanotubes and Hydrophilic Polymers

Carbon nanotubes (CNTs) are a promising material for humidity sensors and wearable electronics due to their solution capability, good flexibility, and high conductivity. However, the performance of CNT-based humidity sensors is limited by their low sensitivity and slow response. Herein CNTs and hydrophilic polymers were mixed to form a composite. The hydrophilicity of the polymers and the network structure of the CNTs empowered the humidity sensors with a high response of 171% and a fast response/recovery time of 23 s/10 s. Owing to the sticky and flexible polymers, the humidity sensors showed strong adhesion to the PET substrate and exhibited outstanding bending durability. Furthermore, the flexible humidity sensor was applied to monitor human breathing and detect finger movements and handshaking.

Fully integrated wearable humidity sensor for respiration monitoring

Respiration monitoring is a promising alternative to medical diagnosis of several diseases. However, current techniques of respiration monitoring often require expensive and cumbersome devices which greatly limit their medical applications. Here, we present a fully integrated wearable device consisting of a flexible LCP-copper interdigital electrode, a sensing layer and a wireless electrochemical analysis system. The developed humidity sensor exhibits a high sensitivity, a good repeatability and a rapid response/recover time. The long-term stability is over 30 days at different relative humidity. By integrating the flexible humidity sensor with miniaturized electrochemical analysis system (0.8 cm × 1.8 cm), response current concerning respiration can be wirelessly transmitted to App-assisted smartphone in real time. Furthermore, the fabricated humidity sensor can realize skin moisture monitoring in a touch-less way. The large-scale production of miniaturized flexible sensor (4 mm × 6 mm) has significantly contributed to commercial deployment.

Ion-in-Conjugation: A Promising Concept for Multifunctional Organic Semiconductors

Most organic semiconductors (OSCs) consist of conjugated skeletons with flexible peripheral chains. Their weak intermolecular interactions from dispersion and induction forces result in environmental susceptibilities and are unsuitable for many multifunctional applications where direct exposure to external environments is unavoidable, such as gas absorption, chemical sensing, and catalysis. To exploit the advantages of inorganic semiconductors in OSCs, ion-in-conjugation (IIC) materials are proposed. An IIC material refers to any conjugated material (molecules, polymers, and crystals) in Kekule's structural formula containing stoichiometric ionic states in its conjugated backbone in the electronic ground state. In this review, the definitions, structures, synthesis, properties, and applications of IIC materials are described briefly. Four types of IIC material, including zwitterionic conjugated molecules/polymers, conjugated ionic dyes, π-d conjugated molecules and polymers, and coordinatively doped polymers, are reported. Their applications in gas sensing, humidity sensing, resistive memory devices, and thermal/photo-/electro-catalysis are demonstrated. The challenges and opportunities for future research are also discussed. It is expected that this work will inspire the design of new organic electronic information materials.

Lamellar Nanocomposite Based on a 1D Crayfish-like CeIII-Substituted Phospho(III)tungstate Semiconductor and Polyaniline Used as a High-Performance Humidity Sensing Device

In order to meet people's demand for intelligent management of daily life and health, manufacturing and developing humidity monitoring equipment with convenience, high sensitivity, easy miniaturization, and low cost is particularly important in the era of rapid development of artificial intelligence and the Internet of Things. Polyaniline (PANI) is an attractive humidity sensing material due to its designable functional properties. However, PANI modified polyoxometalates (POMs) for humidity sensing are still rare. As a proof of concept, a novel moisture sensing composite material was obtained based on PANI and a novel 1D rare-earth-substituted phospho(III)tungstate [H₂N(CH₃)₂]₉Na₃H₆[Ce₂(H₂O)₃W₅O₁₃(C₂O₄)][HP^IIIW₉O₃₃]₂[(HP^III)₂W₁₅O₅₄]·42H₂O (1). Notably, the anion structure of 1 contains trivacant Keggin-type [B-α-HP^IIIW₉O₃₃]^8- and Dawson-like [(HP^III)₂W₁₅O₅₄]^10- subunits linked by a heterometallic [Ce₂(H₂O)₃W₅O₃₂(C₂O₄)]^30- cluster. Furthermore, the 1/PANI composite shows a typical semiconductive characteristic with a "band-like" conductive mechanism. The fabricated 1/PANI-based humidity sensing device exhibits a broad sensing range (11∼97% relative humidity), fast response/recovery time (3.45 s/3.24 s), good repeatability, and long-term stability (over 3 months). Additionally, the possible sensing mechanism is proposed. This work offers an enormous possibility for the design of high-performance humidity sensing materials through POM material chemistry.

