Review of Flexible Temperature Sensing Networks for Wearable Physiological Monitoring

Electrospun multifunctional nanofibers for advanced wearable sensors

The multifunctional extension of fiber-based wearable sensors determines their integration and sustainable development, with electrospinning technology providing reliable, efficient, and scalable support for fabricating these sensors. Despite numerous studies on electrospun fiber-based wearable sensors, further attention is needed to leverage composite structural engineering for functionalizing electrospun fibers. This paper systematically reviews the research progress on fiber-based multifunctional wearable sensors in terms of design concept, device fabrication, mechanism exploration, and application potential. Firstly, the basics of electrospinning are briefly introduced, including its development, principles, parameters, and material selection. Tactile sensors, as crucial components of wearable sensors, are discussed in detail, encompassing their performance parameters, transduction mechanisms, and preparation strategies for pressure, strain, temperature, humidity, and bioelectrical signal sensors. The main focus of the article is on the latest research progress in multifunctional sensing design concepts, multimodal decoupling mechanisms, sensing mechanisms, and functional extensions. These extensions include multimodal sensing, self-healing, energy harvesting, personal thermal management, EMI shielding, antimicrobial properties, and other capabilities. Furthermore, the review assesses existing challenges and outlines future developments for multifunctional wearable sensors, highlighting the need for continued research and innovation.

Luminescent Lanthanide Infinite Coordination Polymers for Ratiometric Sensing Applications

Ratiometric lanthanide coordination polymers (Ln-CPs) are advanced materials that combine the unique optical properties of lanthanide ions (e.g., Eu3+, Tb3+, Ce3+) with the structural flexibility and tunability of coordination polymers. These materials are widely used in biological and chemical sensing, environmental monitoring, and medical diagnostics due to their narrow-band emission, long fluorescence lifetimes, and excellent resistance to photobleaching. This review focuses on the composition, sensing mechanisms, and applications of ratiometric Ln-CPs. The ratiometric fluorescence mechanism relies on two distinct emission bands, which provides a self-calibrating, reliable, and precise method for detection. The relative intensity ratio between these bands varies with the concentration of the target analyte, enabling real-time monitoring and minimizing environmental interference. This ratiometric approach is particularly suitable for detecting trace analytes and for use in complex environments where factors like background noise, temperature fluctuations, and light intensity variations may affect the results. Finally, we outline future research directions for improving the design and synthesis of ratiometric Ln-CPs, such as incorporating long-lifetime reference luminescent molecules, exploring near-infrared emission systems, and developing up-conversion or two-photon luminescent materials. Progress in these areas could significantly broaden the scope of ratiometric Ln-CP applications, especially in biosensing, environmental monitoring, and other advanced fields.

Recent Advances in Nanomaterial-Based Biosignal Sensors

Recent research for medical fields, robotics, and wearable electronics aims to utilize biosignal sensors to gather bio-originated information and generate new values such as evaluating user well-being, predicting behavioral patterns, and supporting disease diagnosis and prevention. Notably, most biosignal sensors are designed for body placement to directly acquire signals, and the incorporation of nanomaterials such as metal-based nanoparticles or nanowires, carbon-based or polymer-based nanomaterials-offering stretchability, high surface-to-volume ratio, and tunability for various properties-enhances their adaptability for such applications. This review categorizes nanomaterial-based biosignal sensors into three types and analyzes them: 1) biophysical sensors that detect deformation such as folding, stretching, and even pulse, 2) bioelectric sensors that capture electric signal originating from human body such as heart and nerves, and 3) biochemical sensors that catch signals from bio-originated fluids such as sweat, saliva and blood. Then, limitations and improvements to nanomaterial-based biosignal sensors is depicted. Lastly, it is highlighted on deep learning-based signal processing and human-machine interface applications, which can enhance the potential of biosignal sensors. Through this paper, it is aim to provide an understanding of nanomaterial-based biosignal sensors, outline the current state of the technology, discuss the challenges that be addressed, and suggest directions for development.

Advances in flexible ionic thermal sensors: present and perspectives

Ionic thermal sensors (ITSs) represent a promising frontier in sensing technology, offering unique advantages over conventional electronic sensors. Comprising a polymer matrix and electrolyte, these sensors possess inherent flexibility, stretchability, and biocompatibility, allowing them to establish stable and intimate contact with soft surfaces without inducing mechanical or thermal stress. Through an ion migration/dissociation mechanism similar to biosensing, ITSs ensure low impedance contact and high sensitivity, especially in physiological monitoring applications. This review provides a comprehensive overview of ionic thermal sensing mechanisms, contrasting them with their electronic counterparts. Additionally, it explores the intricacy of the sensor architecture, detailing the roles of active sensing elements, stretchable electrodes, and flexible substrates. The decoupled sensing mechanisms for skin-inspired multimodal sensors are also introduced based on several representative examples. The latest applications of ITS are categorized into ionic skin (i-skin), healthcare, spatial thermal perception, and environment detection, regarding their materials, structures, and operation modes. Finally, the perspectives of ITS research are presented, emphasizing the significance of standardized sensing parameters and emerging requirements for practical applications.

PEDOT:PSS-Based Wearable Flexible Temperature Sensor and Integrated Sensing Matrix for Human Body Monitoring

Flexible temperature sensors have been widely used in electronic skins and health monitoring. Body temperature as one of the key physiological signals is crucial for detecting human body's abnormalities, which necessitates high sensitivity, quick responsiveness, and stable monitoring. In this paper, we reported a resistive temperature sensor designed as an ultrathin laminated structure with a serpentine pattern and a bioinspired adhesive layer, which was fabricated with a composite of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)/single-wall carbon nanotubes/reduced graphene oxide (PEDOT:PSS/SWCNTs/rGO) and polydimethylsiloxane (PDMS). The temperature sensor exhibited a high temperature sensitivity of 0.63% °C-1, coupled with outstanding linearity of 0.98 within 25-45 °C. Furthermore, it showed fast response and recovery speeds of 4.8 and 5.8 s, respectively, between 25 and 36 °C. It also demonstrated exceptional stability when subjected to stress and bending disturbances with the maximum bending interference deviation of 0.03%. Additionally, it displayed good cyclic stability over a broad temperature range from 25 to 85 °C, and the standard deviation at 25 °C is 0.14%. A series of experiments including blowing detection, respiratory monitoring with or without a mask, and during rest or sleep were conducted to show the potential of the flexible temperature sensors in human body monitoring. Furthermore, a 4 × 4 flexible temperature sensor matrix was integrated to detect and map objects such as wrenches and blood vessels through human hand skin. The results were consistent with those of infrared measurements. The flexible temperature sensor is capable of real-time temperature monitoring and has the potential in tracking human respiration, assessing sleep quality, and mapping the temperature of various objects.

