Breathable Strain/Temperature Sensor Based on Fibrous Networks of Ionogels Capable of Monitoring Human Motion, Respiration, and Proximity

Probing the ionic activation enthalpies in anionic polysaccharide xerogel-based single ion conductor for temperature sensing

Ionic charge transport in polymer-based solid electrolytes is significantly affected by thermal perturbations, facilitating the detection of temperature variations. However, the impact of ionic interactions and molecular arrangements in polymeric single-ion conductors (SICs) has not been thoroughly investigated for temperature sensing. By probing the effect of the associated energies for ionic interactions and polymeric rearrangements, the thermal sensing characteristics of alginate have been studied. For the first time, alginate SIC interacting with multivalent ions (viz., Na+, Ca2+ and Fe3+) to form xerogel has been exploited as a temperature-sensing layer by fabricating a xerogel-based ionic thermistor (xIT) as a temperature sensor. The xIT has demonstrated stable functioning from 25 to 70 °C and unveiled enhanced sensing abilities in the physiological state of the human body (35-40 °C), exhibiting a monotonic linear response, high sensitivity (-3.77 % °C-1), and high accuracy (0.1 °C). The sensing characteristic is observed due to the inward ionic flux under thermal and electrical perturbations. The concentration of ionic charge carriers and ionic drift are assumed to be Arrhenius-activated processes. A general microscopic model of ion transport within polysaccharides has been elucidated via hopping mechanisms, and the effects of the associated activation energies on temperature sensitivity have been explained.

Review of ionic liquid and ionogel-based biomaterials for advanced drug delivery

Ionic liquids (ILs) play a crucial role in the design of novel materials. The ionic nature of ILs provides numerous advantages in drug delivery, acting as a green solvent or active ingredient to enhance the solubility, permeability, and binding efficiency of drugs. They could also function as a structuring agent in the development of nano/micro particles for drug delivery, including micelles, vesicles, gels, emulsion, and more. This review summarize the ILs and IL-based gel structures with their advanced drug delivery applications. The first part of review focuses on the role of ILs in drug formulation and the applications of ILs in drug delivery. The second part of review offers a comprehensive overview of recent drug delivery applications of IL-based gel. It aims to offer new perspectives and attract more attention to open up new avenues in the biomedical applications of ILs and IL-based gels.

Smart-Adhesive, Breathable and Waterproof Fibrous Electronic Skins

For the need of direct contact with the skin, electronic skins (E-skins) should not only fulfill electric functions, but also ensure comfort during wearing, including permeability, waterproofness, and easy removal. Herein, the study has developed a self-adhesive, detach-on-demand, breathable, and waterproof E-skin (PDSC) for motion sensing and wearable comfort by electrospinning styrene-isoprene block copolymer rubber with carbon black nanosheets as the sensing layer and liner copolymers of N, N-dimethylacrylamide, n-octadecyl acrylate and lauryl methacrylate as the adhesive layer. The high elasticity and microfiber network structure endow the PDSC with good sensitivity and high linearity for strain sensing. The hydrophobic and crystallizable adhesive layer ensures robust, waterproof, and detaching-on-demand skin adhesion. Meanwhile, the fiber structure enables the PDSC good air and water permeability. The integration of electric and wearable functions endows the PDSC with great potential for motion sensing during human activities as both the sensing and wearable performances.

Acryloyl chitosan as a macro-crosslinker for freezing-resistant, self-healing and self-adhesive ionogels-based multicompetent flexible sensors

Here, a polysaccharide derivative acryloyl chitosan (AcCS) is exploited as macro-crosslinker to synthesize a novel ionogel poly (acrylic acid-co-1-Vinyl-3-butyl imidazolium chloride) (AA-IL/AcCS) via a one-pot method. AcCS provides abundant physical and chemical crosslinking sites contributing to the high mechanical stretchability (elongation at break 600 %) and strength (tensile strength 137 kPa) of AA-IL/AcCS. The high-density of dynamic bonds (hydrogen bonds and electrostatic interactions) in the network of ionogels enables self-healing and self-adhesive features of AA-IL/AcCS. Meanwhile, AA-IL/AcCS exhibits high ionic conductivity (0.1 mS/cm) at room temperature and excellent antifreeze ability (-58 °C). The AA-IL/AcCS-based sensor shows diverse sensory capabilities towards temperature and humidity, moreover, it could precisely detect human motions and handwritings signals. Furthermore, AA-IL/AcCS exhibits excellent bactericidal properties against both gram-positive and gram-negative bacteria. This work opens the possibility of polysaccharides as a macro-crosslinkers for preparing ionogel-based sensors for wearable electronics.