Feasibility Analysis and Implementation of Head-Mounted Electrical Impedance Respiratory Monitoring

The respiratory rate is one of the crucial indicators for monitoring human physiological health. The purpose of this paper was to introduce a head-mounted respiratory monitoring solution based on electrical impedance sensing. Firstly, we constructed a finite element model to analyze the feasibility of using head impedance for respiratory sensing based on the physiological changes in the pharynx. After that, we developed a circuit module that could be integrated into a head-mounted respiratory monitoring device using a bioelectrical impedance sensor. Furthermore, we combined adaptive filtering and respiratory tracking algorithms to develop an app for a mobile phone. Finally, we conducted controlled experiments to verify the effectiveness of this electrical impedance sensing system for extracting respiratory rate. We found that the respiration rates measured by the head-mounted electrical impedance respiratory monitoring system were not significantly different from those of commercial respiratory monitoring devices by a paired t-test (p > 0.05). The results showed that the respiratory rates of all subjects were within the 95% confidence interval. Therefore, the head-mounted respiratory monitoring scheme proposed in this paper was able to accurately measure respiratory rate, indicating the feasibility of this solution. In addition, this respiratory monitoring scheme helps to achieve real-time continuous respiratory monitoring, which can provide new insights for personalized health monitoring.

A Low-Cost Flexible Perforated Respiratory Sensor Based on Platinum for Continuous Respiratory Monitoring

Monitoring sleep conditions is of importance for sleep quality evaluation and sleep disease diagnosis. Accurate respiration detection provides key information about sleep conditions. Here, we propose a perforated temperature sensor that can be worn below the nasal cavity to monitor breath. The sensing system consists of two perforated temperature sensors, signal conditioning circuits, a transmission module, and a supporting analysis algorithm. The perforated structure effectively enhances the sensitivity of the system and shortens the response time. The sensor's response time is 0.07 s in air and sensitivity is 1.4‱°C^-1. The device can achieve a monitoring respiratory temperature range between normal room temperature and 40 °C. The simple and standard micromachining process ensures low cost and high reproducibility. We achieved the monitoring of different breathing patterns, such as normal breathing, panting, and apnea, which can be applied to sleep breath monitoring and exercise information recording.

Dynamic Compensation Method for Humidity Sensors Based on Temperature and Humidity Decoupling

Currently, integrated humidity sensors with fast-response time are widely needed. The most commonly used polyimide capacitive humidity sensor has a long response time, which is difficult to meet the need for a fast response. Most studies focusing on technology and materials have a high cost and are difficult to ensure compatability with the CMOS process. The dynamic compensation method can shorten the response time by only adding digital circuits or software processing. However, conventional compensation technology is not suitable for humidity sensors due to temperature coupling. This paper proposes a new dynamic compensation method for humidity sensors based on the decoupling of temperature factors by analyzing the coupling relationship between sensor dynamic characteristics and temperature. Simulations and experiments were used to verify the proposed method. The experimental results show that the proposed method reduces the humidity response time of the sensor by 85.6%. The proposed method can effectively shorten the response time of humidity sensors.

Porous and conductive cellulose nanofiber/carbon nanotube foam as a humidity sensor with high sensitivity

In this study, we developed a humidity sensor with high sensitivity based on cellulose nanofiber/carbon nanotube (CNF/CNT) hybrid foam. The porous structure of the foam not only provides more contact interface for water molecules adsorption, but also tunes the conductivity of the CCF closed to the point where the sensor is most sensitive to the change in humidity. With this porous structural design, the obtained foam sensor shows a high humidity sensitivity of 87.3% (ΔI/I₀, and the response limit is 100%), excellent linearity (R² = 0.996) within the humidity range from 29 to 95% relative humidity (RH), and good long-time stability (more than two months). Furthermore, the water vapor adsorption behavior of the CNF/CNT foam sensor can be well described by the pseudo-first-order kinetic model. Finally, a simple humidity measuring device based on the CNF/CNT foam is presented, which can find good applications for human breath and fingertip humidity monitoring.