Toward Integrated Multifunctional Laser-Induced Graphene-Based Skin-Like Flexible Sensor Systems

The burgeoning demands for health care and human-machine interfaces call for the next generation of multifunctional integrated sensor systems with facile fabrication processes and reliable performances. Laser-induced graphene (LIG) with highly tunable physical and chemical characteristics plays vital roles in developing versatile skin-like flexible or stretchable sensor systems. This Progress Report presents an in-depth overview of the latest advances in LIG-based techniques in the applications of flexible sensors. First, the merits of the LIG technique are highlighted especially as the building blocks for flexible sensors, followed by the description of various fabrication methods of LIG and its variants. Then, the focus is moved to diverse LIG-based flexible sensors, including physical sensors, chemical sensors, and electrophysiological sensors. Mechanisms and advantages of LIG in these scenarios are described in detail. Furthermore, various representative paradigms of integrated LIG-based sensor systems are presented to show the capabilities of LIG technique for multipurpose applications. The signal cross-talk issues are discussed with possible strategies. The LIG technology with versatile functionalities coupled with other fabrication strategies will enable high-performance integrated sensor systems for next-generation skin electronics.

A Review of Conductive Hydrogel-Based Wearable Temperature Sensors

Conductive hydrogel has garnered significant attention as an emergent candidate for diverse wearable sensors, owing to its remarkable and tailorable properties such as flexibility, biocompatibility, and strong electrical conductivity. These attributes make it highly suitable for various wearable sensor applications (e.g., biophysical, bioelectrical, and biochemical sensors) that can monitor human health conditions and provide timely interventions. Among these applications, conductive hydrogel-based wearable temperature sensors are especially important for healthcare and disease surveillance. This review aims to provide a comprehensive overview of conductive hydrogel-based wearable temperature sensors. First, this work summarizes different types of conductive fillers-based hydrogel, highlighting their recent developments and advantages as wearable temperature sensors. Next, this work discusses the sensing characteristics of conductive hydrogel-based wearable temperature sensors, focusing on sensitivity, dynamic stability, stretchability, and signal output. Then, state-of-the-art applications are introduced, ranging from body temperature detection and wound temperature detection to disease monitoring. Finally, this work identifies the remaining challenges and prospects facing this field. By addressing these challenges with potential solutions, this review hopes to shed some light on future research and innovations in this promising field.

Recent Advances in Flexible Temperature Sensors: Materials, Mechanism, Fabrication, and Applications

Flexible electronics is an emerging and cutting-edge technology which is considered as the building blocks of the next generation micro-nano electronics. Flexible electronics integrate both active and passive functions in devices, driving rapid developments in healthcare, the Internet of Things (IoT), and industrial fields. Among them, flexible temperature sensors, which can be directly attached to human skin or curved surfaces of objects for continuous and stable temperature measurement, have attracted much attention for applications in disease prediction, health monitoring, robotic signal sensing, and curved surface temperature measurement. Preparing flexible temperature sensors with high sensitivity, fast response, wide temperature measurement interval, high flexibility, stretchability, low cost, high reliability, and stability has become a research target. This article reviewed the latest development of flexible temperature sensors and mainly discusses the sensitive materials, working mechanism, preparation process, and the applications of flexible temperature sensors. Finally, conclusions based on the latest developments, and the challenges and prospects for research in this field are presented.

Functional fibers/textiles for smart sensing devices and applications in personal healthcare systems

Personalized medical diagnostics and monitoring have become increasingly important due to inefficient and delayed medical services of traditional centralized healthcare systems. To enhance the comfort and portability, flexible health monitoring systems have been developed in recent years. In particular, smart fiber/textile-based sensing devices show superiority for continuously monitoring personal health and vital physiological parameters owing to their light weight, good flexibility and inherent miniaturization. This review focuses on the recent advances in smart fiber/textile-based sensing devices for wearable electronic applications. First, fabrication strategies of smart sensing fibers/textiles are introduced in detail. In addition, sensing performances, working principles and applications of smart sensing fibers/textiles such as pressure sensing fibers/textiles, stretchable strain sensing fibers/textiles, temperature sensing fibers/textiles, and biofluid, gas and humidity sensing fibers/textiles in health monitoring are also reviewed systematically. Finally, we propose current challenges and future prospects in the area of fiber/textile-based sensors for wearable healthcare monitoring and diagnosis systems. In general, this review aims to give an overall perspective of the promising field by reviewing various fiber/textile-based sensing devices and highlighting the importance for researchers to keep up with the sequential exploration of soft sensing fibers/textiles for applications in wearable smart systems.

Temperature dependent Raman spectroscopy and sensing performance of 2D black phosphorus

Temperature is an important parameter to be monitored in new wearable electronic devices. Layered black phosphorus (BP) has inherent good thermal stability and semiconductor properties and has a promising application as a temperature sensing layer. Here, we investigate the temperature sensing properties of BP, using in situ Raman spectroscopy and x-ray diffraction techniques. Flexible sensors are constructed, and temperature response is investigated in the range of 6-38 °C. The prospect application for monitoring the temperature of human body parts is demonstrated. The results show that the BP-based temperature sensors demonstrate good negative temperature coefficient characteristics and display high sensitivity and reproducible sensing performance. The temperature-dependent performance suggests the feasibility of BP as a sensitive layer in a wide temperature range. This work paves the way for exploring new applications of amazing layered materials, such as BP, in wearable electronic devices.

Microneedle Patches-Integrated Transdermal Bioelectronics for Minimally Invasive Disease Theranostics

Wearable epidermal electronics with non- or minimally-invasive characteristics can collect, transduce, communicate, and interact with accessible physicochemical health indicators on the skin. However, due to the stratum corneum layer, rich information about body health is buried under the skin stratum corneum layer, for example, in the skin interstitial fluid. Microneedle patches are typically designed with arrays of special microsized needles of length within 1000 µm. Such characteristics potentially enable the access and sample of biomolecules under the skin or give therapeutical treatment painlessly and transdermally. Integrating microneedle patches with various electronics allows highly efficient transdermal bioelectronics, showing their great promise for biomedical and healthcare applications. This comprehensive review summarizes and highlights the recent progress on integrated transdermal bioelectronics based on microneedle patches. The design criteria and state-of-the-art fabrication techniques for such devices are initially discussed. Next, devices with different functions, including but not limited to health monitoring, drug delivery, and therapeutical treatment, are highlighted in detail. Finally, key issues associated with current technologies and future opportunities are elaborated to sort out the state of recent research, point out potential bottlenecks, and provide future research directions.

Living Synthelectronics: A New Era for Bioelectronics Powered by Synthetic Biology

Bioelectronics, which converges biology and electronics, has attracted great attention due to their vital applications in human-machine interfaces. While traditional bioelectronic devices utilize nonliving organic and/or inorganic materials to achieve flexibility and stretchability, a biological mismatch is often encountered because human tissues are characterized not only by softness and stretchability but also by biodynamic and adaptive properties. Recently, a notable paradigm shift has emerged in bioelectronics, where living cells, and even viruses, modified via gene editing within synthetic biology, are used as core components in a new hybrid electronics paradigm. These devices are defined as "living synthelectronics," and they offer enhanced potential for interfacing with human tissues at informational and substance exchange levels. In this Perspective, the recent advances in living synthelectronics are summarized. First, opportunities brought to electronics by synthetic biology are briefly introduced. Then, strategic approaches to designing and making electronic devices using living cells/viruses as the building blocks, sensing components, or power sources are reviewed. Finally, the challenges faced by living synthelectronics are raised. It is believed that this paradigm shift will significantly contribute to the real integration of bioelectronics with human tissues.