Flexible iontronics with super stretchability, toughness and enhanced conductivity based on collaborative design of high-entropy topology and multivalent ion-dipole interactions

All-solid-state ionic conductive elastomers (ASSICEs) are emerging as a promising alternative to hydrogels and ionogels in flexible electronics. Nevertheless, the synthesis of ASSICEs with concomitant mechanical robustness, superior ionic conductivity, and cost-effective recyclability poses a formidable challenge, primarily attributed to the inherent contradiction between mechanical strength and ionic conductivity. Herein, we present a collaborative design of high-entropy topological network and multivalent ion-dipole interaction for ASSICEs, and successfully mitigate the contradiction between mechanical robustness and ionic conductivity. Benefiting from the synergistic effect of this design, the coordination, de-coordination, and intrachain transfer of Li+ are effectively boomed. The resultant ASSICEs display exceptional mechanical robustness (breaking strength: 7.45 MPa, fracture elongation: 2621%, toughness: 107.19 MJ m-3) and impressive ionic conductivity (1.15 × 10-2 S m-1 at 25 °C). Furthermore, these ASSICEs exhibit excellent environmental stability (fracture elongation exceeding 1400% at 50 °C or -60 °C) and recyclability. Significantly, the application of these ASSICEs in a strain sensor highlights their potential in various fields, including human-interface communication, aerospace vacuum measurement, and medical balloon monitoring.

An Ionic Assisted Enhancement Strategy Enabled High Performance Flexible Pressure-Temperature Dual Sensor

Flexible pressure sensors with a broad range and high sensitivity are greatly desired yet challenging to build. Herein, we have successfully fabricated a pressure-temperature dual sensor via an ionic assisted charge enhancement strategy. Benefiting from the immobilization effect for [EMIM+] [TFSI-] ion pairs and charge transfer between ionic liquid (IL) and HFMO (H10Fe3Mo21O51), the formed IL-HFMO-TPU pressure sensor shows a high sensitivity of 25.35 kPa-1 and broad sensing range (∼10 MPa), respectively. Furthermore, the sensor device exhibits high durability and stability (5000 cycles@1 MPa). The IL-HFMO-TPU sensor also shows the merit of good temperature sensing properties. Attributed to these superior properties, the proposed sensor device could detect pressure in an ultrawide sensing range (from Pa to MPa), including breathe and biophysical signal monitoring etc. The proposed ionic assisted enhancement approach is a generic strategy for constructing high performance flexible pressure-temperature dual sensor.

Recent Advances in Electrospinning Techniques for Precise Medicine

In the realm of precise medicine, the advancement of manufacturing technologies is vital for enhancing the capabilities of medical devices such as nano/microrobots, wearable/implantable biosensors, and organ-on-chip systems, which serve to accurately acquire and analyze patients' physiopathological information and to perform patient-specific therapy. Electrospinning holds great promise in engineering materials and components for advanced medical devices, due to the demonstrated ability to advance the development of nanomaterial science. Nevertheless, challenges such as limited composition variety, uncontrollable fiber orientation, difficulties in incorporating fragile molecules and cells, and low production effectiveness hindered its further application. To overcome these challenges, advanced electrospinning techniques have been explored to manufacture functional composites, orchestrated structures, living constructs, and scale-up fabrication. This review delves into the recent advances of electrospinning techniques and underscores their potential in revolutionizing the field of precise medicine, upon introducing the fundamental information of conventional electrospinning techniques, as well as discussing the current challenges and future perspectives.