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.

Recent Developments and Implementations of Conductive Polymer-Based Flexible Devices in Sensing Applications

Flexible sensing devices have attracted significant attention for various applications, such as medical devices, environmental monitoring, and healthcare. Numerous materials have been used to fabricate flexible sensing devices and improve their sensing performance in terms of their electrical and mechanical properties. Among the studied materials, conductive polymers are promising candidates for next-generation flexible, stretchable, and wearable electronic devices because of their outstanding characteristics, such as flexibility, light weight, and non-toxicity. Understanding the interesting properties of conductive polymers and the solution-based deposition processes and patterning technologies used for conductive polymer device fabrication is necessary to develop appropriate and highly effective flexible sensors. The present review provides scientific evidence for promising strategies for fabricating conductive polymer-based flexible sensors. Specifically, the outstanding nature of the structures, conductivity, and synthesis methods of some of the main conductive polymers are discussed. Furthermore, conventional and innovative technologies for preparing conductive polymer thin films in flexible sensors are identified and evaluated, as are the potential applications of these sensors in environmental and human health monitoring.

A Dual-Function Sensor for Highly Sensitive Detection of Flame and Humidity

Early warning sensors rapidly monitor critical temperatures, humidity, and fires, which are crucial to reduce or avoid natural disasters in complex environments, such as fire or water disasters. Here, a highly sensitive, readable, and dual-functional sensor is designed for a fast-response fire alarm and rapid humidity detection based on sustainable biological films (named MSCG films). The MSCG films are composed of grafted sisal nanofibers (MgC), silk nanofibers, graphene, and citric acid (CA). After crosslinking with CA, MSCG films exhibit good wet strength (i.e., 128.8 MPa) after soaking in 100 °C water, thus confirming that the films would be applicable to a broad temperature range in humid environments. After flame ignition, the MSCG films are rapidly carbonized to activate an alarm sound and a light in the circuit with a fire response time as short as 1 s. It exhibits ultrafast temperature response/recovery time (i.e., 0.1 s/0.3 s) and rapid humidity response time (i.e., 0.9 s). The dual-functional sensor is further assembled into a versatile sensor system for real-time monitoring of fire accidents and environmental humidity, which can be integrated into consumer electronics, such as portable laptops and mobile phones.

NaCl Ionization-Based Moisture Sensor Prepared by Aerosol Deposition for Monitoring Respiratory Patterns

A highly polarizable moisture sensor with multimodal sensing capabilities has great advantages for healthcare applications such as human respiration monitoring. We introduce an ionically polarizable moisture sensor based on NaCl/BaTiO₃ composite films fabricated using a facile aerosol deposition (AD) process. The proposed sensing model operates based on an enormous NaCl ionization effect in addition to natural moisture polarization, whereas all previous sensors are based only on the latter. We obtained an optimal sensing performance in a 0.5 µm-thick layer containing NaCl-37.5 wt% by manipulating the sensing layer thickness and weight fraction of NaCl. The NaCl/BaTiO₃ sensing layer exhibits outstanding sensitivity over a wide humidity range and a fast response/recovery time of 2/2 s; these results were obtained by performing the one-step AD process at room temperature without using any auxiliary methods. Further, we present a human respiration monitoring system using a sensing device that provides favorable and stable electrical signals under diverse respiratory scenarios.

Recent Advances in Intelligent Wearable Medical Devices Integrating Biosensing and Drug Delivery

The primary roles of precision medicine are to perform real-time examination, administer on-demand medication, and apply instruments continuously. However, most current therapeutic systems implement these processes separately, leading to treatment interruption and limited recovery in patients. Personalized healthcare and smart medical treatment have greatly promoted research on and development of biosensing and drug-delivery integrated systems, with intelligent wearable medical devices (IWMDs) as typical systems, which have received increasing attention because of their non-invasive and customizable nature. Here, the latest progress in research on IWMDs is reviewed, including their mechanisms of integrating biosensing and on-demand drug delivery. The current challenges and future development directions of IWMDs are also discussed.

