Self-Powered Multifunctional Electronic Skin Based on Carbon Nanotubes/Poly(dimethylsiloxane) for Health Monitoring

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.

Recent Progress on Flexible Self-Powered Tactile Sensing Platforms for Health Monitoring and Robotics

Over the past decades, tactile sensing technology has made significant advances in the fields of health monitoring and robotics. Compared to conventional sensors, self-powered tactile sensors do not require an external power source to drive, which makes the entire system more flexible and lightweight. Therefore, they are excellent candidates for mimicking the tactile perception functions for wearable health monitoring and ideal electronic skin (e-skin) for intelligent robots. Herein, the working principles, materials, and device fabrication strategies of various self-powered tactile sensing platforms are introduced first. Then their applications in health monitoring and robotics are presented. Finally, the future prospects of self-powered tactile sensing systems are discussed.

Recent advances in wearable flexible electronic skin: types, power supply methods, and development prospects

In recent years, wearable e-skin has emerged as a prominent technology with a wide range of applications in healthcare, health surveillance, human-machine interface, and virtual reality. Inspired by the properties of human skin, arrayed wearable e-skin is a novel technology that offers multifunctional sensing capabilities. It can detect and quantify various stimuli, mimicking the human somatosensory system, and record a wide range of physical and physiological parameters in real time. By combining flexible electronic device units with a data acquisition system, specific functional sensors can be distributed in targeted areas to achieve high sensitivity, resolution, adjustable sensing range, and large-area expandability. This review provides a comprehensive overview of recent advances in wearable e-skin technology, including its development status, types of applications, power supply methods, and prospects for future development. The emphasis of current research is on enhancing the sensitivity and stability of sensors, improving the comfort and reliability of wearable devices, and developing intelligent data processing and application algorithms. This review aims to serve as a scientific reference for the intelligent development of wearable e-skin technology.

Research Progress and Application of Multimodal Flexible Sensors for Electronic Skin

In recent years, wearable electronic skin has garnered significant attention due to its broad range of applications in various fields, including personal health monitoring, human motion perception, human-computer interaction, and flexible display. The flexible multimodal sensor, as the core component of electronic skin, can mimic the multistimulus sensing ability of human skin, which is highly significant for the development of the next generation of electronic devices. This paper provides a summary of the latest advancements in multimodal sensors that possess two or more response capabilities (such as force, temperature, humidity, etc.) simultaneously. It explores the relationship between materials and multiple sensing capabilities, focusing on both active materials that are the same and different. The paper also discusses the preparation methods, device structures, and sensing properties of these sensors. Furthermore, it introduces the applications of multimodal sensors in human motion and health monitoring, as well as intelligent robots. Finally, the current limitations and future challenges of multimodal sensors will be presented.

An ultra-soft conductive elastomer for multifunctional tactile sensors with high range and sensitivity

Flexible tactile sensors have become important as essential tools for facilitating human and object interactions. However, the materials utilized for the electrodes of capacitive tactile sensors often cannot simultaneously exhibit high conductivity, low modulus, and strong adhesiveness. This limitation restricts their application on flexible interfaces and results in device failure due to mechanical mismatch. Herein, we report an ultra-low modulus, highly conductive, and adhesive elastomer and utilize it to fabricate a microstructure-coupled multifunctional flexible tactile sensor. We prepare a supramolecular conductive composite film (SCCF) as the electrode of the tactile sensor using a supramolecular deep eutectic solvent, polyvinyl alcohol (PVA) solution, poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), and MXene suspension. We employ a polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) film containing 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM:TFSI) as the dielectric layer to fabricate capacitive sensors with an electrical double layer structure. Furthermore, we enhance the performance of the device by incorporating coupled pyramid and dome microstructures, which endow the sensor with multi-directional force detection. Our SCCF exhibits extremely high conductivity (reaching 710 S cm-1), ultra-low modulus (0.8 MPa), and excellent interface adhesion strength (>120 J m-2). Additionally, due to the outstanding conductivity and unique structure of the SCCF, it possesses remarkable electromagnetic shielding ability (>50 dB). Moreover, our device demonstrates a high sensitivity of up to 1756 kPa-1 and a wide working range reaching 400 kPa, combining these attributes with the requirements of an ultra-soft human-machine interface to ensure optimal contact between the sensor and interface materials. This innovative and flexible tactile sensor holds great promise and potential for addressing various and complex demands of human-machine interaction.

