Ti3C2Tx MXene-Based Flexible Piezoresistive Physical Sensors

Robust Treble-Weaving Wearable Textiles for Pressure and Temperature Monitoring in Harsh Environments

Wearable sensing textiles with continuous temperature monitoring, tactile feedback, and motion perception are highly desirable for personal safeguarding in extreme environments, such as fire scenes and extreme sports. However, it remains challenging for current wearable sensors to maintain reliable performance and provide point-of-care monitoring in harsh environments, such as high- and low-temperature or high-humidity conditions. Herein, a robust temperature and pressure sensing textile (TPST) with a hierarchical triple-weaving structure is developed using industrial weaving technology. The well-engineered interlacing configuration of the polyimide binding yarns in the triple-weaving structure tightly solidifies the carbon-based sensing yarns between two weaving layers, forming an integrated textile sensing array. The TPST not only exhibits excellent sensing sensitivity, reliability, and rapid response to pressure and temperature stimuli but also shows robust mechanical properties, flame resistance, and wearing comfort. Moreover, we demonstrate the application of the TPST for continuous temperature monitoring, human motion mapping, and vital sign monitoring. This technology offers significant potential for enhancing autonomous rescue operations and defense wearables.

MXenes for Wearable Physical Sensors toward Smart Healthcare

The gradual rise of personal healthcare awareness is accelerating the deployment of wearable sensors, whose ability of acquiring physiological vital signs depends on sensing materials. MXenes have distinct chemical and physical superiorities over other 2D nanomaterials for wearable sensors. This review presents a comprehensive summary of the latest advancements in MXenes-based materials for wearable physical sensors. It begins with an introduction to special structural features of MXenes for sensing performance, followed by an in-depth exploration of versatile functionalities. A detailed description of different sensing mechanisms is also included to illustrate the contribution of MXenes to the sensing performance and its improvement. In addition, the real-world applications of MXenes-based physical sensors for monitoring different physiological signs are included as well. The remaining challenges of MXenes-based materials for wearable physical sensors and their promising opportunities are finally narrated, in conjunction with a prospective for future development.

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.

MXene Crosslinked Hydrogels with Low Hysteresis Conferred by Sliding Tangle Island Strategy

Single-network hydrogels are often too fragile to withstand mechanical loading, whereas double-network hydrogels typically exhibit significant hysteresis during cyclic stretching-releasing process due to the presence of a sacrificial network. Consequently, it is a considerable challenge for designing hydrogels that are both low in hysteresis and high in toughness for applications requiring dynamic mechanical loads. Herein, the study introduced a novel "sliding tangle island" strategy for creating tough and low-hysteresis hydrogels, which are prepared through in situ polymerization of highly concentrated acrylamides (AM) to form numerous entanglements within the MXene spacing without any chemical crosslinker. The MXene entangled with long polyacrylamide (PAM) chains to form tangle island that served as a relay station to transmit stress to neighboring molecular chains. This mechanism helps alleviate stress concentration and enhances energy dissipation efficiency, thereby reducing mechanical hysteresis. The resulting hydrogel exhibited exceptional properties, including high stretchability (≈900%), low hysteresis (less than 7%), high toughness (1.34 MJ m-3), and excellent sensing performance to rival the commercial hydrogel electrode. Therefore, this work sheds light on feasible design of energy dissipation structure to reduce the hysteresis of the composite hydrogels.

Innervate Commercial Fabrics with Spirally-Layered Iontronic Fibrous Sensors Toward Dual-Functional Smart Garments

Electronic fabrics exhibit desirable breathability, wearing comfort, and easy integration with garments. However, surficial deposition of electronically functional materials/compounds onto fabric substrates would consequentially alter their intrinsic properties (e.g., softness, permeability, biocompatibility, etc.). To address this issue, here, a strategy to innervate arbitrary commercial fabrics with unique spirally-layered iontronic fibrous (SLIF) sensors is presented to realize both mechanical and thermal sensing functionalities without sacrificing the intrinsic fabric properties. The mechanical sensing function is realized via mechanically regulating the interfacial ionic supercapacitance between two perpendicular SLIF sensors, while the thermal sensing function is achieved based on thermally modulating the intrinsic ionic impedance in a single SLIF sensor. The resultant SLIF sensor-innervated electronic fabrics exhibit high mechanical sensitivity of 81 N-1, superior thermal sensitivity of 34,400 Ω °C-1, and more importantly, greatly minimized mutual interference between the two sensing functions. As demonstrations, various smart garments are developed for the precise monitoring of diverse human physiological signals. Moreover, artificial intelligence-assisted object recognition with high-accuracy (97.8%) is demonstrated with a SLIF sensor-innervated smart glove. This work opens up a new path toward the facile construction of versatile smart garments for wearable healthcare, human-machine interfaces, and the Internet of Things.

Advancements in MXene Composite Materials for Wearable Sensors: A Review

In recent years, advancements in the Internet of Things (IoT), manufacturing processes, and material synthesis technologies have positioned flexible sensors as critical components in wearable devices. These developments are propelling wearable technologies based on flexible sensors towards higher intelligence, convenience, superior performance, and biocompatibility. Recently, two-dimensional nanomaterials known as MXenes have garnered extensive attention due to their excellent mechanical properties, outstanding electrical conductivity, large specific surface area, and abundant surface functional groups. These notable attributes confer significant potential on MXenes for applications in strain sensing, pressure measurement, gas detection, etc. Furthermore, polymer substrates such as polydimethylsiloxane (PDMS), polyurethane (PU), and thermoplastic polyurethane (TPU) are extensively utilized as support materials for MXene and its composites due to their light weight, flexibility, and ease of processing, thereby enhancing the overall performance and wearability of the sensors. This paper reviews the latest advancements in MXene and its composites within the domains of strain sensors, pressure sensors, and gas sensors. We present numerous recent case studies of MXene composite material-based wearable sensors and discuss the optimization of materials and structures for MXene composite material-based wearable sensors, offering strategies and methods to enhance the development of MXene composite material-based wearable sensors. Finally, we summarize the current progress of MXene wearable sensors and project future trends and analyses.