Humidity Sensing Applications of Lead-Free Halide Perovskite Nanomaterials

Over the past decade, perovskite-based nanomaterials have gained notoriety within the scientific community and have been used for a variety of viable applications. The unique structural properties of these materials, namely good direct bandgap, low density of defects, large absorption coefficient, high sensitivity, long charge carrier lifetime, good selectivity, acceptable stability at room temperature, and good diffusion length have prompted researchers to explore their potential applications in photovoltaics, light-emitting devices, transistors, sensors, and other areas. Perovskite-based devices have shown very excellent sensing performances to numerous chemical and biological compounds in both solid and liquid mediums. When used in sensing devices, Perovskite nanomaterials are for the most part able to detect O₂, NO₂, CO₂, H₂O, and other smaller molecules. This review article looks at the use of lead-free halide perovskite materials for humidity sensing. A complete description of the underlying mechanisms and charge transport characteristics that are necessary for a thorough comprehension of the sensing performance will be provided. An overview of considerations and potential recommendations for the creation of new lead-free perovskite nanostructure-based sensors is presented.

An All-Fabric Tactile-Sensing Keypad with Uni-Modal and Ultrafast Response/Recovery Time for Smart Clothing Applications

Keypads constructed from fabric materials are the ideal input devices for smart clothing applications. However, multi-modal reaction problems have to be addressed before they can be of practical use on apparels, i.e., the fabric-based keypads need to distinguish between the legitimate actions by the fingertips and the illegitimate deformations and stresses caused by human movements. In this paper, we propose to use the humidity sensor functionalized from graphene oxide (GO)-coated polyester fibers to construct the e-textile keypads. As the moisture level in the proximity of human fingertips is much higher (over 70%) than other parts of the human body, humidity sensing has many advantages over other tactility mechanisms. Experiments have demonstrated that the GO-functionalized fabric keypad has a stable uni-modal tactility only to fingertip touches, and it is not sensitive to deformation, pressure, temperature variation, and other ambient interferences. With biasing and sensing circuits, the keypad exhibits a quick response and recovery time (around 0.1 s), comparable to mechanical keyboards. To demonstrate its application on smart clothing, the keypad was sewn on a sweater and embroidered conductive yarns were used to control an MP3 player in the pocket.

Environment-Resistant Organohydrogel-Based Sensor Enables Highly Sensitive Strain, Temperature, and Humidity Responses

Conductive hydrogels have been extensively used in wearable skin sensors owing to their outstanding flexibility, tissuelike compliance, and biocompatibility. However, the dehydration and embrittlement of hydrogels can result in sensitivity loss or even invalidation, restraining their wearable applications in external environments, especially at low temperatures and in arid environments. Herein, an environment-resistant organohydrogel is developed for multifunctional sensors. A double-network organohydrogel based on hyaluronic acid and poly(acrylic acid-co-acrylamide) is developed, and glycerol is introduced into the organohydrogel network via a solvent displacement strategy. Owing to the water-locking effects of glycerol and tough polymeric backbone, the resultant organohydrogel not only exhibits stable tensibility but also maintains excellent flexibility and stable conductivity with the environment-resistant properties, including freezing resistance against -30 °C and moisture retention at 4% relative humidity in a high temperature of 60 °C. Moreover, a series of organohydrogel-based sensors and an array device are developed to achieve highly sensitive strain, temperature, and humidity responses and exhibit a high gauge factor of 10.79 in the strain-sensitive test. This work develops a universal ionic skin based on organohydrogels to be applied to wearable sensors for health monitoring.

A nanoforest-based humidity sensor for respiration monitoring

Traditional humidity sensors for respiration monitoring applications have faced technical challenges, including low sensitivity, long recovery times, high parasitic capacitance and uncalibrated temperature drift. To overcome these problems, we present a triple-layer humidity sensor that comprises a nanoforest-based sensing capacitor, a thermistor, a microheater and a reference capacitor. When compared with traditional polyimide-based humidity sensors, this novel device has a sensitivity that is improved significantly by 8 times within a relative humidity range of 40–90%. Additionally, the integration of the microheater into the sensor can help to reduce its recovery time to 5 s. The use of the reference capacitor helps to eliminate parasitic capacitance, and the thermistor helps the sensor obtain a higher accuracy. These unique design aspects cause the sensor to have an excellent humidity sensing performance in respiration monitoring applications. Furthermore, through the adoption of machine learning algorithms, the sensor can distinguish different respiration states with an accuracy of 94%. Therefore, this humidity sensor design is expected to be used widely in both consumer electronics and intelligent medical instrument applications.