Synergistic advancements in high-performance flexible capacitive pressure sensors: structural modifications, AI integration, and diverse applications

The development of flexible pressure sensors for monitoring human motion and physiological signals has attracted extensive scientific research. However, achieving low monitoring limits, a wide detection range, large bending stresses, and excellent mechanical stability simultaneously remains a serious challenge. With the aim of developing a high-performance capacitive pressure sensor (CPS), this paper introduces the successful preparation of a single-walled carbon nanotube (SWNT)/polydimethylsiloxane (S-PDMS) composite dielectric with a foam-like structure (high permittivity and low elasticity modulus) and MXene/SWNT (S-MXene) composite film electrodes with a micro-crumpled structure. The above structurally modified CPS (SMCPS) demonstrated an excellent response output during pressure loading, achieving a wide pressure detection range (up to 700 kPa), a low detection limit (16.55 Pa), fast response/recovery characteristics (48/60 ms), enhanced sensitivity across a wide pressure range, long-term stability under repeated heavy loading and unloading (40 kPa, >2000 cycles), and reliable performance under various temperature and humidity conditions. The SMCPS demonstrated a precise and stable capacitive response in monitoring subtle physiological signals and detecting motion, owing to its unique electrode structure. The flexible device was integrated with an Internet of Things module to create a smart glove system that enables real-time tracking of dynamic gestures. This system demonstrates exceptional performance in gesture recognition and prediction with artificial intelligence analysis, highlighting the potential of the SMCPS in human-machine interface applications.

Two-Stage Micropyramids Enhanced Flexible Piezoresistive Sensor for Health Monitoring and Human-Computer Interaction

High-performance flexible piezoresistive sensors are becoming increasingly essential in various novel applications such as health monitoring, soft robotics, and human-computer interaction. The evolution of the interfacial contact morphology determines the sensing properties of piezoresistive devices. The introduction of microstructures enriches the interfacial contact morphology and effectively boosts the sensitivity; however, the limited compressibility of conventional microstructures leads to rapid saturation of the sensitivity in the low-pressure range, which hinders their application. Herein, we present a flexible piezoresistive sensor featuring a two-stage micropyramid array structure, which effectively enhances the sensitivity while widening the sensing range. Owing to the synergistic enhancement effect resulting from the sequential contact of micropyramids of various heights, the devices demonstrate remarkable performance, including boosting sensitivity (30.8 kPa-1) over a wide sensing range (up to 200 kPa), a fast response/recovery time (75/50 ms), and an ultralong durability of 15,000 loading-unloading cycles. As a proof of concept, the sensor is applied to detect human physiological and motion signals, further demonstrating a real-time spatial pressure distribution sensing system and a game control system, showing great potential for applications in health monitoring and human-computer interaction.

Synergistic Effect and Electrical Properties of Microporous Liquid Metal/Carbon Nanotube Polymer Sponge

Sensing sponge materials with light weight, high elasticity, and electrical sensing properties are in enormous demand in electronic fields, but there is an imminent need to develop a scalable and facile method for the manufacture of the sensing material. Herein, an efficient in situ polymerization and convenient preparation process is reported to manufacture the microporous liquid metal/carbon nanotube-polysulfide rubber (LM/CNT-PSR) sponges with excellent mechanical and electrical properties, based on fluidic LMs and rigid CNTs with unique synergistic effect for sponge composites. Excellent mechanical properties of LM/CNT-PSR sponges, such as low density, excellent elasticity, remarkable mechanical recoverability, and self-healing property, are endowed by the interconnected microporous structure of sponge and flexible polysulfide rubber matrix with disulfide bonds. In addition, the synergistic effect of LMs and CNTs leads to excellent conductivity and unique electrical sensing property under mechanical pressure. Microporous LM/CNT-PSR sponges with high performance and simple fabrication process are promising sensing materials for various electronic devices, such as human motion monitoring, and weighing sensing.

Visible-Light Transparent, Ultrastretchable, and Self-Healable Semicrystalline Fluorinated Ionogels for Underwater Strain Sensing

The development of ultrastretchable ionogels with a combination of high transparency and unique waterproofness is central to the development of emerging skin-inspired sensors. In this study, an ultrastretchable semicrystalline fluorinated ionogel (SFIG) with visible-light transparency and underwater stability is prepared through one-pot copolymerization of acrylic acid and fluorinated acrylate monomers in a mixed solution of poly(ethylene oxide) (PEO) and fluorinated ionic liquids. Benefiting from the formation of the PEO-chain semicrystalline microstructures and the abundant noncovalent interactions (reversible hydrogen bonds and ion-dipole interactions) in an ionogel, SFIG is rendered with room-temperature stable cross-linking structures, providing high mechanical elasticity as well as high chain segment dynamics for self-healing and efficient energy absorption during the deformation. The resultant SFIG exhibits excellent stretchability (>2500%), improved mechanical toughness (7.4 MJ m^-3), and room-temperature self-healability. Due to the high compatibility and abundance of hydrophobic fluorinated moieties in the ionogel, the SFIG demonstrates high visible-light transparency (>97%) and excellent waterproofness. Due to these unique advantages, the as-prepared SFIG is capable of working as an ultrastretchable ionic conductor in capacitive-type strain sensors, demonstrating excellent underwater strain-sensing performances with high sensitivity, large detecting range, and exceptional durability. This work might provide a straightforward and efficient method for obtaining waterproof ionogel elastomers for application in next-generation underwater sensors and communications.