Nitroxyl radical triggered the construction of a molecular protective layer for achieving durable Zn metal anodes

The issues of dendrite growth, hydrogen evolution reaction, and zinc anode corrosion have significantly hindered the widespread implementation of aqueous zinc-ion batteries (AZIBs). Herein, trace amounts of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) additive is introduced into AZIBs to protect the zinc metal anode. Trace amounts of the TEMPO additive with nitroxyl radical can provide fast Zn2+ transport and anode protection ability by forming an adsorbed molecular layer via Zn-O bond. This interface not only provides strong interfacial compatibility and promotes dynamic transport of Zn2+, but also induces deposition of Zn2+ along Zn (002) plane. Additionally, the molecular protective layer significantly inhibits hydrogen evolution reaction (HER) and corrosion. The Zn anodes achieve high Coulombic efficiency of up to 99.75 % and long-term plating/stripping of more than 1400 h at 1 mA cm-2 and 0.5 mAh cm-2. The Zn//Zn symmetric cell can operate continuously for 2500 h at a current density of 1 mA cm-2 and 1 mAh cm-2, and it can still last for nearly 1400 h even when the current density is increased to 5 mA cm-2. Furthermore, the Zn//V2O5 full cell using TEMPO/ZnSO4 electrolyte effectively maintains a maximum capacity retention rate of 53.4 % even after 1500 cycles at 5 A/g. This innovative strategy introduces trace additive with free radicals into the electrolyte, which may help to achieve large-scale, ultra-long-life, and low-cost AZIBs.

Selenium-enriched hollow NiCo2O4/NiO heterostructured nanocages as an efficient electrocatalyst for oxygen evolution reaction

Finding clean, sustainable, and environmentally friendly technologies is especially crucial in addressing both energy and environmental challenges. To accelerate the oxygen evolution reaction (OER) and to overcome the obstacle of high energy consumption, exploring high-performance electrocatalysts is imperative to maximize the practical applicability of water splitting. Developing electrocatalyst through strategic surface modifications represents a significant approach for the construction of active catalytic centers. In the present work, we successfully synthesized selenium-incorporated hollow NiCo2O4/NiO heterostructured nanocages as electrocatalysts for the OER by precisely controlling the structure and composition of the material. The findings demonstrated that the surface-reconstructed hollow 5 wt% Se-NiCo2O4/NiO heterostructured nanocages resulted in an increased number of active sites through interfacial engineering. Benefiting from the structural control, mass transport was further expedited and due to increased conductivity, accelerated the charge transfer processes within the system. The electrocatalyst exhibited remarkable activity for the OER and displayed a low overpotential (η = 288 mV) at a current density (j) of 10 mA cm-2, small Tafel slope (66.7 mV dec-1) and better stability. This work offers a viable and adaptable method for fabricating a range of functional coordinated MOF compounds that are capable of utilization across diverse energy applications, including storage, conversion and environmental purposes.

Multifunctional Conductive Hydrogel Composites with Nickel Nanowires and Liquid Metal Conductive Highways

The dramatic growth of smart wearable electronics has generated a demand for conductive hydrogels due to their tunability, stimulus responsiveness, and multimodal sensing capabilities. However, the substantial trade-off between mechanical and electrical properties hinders their multifunctionality. Here, we report a double-network hydrogel composite that features a conductive "highway" constructed using magnetic-field-aligned nickel nanowires and liquid metal. The liquid metal fills the gaps between the aligned nickel nanowires. Such interconnected structures can form efficient conductive paths at low filler content, resulting in high conductivity (1.11 × 104 S/m) and mechanical compliance (Young's modulus, 89 kPa; toughness, 721 kJ/m3). When used as a wearable sensor, the hydrogel displays a high sensitivity and fast response for wireless motion detection and human-machine interaction. Furthermore, by exploiting its outstanding conductivity and electrical heating capacity, the hydrogel integrates electromagnetic shielding and thermal management functionalities. Owing to these all-around properties, our design offers a broader platform for expanding hydrogel applications.

Recent Advances in Flexible Pressure Sensors Based on MXene Materials

In the past decade, with the rapid development of wearable electronics, medical health monitoring, the Internet of Things, and flexible intelligent robots, flexible pressure sensors have received unprecedented attention. As a very important kind of electronic component for information transmission and collection, flexible pressure sensors have gained a wide application prospect in the fields of aerospace, biomedical and health monitoring, electronic skin, and human-machine interface. In recent years, MXene has attracted extensive attention because of its unique 2D layered structure, high conductivity, rich surface terminal groups, and hydrophilicity, which has brought a new breakthrough for flexible sensing. Thus, it has become a revolutionary pressure-sensitive material with great potential. In this work, the recent advances of MXene-based flexible pressure sensors are reviewed from the aspects of sensing type, sensing mechanism, material selection, structural design, preparation strategy, and sensing application. The methods and strategies to improve the performance of MXene-based flexible pressure sensors are analyzed in details. Finally, the opportunities and challenges faced by MXene-based flexible pressure sensors are discussed. This review will bring the research and development of MXene-based flexible sensors to a new high level, promoting the wider research exploitation and practical application of MXene materials in flexible pressure sensors.