Humidity-Based Human-Machine Interaction System for Healthcare Applications

Human-machine interaction (HMI) systems are widely used in the healthcare field, and they play an essential role in assisting the rehabilitation of patients. Currently, a large number of HMI-related research studies focus on piezoresistive sensors, self-power sensors, visual and auditory receivers, and so forth. These sensing modalities do not possess high reliability with regard to breathing condition detection. The humidity signal conveyed by breathing provides excellent stability and a fast response; however, humidity-based HMI systems have rarely been studied. Herein, we integrate a humidity sensor and a graphene thermoacoustic device into a humidity-based HMI system (HHMIS), which is capable of monitoring respiratory signals and emitting acoustic signals. HHMIS has a practical value in healthcare to assist patients. For example, it works as a prewarning system for respiratory-related disease patients with abnormal respiratory rates, and as an artificial throat device for aphasia patients. Achieved based on a laser direct writing technology, this wearable device features low cost, high flexibility, and can be prepared on a large scale. This portable non-contact HMMIS has broad application prospects in many fields such as medical health and intelligent control.

A Wearable Bioimpedance Chest Patch for Real-Time Ambulatory Respiratory Monitoring

OBJECTIVE

This paper aims to introduce a wearable solution and a low-complexity algorithm for real-time continuous ambulatory respiratory monitoring.

METHODS

A wearable chest-worn patch is designed using a bioimpedance (BioZ) sensor to measure the changes in chest impedance caused by breathing. Besides, a medical-grade infrared temperature sensor is utilized to monitor body temperature. The computing algorithm implemented on the patch enables computation of breath-by-breath respiratory rate and chest temperature in real-time. Two wireless communication protocols are included in the system, namely Bluetooth and Long Range (LoRa), which enable both short-range and long-range data transmission.

RESULTS

The breathing rate measured in static (i.e., standing, sitting, supine, and lateral lying) and dynamic (i.e., walking, running, and cycling) positions by our device yielded an accuracy of more than 97.8% and 98.5% to the ground truth, respectively. Additionally, the devices performance is evaluated in real-world scenarios both indoors and outdoors.

CONCLUSION

The proposed system is capable of measuring breathing rate throughout a variety of daily activities. To the best of our knowledge, this is the first BioZ-based wearable patch capable of detecting breath-by-breath respiratory rate in real-time remotely under unrestricted ambulatory conditions.

SIGNIFICANCE

This study establishes a strategy for continuous respiratory monitoring that could aid in the early detection of cardiopulmonary disorders in everyday life.

Polyimide-Sputtered and Polymerized Films with Ultrahigh Moisture Sensitivity for Respiratory Monitoring and Contactless Sensing

Respiratory monitoring and contactless sensing using the moisture produced by respiration and perspiration have garnered considerable attention in recent years. In this study, we fabricated polyimide-sputtered and polymerized (PSP) humidity sensors with ultrahigh capacitive sensitivity, fast response, and a wide working range of relative humidity (RH). The sensors produced >40 000 times increment in the sensing signal over the 10-95% RH range at 10 Hz and exhibited good performance at low RH levels (<40%) as well. These sensors displayed excellent sensing properties with small hysteresis, long-time stability, and fast response and recovery times (2.4 and 1.2 s, respectively). In the mechanism study of PSP humidity sensors, we found that the high sensitivity can be attributed to massive hydrophilic functional groups formed on the polymer chains by moist aging with oxidation and the fast response speed is due to the mesoporous structure of PSP films. We also fabricated a 5 × 5 array of PSP humidity sensors to identify the shapes of wet objects and of leaves during transpiration. Thus, we reported a novel and effective method for fabricating high-performance humidity polymer films, channeling new pathways for the development of advanced humidity and gas sensors.

Simultaneous Biomechanical and Biochemical Monitoring for Self-Powered Breath Analysis

The high moisture level of exhaled gases unavoidably limits the sensitivity of breath analysis via wearable bioelectronics. Inspired by pulmonary lobe expansion/contraction observed during respiration, a respiration-driven triboelectric sensor (RTS) was devised for simultaneous respiratory biomechanical monitoring and exhaled acetone concentration analysis. A tin oxide-doped polyethyleneimine membrane was devised to play a dual role as both a triboelectric layer and an acetone sensing material. The prepared RTS exhibited excellent ability in measuring respiratory flow rate (2-8 L/min) and breath frequency (0.33-0.8 Hz). Furthermore, the RTS presented good performance in biochemical acetone sensing (2-10 ppm range at high moisture levels), which was validated via finite element analysis. This work has led to the development of a novel real-time active respiratory monitoring system and strengthened triboelectric-chemisorption coupling sensing mechanism.