Design of Stretchable and Conductive Self-Adhesive Hydrogels as Flexible Sensors by Guar-Gum-Enabled Dynamic Interactions

The limited elasticity and inadequate bonding of hydrogels made from guar gum (GG) significantly hinder their widespread implementation in personalized wearable flexible electronics. In this study, we devise GG-based self-adhesive hydrogels by creating an interpenetrating network of GG cross-linked with acrylic, 4-vinylphenylboronic acid, and Ca2+. With the leverage of the dynamic interactions (hydrogen bonds, borate ester bonds, and coordination bonds) between -OH in GG and monomers, the hydrogel exhibits a high stretchability of 700%, superior mechanical stress of 110 kPa, and robust adherence to several substrates. The adhesion strength of 54 kPa on porcine skin is obtained. Furthermore, the self-adhesive hydrogel possesses stable conductivity, an elevated gauge factor (GF), and commendable durability. It can be affixed to the human body as a strain sensor to obtain precise monitoring of human movement behavior. Our research offers possibilities for the development of GG-based hydrogels and applications in wearable electronics and medical monitoring.

Bio-Inspired Sensory Receptors for Artificial-Intelligence Perception

In the era of artificial intelligence (AI), there is a growing interest in replicating human sensory perception. Selective and sensitive bio-inspired sensory receptors with synaptic plasticity have recently gained significant attention in developing energy-efficient AI perception. Various bio-inspired sensory receptors and their applications in AI perception are reviewed here. The critical challenges for the future development of bio-inspired sensory receptors are outlined, emphasizing the need for innovative solutions to overcome hurdles in sensor design, integration, and scalability. AI perception can revolutionize various fields, including human-machine interaction, autonomous systems, medical diagnostics, environmental monitoring, industrial optimization, and assistive technologies. As advancements in bio-inspired sensing continue to accelerate, the promise of creating more intelligent and adaptive AI systems becomes increasingly attainable, marking a significant step forward in the evolution of human-like sensory perception.

Flexible thermoelectric CMTs/KCl/gelatin composite for a wearable pressure and temperature sensor

Flexible sensors have promising applications in the fields of health monitoring and artificial intelligence, which have attracted much attention from researchers. However, the design and manufacture of sensors with multiple sensing functions (like simultaneously having both temperature and pressure sensing capabilities) still present a significant challenge. Here, an ionic thermoelectric sensor for synchronous temperature and pressure sensing was developed on the basis of a carbon microtubes (CMTs)/potassium chloride (KCl)/gelatin composite consisting of gelatin as the polymer matrix, CMTs as the conductive material and KCl as the ion source. The designed CMTs/KCl/gelatin composite with the good ductility (830%) and flexibility can achieve a Seebeck coefficient of 4 mV K-1 and a dual stimulus responsiveness to pressure and temperature. In addition, not only the movement of the human body (e.g., fingers, arms), but also the temperature difference between the human body and the environment, were able to be monitored by the designed CMTs/KCl/gelatin sensors. This study provides a novel strategy for the design and preparation of high-performance flexible sensors by utilizing ion-gel thermoelectric materials and promotes the research of temperature and pressure sensing technologies.

Inkjet-printed sub-zero temperature sensor for real-time monitoring of cold environments

Real-time monitoring of low temperatures (usually below 0 °C) or cold environments is a specific requirement that finds its high demand in the aerospace, pharmaceutical, food, and beverage industries to maintain the temperature at high altitudes or in refrigerators and cold storage. In general, this purpose is achieved by using a sub-zero temperature sensor coupled with a control system. However, the market available such temperature sensors are very expensive, and bulky, thus not being suitable for portable operation, and also they suffer from poor accuracy. Therefore, the development of high-performance, low-cost, lightweight, and portable sub-zero temperature sensors is highly desired. In our recent work, we developed such sensors and integrated them with auxiliary electronics to demonstrate their wireless operation for the continuous and real-time monitoring of cold environments. So, in order to obtain low-cost sensors a cost-effective inkjet printing technology was employed for the fabrication of devices. A lightweight polydimethylsiloxane (PDMS) was used as the substrate and an electrically conducting graphene nanocomposite was used as the temperature-sensing material. To obtain a functional graphene nanocomposite film with a thickness of 530 nm and a conductivity of ~189 S m-1, the printed graphene nanocomposite was photonically sintered using a xenon flash lamp. This step was crucial for obtaining a sensor on the soft PDMS platform. The graphene nanocomposite film exhibited a positive temperature coefficient resistance value of approximately 0.119 %/°C, and its resistance values varied almost linearly (with an Adjusted R2 value (model accuracy) of 0.99) with temperature within the operating range of -30 °C to 80 °C. The sensor was properly encapsulated for protection without significantly affecting its performance. The sensors demonstrated sufficient flexibility, with a bending radius of 20 mm, and sustained 500 continuous bending cycles. Finally, the real-time operation of the sensors was demonstrated by wirelessly transmitting and monitoring the temperature over a smartphone platform.

Ultrasensitive Colorimetric Luminescence Thermometry by Progressive Phase Transition

Luminescent materials that display quick spectral responses to thermal stimuli have attracted pervasive attention in sensing technologies. Herein, a programmable luminescence color switching in lanthanide-doped LiYO2 under thermal stimuli, based on deliberate control of the monoclinic (β) to tetragonal (α) phase transition in the crystal lattice, is reported. Specifically, a lanthanide-doping (Ln3+ ) approach to fine-tune the phase-transition temperature in a wide range from 294 to 359 K is developed. Accordingly, an array of Ln3+ -doped LiYO2 crystals that exhibit progressive phase transition, and thus sequential color switching at gradually increasing temperatures, is constructed. The tunable optical response to thermal stimuli is harnessed for colorimetric temperature indication and quantitative detection, demonstrating superior sensitivity and temperature resolution (Sr = 26.1% K-1 , δT = 0.008 K). The advances in controlling the phase-transition behavior of luminescent materials also offer exciting opportunities for high-performance personalized health monitoring.

Materials-Driven Soft Wearable Bioelectronics for Connected Healthcare

In the era of Internet-of-things, many things can stay connected; however, biological systems, including those necessary for human health, remain unable to stay connected to the global Internet due to the lack of soft conformal biosensors. The fundamental challenge lies in the fact that electronics and biology are distinct and incompatible, as they are based on different materials via different functioning principles. In particular, the human body is soft and curvilinear, yet electronics are typically rigid and planar. Recent advances in materials and materials design have generated tremendous opportunities to design soft wearable bioelectronics, which may bridge the gap, enabling the ultimate dream of connected healthcare for anyone, anytime, and anywhere. We begin with a review of the historical development of healthcare, indicating the significant trend of connected healthcare. This is followed by the focal point of discussion about new materials and materials design, particularly low-dimensional nanomaterials. We summarize material types and their attributes for designing soft bioelectronic sensors; we also cover their synthesis and fabrication methods, including top-down, bottom-up, and their combined approaches. Next, we discuss the wearable energy challenges and progress made to date. In addition to front-end wearable devices, we also describe back-end machine learning algorithms, artificial intelligence, telecommunication, and software. Afterward, we describe the integration of soft wearable bioelectronic systems which have been applied in various testbeds in real-world settings, including laboratories that are preclinical and clinical environments. Finally, we narrate the remaining challenges and opportunities in conjunction with our perspectives.