Soft Electronics for Health Monitoring Assisted by Machine Learning

Due to the development of the novel materials, the past two decades have witnessed the rapid advances of soft electronics. The soft electronics have huge potential in the physical sign monitoring and health care. One of the important advantages of soft electronics is forming good interface with skin, which can increase the user scale and improve the signal quality. Therefore, it is easy to build the specific dataset, which is important to improve the performance of machine learning algorithm. At the same time, with the assistance of machine learning algorithm, the soft electronics have become more and more intelligent to realize real-time analysis and diagnosis. The soft electronics and machining learning algorithms complement each other very well. It is indubitable that the soft electronics will bring us to a healthier and more intelligent world in the near future. Therefore, in this review, we will give a careful introduction about the new soft material, physiological signal detected by soft devices, and the soft devices assisted by machine learning algorithm. Some soft materials will be discussed such as two-dimensional material, carbon nanotube, nanowire, nanomesh, and hydrogel. Then, soft sensors will be discussed according to the physiological signal types (pulse, respiration, human motion, intraocular pressure, phonation, etc.). After that, the soft electronics assisted by various algorithms will be reviewed, including some classical algorithms and powerful neural network algorithms. Especially, the soft device assisted by neural network will be introduced carefully. Finally, the outlook, challenge, and conclusion of soft system powered by machine learning algorithm will be discussed.

Flexible and Stretchable Carbon-Based Sensors and Actuators for Soft Robots

In recent years, the emergence of low-dimensional carbon-based materials, such as carbon dots, carbon nanotubes, and graphene, together with the advances in materials science, have greatly enriched the variety of flexible and stretchable electronic devices. Compared with conventional rigid devices, these soft robotic sensors and actuators exhibit remarkable advantages in terms of their biocompatibility, portability, power efficiency, and wearability, thus creating myriad possibilities of novel wearable and implantable tactile sensors, as well as micro-/nano-soft actuation systems. Interestingly, not only are carbon-based materials ideal constituents for photodetectors, gas, thermal, triboelectric sensors due to their geometry and extraordinary sensitivity to various external stimuli, but they also provide significantly more precise manipulation of the actuators than conventional centimeter-scale pneumatic and hydraulic robotic actuators, at a molecular level. In this review, we summarize recent progress on state-of-the-art flexible and stretchable carbon-based sensors and actuators that have creatively added to the development of biomedicine, nanoscience, materials science, as well as soft robotics. In the end, we propose the future potential of carbon-based materials for biomedical and soft robotic applications.

An Ultra-Sensitive and Multifunctional Electronic Skin with Synergetic Network of Graphene and CNT

Electronic skin (e-skin) has attracted tremendous interest due to its diverse potential applications, including in physiological signal detection, health monitoring, and artificial throats. However, the major drawbacks of traditional e-skin are the weak adhesion of substrates, incompatibility between sensitivity and stretchability, and its single function. These shortcomings limit the application of e-skin and increase the complexity of its multifunctional integration. Herein, the synergistic network of crosslinked SWCNTs within and between multilayered graphene layers was directly drip coated onto the PU thin film with self-adhesion to fabricate versatile e-skin. The excellent mechanical properties of prepared e-skin arise from the sufficient conductive paths guaranteed by SWCNTs in small and large deformation under various strains. The prepared e-skin exhibits a low detection limit, as small as 0.5% strain, and compatibility between sensitivity and stretchability with a gauge factor (GF) of 964 at a strain of 0-30%, and 2743 at a strain of 30-60%. In physiological signals detection application, the e-skin demonstrates the detection of subtle motions, such as artery pulse and blinking, as well as large body motions, such as knee joint bending, elbow movement, and neck movement. In artificial throat application, the e-skin integrates sound recognition and sound emitting and shows clear and distinct responses between different throat muscle movements and different words for sound signal acquisition and recognition, in conjunction with superior sound emission performance with a sound spectrum response of 71 dB (f = 12.5 kHz). Overall, the presented comprehensive study of novel materials, structures, properties, and mechanisms offers promising potential in physiological signals detection and artificial throat applications.