Azobenzene/Acid Binary Systems for Colorimetric Humidity Sensing with Reversibility, High Sensitivity, and Tunable Colors

Humidity sensors are important for humidity detection in many storage and manufacturing processes. Issues like sensibility, response rate, controllability, and material and preparation process costs need to be taken into consideration for practical applications. Herein, we report an investigation on a series of azobenzene/acid binary systems using easily accessible compounds, whose thin films display reversible and widely tunable color changes in response to humidity stimulation, with high sensitivity, fast color change, and recovery speed. The interesting properties for colorimetric humidity sensing are showcased with potential applications in dynamic art painting, smart windows, and respiration monitoring.

Respiratory Monitoring by Ultrafast Humidity Sensors with Nanomaterials: A Review

Respiratory monitoring is a fundamental method to understand the physiological and psychological relationships between respiration and the human body. In this review, we overview recent developments on ultrafast humidity sensors with functional nanomaterials for monitoring human respiration. Key advances in design and materials have resulted in humidity sensors with response and recovery times reaching 8 ms. In addition, these sensors are particularly beneficial for respiratory monitoring by being portable and noninvasive. We systematically classify the reported sensors according to four types of output signals: impedance, light, frequency, and voltage. Design strategies for preparing ultrafast humidity sensors using nanomaterials are discussed with regard to physical parameters such as the nanomaterial film thickness, porosity, and hydrophilicity. We also summarize other applications that require ultrafast humidity sensors for physiological studies. This review provides key guidelines and directions for preparing and applying such sensors in practical applications.

A Camel Nose-Inspired Highly Durable Neuromorphic Humidity Sensor with Water Source Locating Capability

Numerous emerging applications in modern society require humidity sensors that are not only sensitive and specific but also durable and intelligent. However, conventional humidity sensors do not have all of these simultaneously because they require very different or even contradictory design principles. Here, inspired by camel noses, we develop a porous zwitterionic capacitive humidity sensor. Relying on the synergistic effect of a porous structure and good chemical and thermal stabilities of hygroscopic zwitterions, this sensor simultaneously exhibits high sensitivity, discriminability, excellent durability, and, in particular, the highest respond speed among reported capacitive humidity sensors, with demonstrated applications in the fast discrimination between fresh, stale, and dry leaves, high-resolution touchless human-machine interactive input devices, and the real-time monitoring humidity level of a hot industrial exhaust. More importantly, this sensor exhibits typical synapse behaviors such as paired-pulse facilitation due to the strong binding interactions between water and zwitterions. This leads to learning and forgetting features with a tunable memory, thus giving the sensor artificial intelligence and enabling the location of water sources. This work offers a general design principle expected to be applied to develop other high-performance biochemical sensors and the next-generation intelligent sensors with much broader applications.

Polymeric piezoresistive airflow sensor to monitor respiratory patterns

Monitoring human respiratory patterns is of great importance as it gives essential information for various medical conditions, e.g. sleep apnoea syndrome and chronic obstructive pulmonary disease and asthma, etc. Herein, we have developed a polymeric airflow sensor based on nanocomposites of vertically grown graphene nanosheets (VGNs) with polydimethylsiloxane (PDMS) and explored their applications in monitoring human respiration. The sensing performance of the VGNs/PDMS nanocomposite was characterized by exposing to a range of airflow rates (20-130 l min-1), and a linear performance with high sensitivity and low response time (mostly below 1 s) was observed. To evaluate the experimental results, finite-element simulation models were developed in the COMSOL Multiphysics package. The piezoresistive properties of VGNs/PDMS thin film and fluid-solid interaction were thoroughly studied. Laser Doppler vibrometry measures of sensor tip displacement closely approximated simulated deflection results and validated the dynamic response of the sensor. By comparing the proposed sensor and some other airflow sensors in the literature, it is concluded that the VGNs/PDMS airflow sensor has excellent features in terms of sensor height, detection range and sensitivity. The potential application of the VGNs/PDMS airflow sensor in detecting the respiration pattern of human exercises like walking, jogging and running has been demonstrated.