The Interconnecting Process and Sensing Performance of Stretchable Hybrid Electronic Yarn for Body Temperature Monitoring

Flexible and stretchable electronic yarn containing electronic components (i.e., hybrid electronic yarn) are essential for manufacturing smart textile garments or fabrics. Due to their low stretchability and easy interconnection fracture, previously reported hybrid electronic sensing yarns have poor mechanical durability and washability. In order to address this issue, a stretchable hybrid electronic yarn for body temperature monitoring was designed and prepared using a spandex filament as the core yarn and a thin enameled copper wire connected with a thermal resistor as the wrapping fiber. The temperature sensing performance of different hybrid electronic yarn samples was evaluated using the following three types of interconnection methods: conductive adhesive bonding, melt soldering, and hot pressure bonding. The optimal interconnection method with good sensing performance was determined. Furthermore, in order to improve the mechanical durability of the hybrid electronic yarn made using the optimal interconnection method, the interconnection area was encapsulated with polymers, and the effect of polymer materials and structures on the temperature-sensing properties was evaluated. The results show that traditional wrapping combined with hot pressing interconnection followed by tube encapsulating technology is beneficial for achieving high stretchability and good temperature-sensing performance of hybrid electronic yarn.

Doping Engineering of Conductive Polymers and Their Application in Physical Sensors for Healthcare Monitoring

Physical sensors have emerged as a promising technology for real-time healthcare monitoring, which tracks various physical signals from the human body. Accurate acquisition of these physical signals from biological tissue requires excellent electrical conductivity and long-term durability of the sensors under complex mechanical deformation. Conductive polymers, combining the advantages of conventional polymers and organic conductors, are considered ideal conductive materials for healthcare physical sensors due to their intrinsic conductive network, tunable mechanical properties, and easy processing. Doping engineering has been proposed as an effective approach to enhance the sensitivity, lower the detection limit, and widen the operational range of sensors based on conductive polymers. This approach enables the introduction of dopants into conductive polymers to adjust and control the microstructure and energy levels of conductive polymers, thereby optimizing their mechanical and conductivity properties. This review article provides a comprehensive overview of doping engineering methods to improve the physical properties of conductive polymers and highlights their applications in the field of healthcare physical sensors, including temperature sensors, strain sensors, stress sensors, and electrophysiological sensing. Additionally, the challenges and opportunities associated with conductive polymer-based physical sensors in healthcare monitoring are discussed.

Thermoregulation during Field Exercise in Horses Using Skin Temperature Monitoring

Hyperthermia and exertional heat illness (EHI) are performance and welfare issues for all exercising horses. Monitoring the thermoregulatory response allows for early recognition of metabolic heat accumulation during exercise and the possibility of taking prompt and effective preventative measures to avoid a further increase in core body temperature (Tc) leading to hyperthermia. Skin temperature (Tsk) monitoring is most used as a non-invasive tool to assess the thermoregulatory response pre- and post-exercise, particularly employing infrared thermographic equipment. However, only a few studies have used thermography to monitor skin temperature continuously during exercise. This commentary provides an overview of studies investigating surface skin temperature mainly by infrared thermography (IRT) during exercise. The scientific evidence, including methodologies, applications, and challenges associated with (continuous) skin temperature monitoring in horses during field exercise, is discussed. The commentary highlights that, while monitoring Tsk is straightforward, continuous Tsk alone does not always reliably estimate Tc evolvement during field exercise. In addition, inter-individual differences in thermoregulation need to be recognized and accounted for to optimize individual wellbeing. With the ongoing development and application of advanced wearable monitoring technology, there may be future advances in equipment and modeling for timely intervention with horses at hyperthermic risk to improve their welfare. However, at this point, infrared thermographic assessment of Tsk should always be used in conjunction with other clinical assessments and veterinary examinations for a reliable monitoring of the welfare of the horse.

Recent advances in wearable iontronic sensors for healthcare applications

Iontronic sensors have garnered significant attention as wearable sensors due to their exceptional mechanical performance and the ability to maintain electrical performance under various mechanical stimuli. Iontronic sensors can respond to stimuli like mechanical stimuli, humidity, and temperature, which has led to exploration of their potential as versatile sensors. Here, a comprehensive review of the recent researches and developments on several types of iontronic sensors (e.g., pressure, strain, humidity, temperature, and multi-modal sensors), in terms of their sensing principles, constituent materials, and their healthcare-related applications is provided. The strategies for improving the sensing performance and environmental stability of iontronic sensors through various innovative ionic materials and structural designs are reviewed. This review also provides the healthcare applications of iontronic sensors that have gained increased feasibility and broader applicability due to the improved sensing performance. Lastly, outlook section discusses the current challenges and the future direction in terms of the applicability of the iontronic sensors to the healthcare.

Self-Assembled and Multilayer-Overlapped ESM-PDA@rGO Nanofilm-Based Flexible Wearable Sensor for Real-Time Body Temperature Monitoring

There is an urgent need for wearable sensors that continuously monitor human physiological conditions in real time. Herein, an ESM-PDA@rGO-based flexible wearable temperature sensor was successfully constructed by integrating an eggshell membrane (ESM) with reduced graphene oxide (rGO) through dopamine (DA) polymerization. Depending on the "bridge effect" of diversified polydopamine (PDA) chains, on the one hand, a staggered arrangement of PDA-rGO frameworks and a lot of conductive pathways were produced and acted as an active layer. On the other hand, PDA-rGO frameworks were linked with ESM by PDA chains as a flexible sensing nanofilm. As a result, these mechanical merits of the ESM-PDA@rGO exhibited a 1.8-fold increase in Young' s modulus and 1.4-fold increase in tensile strength. Thereby, the conformability and performance of the temperature sensor were greatly enhanced, showing excellent sensitivity (-2.23%/°C), good linearity (R2 = 0.979), as well as stability. Especially, the flexible sensing nanofilm is evolved from the staggered arrangement of PDA-rGO frameworks, which endows it with rapid response (only 4-8 s), high resolution (0.1 °C), as well as excellent long-term durability (10 weeks). More importantly, the temperature sensor demonstrates insensitivity to bending deformation, ensuring reliable wearing stability. The sensor allows for online, real-time monitoring of human body temperature, encompassing both core (forehead, temple, cochlea, and exhale gas) and shell (palm and back of the hand, fingertip, and instep) temperatures. Particularly, it can accurately assess minor changes in peripheral body temperature before and after exercise, and it is capable of mapping daily patterns of body temperatures. The developed temperature sensor will provide us new materials design concepts and holds considerable promise in the fields of e-skin, disease surveillance, prediction, and diagnose.

Current and future technologies for monitoring cultured meat: A review

The high population growth rate, massive animal food consumption, fast economic progress, and limited food resources could lead to a food crisis in the future. There is a huge requirement for dietary proteins including cultured meat is being progressed to fulfill the need for meat-derived proteins in the diet. However, production of cultured meat requires monitoring numerous bioprocess parameters. This review presents a comprehensive overview of various widely adopted techniques (optical, spectroscopic, electrochemical, capacitive, FETs, resistive, microscopy, and ultrasound) for monitoring physical, chemical, and biological parameters that can improve the bioprocess control in cultured meat. The methods, operating principle, merits/demerits, and the main open challenges are reviewed with the aim to support the readers in advancing knowledge on novel sensing systems for cultured meat applications.