Permeable Bioelectronics toward Biointegrated Systems

Bioelectronics integrates electronics with biological organs, sustaining the natural functions of the organs. Organs dynamically interact with the external environment, managing internal equilibrium and responding to external stimuli. These interactions are crucial for maintaining homeostasis. Additionally, biological organs possess a soft and stretchable nature; encountering objects with differing properties can disrupt their function. Therefore, when electronic devices come into contact with biological objects, the permeability of these devices, enabling interactions and substance exchanges with the external environment, and the mechanical compliance are crucial for maintaining the inherent functionality of biological organs. This review discusses recent advancements in soft and permeable bioelectronics, emphasizing materials, structures, and a wide range of applications. The review also addresses current challenges and potential solutions, providing insights into the integration of electronics with biological organs.

Electrospinning Nanofibers as Stretchable Sensors for Wearable Devices

Wearable devices attract great attention in intelligent medicine, electronic skin, artificial intelligence robots, and so on. However, boundedness of traditional sensors based on rigid materials unconstrained self-multilayer structure assembly and dense substrate in stretchability and permeability limits their applications. The network structure of the elastomeric nanofibers gives them excellent air permeability and stretchability. By introducing metal nanofillers, intrinsic conductive polymers, carbon materials, and other methods to construct conductive paths, stretchable conductors can be effectively prepared by elastomeric nanofibers, showing great potential in the field of flexible sensors. This perspective briefly introduces the representative preparations of conductive thermoplastic polyurethane, nylon, and hydrogel nanofibers by electrospinning and the application of integrated electronic devices in biological signal detection. The main challenge is to unify the stretchability and conductivity of the fiber structure.

Ionogels for flexible conductive substrates and their application in biosensing

Ionogels are highly conductive gels made from ionic liquids dispersed in a matrix made of organic or inorganic materials. Ionogels are known for high ionic conductivity, flexibility, high thermal and electrochemical stability. These characteristics make them suitable for sensing and biosensing applications. This review discusses about the two main constituents, ionic liquids and matrix, used to make ionogels and effect of these materials on the characteristics of ionogels. Here, the material properties like mechanical, electrochemical and stability are discussed for both polymer matrix and ionic liquid. We have briefly described about the fabrication methods like 3D printing, sol-gel, blade coating, spin coating, aerosol jet printing etc., used to make films or coating of these ionogels. The advantages and disadvantages of each method are also briefly summarized. Finally, the last section provides a few examples of application of flexible ionogels in areas like wearables, human-machine interface, electronic skin and detection of biological molecules.

Fully Integrated Patch Based on Lamellar Porous Film Assisted GaN Optopairs for Wireless Intelligent Respiratory Monitoring

Respiratory pattern is one of the most crucial indicators for accessing human health, but there has been limited success in implementing fast-responsive, affordable, and miniaturized platforms with the capability for smart recognition. Herein, a fully integrated and flexible patch for wireless intelligent respiratory monitoring based on a lamellar porous film functionalized GaN optoelectronic chip with a desirable response to relative humidity (RH) variation is reported. The submillimeter-sized GaN device exhibits a high sensitivity of 13.2 nA/%RH at 2-70%RH and 61.5 nA/%RH at 70-90%RH, and a fast response/recovery time of 12.5 s/6 s. With the integration of a wireless data transmission module and the assistance of machine learning based on 1-D convolutional neural networks, seven breathing patterns are identified with an overall classification accuracy of >96%. This integrated and flexible on-mask sensing platform successfully demonstrates real-time and intelligent respiratory monitoring capability, showing great promise for practical healthcare applications.

High strength, self-healing sensitive ionogel sensor based on MXene/ionic liquid synergistic conductive network for human-motion detection