Enabling Longitudinal Respiration Monitoring Using Vapor-Coated Conducting Textiles

Wearable sensors allow for portable, long-term health monitoring in natural environments. Recently, there has been an increase in demand for technology that can reliably monitor respiration, which can be indicative of cardiac diseases, asthma, and infection by respiratory viruses. However, to date, the most reliable respiration monitoring system involves a tightly worn chest belt that is not conducive to longitudinal monitoring. Herein, we report that accurate respiration monitoring can be effected using a fabric-based humidity sensor mounted within a face mask. Our humidity sensor is created using cotton fabrics coated with a persistently p-doped conjugated polymer, poly(3,4-ethylenedioxythiophene):chloride (PEDOT-Cl), using a previously reported chemical vapor deposition process. The vapor-deposited polymer coating displays a stable, rapid, and reversible change in conductivity with an increase in local humidity, such as the humidity changes experienced within a face mask as the wearer breathes. Thus, when integrated into a face mask, the PEDOT-Cl-coated cotton humidity sensor is able to transduce breaths into an electrical signal. The humidity sensor-incorporated face mask is able to differentiate between deep and shallow breathing, as well as breathing versus talking. The sensor-incorporated face mask platform also functions both while walking and sitting, providing equally high signal quality in both indoor and outdoor contexts. Additionally, we show that the face mask can be worn for long periods of time with a negligible decline in the signal quality.

Humidity sensors based on metal organic frameworks derived polyelectrolyte films

Application of metal organic frameworks (MOFs) on sensors is of great interest for researchers. Film forming ability of the sensing material is very important for both the preparation process and sensing properties of the devices. Humidity sensors based on UIO-66 derived polyelectrolyte films were well prepared by in situ thiol-ene click cross-linking polymerization in this work. The hydrophilicity of the sensing film could be controlled by the feed ratios. The optimized humidity sensor shows a fast response to RHs change (Res/Rec time is 3.1 s/1.5 s, respectively) with ∼1.2% RH of humidity hysteresis. The water molecules adsorption behavior of the film and the sensing mechanism were also be investigated. The humidity sensor with good water and thermal stability and repeatability was applied in breath monitoring, which can well distinguish different breath states.

A High-Wet-Strength Biofilm for Readable and Highly Sensitive Humidity Sensors

Low-cost and flexible biofilm humidity sensors with good wet strength are crucial for humidity detection. However, it remains a great challenge to integrate good reversibility, rapid humidity response, and robust humid mechanical strength in one sensor. In this respect, we report a facile method to prepare a sustainable biofilm (named MC film) from sisal cellulose microcrystals (MSF-g-COOH) and citric acid (CA). After cross-linking with CA, the MC film exhibits excellent wet strength and rapid humidity response. More importantly, MC film can be used over a wide temperature range with excellent durability and reversibility for humidity detection. A highly sensitive humidity sensor fabricated from the MC film exhibits high reversibility and excellent water resistance and can be applied in humidity and personalized breath health monitoring. Our work fills the gap between biomaterial design and high-performance sensing devices.

Enhanced Selectivity in Volatile Organic Compound Gas Sensors Based on ReS2-FETs under Light-Assisted and Gate-Bias Tunable Operation

Using a single-device two-dimensional (2D) rhenium disulfide (ReS₂) field-effect transistor (FET) with enhanced gas species selectivity by light illumination, we reported a selective and sensitive detection of volatile organic compound (VOC) gases. 2D materials have the advantage of a high surface-area-to-volume ratio for high sensitivity to molecules attached to the surface and tunable carrier concentration through field-effect control from the back-gate of the channel, while keeping the top surface open to the air for chemical sensing. In addition to these advantages, ReS₂ has a direct band gap also in multilayer cases, which sets it apart from other transition-metal dichalcogenides (TMDCs). We take advantage of the effective response of ReS₂ to light illumination to improve the selectivity and gas-sensing efficiency of a ReS₂-FET device. We found that light illumination modulates the drain current response in a ReS₂-FET to adsorbed molecules, and the sensing activity differs depending on the gas species used, such as acetone, ethanol, and methanol. Furthermore, wavelength and carrier density rely on certain variations in light-modulated sensing behaviors for each chemical. The device will distinguish the gas concentration in a mixture of VOCs using the differences induced by light illumination, enhancing the selectivity of the sensor device. Our results shed new light on the sensing technologies for realizing a large-scale sensor network in the Internet-of-Things era.