Flexible thin film thermocouples: From structure, material, fabrication to application

Flexible thin-film thermocouples (TFTCs) have been garnering interest as temperature sensors due to the advantages of being flexible, ultrathin, and ultralight. Additionally, they have fast response times and enable detection of temperature. These properties have made them suitable for applications such as wearable electronics, healthcare, portable personal devices, and smart detection systems. This review presents the progress in the development of flexible TFTCs. The mechanism, structural design, materials, fabrication methods, and related applications of flexible TFTCs are also elaborated. Finally, future development directions of flexible TFTCs are discussed such as wide-range temperature measurement, multiple sensor integration, and achieving reliable cold-end compensation systems.

Smart Detecting and Versatile Wearable Electrical Sensing Mediums for Healthcare

A rapidly expanding global population and a sizeable portion of it that is aging are the main causes of the significant increase in healthcare costs. Healthcare in terms of monitoring systems is undergoing radical changes, making it possible to gauge or monitor the health conditions of people constantly, while also removing some minor possibilities of going to the hospital. The development of automated devices that are either attached to organs or the skin, continually monitoring human activity, has been made feasible by advancements in sensor technologies, embedded systems, wireless communication technologies, nanotechnologies, and miniaturization being ultra-thin, lightweight, highly flexible, and stretchable. Wearable sensors track physiological signs together with other symptoms such as respiration, pulse, and gait pattern, etc., to spot unusual or unexpected events. Help may therefore be provided when it is required. In this study, wearable sensor-based activity-monitoring systems for people are reviewed, along with the problems that need to be overcome. In this review, we have shown smart detecting and versatile wearable electrical sensing mediums in healthcare. We have compiled piezoelectric-, electrostatic-, and thermoelectric-based wearable sensors and their working mechanisms, along with their principles, while keeping in view the different medical and healthcare conditions and a discussion on the application of these biosensors in human health. A comparison is also made between the three types of wearable energy-harvesting sensors: piezoelectric-, electrostatic-, and thermoelectric-based on their output performance. Finally, we provide a future outlook on the current challenges and opportunities.

UV polymerization fabrication method for polymer composite based optical fiber sensors

Optical fiber (OF) sensors are critical optical devices with excellent sensing capabilities and the capacity to operate in remote and hostile environments. However, integrating functional materials and micro/nanostructures into the optical fiber systems for specific sensing applications has limitations of compatibility, readiness, poor control, robustness, and cost-effectiveness. Herein, we have demonstrated the fabrication and integration of stimuli-responsive optical fiber probe sensors using a novel, low-cost, and facile 3D printing process. Thermal stimulus-response of thermochromic pigment micro-powders was integrated with optical fibers by incorporating them into ultraviolet-sensitive transparent polymer resins and then printed via a single droplet 3D printing process. Hence, the thermally active polymer composite fibers were grown (additively manufactured) on top of the commercial optical fiber tips. Then, the thermal response was studied within the temperature range of (25-35 °C) and (25-31 °C) for unicolor and dual color pigment powders-based fiber-tip sensors, respectively. The unicolor (with color to colorless transition) and dual color (with color to color transition) powders-based sensors exhibited substantial variations in transmission and reflection spectra by reversibly increasing and decreasing temperatures. The sensitivities were calculated from the transmission spectra where average change in transmission spectra was recorded as 3.5% with every 1 °C for blue, 3% for red and 1% for orange-yellow thermochromic powders based optical fiber tip sensors. Our fabricated sensors are cost-effective, reusable, and flexible in terms of materials and process parameters. Thus, the fabrication process can potentially develop transparent and tunable thermochromic sensors for remote sensing with a much simpler manufacturing process compared to conventional and other 3D printing processes for optical fiber sensors. Moreover, this process can integrate micro/nanostructures as patterns on the optical fiber tips to increase sensitivity. The developed sensors may be employed as remote temperature sensors in biomedical and healthcare applications.

Flexible Passive Sensor Patch with Contactless Readout for Measurement of Human Body Temperature

A passive flexible patch for human skin temperature measurement based on contact sensing and contactless interrogation is presented. The patch acts as an RLC resonant circuit embedding an inductive copper coil for magnetic coupling, a ceramic capacitor as the temperature-sensing element and an additional series inductor. The temperature affects the capacitance of the sensor and consequently the resonant frequency of the RLC circuit. Thanks to the additional inductor, the dependency of the resonant frequency from the bending of the patch has been reduced. Considering a curvature radius of the patch of up to 73 mm, the maximum relative variation in the resonant frequency has been reduced from 812 ppm to 7.5 ppm. The sensor has been contactlessly interrogated by a time-gated technique through an external readout coil electromagnetically coupled to the patch coil. The proposed system has been experimentally tested within the range of 32-46 °C, giving a sensitivity of -619.8 Hz/°C and a resolution of 0.06 °C.

Inkjet-Printed Polyelectrolyte Seed Layer-Based, Customizable, Transparent, Ultrathin Gold Electrodes and Facile Implementation of Photothermal Effect

Recently, interest in transparent electrodes has been increasing in biomedical engineering applications for such as electro-optical hybrid neuro-technologies. However, conventional photolithography-based electrode fabrication methods have limited design customization and large-area applicability. For biomedical engineering applications, it is crucial that we can easily customize the electrode design for different patients over a large body area. In this paper, we propose a novel method to fabricate customization-friendly, transparent, ultrathin, gold microelectrodes using inkjet printing technology. Unlike with typical direct printing of conductive inks, we inkjet-printed a polymer nucleation-inducing seed layer, followed by mask-less vacuum deposition of ultrathin gold (<6 nm) to produce selectively, high-transparency electrodes in the predefined shapes of the inkjet-printed polymer. Owing to the design flexibility of inkjet printing, the transparent ultrathin gold electrodes can be highly efficient in design customization over a large area. Simultaneously, a layer of nonconductive gold islands is formed in the nonprinted region, and this nanostructured layer can implement a photothermal effect that offers versatility for novel biomedical applications. As a demonstration of the effectiveness of these transparent electrodes, and the facile implementation of the photothermal effect for biomedical applications, we successfully fabricated transparent resistive temperature detectors. We used these to directly sense the photothermal effect and to demonstrate their bioimaging capabilities.

Nursing Management of Temperature in a Patient with Stroke

Fever is common in patients with stroke and is associated with worse outcomes. Studies in brain injury informed interventions commonly termed therapeutic temperature management (TTM) to improve the monitoring and management of fever. While the role and benefit of TTM in stroke patients has not been well studied, the nurse and healthcare team must extrapolate existing data to determine how to best monitor and apply TTM after stroke. Nurses should be knowledgeable about interventions to monitor and manage complications of TTM (eg, shivering), the studies underway to quantify the impact of fever treatment and emerging technology expected to improve TTM.

Wearable Two-Dimensional Nanomaterial-Based Flexible Sensors for Blood Pressure Monitoring: A Review

Flexible sensors have been extensively employed in wearable technologies for physiological monitoring given the technological advancement in recent years. Conventional sensors made of silicon or glass substrates may be limited by their rigid structures, bulkiness, and incapability for continuous monitoring of vital signs, such as blood pressure (BP). Two-dimensional (2D) nanomaterials have received considerable attention in the fabrication of flexible sensors due to their large surface-area-to-volume ratio, high electrical conductivity, cost effectiveness, flexibility, and light weight. This review discusses the transduction mechanisms, namely, piezoelectric, capacitive, piezoresistive, and triboelectric, of flexible sensors. Several 2D nanomaterials used as sensing elements for flexible BP sensors are reviewed in terms of their mechanisms, materials, and sensing performance. Previous works on wearable BP sensors are presented, including epidermal patches, electronic tattoos, and commercialized BP patches. Finally, the challenges and future outlook of this emerging technology are addressed for non-invasive and continuous BP monitoring.