Ionogels with both high strength and high conductivity for wearable strain and pressure dual-mode sensors are needed for human motion and health monitoring. Here, multiple hydrogen bonds are introduced through imidazolidinyl urea (IU) as a chain extender to provide high mechanical and self-healing properties for the water-borne polyurethane (WPU). The MXene/ionic liquids synergistic conductive network provides excellent conductivity and also reduces the relative content of ionic liquids to maintain the mechanical properties of the ionogels. The mechanical strength of this ionogel reached 1.81-2.24 MPa and elongation at break reached 570-624%. It also has excellent conductivity (22.7-37.5 mS m-1), gauge factor (GF) (as a strain sensor, GF = 1.8), sensitivity (S) (as a press sensor, S1 = 29.8 kPa-1, S2 = 1.3 kPa-1), and fast response time (as a strain sensor = 185 ms; as a press sensor = 204 ms). The ionogel also exhibits rapid photothermal self-healing capabilities due to the inherent photothermal behavior of MXene. It can maintain good elasticity and conductivity at low temperatures. In addition, this ionogel is able to stretch for 1200 cycles without significant change in the relative change of resistance. The ionogel can be assembled as a strain sensor for monitoring human motion and as a pressure sensor array for obtaining pressure magnitude and position information.

Twisted Optical Micro/Nanofibers Enabled Detection of Subtle Temperature Variation

The detection of subtle temperature variation plays an important role in many applications, including proximity sensing in robotics, temperature measurements in microfluidics, and tumor monitoring in healthcare. Herein, a flexible miniaturized optical temperature sensor is fabricated by embedding twisted micro/nanofibers in a thin layer of polydimethylsiloxane. Enabled by the dramatic change of the coupling ratio under subtle temperature variation, the sensor exhibits an ultrahigh sensitivity (-30 nm/°C) and high resolution (0.0012 °C). As a proof-of-concept demonstration, a robotic arm equipped with our sensor can avoid undesired collisions by detecting the subtle temperature variation caused by the existence of a human. Moreover, benefiting from the miniaturized and engineerable sensing structure, real-time measurement of subtle temperature variation in microfluidic chips is realized. These initial results pave the way toward a category of optical sensing devices ranging from robotic skin to human-machine interfaces and implantable healthcare sensors.

An Ultrastretchable Gradient Ionogel Induced by a Self-Floating Strategy for Strain Sensing

The fabrication of gradient ionogels for flexible strain sensors remains challenging because of the complex preparation procedures, and it is still difficult to prepare highly stretchable ionogels (strain > 10000%). In this study, a strategy is proposed to successfully fabricate gradient ionogels and apply them to flexible strain sensors by utilizing the self-floating character of the polysiloxane cross-linker. A gradient ionogel with ultrahigh stretchability (>14000%) is prepared via a one-step in situ photopolymerization process of the precursor with long-chain poly(dimethylsiloxane) bis(2-methyl acrylate) (PDMSMA). PDMSMA, which has a self-floating ability and excellent flexibility, induces a gradient composition distribution in the ionogel, thereby endowing the ionogel with superior stretchability and gradient changes in conductivity and adhesivity from the top to the bottom layer. Because of multiple molecular interactions, the bottom surface of the ionogel possesses good resilience and self-adhesion, whereas the top surface, which has a high PDMSMA content, shows a nonsticky performance. As a result, a singular gradient ionogel having both a sticky bottom surface and a nonsticky top surface is achieved. Furthermore, the flexible strain sensor that is created based on these gradient ionogels exhibits high sensitivity (its gauge factor reaching 5.08), a wide detection range (1-1500%), fast response times, and good linearity. Notably, the detection signal remains repeatable over 1000 uninterrupted strain cycles. The fabricated strain sensor was further utilized to monitor joint movements and physiological signals. This work provides a facile strategy for fabricating gradient ionogels and shows their application potential in the field of flexible electronics.

Polyurethanes Modified by Ionic Liquids and Their Applications

Polyurethane (PU) refers to the polymer containing carbamate groups in its molecular structure, generally obtained by the reaction of isocyanate and alcohol. Because of its flexible formulation, diverse product forms, and excellent performance, it has been widely used in mechanical engineering, electronic equipment, biomedical applications, etc. Through physical or chemical methods, ionic groups are introduced into PU, which gives PU electrical conductivity, flame-retardant, and antistatic properties, thus expanding the application fields of PU, especially in flexible devices such as sensors, actuators, and functional membranes for batteries and gas absorption. In this review, we firstly introduced the characteristics of PU in chemical and microphase structures and their related physical and chemical performance. To improve the performance of PU, ionic liquids (ILs) were applied in the processing or synthesis of PU, resulting in a new type of PU called ionic PU. In the following part of this review, we mainly summarized the fabrication methods of IL-modified PUs via physical blending and the chemical copolymerization method. Then, we summarized the research progress of the applications for IL-modified PUs in different fields, including sensors, actuators, transistors, antistatic films, etc. Finally, we discussed the future development trends and challenges faced by IL-modified PUs.