An Ultrasensitive Biomimetic Optic Afferent Nervous System with Circadian Learnability

The optic afferent nervous system (OANS) plays a significant role in generating vision and circadian behaviors based on light detection and signals from the endocrine system. However, the bionic simulation of this photochemically mediated behavior is still a challenge for neuromorphic devices. Herein, stimuli of neurotransmitters at ultralow concentrations and illumination are coupled to artificial synapses with the aid of biofunctionalized heterojunction and tunneling to successfully simulate a circadian neural response. Furthermore, the mechanisms underlying the photosensitive synaptic current in response to stimuli are described. Interestingly, this OANS is demonstrated to be capable of mimicking normal and abnormal circadian learnability by combining the measured synaptic current with a three-layer spike neural network. Strong theoretical and experimental evidence, as well as applications, are provided for the proposed biomimetic OANS to demonstrate that it can reproduce biological circadian behavior, thus establishing it as a promising candidate for future neuromorphic intelligent robots.

Silver nanowires/waterborne polyurethane composite film based piezoresistive pressure sensor for ultrasensitive human motion monitoring

Flexible piezoresistive pressure sensors are gaining significant attention, particularly in the realm of flexible wearable electronic skin. Here, a flexible piezoresistive pressure sensor was developed with a broad sensing range and high sensitivity. We achieved this by curing polydimethylsiloxane (PDMS) on sandpaper, creating a PDMS film as the template with a micro-protrusion structure. The core sensing layer was formed using a composite of silver nanowires (AgNWs) and waterborne polyurethane (WPU) with a similar micro-protrusion structure. The sensor stands out with its exceptional sensitivity, showing a value of 1.04 × 106kPa-1with a wide linear range from 0 to 27 kPa. It also boasts a swift response and recovery time of 160 ms, coupled with a low detection threshold of 17 Pa. Even after undergoing more than 1000 cycles, the sensor continues to deliver stable performance. The flexible piezoresistive pressure sensor based on AgNWs/WPU composite film (AWCF) can detect small pressure changes such as pulse, swallowing, etc, which indicates that the sensor has great application potential in monitoring human movement and flexible wearable electronic skin.

Monitoring Ti3C2Tx MXene Degradation Pathways Using Raman Spectroscopy

Extending applications of Ti3C2Tx MXene in nanocomposites and across fields of electronics, energy storage, energy conversion, and sensor technologies necessitates simple and efficient analytical methods. Raman spectroscopy is a critical tool for assessing MXene composites; however, high laser powers and temperatures can lead to the materials' deterioration during the analysis. Therefore, an in-depth understanding of MXene photothermal degradation and changes in its oxidation state is required, but no systematic studies have been reported. The primary aim of this study was to investigate the degradation of the MXene lattice through Raman spectroscopic analysis. Distinct spectral markers were related to structural alterations within the Ti3C2Tx material after subjecting it to thermal- and laser-induced degradation. During the degradation processes, spectral markers were revealed for several specific steps: a decrease in the number of interlayer water molecules, a decrease in the number of -OH groups, formation of C-C bonds, oxidation of the lattice, and formation of TiO2 nanoparticles (first anatase, followed by rutile). By tracking of position shifts and intensity changes for Ti3C2Tx, the spectral markers that signify the initiation of each step were found. This spectroscopic approach enhances our understanding of the degradation pathways of MXene, and facilitating enhanced and dependable integration of these materials into devices for diverse applications, from energy storage to sensors.

Research on high sensitivity piezoresistive sensor based on structural design

With the popularity of smart terminals, wearable electronic devices have shown great market prospects, especially high-sensitivity pressure sensors, which can monitor micro-stimuli and high-precision dynamic external stimuli, and will have an important impact on future functional development. Compressible flexible sensors have attracted wide attention due to their simple sensing mechanism and the advantages of light weight and convenience. Sensors with high sensitivity are very sensitive to pressure and can detect resistance/current changes under pressure, which has been widely studied. On this basis, this review focuses on analyzing the performance impact of device structure design strategies on high sensitivity pressure sensors. The design of structures can be divided into interface microstructures and three-dimensional framework structures. The preparation methods of various structures are introduced in detail, and the current research status and future development challenges are summarized.

Anti-Oxidized Self-Assembly of Multilayered F-Mene/MXene/TPU Composite with Improved Environmental Stability and Pressure Sensing Performances

MXenes, as emerging 2D sensing materials for next-generation electronics, have attracted tremendous attention owing to their extraordinary electrical conductivity, mechanical strength, and flexibility. However, challenges remain due to the weak stability in the oxygen environment and nonnegligible aggregation of layered MXenes, which severely affect the durability and sensing performances of the corresponding MXene-based pressure sensors, respectively. Here, in this work, we propose an easy-to-fabricate self-assembly strategy to prepare multilayered MXene composite films, where the first layer MXene is hydrogen-bond self-assembled on the electrospun thermoplastic urethane (TPU) fibers surface and the anti-oxidized functionalized-MXene (f-MXene) is subsequently adhered on the MXene layer by spontaneous electrostatic attraction. Remarkably, the f-MXene surface is functionalized with silanization reagents to form a hydrophobic protective layer, thus preventing the oxidation of the MXene-based pressure sensor during service. Simultaneously, the electrostatic self-assembled MXene and f-MXene successfully avoid the invalid stacking of MXene, leading to an improved pressure sensitivity. Moreover, the adopted electrospinning method can facilitate cyclic self-assembly and the formation of a hierarchical micro-nano porous structure of the multilayered f-MXene/MXene/TPU (M-fM2T) composite. The gradient pores can generate changes in the conductive pathways within a wide loading range, broadening the pressure detection range of the as-proposed multilayered f-MXene/MXene/TPU piezoresistive sensor (M-fM2TPS). Experimentally, these novel features endow our M-fM2TPS with an outstanding maximum sensitivity of 40.31 kPa-1 and an extensive sensing range of up to 120 kPa. Additionally, our M-fM2TPS exhibits excellent anti-oxidized properties for environmental stability and mechanical reliability for long-term use, which shows only ~0.8% fractional resistance changes after being placed in a natural environment for over 30 days and provides a reproducible loading-unloading pressure measurement for more than 1000 cycles. As a proof of concept, the M-fM2TPS is deployed to monitor human movements and radial artery pulse. Our anti-oxidized self-assembly strategy of multilayered MXene is expected to guide the future investigation of MXene-based advanced sensors with commercial values.