Toward a new generation of permeable skin electronics

Skin-mountable electronics are considered to be the future of the next generation of portable electronics, due to their softness and seamless integration with human skin. However, impermeable materials limit device comfort and reliability for long-term, continuous usage. The recent emergence of permeable skin-mountable electronics has attracted tremendous attention in the soft electronics field. Herein, we provide a comprehensive and systematic review of permeable skin-mountable electronics. Typical porous materials and structures are first highlighted, followed by discussion of important device properties. Then, we review the latest representative applications of breathable skin-mountable electronics, such as bioelectrical sensors, temperature sensors, humidity and hydration sensors, strain and pressure sensors, and energy harvesting and storage devices. Finally, a conclusion and future directions for permeable skin electronics are provided.

Electronic Alert Signal for Early Detection of Tissue Injuries in Patients: An Innovative Pressure Sensor Mattress

Monitoring the early stage of developing tissue injuries requires intact skin for surface detection of cell damage. However, electronic alert signal for early detection is limited due to the lack of accurate pressure sensors for lightly pigmented skin injuries in patients. We developed an innovative pressure sensor mattress that produces an electronic alert signal for the early detection of tissue injuries. The electronic alert signal is developed using a web and mobile application for pressure sensor mattress reporting. The mattress is based on body distributions with reference points, temperature, and a humidity sensor to detect lightly pigmented skin injuries. Early detection of the pressure sensor is linked to an electronic alert signal at 32 mm Hg, a temperature of 37 °C, a relative humidity of 33.5%, a response time of 10 s, a loading time of 30 g, a density area of 1 mA, and a resistance of 7.05 MPa (54 N) at 0.87 m³/min. The development of the innovative pressure sensor mattress using an electronic alert signal is in line with its enhanced pressure detection, temperature, and humidity sensors.

Towards real-time thermal stress prediction systems for workers

Exposure to extreme temperatures in workplaces implies serious physical hazards to workers. In addition, a poorly acclimatized worker can have reduced performance and alertness. It may therefore be more vulnerable to the risk of accidents and injuries. Due to the incompatibility of standards and regulations with some work environments and a lack of thermal exchange in many personal protective equipment, heat stress remains among the most common physical risks in many industrial sectors. Furthermore, conventional methods of measuring physiological parameters in order to calculate personal thermophysiological constraints are not practical to use during work tasks. However, the emergence of wearable technologies can contribute to real-time measurement of body temperature and the biometric signals needed to assess thermophysiological constraints while actively working. Thus, the present study was carried out in order to scrutinize the current knowledge of these types of technologies by analyzing the available systems and the advances made in previous studies, as well as to discuss the efforts required to develop devices for the prevention of the occurrence of heat stress in real time.

Skin-Inspired Thermoreceptors-Based Electronic Skin for Biomimicking Thermal Pain Reflexes

Electronic systems possessing skin-like morphology and functionalities (electronic skins [e-skins]) have attracted considerable attention in recent years to provide sensory or haptic feedback in growing areas such as robotics, prosthetics, and interactive systems. However, the main focus thus far has been on the distributed pressure or force sensors. Herein a thermoreceptive e-skin with biological systems like functionality is presented. The soft, distributed, and highly sensitive miniaturized (≈700 µm2 ) artificial thermoreceptors (ATRs) in the e-skin are developed using an innovative fabrication route that involves dielectrophoretic assembly of oriented vanadium pentoxide nanowires at defined locations and high-resolution electrohydrodynamic printing. Inspired from the skin morphology, the ATRs are embedded in a thermally insulating soft nanosilica/epoxy polymeric layer and yet they exhibit excellent thermal sensitivity (-1.1 ± 0.3% °C-1 ), fast response (≈1s), exceptional stability (negligible hysteresis for >5 h operation), and mechanical durability (up to 10 000 bending and twisting loading cycles). Finally, the developed e-skin is integrated on the fingertip of a robotic hand and a biological system like reflex is demonstrated in response to temperature stimuli via localized learning at the hardware level.

Printed temperature sensor array for high-resolution thermal mapping

Fully-printed temperature sensor arrays—based on a flexible substrate and featuring a high spatial-temperature resolution—are immensely advantageous across a host of disciplines. These range from healthcare, quality and environmental monitoring to emerging technologies, such as artificial skins in soft robotics. Other noteworthy applications extend to the fields of power electronics and microelectronics, particularly thermal management for multi-core processor chips. However, the scope of temperature sensors is currently hindered by costly and complex manufacturing processes. Meanwhile, printed versions are rife with challenges pertaining to array size and sensor density. In this paper, we present a passive matrix sensor design consisting of two separate silver electrodes that sandwich one layer of sensing material, composed of poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS). This results in appreciably high sensor densities of 100 sensor pixels per cm$$^2$$ 2 for spatial-temperature readings, while a small array size is maintained. Thus, a major impediment to the expansive application of these sensors is efficiently resolved. To realize fast and accurate interpretation of the sensor data, a neural network (NN) is trained and employed for temperature predictions. This successfully accounts for potential crosstalk between adjacent sensors. The spatial-temperature resolution is investigated with a specially-printed silver micro-heater structure. Ultimately, a fairly high spatial temperature prediction accuracy of 1.22  °C is attained.

Skin bioelectronics towards long-term, continuous health monitoring

Skin bioelectronics are considered as an ideal platform for personalised healthcare because of their unique characteristics, such as thinness, light weight, good biocompatibility, excellent mechanical robustness, and great skin conformability. Recent advances in skin-interfaced bioelectronics have promoted various applications in healthcare and precision medicine. Particularly, skin bioelectronics for long-term, continuous health monitoring offer powerful analysis of a broad spectrum of health statuses, providing a route to early disease diagnosis and treatment. In this review, we discuss (1) representative healthcare sensing devices, (2) material and structure selection, device properties, and wireless technologies of skin bioelectronics towards long-term, continuous health monitoring, (3) healthcare applications: acquisition and analysis of electrophysiological, biophysical, and biochemical signals, and comprehensive monitoring, and (4) rational guidelines for the design of future skin bioelectronics for long-term, continuous health monitoring. Long-term, continuous health monitoring of advanced skin bioelectronics will open unprecedented opportunities for timely disease prevention, screening, diagnosis, and treatment, demonstrating great promise to revolutionise traditional medical practices.