Single-sided and integrated polyaniline/ poly(vinylidene fluoride) flexible membrane with micro/nanostructures as breathable, nontoxic and fast response wearable humidity sensor

Harmless and breathable flexible humidity sensor has important applications in continuous and real-time detection of human physiological activities. In this work, with hydrophobic poly (vinylidene fluoride) (PVDF) membrane as both the template and substrate and cetyltrimethylammonium bromide as a structure regulator, polyaniline (PANI) was unilaterally deposited on a PVDF microporous membrane to facilely fabricate a single-sided integrated flexible humidity sensor (IFHS). Such IFHS is featured with unique micro/nano structure and good air permeability. Moreover, it exhibits good humidity sensing properties at room temperature including fast response, small hysteresis and stable response even under bending deformation. The flexible sensor could realize non-contact monitoring of human respiration and speaking activities. Unilateral deposition of PANI and good breathability of IFHS avoids direct contact between PANI and human skin, thus averting harms to human and minimizing the deterioration of humidity sensing properties of PANI layer. The simple method is universal to the preparation of single-sided, integrated, breathable, nontoxic and fast response wearable humidity sensors based on PANI and hydrophobic microporous polymer membranes, offering useful references for the construction of advanced flexible sensors.

Review: Sensors for Biosignal/Health Monitoring in Electronic Skin

Skin is the largest sensory organ and receives information from external stimuli. Human body signals have been monitored using wearable devices, which are gradually being replaced by electronic skin (E-skin). We assessed the basic technologies from two points of view: sensing mechanism and material. Firstly, E-skins were fabricated using a tactile sensor. Secondly, E-skin sensors were composed of an active component performing actual functions and a flexible component that served as a substrate. Based on the above fabrication processes, the technologies that need more development were introduced. All of these techniques, which achieve high performance in different ways, are covered briefly in this paper. We expect that patients' quality of life can be improved by the application of E-skin devices, which represent an applied advanced technology for real-time bio- and health signal monitoring. The advanced E-skins are convenient and suitable to be applied in the fields of medicine, military and environmental monitoring.

Quaternary Oxidized Carbon Nanohorns—Based Nanohybrid as Sensing Coating for Room Temperature Resistive Humidity Monitoring

We report the relative humidity (RH) sensing response of a resistive sensor, employing sensing layers, based on a quaternary organic–inorganic hybrid nanocomposite comprising oxidized carbon nanohorns (CNHox), graphene oxide (GO), tin dioxide, and polyvinylpyrrolidone (PVP), at 1/1/1/1 and 0.75/0.75/1/1/1 mass ratios. The sensing structure comprises a silicon substrate, a SiO2 layer, and interdigitated transducer (IDT) electrodes. The sensing film was deposited via the drop-casting method on the sensing structure. The morphology and the composition of the sensing layers were investigated through Scanning Electron Microscopy (SEM), X-ray diffraction (XRD), and RAMAN spectroscopy. The organic–inorganic quaternary hybrid-based thin film’s resistance increased when the sensors were exposed to relative humidity ranging from 0 to 100%. The manufactured devices show a room temperature response comparable to that of a commercial capacitive humidity sensor and characterized by excellent linearity, rapid response and recovery times, and good sensitivity. While the sensor with CNHox/GO/SnO2/PVP at 0.75/0.75/1/1 as the sensing layer has the best performance in terms of linearity and recovery time, the structures employing the CNHox/GO/SnO2/PVP at 1/1/1/1 (mass ratio) have a better performance in terms of relative sensitivity. We explained each constituent of the quaternary hybrid nanocomposites’ sensing role based on their chemical and physical properties, and mutual interactions. Different alternative mechanisms were taken into consideration and discussed. Based on the sensing results, we presume that the effect of the p-type semiconductor behavior of CNHox and GO, correlated with swelling of PVP, dominates and leads to the overall increasing resistance of the sensing layer. The hard–soft acid–base (HSAB) principle also supports this mechanism.

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.