Facile Multiple Graded Wrinkle Construction Strategy for Vastly Boosting the Sensing Performance of Ionic Skins

The construction of surface microstructures (e.g., micropyramids and wrinkles) has been proven as the most effective means to boost the sensitivity of ionic skins (I-skins). However, the single-scale micronano patterns constructed by the common fabrication strategy generally lead to a limited pressure-response range. Here, a convenient repeated stretching/coordinating/releasing strategy is developed to controllably construct multiple graded wrinkles on the polyelectrolyte hydrogel-based I-skins for increasing their sensitivity over a broad pressure range. We find that the small wrinkles allow for high sensitivity yet small pressure detection range, while the large wrinkles can reduce structural stiffening to generate large pressure-response range but incur limited sensitivity. The multiple graded wrinkles can combine the merits of both the small and large wrinkles to simultaneously improve the sensitivity and broaden the pressure-response range. In particular, the sensing performance of multiple-wrinkle-based I-skins substantially outperforms the superposition of the sensing performance of different single-wrinkle-based I-skins. As a proof of concept, the triple-wrinkle-based I-skins can provide an extremely high sensitivity of 17,309 kPa-1 and an ultrawide pressure detection range of 0.38 Pa to 372 kPa. The approach and insight contribute to the future development of I-skins with a broader pressure-response range and higher sensitivity.

Integrated Sensors for Soft Medical Robotics

Minimally invasive procedures assisted by soft robots for surgery, diagnostics, and drug delivery have unprecedented benefits over traditional solutions from both patient and surgeon perspectives. However, the translation of such technology into commercialization remains challenging. The lack of perception abilities is one of the obstructive factors paramount for a safe, accurate and efficient robot-assisted intervention. Integrating different types of miniature sensors onto robotic end-effectors is a promising trend to compensate for the perceptual deficiencies in soft robots. For example, haptic feedback with force sensors helps surgeons to control the interaction force at the tool-tissue interface, impedance sensing of tissue electrical properties can be used for tumor detection. The last decade has witnessed significant progress in the development of multimodal sensors built on the advancement in engineering, material science and scalable micromachining technologies. This review article provides a snapshot on common types of integrated sensors for soft medical robots. It covers various sensing mechanisms, examples for practical and clinical applications, standard manufacturing processes, as well as insights on emerging engineering routes for the fabrication of novel and high-performing sensing devices.

Monolayer Vacancy-Induced MXene Memory for Write-Verify-Free Programming

The fundamental logic states of 1 and 0 in Complementary Metal-Oxide-Semiconductor (CMOS) are essential for modern high-speed non-volatile solid-state memories. However, the accumulated storage signal in conventional physical components often leads to data distortion after multiple write operations. This necessitates a write-verify operation to ensure proper values within the 0/1 threshold ranges. In this work, a non-gradual switching memory with two distinct stable resistance levels is introduced, enabled by the asymmetric vertical structure of monolayer vacancy-induced oxidized Ti3C2Tx MXene for efficient carrier trapping and releasing. This non-cumulative resistance effect allows non-volatile memories to attain valid 0/1 logic levels through direct reprogramming, eliminating the need for a write-verify operation. The device exhibits superior performance characteristics, including short write/erase times (100 ns), a large switching ratio (≈3 × 104), long cyclic endurance (>104 cycles), extended retention (>4 × 106 s), and highly resistive stability (>104 continuous write operations). These findings present promising avenues for next-generation resistive memories, offering faster programming speed, exceptional write performance, and streamlined algorithms.

Flexible tactile sensors with interlocking serrated structures based on stretchable multiwalled carbon nanotube/silver nanowire/silicone rubber composites

Flexible tactile sensors have attracted significant interest because of their application scope in the fields of biomedicine, motion detection, and human-computer interaction. However, the development of tactile sensors with high sensitivity and flexibility remains a critical challenge. This study develops a patterned, stretchable, and fully elastomeric multiwalled carbon nanotube (MWCNT)/silver nanowire (Ag NW)/silicone rubber (SR) composite. The addition of Ag NWs to MWCNTs enhances the transmission path of the conductive network, yielding a CNT/Ag NW/SR composite with a sensitivity coefficient of 40. This characteristic renders it suitable for use as a piezoresistive sensing material. The interlocking sawtooth structure can convert the mechanical stimuli of the sensor to the tensile strain of the composite, thereby enhancing its sensitivity and flexibility. Experimental results indicate that the developed tactile sensor exhibited a sensitivity of 2.82 N-1 at 0-0.5 N and 1.51 N-1 at 0.5-2 N. These haptic sensors also demonstrate good dynamic response, repeatability, and long life. Furthermore, experimental results show that these haptic sensors exhibit high reproducibility, fast dynamic response, and good mechanical and electrical stability. Because of these exceptional properties, the as-prepared sensor can be applied in the development of smart robots, prosthetics, and wearable devices.

Integrated Actuation and Sensing: Toward Intelligent Soft Robots

Soft robotics has received substantial attention due to its remarkable deformability, making it well-suited for a wide range of applications in complex environments, such as medicine, rescue operations, and exploration. Within this domain, the interaction of actuation and sensing is of utmost importance for controlling the movements and functions of soft robots. Nonetheless, current research predominantly focuses on isolated actuation and sensing capabilities, often neglecting the critical integration of these 2 domains to achieve intelligent functionality. In this review, we present a comprehensive survey of fundamental actuation strategies and multimodal actuation while also delving into advancements in proprioceptive and haptic sensing and their fusion. We emphasize the importance of integrating actuation and sensing in soft robotics, presenting 3 integration methodologies, namely, sensor surface integration, sensor internal integration, and closed-loop system integration based on sensor feedback. Furthermore, we highlight the challenges in the field and suggest compelling directions for future research. Through this comprehensive synthesis, we aim to stimulate further curiosity among researchers and contribute to the development of genuinely intelligent soft robots.