Wearable Graphene-based smart face mask for Real-Time human respiration monitoring

After the pandemic of SARS-CoV-2, the use of face-masks is considered the most effective way to prevent the spread of virus-containing respiratory fluid. As the virus targets the lungs directly, causing shortness of breath, continuous respiratory monitoring is crucial for evaluating health status. Therefore, the need for a smart face mask (SFM) capable of wirelessly monitoring human respiration in real-time has gained enormous attention. However, some challenges in developing these devices should be solved to make practical use of them possible. One key issue is to design a wearable SFM that is biocompatible and has fast responsivity for non-invasive and real-time tracking of respiration signals. Herein, we present a cost-effective and straightforward solution to produce innovative SFMs by depositing graphene-based coatings over commercial surgical masks. In particular, graphene nanoplatelets (GNPs) are integrated into a polycaprolactone (PCL) polymeric matrix. The resulting SFMs are characterized morphologically, and their electrical, electromechanical, and sensing properties are fully assessed. The proposed SFM exhibits remarkable durability (greater than1000 cycles) and excellent fast response time (∼42 ms), providing simultaneously normal and abnormal breath signals with clear differentiation. Finally, a developed mobile application monitors the mask wearer's breathing pattern wirelessly and provides alerts without compromising user-friendliness and comfort.

Environmentally Stable, Robust, Adhesive, and Conductive Supramolecular Deep Eutectic Gels as Ultrasensitive Flexible Temperature Sensor

It is essential and of great significance to impart high mechanical performance, environmental stability, and high sensitivity to emerging flexible temperature sensors. In this work, polymerizable deep eutectic solvents are designed and prepared by simply mixing N-cyanomethyl acrylamide (NCMA) containing an amide group and a cyano group in the same side chain with lithium bis(trifluoromethane) sulfonimide (LiTFSI), and obtain supramolecular deep eutectic polyNCMA/LiTFSI gels after polymerization. These supramolecular gels exhibit excellent mechanical performance (tensile strength of 12.9 MPa and fracture energy of 45.3 kJ m^-2 ), strong adhesion force, high-temperature responsiveness, self-healing ability, and shape memory behavior due to the reversible reconstruction ability of amide hydrogen bonds and cyano-cyano dipole-dipole interactions in the gel network. In addition, the gels also demonstrate good environmental stability and 3D printability. To verify its application potential as a flexible temperature sensor, the polyNCMA/LiTFSI gel-based wireless temperature monitor is developed and displays outstanding thermal sensitivity (8.4%/K) over a wide detection range. The preliminary result also suggests the promising potential of PNCMA gel as a pressure sensor.

High-Sensitivity Wearable Sensor Based On a MXene Nanochannel Self-Adhesive Hydrogel

To address the shortcomings of traditional filler-based wearable hydrogels, a new type of nanochannel hydrogel sensor is fabricated in this work through a combination of the unique structure of electrospun fiber textile and the properties of a double network hydrogel. Unlike the traditional Ti₃C₂T_x MXene-based hydrogels, the continuously distributed Ti₃C₂T_x MXene in the nanochannels of the hydrogel forms a tightly interconnected structure similar to the neuron network. As a result, they have more free space to flip and perform micromovements, which allows one to significantly increase the electrical conductivity and sensitivity of the hydrogel. According to the findings, the Ti₃C₂T_x MXene nanochannel hydrogel has excellent mechanical properties as well as self-adhesion and antifreezing characteristics. The hydrogel sensor successfully detects different human motions and physiological signals (e.g., low pulse signals) with high stability and sensitivity. Therefore, the proposed Ti₃C₂T_x MXene-based hydrogel with a unique structure and properties is very promising in the field of flexible wearable devices.