A Fully Flexible Intelligent Thermal Touch Panel Based on Intrinsically Plastic Ag2 S Semiconductor

Wearable touch panels, a typical flexible electronic device, can recognize and feed back the information of finger touch and movement. Excellent wearable touch panels are required to accurately and quickly monitor the signals of finger movement as well as the capacity of bearing various types of deformation. High-performance thermistor materials are one of the key functional components, but to date, a long-standing bottleneck is that inorganic semiconductors are typically brittle while the electrical properties of organic semiconductors are quite low. Herein, a high-performance flexible temperature sensor is reported by using plastic Ag₂ S with ultrahigh temperature coefficient of resistance of -4.7% K^-1 and resolution of 0.05 K, and rapid response/recovery time of 0.11/0.11 s. Moreover, the temperature sensor shows excellent durability without performance damage or loss during force stimuli tests. In addition, a fully flexible intelligent touch panel composed of a 16 × 10 Ag₂ S-film-based temperature sensor array, as well as a flexible printed circuit board and a deep-learning algorithm is designed for perceiving finger touch signals in real-time, and intelligent feedback of Chinese characters and letters on an app. These results strongly show that high-performance flexible inorganic semiconductors can be widely used in flexible electronics.

Emerging Role of Liquid Metals in Sensing

Low melting point metals and alloys are the group of materials that combine metallic and liquid properties, simultaneously. The fascinating characteristics of liquid metals (LMs) including softness and high electrical and thermal conductivity, as well as their unique interfacial chemistry, have started to dominate various research disciplines. Utilization of LMs as responsive interfaces, enabling sensing in a flexible and versatile manner, is one of the most promising traits demonstrated for LMs. In the context of LMs-enabled sensors, gallium (Ga) and its alloys have emerged as multipurpose functional materials with many compelling physical and chemical properties. Responsiveness to different stimuli and easy-to-functionalize interfaces of Ga-based LMs make them ideal candidates for a variety of sensing applications. However, despite the vast capabilities of Ga-based LMs in sensing, applications of these materials for developing different sensors have not been fully explored. In the present review, we provide a comprehensive overview regarding the applications of Ga-based LMs in a wide range of sensing approaches that cover different physical and chemical sensors. The unique features of Ga-based LMs, which make them promising materials for sensing, are discussed in subsections followed by relevant case studies. Finally, challenges as well as the prospected future and developing motifs are highlighted for each type of LM-based sensors.

Highly sensitive, flexible and biocompatible temperature sensor utilizing ultra-long Au@AgNW-based polymeric nanocomposites

Owing to their excellent sensitivity, stretchability, flexibility and conductivity, polymeric nanocomposites with conductive fillers have shown promise for a wide range of applications in bioelectronics and wearable devices. Herein, we report on the development of a flexible and biocompatible polymeric nanocomposite comprising ultra-long Ag-Au core-sheath nanowires (Au@AgNWs) dispersed in elastomeric media to fabricate a high-resolution wearable temperature sensor. Ultra-long AgNWs with an aspect ratio of about 1500 were synthesized using a Ca²⁺ ion-mediated facile one-pot polyol process. To enhance the biocompatibility and anti-oxidative property of the AgNWs, a 10-20 nm gold (Au) layer was conformably deposited without affecting the original nanowire morphology. The core-sheath structure of Au@AgNWs was characterized using HRTEM and EDS elemental mapping while the biocompatibility and anti-oxidative properties were tested using hydrogen peroxide (H₂O₂) etching in solution phase. Finally, the fabricated nanowires were used to prepare the Au@AgNW-poly-ethylene glycol (PEG)-polyurethane (PU)-based nanocomposite ink which can be printed on interdigitated electrodes to fabricate a thermoresistive temperature sensor with negative temperature coefficient (NTC) of resistance and quick response time (<100 s). The Au@AgNW-PEG-PU nanocomposite was characterized in detail and a novel temperature sensing mechanism based on controlling the internanowire distance of the PEG coated Au@AgNWs percolation by means of capillarity force among the nanowires as a result of the glass transition temperature of thermosensitive PEG was demonstrated. The proposed printable temperature sensor is flexible and biocompatible and shows promise for a range of wearable applications.

Recent Advances in Multiresponsive Flexible Sensors towards E-skin: A Delicate Design for Versatile Sensing

Multiresponsive flexile sensors with strain, temperature, humidity, and other sensing abilities serving as real electronic skin (e-skin) have manifested great application potential in flexible electronics, artificial intelligence (AI), and Internet of Things (IoT). Although numerous flexible sensors with sole sensing function have already been reported since the concept of e-skin, that mimics the sensing features of human skin, was proposed about a decade ago, the ones with more sensing capacities as new emergences are urgently demanded. However, highly integrated and highly sensitive flexible sensors with multiresponsive functions are becoming a big thrust for the detection of human body motions, physiological signals (e.g., skin temperature, blood pressure, electrocardiograms (ECG), electromyograms (EMG), sweat, etc.) and environmental stimuli (e.g., light, magnetic field, volatile organic compounds (VOCs)), which are vital to real-time and all-round human health monitoring and management. Herein, this review summarizes the design, manufacturing, and application of multiresponsive flexible sensors and presents the future challenges of fabricating these sensors for the next-generation e-skin and wearable electronics.

Temperature sensing using junctions between mobile ions and mobile electrons

We develop temperature sensors on the basis of charges accumulated at the electrolyte/dielectric interface and dielectric/electrode interface. The accumulated charges make the temperature sensors self-powered, which simplifies circuit design and enables portable sensing. The sensors are stretchable, but deformation does not affect temperature sensing. The sensors have high sensitivity and fast response. They can be made small and transparent. Such temperature sensors open new possibilities to create human–machine interfaces and soft robots in healthcare and engineering.

Sensing technology is under intense development to enable the Internet of everything and everyone in new and useful ways. Here we demonstrate a method of stretchable and self-powered temperature sensing. The basic sensing element consists of three layers: an electrolyte, a dielectric, and an electrode. The electrolyte/dielectric interface accumulates ions, and the dielectric/electrode interface accumulates electrons (in either excess or deficiency). The ions and electrons at the two interfaces are usually not charge-neutral, and this charge imbalance sets up an ionic cloud in the electrolyte. The design functions as a charged temperature-sensitive capacitor. When temperature changes, the ionic cloud changes thickness, and the electrode changes open-circuit voltage. We demonstrate high sensitivity (∼1 mV/K) and fast response (∼10 ms). Such temperature sensors can be made small, stable, and transparent. Depending on the arrangement of the electrolyte, dielectric, and electrode, we develop four designs for the temperature sensor. In addition, the temperature sensor has good linearity in the range of tens of Kelvin. We further show that the temperature sensors can be integrated into stretchable electronics and soft robots.

Flexible Pyroresistive Graphene Composites for Artificial Thermosensation Differentiating Materials and Solvent Types

When we touch an object, thermosensation allows us to perceive not only the temperature but also wetness and types of materials with different thermophysical properties (i.e., thermal conductivity and heat capacity) of objects. Emulation of such sensory abilities is important in robots, wearables, and haptic interfaces, but it is challenging because they are not directly perceptible sensations but rather learned abilities via sensory experiences. Emulating the thermosensation of human skin, we introduce an artificial thermosensation based on an intelligent thermo-/calorimeter (TCM) that can objectively differentiate types of contact materials and solvents with different thermophysical properties. We demonstrate a TCM based on pyroresistive composites with ultrahigh sensitivity (11.2% °C^-1) and high accuracy (<0.1 °C) by precisely controlling the melt-induced volume expansion of a semicrystalline polymer, as well as the negative temperature coefficient of reduced graphene oxide. In addition, the ultrathin TCM with coplanar electrode design shows deformation-insensitive temperature sensing, facilitating wearable skin temperature monitoring with accuracy higher than a commercial thermometer. Moreover, the TCM with a high pyroresistivity can objectively differentiate types of contact materials and solvents with different thermophysical properties. In a proof-of-principle application, our intelligent TCM, coupled with a machine-learning algorithm, enables objective evaluation of the thermal attributes (coolness and wetness) of skincare products.