Flexible Piezoresistive Sensors Based on PPy Granule-Anchored Multilayer Fibrous Membranes with a Wide Operating Range and High Sensitivity

The employment of flexible piezoresistive sensors has sparked growing interest within the realm of wearable electronic devices, specifically in the fields of health detection and e-skin. Nevertheless, the advancement of piezoresistive sensors has been impeded by their limited sensitivity and restricted operating ranges. Consequently, it is imperative to fabricate sensors with heightened sensitivity and expanded operating ranges through the utilization of the appropriate methodologies. In this paper, piezoresistive sensors were fabricated utilizing electrospun polyvinylidene fluoride/polyacrylonitrile/polyethylene-polypropylene glycol multilayer fibrous membranes anchored with polypyrrole granules as the sensing layer, while electrospun thermoplastic polyurethane (TPU) fibers were employed as the flexible substrate. The sensitivity of the sensor is investigated by varying the fiber diameter of the sensing layer. The experimental findings reveal that a concentration of 14 wt % in the spinning solution exhibits high sensitivity (996.7 kPa-1) within a wide working range (0-10 kPa). This is attributed to the favorable diameter of the fibers prepared at this concentration, which facilitates the uniform in situ growth of pyrrole. The highly deformable TPU flexible fibers and multilayer sensing layer structure enable different linear responses across a broad pressure range (0-1 MPa). Furthermore, the sensor demonstrates good cyclic stability and can detect human movements under different pressures. These results suggest that the piezoresistive sensor with a wide operating range and high sensitivity has significant potential for future health monitoring and artificial intelligence applications.

Age of Flexible Electronics: Emerging Trends in Soft Multifunctional Sensors

With the commercialization of first-generation flexible mobiles and displays in the late 2010s, humanity has stepped into the age of flexible electronics. Inevitably, soft multifunctional sensors, as essential components of next-generation flexible electronics, have attracted tremendous research interest like never before. This review is dedicated to offering an overview of the latest emerging trends in soft multifunctional sensors and their accordant future research and development (R&D) directions for the coming decade. First, key characteristics and the predominant target stimuli for soft multifunctional sensors are highlighted. Second, important selection criteria for soft multifunctional sensors are introduced. Next, emerging materials/structures and trends for soft multifunctional sensors are identified. Specifically, the future R&D directions of these sensors are envisaged based on their emerging trends, namely i) decoupling of multiple stimuli, ii) data processing, iii) skin conformability, and iv) energy sources. Finally, the challenges and potential opportunities for these sensors in future are discussed, offering new insights into prospects in the fast-emerging technology.

High-sensitivity piezoresistive sensors based on cellulose handsheets using origami-inspired corrugated structures

Cellulose-based composites have attracted significant attention in the fabrication and advancement of wearable devices due to their sustainable, degradable, and cost-effective properties. However, achieving a cellulosic sensor with reliable sensory feedback remains challenging owing to the deficiency in reversible microstructures during response processes. In this study, we developed a piezoresistive sensor consisting of nearly pure cellulose handsheets using origami-inspired corrugated structures to achieve durable and sensitive piezoresistive responses. Multi-walled carbon nanotubes (MWCNTs) were used as conducting agents. With the addition of 7 wt% MWCNTs, 36.27 % of the cellulose fiber surface was covered and the conductivity of cellulose handsheets was increased to 8.7 S/m. The obtained conductive cellulose handsheets were transformed into corrugated structures and integrated orthogonally to construct the piezoresistive sensors with reversible electrical paths for electrons. The restorable corrugated structure endowed the sensors with a wide workable pressure range (0-10 kPa), high sensitivity (6.09 kPa-1 in a range of 0-0.92 kPa), fast response time (<280 ms), and good durability (>1000 cycles). Furthermore, the practical applications of the proposed sensors as wearable devices were demonstrated through phonation, real-time sports monitoring, and step pressure tests.

Constructing high-density crack-microstructures within MXene interlayers for ultrasensitive and superhydrophobic cellulosic fibers-based sensors

Incorporating biopolymers into two-dimensional transition metal carbides and/or nitrides (2D MXene) has been demonstrated as an effective strategy to improve the mechanical behaviors of MXene-based composites. However, the insulate nature of biopolymers inevitably deteriorated the electrical conductivity and the sensitivity of assembled sensors. Herein, a novel cellulose nanofiber (CNF)/MXene/carbon black (CB) composite was demonstrated as the conductive layer in eco-friendly cellulose paper-based sensors by intercalating the CB into the MXene/CNF interlayer, followed by coating hydrophobic SiO2 for encapsulation. Befitting from the high-density crack-microstructures between CB and MXene, the fabricated superhydrophobic paper CB/CNF/MXene/SiO2 sensor delivered ultrahigh sensitivity of 729.52 kPa-1, low detect limit of 0.29 Pa, rapid response time of 80 ms and excellent stability over 10,000 cycles. Moreover, the fabricated sensor was capable of detecting the physiological parameter of human (e.g. huge/subtle movements) and spatial pressure distribution. Furthermore, the presence of SiO2 layer endowed the sensor with superhydrophobic performance (water contact angle ∼158.2 o) and stable electrical signals under high moisture conditions or even under water. Our work proposed a novel strategy to boost the sensitivity of MXene-based conductive layer in flexible electronic devices.

MXene-Based Elastomer Mimetic Stretchable Sensors: Design, Properties, and Applications

Flexible sensors based on MXene-polymer composites are highly prospective for next-generation wearable electronics used in human-machine interfaces. One of the motivating factors behind the progress of flexible sensors is the steady arrival of new conductive materials. MXenes, a new family of 2D nanomaterials, have been drawing attention since the last decade due to their high electronic conductivity, processability, mechanical robustness and chemical tunability. In this review, we encompass the fabrication of MXene-based polymeric nanocomposites, their structure-property relationship, and applications in the flexible sensor domain. Moreover, our discussion is not only limited to sensor design, their mechanism, and various modes of sensing platform, but also their future perspective and market throughout the world. With our article, we intend to fortify the bond between flexible matrices and MXenes thus promoting the swift advancement of flexible MXene-sensors for wearable technologies.