Flexible, Permeable, and Recyclable Liquid-Metal-Based Transient Circuit Enables Contact/Noncontact Sensing for Wearable Human-Machine Interaction

The past several years have witnessed a rapid development of intelligent wearable devices. However, despite the splendid advances, the creation of flexible human-machine interfaces that synchronously possess multiple sensing capabilities, wearability, accurate responsivity, sensitive detectivity, and fast recyclability remains a substantial challenge. Herein, a convenient yet robust strategy is reported to craft flexible transient circuits via stencil printing liquid metal conductor on the water-soluble electrospun film for human-machine interaction. Due to the inherent liquid conductor within porous substrate, the circuits feature high-resolution, customized patterning viability, attractive permeability, excellent electroconductivity, and superior mechanical stability. More importantly, such circuits display appealing noncontact proximity capabilities while maintaining compelling tactile sensing performance, which is unattainable by traditional systems with compromised contact sensing. As such, the flexible circuit is utilized as wearable sensors with practical multifunctionality, including information transfer, smart identification, and trajectory monitoring. Furthermore, an intelligent human-machine interface composed of the flexible sensors is fabricated to realize specific goals such as wireless object control and overload alarm. The transient circuits are quickly and efficiently recycled toward high economic and environmental values. This work opens vast possibilities of generating high-quality flexible and transient electronics for advanced applications in soft and intelligent systems.

Ionogels: recent advances in design, material properties and emerging biomedical applications

Ionic liquid (IL)-based gels (ionogels) have received considerable attention due to their unique advantages in ionic conductivity and their biphasic liquid-solid phase property. In ionogels, the negligibly volatile ionic liquid is retained in the interconnected 3D pore structure. On the basis of these physical features as well as the chemical properties of well-chosen ILs, there is emerging interest in the anti-bacterial and biocompatibility aspects. In this review, the recent achievements of ionogels for biomedical applications are summarized and discussed. Following a brief introduction of the various types of ILs and their key physicochemical and biological properties, the design strategies and fabrication methods of ionogels are presented by means of different confining networks. These sophisticated ionogels with diverse functions, aimed at biomedical applications, are further classified into several active domains, including wearable strain sensors, therapeutic delivery systems, wound healing and biochemical detections. Finally, the challenges and possible strategies for the design of future ionogels by integrating materials science with a biological interface are proposed.

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.

Multifunctional Iontronic Sensor Based on Liquid Metal-Filled Ho llow Ionogel Fibers in Detecting Pressure, Temperature, and Proximity

Fiber-based pressure/temperature sensors are highly desired in wearable electronics because of their natural advantages of good breathability and easy integrability. However, it is still a great challenge to fabricate reliable and highly sensitive fiber-based pressure/temperature sensors via a scalable and facile strategy. Herein, a novel fiber-based iontronic sensor with excellent pressure- and temperature-sensing capabilities is designed by assembling two crossed hollow and porous ionogel fibers filled with liquid metal. Serving as a pressure sensor, a high detection resolution (1.16 Pa), a high sensitivity of 13.30 kPa^-1 (0-2 kPa), and a wide detection range (∼207 kPa) are realized owing to its novel hierarchical structure and the selection of deformable liquid electrodes. As a temperature sensor, it exhibits a high temperature sensitivity of 25.99% °C^-1 (35-40 °C), high resolution of 0.02 °C, and good repeatability and reliability. On the basis of these excellent sensing capabilities, the as-prepared sensor can detect not only pressure signals varied from weak pulse to large joint movements but also the proximity of different objects. Furthermore, a large-area fiber array can be easily woven for acquiring the pressure mapping to intuitively distinguish the location, magnitude, and shape of the loaded object. This work provides a universal strategy to design fiber-shaped iontronic sensors for wearable electronics.