Mutually Noninterfering Flexible Pressure-Temperature Dual-Modal Sensors Based on Conductive Metal-Organic Framework for Electronic Skin

Pressure and temperature are two important indicators for human skin perception. Electronic skin (E-skin) that mimics human skin within one single flexible sensor is beneficial for detecting and differentiating pressure and temperature and showing immunity from tensile strain disruptions. However, few studies have simultaneously realized these conditions. Herein, a flexible and strain-suppressed pressure-temperature dual-modal sensor based on conductive and microstructured metal-organic framework (MOF) films was reported and mainly prepared by in situ growing Ni₃(HiTP)₂ onto microstructured mixed cellulose (MSMC) substrates. The sensor exhibits distinguishable and strain-suppressed properties for pressure (sensing range up to 300 kPa, sensitivity of 61.61 kPa^-1, response time of 20 ms, and ultralow detection limit of 1 Pa) and temperature sensing (sensitivity of 57.1 μV/K). Theoretical calculations successfully analyzed the mutually noninterfering mechanism between pressure and temperature. Owing to its effective perception in static and dynamic surroundings, this sensor has great potential applications, such as in electronic skin and smart prosthetics.

Binder-free printed PEDOT wearable sensors on everyday fabrics using oxidative chemical vapor deposition

Polymeric sensors on fabrics have vast potential toward the development of versatile applications, particularly when the ready-made wearable or fabric can be directly coated. However, traditional coating approaches, such as solution-based methods, have limitations in achieving uniform and thin films because of the poor surface wettability of fabrics. Herein, to realize a uniform poly(3,4-ethylenedioxythiophene) (PEDOT) layer on various everyday fabrics, we use oxidative chemical vapor deposition (oCVD). The oCVD technique is a unique method capable of forming patterned polymer films with controllable thicknesses while maintaining the inherent advantages of fabrics, such as exceptional mechanical stability and breathability. Utilizing the superior characteristics of oCVD PEDOT, we succeed in fabricating blood pressure– and respiratory rate–monitoring sensors by directly depositing and patterning PEDOT on commercially available disposable gloves and masks, respectively. Those results are expected to pave efficient and facile ways for skin-compatible and affordable sensors for personal health care monitoring.

Pressure-Based Immunoassays with Versatile Electronic Sensors for Carcinoembryonic Antigen Detection

Pressure-based immunoassays have been studied for point-of-care testing for which increasing the sensitivity is still a challenge. In this study, we described an enhanced pressure-based immunoassay with a versatile electronic sensor for the sensitive biological analysis. The versatile electronic sensor had multifunctional sensing capabilities with temperature and pressure recording. Magnetic bead-modified capture antibody and platinum nanoparticle-labeled detection antibody were used as the biorecognition element of the target carcinoembryonic antigen (CEA) (as a model analyte) and would form a sandwich-type immune complex with CEA. After simple magnetic separation, this complex was transferred into the detection chamber, which contained both hydrogen peroxide (H₂O₂) and 3,3',5,5'-tetramethylbenzidine (TMB). With the catalytic ability of PtNPs, the "H₂O₂-TMB-PtNPs" system was catalyzed to generate a large amount of oxygen (O₂) and photothermal agent of oxidizer TMB (ox-TMB). Meanwhile, in a sealed chamber, further irradiation with an 808 nm near-infrared laser led to a triple-step signal amplification strategy of pressure increase, temperature increase, and gas thermal expansion to receive a strong electrical signal through the electronic sensor in real time. Thus, the amplified electrical signal from the electronic sensor could reveal the target concentration. In addition, we also verified that the synergistic system with two physical quantities had a lower limit of detection and a wider detection range compared to the detection system with a single physical quantity. In general, this immunoassay not only helped in exploring an effective signal amplification pathway but also offered an opportunity for the development of versatile electronic sensors in point-of-care settings.

Biomass-Derived Carbon Materials: Controllable Preparation and Versatile Applications

Biomass-derived carbon materials (BCMs) are encountering the most flourishing moment because of their versatile properties and wide potential applications. Numerous BCMs, including 0D carbon spheres and dots, 1D carbon fibers and tubes, 2D carbon sheets, 3D carbon aerogel, and hierarchical carbon materials have been prepared. At the same time, their structure-property relationship and applications have been widely studied. This paper aims to present a review on the recent advances in the controllable preparation and potential applications of BCMs, providing a reference for future work. First, the chemical compositions of typical biomass and their thermal degradation mechanisms are presented. Then, the typical preparation methods of BCMs are summarized and the relevant structural management rules are discussed. Besides, the strategies for improving the structural diversity of BCMs are also presented and discussed. Furthermore, the applications of BCMs in energy, sensing, environment, and other areas are reviewed. Finally, the remaining challenges and opportunities in the field of BCMs are discussed.

Conductive Polymer Nanocomposites for Stretchable Electronics: Material Selection, Design, and Applications

Stretchable electronics that can elongate elastically as well as flex are crucial to a wide range of emerging technologies, such as wearable medical devices, electronic skin, and soft robotics. Critical to stretchable electronics is their ability to withstand large mechanical strain without failure while retaining their electrical conduction properties, a feat significantly beyond traditional metals and silicon-based semiconductors. Herein, we present a review of the recent advances in stretchable conductive polymer nanocomposites with exceptional stretchability and electrical properties, which have the potential to transform a wide range of applications, including wearable sensors for biophysical signals, stretchable conductors and electrodes, and deformable energy-harvesting and -storage devices. Critical to achieving these stretching properties are the judicious selection and hybridization of nanomaterials, novel microstructure designs, and facile fabrication processes, which are the focus of this Review. To highlight the potentials of conductive nanocomposites, a summary of some recent important applications is presented, including COVID-19 remote monitoring, connected health, electronic skin for augmented intelligence, and soft robotics. Finally, perspectives on future challenges and new research opportunities are also presented and discussed.

Scaling Metal-Elastomer Composites toward Stretchable Multi-Helical Conductive Paths for Robust Responsive Wearable Health Devices

Stretchable electronics have advanced rapidly and many applications require high repeatability and robustness under various mechanical deformations. It has been described here that how a highly stretchable and reliable conductor composite made from helical copper wires and a soft elastomer, named eHelix, can provide mechanically robust and strain-insensitive electronic conductivity for wearable devices. The reversibility of the mechanical behavior of the metal-elastomer system has been studied using finite element modeling methods. Optimal design parameters of such helical metal-elastomer structures are found. The scaling of multiple copper wires into such helical shapes to form a Multi-eHelix system is further shown. With the same elastomer volume, Multi-eHelix has more conductive paths and a higher current density than the single-eHelix. Integrations of these eHelix stretchable conductors with fabrics showed wearable displays that can survive machine-washes and hundreds of mechanical loading cycles. The integration of the eHelix developed by us with a wearable optical heart rate sensor enabled a wearable health monitoring system that can display measured heart rates on clothing. Furthermore, Multi-eHelix conductors are used to connect flexible printed circuit boards and piezoresistive sensors on a tactile sensing glove for the emerging sensorized prosthetics.