Intelligent, Flexible Artificial Throats with Sound Emitting, Detecting, and Recognizing Abilities

In recent years, there has been a notable rise in the number of patients afflicted with laryngeal diseases, including cancer, trauma, and other ailments leading to voice loss. Currently, the market is witnessing a pressing demand for medical and healthcare products designed to assist individuals with voice defects, prompting the invention of the artificial throat (AT). This user-friendly device eliminates the need for complex procedures like phonation reconstruction surgery. Therefore, in this review, we will initially give a careful introduction to the intelligent AT, which can act not only as a sound sensor but also as a thin-film sound emitter. Then, the sensing principle to detect sound will be discussed carefully, including capacitive, piezoelectric, electromagnetic, and piezoresistive components employed in the realm of sound sensing. Following this, the development of thermoacoustic theory and different materials made of sound emitters will also be analyzed. After that, various algorithms utilized by the intelligent AT for speech pattern recognition will be reviewed, including some classical algorithms and neural network algorithms. Finally, the outlook, challenge, and conclusion of the intelligent AT will be stated. The intelligent AT presents clear advantages for patients with voice impairments, demonstrating significant social values.

Bubbles-Induced Porous Structure-Based Flexible Piezoresistive Sensors for Speech Recognition

Flexible piezoresistive sensors with a porous structure that are used in the field of speech recognition are seldom characterized by both high sensitivity and ease of preparation. In this study, a piezoresistive sensor with a porous structure that is both highly sensitive and can be prepared by using a simple method is proposed for speech recognition. The preparation process utilizes the interaction of bubbles generated by ethanol evaporation and active agents with polydimethylsiloxane to produce a porous flexible substrate. This preparation process requires neither templates nor harsh experimental conditions such as a low temperature and a low pressure. Furthermore, the prepared piezoresistive sensor has excellent properties, such as a high sensitivity (27.6 kPa-1), a satisfactory response time (800 μs), and a good stability (10,000 cycles). When used for speech recognition, more than 1500 vocalizations and silent speech signals obtained from subjects saying numbers from "0" to "9" were collected by the sensor for training a convolutional neural network model. The average accuracy of the recognition reached 94.8%. The simple preparation process and the excellent performance of the prepared flexible piezoresistive sensor endow it with a wide application prospect in the field of speech recognition.

Recent advances in two-dimensional nanomaterials for sustainable wearable electronic devices

The widespread adoption of smart terminals has significantly boosted the market potential for wearable electronic devices. Two-dimensional (2D) nanomaterials show great promise for flexible, wearable electronics of next-generation electronic materials and have potential in energy, optoelectronics, and electronics. First, this review focuses on the importance of functionalization/defects in 2D nanomaterials, a discussion of different kinds of 2D materials for wearable devices, and the overall structure-property relationship of 2D materials. Then, in this comprehensive review, we delve into the burgeoning realm of emerging applications for 2D nanomaterial-based flexible wearable electronics, spanning diverse domains such as energy, medical health, and displays. A meticulous exploration is presented, elucidating the intricate processes involved in tailoring material properties for specific applications. Each research direction is dissected, offering insightful perspectives and dialectical evaluations that illuminate future trajectories and inspire fruitful investigations in this rapidly evolving field.

Highly Bioactive MXene-M2-Exosome Nanocomposites Promote Angiogenic Diabetic Wound Repair through Reconstructing High Glucose-Derived Immune Inhibition

The repair of diabetic wounds remains challenging, primarily due to the high-glucose-derived immune inhibition which often leads to the excessive inflammatory response, impaired angiogenesis, and heightened susceptibility to infection. However, the means to reduce the immunosuppression and regulate the conversion of M2 phenotype macrophages under a high-glucose microenvironment using advanced biomaterials for diabetic wounds are not yet fully understood. Herein, we report two-dimensional carbide (MXene)-M2 macrophage exosome (Exo) nanohybrids (FM-Exo) for promoting diabetic wound repair by overcoming the high-glucose-derived immune inhibition. FM-Exo showed the sustained release of M2 macrophage-derived exosomes (M2-Exo) up to 7 days and exhibited broad-spectrum antibacterial activity. In the high-glucose microenvironment, relative to the single Exo, FM-Exo could significantly induce the optimized M2a/M2c polarization ratio of macrophages by activating the PI3K/Akt signaling pathway, promoting the proliferation, migration of fibroblasts, and angiogenic ability of endothelial cells. In the diabetic full-thickness wound model, FM-Exo effectively regulated the polarization status of macrophages and promoted their transition to the M2 phenotype, thereby inhibiting inflammation, promoting angiogenesis through VEGF secretion, and improving proper collagen deposition. As a result, the healing process was accelerated, leading to a better healing outcome with reduced scarring. Therefore, this study introduced a promising approach to address diabetic wounds by developing bioactive nanomaterials to regulate immune inhibition in a high-glucose environment.