Integrated Mechano-Electrochemical Harvesting Fiber and Thermally Responsive Artificial Muscle for Self-Powered Temperature-Strain Dual-Parameter Sensor

Significant progress in healthcare fields around the world has inspired us to develop a wearable strain−temperature sensor that can monitor biomedical signals in daily life. This novel self-powered temperature−strain dual-parameter sensor comprises a mechano-electrochemical harvester (MEH) and a thermally responsive artificial muscle (TAM). The MEHTAM system generates electricity from strain and thermal fluctuations. In addition, the sensor is comfortable to wear, owing to its stretchability (>100%), softness (<3 MPa), and one-dimensional fibers (diameter 230 μm). The MEH induces a change in the electrochemical capacitance, resulting in an electrical signal under applied strain (34 μA/m) and stress (20 μA/(m·MPa)). The TAM can be used as a mechanical temperature sensor, because the tensile stroke responds linearly to changes in temperature. As the harvester and artificial muscle are combined, the MEHTAM system generates electricity, owing to external and internal mechanical stimuli caused by muscle contractions as a response to temperature changes. The MEHTAM system that we have developed—a self-powered, strain−temperature dual-parameter sensor that is soft, stretchable, and fiber-shaped—is an interesting candidate for the production of comfortable, wearable, dual-parameter sensors.

Electrospinning-Based Biosensors for Health Monitoring

In recent years, many different biosensors are being used to monitor physical health. Electrospun nanofiber materials have the advantages of high specific surface area, large porosity and simple operation. These properties play a vital role in biosensors. However, the mechanical properties of electrospun nanofibers are poor relative to other techniques of nanofiber production. At the same time, the organic solvents used in electrospinning are generally toxic and expensive. Meanwhile, the excellent performance of electrospun nanofibers brings about higher levels of sensitivity and detection range of biosensors. This paper summarizes the principle and application of electrospinning technology in biosensors and its comparison with other technologies.

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.

Transparent Conducting Films Based on Carbon Nanotubes: Rational Design toward the Theoretical Limit

Electrically conductive thin-film materials possessing high transparency are essential components for many optoelectronic devices. The advancement in the transparent conductor applications requires a replacement of indium tin oxide (ITO), one of the key materials in electronics. ITO and other transparent conductive metal oxides have several drawbacks, including poor flexibility, high refractive index and haze, limited chemical stability, and depleted raw material supply. Single-walled carbon nanotubes (SWCNTs) are a promising alternative for transparent conducting films (TCFs) because of their unique and excellent chemical and physical properties. Here, the latest achievements in the optoelectronic performance of TCFs based on SWCNTs are analyzed. Various approaches to evaluate the performance of transparent electrodes are briefly reviewed. A roadmap for further research and development of the transparent conductors using "rational design," which breaks the deadlock for obtaining the TCFs with a performance close to the theoretical limit, is also described.

All electrospun fabrics based piezoelectric tactile sensor

Tactile sensors have been widely used in the areas of health monitoring and intelligent human-machine interface. Flexible tactile sensors based on nanofiber mats made by electrospinning can meet the requirements of comfortability and breathability for wearing the body very well. Here, we developed a flexible and self-powered tactile sensor that was sandwich assembled by electrospun organic electrodes and a piezoelectric layer. The metal-free organic electrodes of thermal plastic polyurethane (PU) nanofibers decorated with multi-walled carbon nanotubes were fabricated by electrospinning followed by ultrasonication treatment. The electrospun polyvinylidene fluoride-trifluoroethylene (PVDF-TrFE) mat was utilized as the piezoelectric layer, and it was found that the piezoelectric performance of PVDF-TrFE nanofiber mat added with barium titanate (BaTiO₃) nanoparticles was enhanced about 187% than that of the pure PVDF-TrFE nanofiber mat. For practical application, the as-prepared piezoelectric tactile sensor exhibited an approximative linear relationship between the external force and the electrical output. Then the array of fabricated sensors was attached to the fingertips of a glove to grab a cup of water for tactile sensing, and the mass of water can be directly estimated according to the outputs of the sensor array. Attributed to the integrated merits of good flexibility, enhanced piezoelectric performance, light weight, and efficient gas permeability, the developed tactile sensor could be widely used as wearable devices for robot execution end or prosthesis for tactile feedback.