Low-dimensional nanostructures for monolithic 3D-integrated flexible and stretchable electronics

Flexible/stretchable electronics, which are characterized by their ultrathin design, lightweight structure, and excellent mechanical robustness and conformability, have garnered significant attention due to their unprecedented potential in healthcare, advanced robotics, and human-machine interface technologies. An increasing number of low-dimensional nanostructures with exceptional mechanical, electronic, and/or optical properties are being developed for flexible/stretchable electronics to fulfill the functional and application requirements of information sensing, processing, and interactive loops. Compared to the traditional single-layer format, which has a restricted design space, a monolithic three-dimensional (M3D) integrated device architecture offers greater flexibility and stretchability for electronic devices, achieving a high-level of integration to accommodate the state-of-the-art design targets, such as skin-comfort, miniaturization, and multi-functionality. Low-dimensional nanostructures possess small size, unique characteristics, flexible/elastic adaptability, and effective vertical stacking capability, boosting the advancement of M3D-integrated flexible/stretchable systems. In this review, we provide a summary of the typical low-dimensional nanostructures found in semiconductor, interconnect, and substrate materials, and discuss the design rules of flexible/stretchable devices for intelligent sensing and data processing. Furthermore, artificial sensory systems in 3D integration have been reviewed, highlighting the advancements in flexible/stretchable electronics that are deployed with high-density, energy-efficiency, and multi-functionalities. Finally, we discuss the technical challenges and advanced methodologies involved in the design and optimization of low-dimensional nanostructures, to achieve monolithic 3D-integrated flexible/stretchable multi-sensory systems.

A Dual-Mode Pressure and Temperature Sensor

The emerging field of flexible tactile sensing systems, equipped with multi-physical tactile sensing capabilities, holds vast potential across diverse domains such as medical monitoring, robotics, and human-computer interaction. In response to the prevailing challenges associated with the limited integration and sensitivity of flexible tactile sensors, this paper introduces a versatile tactile sensing system capable of concurrently monitoring temperature and pressure. The temperature sensor employs carbon nanotube/graphene conductive paste as its sensitive material, while the pressure sensor integrates an ionic gel containing boron nitride as its sensitive layer. Through the application of cost-effective screen printing technology, we have successfully manufactured a flexible dual-mode sensor with exceptional performance, featuring high sensitivity (804.27 kPa-1), a broad response range (50 kPa), rapid response time (17 ms), and relaxation time (34 ms), alongside exceptional durability over 5000 cycles. Furthermore, the resistance temperature coefficient of the sensor within the temperature range of 12.5 °C to 93.7 °C is -0.17% °C-1. The designed flexible dual-mode tactile sensing system enables the real-time detection of pressure and temperature information, presenting an innovative approach to electronic skin with multi-physical tactile sensing capabilities.

Future prospects of MXenes: synthesis, functionalization, properties, and application in field effect transistors

MXenes are a family of two-dimensional (2D) materials that have drawn a lot of interest recently because of their distinctive characteristics and possible uses in a variety of industries. This review emphasizes the bright future prospects of MXene materials in the realm of FETs. Their remarkable properties, coupled with their tunability and compatibility, position MXenes as promising candidates for the development of high-performance electronic devices. As research in this field continues to evolve, the potential of MXenes to drive innovation in electronics becomes increasingly evident, fostering excitement for their role in shaping the future of electronic technology. This paper presents a comprehensive overview of MXene materials, focusing on their synthesis methods, functionalization strategies, intrinsic properties, and their promising application in Field Effect Transistors (FETs).

Dual-Stage Surficial Microstructure to Enhance the Sensitivity of MXene Pressure Sensors for Human Physiological Signal Acquisition

Accompanying the rapid growth of wearable electronics, flexible pressure sensors have received great interest due to their promising application in health monitoring, human-machine interfaces, and intelligent robotics. The high sensitivity over a wide responsive range, integrated with excellent repeatability, is a crucial requirement for the fabrication of reliable pressure sensors for various wearable scenes. In this work, we developed a highly sensitive and long-life flexible pressure sensor by constructing surficial microarrayed architecture polydimethylsiloxane (PDMS) film as a substrate and Ti3C2TX MXene/bacterial cellulose (BC) hybrid as an active sensing layer. The specific surficial morphology of PDMS couples with nanointercalated structure of Ti3C2Tx MXene/BC can effectively improve the sensitivity through controlling the stress distribution and layer spacing under different levels of pressure loading. In addition, abundant spontaneous hydrogen bonds between BC and Ti3C2Tx MXene nanosheets endow the MXene coating with highly adhesive strength on the PDMS surface; hence, the cyclic stability of the pressure sensor is greatly boosted. As a result, the obtained MXene/BC/PDMS (MBP) pressure sensor delivers high sensitivity (528.87 kPa-1), fast response/recovery time (45 ms/29 ms), low detection limit (0.6 Pa), and outstanding repeatability of up to 8000 cycles. Those excellent sensing properties of the MBP sensor allow it to serve as a reliable wearable device to monitor full-range human physiological motions, and it is expected to be applied in next-generation portable electronics, such as E-skins, smart healthcare, and the Internet of Things technology.

MXene-Integrated Perylene Anode with Ultra-Stable and Fast Ammonium-Ion Storage for Aqueous Micro Batteries

The aqueous micro batteries (AMBs) are expected to be one of the most promising micro energy storage devices for its safe operation and cost-effectiveness. However, the performance of the AMBs is not satisfactory, which is attributed to strong interaction between metal ions and the electrode materials. Here, the first AMBs are developed with NH4 + as charge carrier. More importantly, to solve the low conductivity and the dissolution during the NH4 + intercalation/extraction problem of perylene material represented by perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA), the Ti3 C2 Tx MXene with high conductivity and polar surface terminals is introduced as a conductive skeleton (PTCDA/Ti3 C2 Tx MXene). Benefitting from this, the PTCDA/Ti3 C2 Tx MXene electrodes exhibit ultra-high cycle life and rate capability (74.31% after 10 000 galvanostatic chargedischarge (GCD) cycles, and 91.67 mAh g-1 at 15.0 A g-1 , i.e., capacity retention of 45.2% for a 30-fold increase in current density). More significantly, the AMBs with NH4 + as charge carrier and PTCDA/Ti3 C2 Tx MXene anode provide excellent energy density and power density, cycle life, and flexibility. This work will provide strategy for the development of NH4 + storage materials and the design of AMBs.