Wearable Ionogel-Based Fibers for Strain Sensors with Ultrawide Linear Response and Temperature Sensors Insensitive to Strain

Fiber-shaped stretchable strain and temperature sensors are highly desirable for wearable electronics due to their excellent flexibility, comfort, air permeability, and easiness to be weaved into fabric. Herein, we prepare a smart ionogel-based fiber composed of thermoplastic polyurethane (TPU) and ionic liquid (IL) by the facile and scalable wet-spinning technique, which can serve as a wearable strain sensor with good linearity (a correlation coefficient of 0.997) in an ultrawide sensing range (up to 700%), ultralow-detection limit (0.05%), fast response (173 ms) and recovery (120 ms), and high reproducibility. Attributed to these outstanding strain sensing performances, the designed TPU/IL ionogel fiber-shaped sensor is able to monitor both subtle physiological activities and large human motions. More interestingly, because of the fast response and high resolution to strain, the fiber-shaped sensor can be sewn into the fabric to secretly encrypt and wirelessly translate message according to the principle of Morse code. More importantly, a wearable strain-insensitive temperature sensor can be obtained from the ionogel fiber if it is designed into an "S" shape, which can effectively eliminate the interference of strain on temperature sense. It is found that the inaccuracy of temperature sense is within 0.15 °C when the sensor is subjected to 30% tensile strain simultaneously. Moreover, this strain-insensitive temperature sensor shows a monotonic temperature response over a wide temperature range (-15 to 100 °C) with an ultrahigh detecting accuracy of 0.1 °C and good reliability, owing to the fast and stable thermal response of IL. This temperature sensor can realize the detection of thermal radiation, proximity, and respiration, exhibiting enormous potential in smart skin, personal healthcare, and wearable electronics. This work proposes a simple but effective strategy to realize the essential strain and temperature sensing capabilities of wearable electronics and smart fabrics without mutual interference.

Dual Thermo-Responsive and Strain-Responsive Ionogels for Smart Windows and Temperature/Motion Monitoring

In this work, a stretchable, dual thermo-responsive and strain-responsive ionogel has been synthesized by one-step photopolymerization. The obtained ionogel shows an ultrahigh stretchability (∼3000%), a high ionic conductivity (up to 3.1 mS/cm), and a good temperature tolerance (-40 to 300 °C). Importantly, these ionogels show an upper critical solution temperature-type phase transition with a wide tunable phase-transition temperature (17.5-42.5 °C) and reversible color/transparency switching. In particular, the as-prepared ionogel-based flexible/wearable temperature monitors and smart windows show an excellent designability and programmability, temperature modulation ability, and thermal responsiveness. Moreover, the ionogels-based strain sensors have temperature- and strain-dual responsibility and a broad strain-sensing range (1-700%), which can effectively monitor various motions. This strategy of fabricating dual thermo- and strain-responsive ionogels by using a one-step method and only one polymer holds great promise for the next generation of multifunctional stimuli-responsive materials.

Macromolecule Relaxation Directed 3D Nanofiber Architecture in Stretchable Fibrous Mats for Wearable Multifunctional Sensors

Elastomer fiber mat sensors, which are capable of perceiving mechanical stimuli, temperature, and vapor of chemicals, are highly desirable for designing wearable electronics and human-robot interfacing devices due to good wearability, skin affinity, and durability, and so on. However, it is still challenging to fabricate multiresponsive flexible wearable sensors with three-dimensional (3D) architecture using simple material and structure design. Herein, we report an all-in-one multiresponsive thermoplastic polyurethane (TPU) nanofiber mat sensors composed of crimped elastomer fibers with deposited platinum nanoparticles (PtNPs) on the fiber surface. The 1D TPU nanofibers could be transferred to nanofibers with different 3D nanofiber architectures by controllable macromolecular chain relaxation of aligned elastomer polymers upon poor solvent annealing. The conductive networks of PtNPs on wavy TPU fibers enable the sensor susceptible to multiple stimuli like strain/pressure, humidity, and organic vapors. Besides, the 3D nanofiber architectures allow the strain sensor to detect wider tensile strain and pressure with higher sensitivity due to delicate fiber morphology and structure control. Therefore, this work provides new insights into the fabrication of multifunctional flexible sensors with 3D architecture in an easy way, advancing the establishment of a multiple signal monitoring platform for the health care and human-machine interfacing.