MXenes vs. clays: emerging and traditional 2D layered nanoarchitectonics

Although MXene materials are considered an emerging research topic, they are receiving considerable interest because, like metals and graphene, they are good electronic conductors but with the particularity that they have a marked hydrophilic character. Having a structural organization and properties close to those of clay minerals (natural silicates typically with a lamellar morphology), they are sometimes referred to as "conducting clays" and exhibit colloidal, surface and intercalation properties also similar to those of clay minerals. The present contribution aims to inform and discuss the nature of MXenes in comparison with clay phyllosilicates, taking into account their structural analogies, outstanding surface properties and advanced applications. The current in-depth understanding of clay minerals may represent a basis for the future development of MXene-derived nanoarchitectures. Comparative examples of the preparation, and studies on the properties and applications of various nanoarchitectures based on clays and MXenes have been included in the present work.

Multilayer Perceptron Algorithm-Assisted Flexible Piezoresistive PDMS/Chitosan/cMWCNT Sponge Pressure Sensor for Sedentary Healthcare Monitoring

Recently, the health problems faced by sedentary workers have received increasing attention. In this study, a pressure sensor based on a poly(dimethylsiloxane) (PDMS)/carboxylated chitosan (CCS)/carboxylated multiwalled carbon nanotube (cMWCNT) sponge was prepared to realize a portable, sensitive, comfortable, and noninvasive healthcare monitoring system for sedentary workers. The proposed piezoresistive pressure sensor exhibited exceptional sensing performances with high sensitivity (147.74 kPa-1), an ultrawide detection range (22 Pa to 1.42 MPa), and reliable stability (over 3000 cycles). Furthermore, the obtained sensor displayed superior capability in detecting various human motion signals. Based on the 4 × 4 sensing array and multilayer perceptron (MLP) algorithm model, a smart cushion was developed to recognize five types of sitting postures and supply timely reminders to sedentary workers. The piezoresistive sponge pressure sensor proposed in this study reveals promising potential in the fields of wearable electronics, healthcare monitoring, and human-machine interface applications.

A highly sensitive flexible capacitive pressure sensor with hierarchical pyramid micro-structured PDMS-based dielectric layer for health monitoring

Herein, a flexible pressure sensor with high sensitivity was created using a dielectric layer featuring a hierarchical pyramid microstructure, both in simulation and fabrication. The capacitive pressure sensor comprises a hierarchically arranged dielectric layer made of polydimethylsiloxane (PDMS) with pyramid microstructures, positioned between copper electrodes at the top and bottom. The achievement of superior sensing performance is highly contingent upon the thickness of the dielectric layer, as indicated by both empirical findings and finite-element analysis. Specifically, the capacitive pressure sensor, featuring a dielectric layer thickness of 0.5 mm, exhibits a remarkable sensitivity of 0.77 kPa-1 within the pressure range below 1 kPa. It also demonstrates an impressive response time of 55 ms and recovery time of 42 ms, along with a low detection limit of 8 Pa. Furthermore, this sensor showcases exceptional stability and reproducibility with up to 1,000 cycles. Considering its exceptional achievements, the pressure sensor has been effectively utilized for monitoring physiological signals, sign language gestures, and vertical mechanical force exerted on objects. Additionally, a 5 × 5 sensor array was fabricated to accurately and precisely map the shape and position of objects. The pressure sensor with advanced performance shows broad potential in electronic skin applications.

Self-Assembled 2D VS2 /Ti3 C2 Tx MXene Nanostructures with Ultrafast Kinetics for Superior Electrochemical Sodium-Ion Storage

Constructing nanostructures with high structural stability and ultrafast electrochemical reaction kinetics as anodes for sodium-ion batteries (SIBs) is a big challenge. Herein, the robust 2D VS2 / Ti3 C2 Tx MXene nanostructures with the strong Ti─S covalent bond synthesized by a one-pot self-assembly approach are developed. The strong interfacial interaction renders the material of good structural durability and enhanced reaction kinetics. Meanwhile, the enlarged and few-layered MXene nanosheets can be easily obtained according to this interaction, providing a conductive network for sufficient electrolyte penetration and rapid charge transfer. As predicted, the VS2 /MXene nanostructures exhibit an extremely low sodium diffusion barrier confirmed by DFT calculations and small charge transfer impedance evidenced by electrochemical impedance spectroscopy (EIS) analysis. Therefore, the SIBs based on the VS2 /MXene electrode present first-class electrochemical performance with the ultrahigh average initial columbic efficiency of 95.08% and excellent sodium-ion storage capacity of 424.6 mAh g-1 even at 10 A g-1 . It also shows an outstanding sodium-ion storage capacity of 514.2 mAh g-1 at 1 A g-1 with a capacity retention of nearly 100% within 500 times high-rate cycling. Such impressive performance demonstrates the successful synthesis strategy and the great potential of interfacial interactions for high-performance energy storage devices.

Temperature-Immune, Wide-Range Flexible Robust Pressure Sensors for Harsh Environments

Flexible pressure sensors possess vast potential for various applications such as new energy batteries, aerospace engines, and rescue robots owing to their exceptional flexibility and adaptability. However, the existing sensors face significant challenges in maintaining long-term reliability and environmental resilience when operating in harsh environments with variable temperatures and high pressures (∼MPa), mainly due to possible mechanical mismatch and structural instability. Here, we propose a composite scheme for a flexible piezoresistive pressure sensor to improve its robustness by utilizing material design of near-zero temperature coefficient of resistance (TCR), radial gradient pressure-dividing microstructure, and flexible interface bonding process. The sensing layer comprising multiwalled carbon nanotubes (MWCNTs), graphite (GP), and thermoplastic polyurethane (TPU) was optimized to achieve a near-zero temperature coefficient of resistance over a temperature range of 25-70 °C, while the radial gradient microstructure layout based on pressure division increases the range of pressure up to 2 MPa. Furthermore, a flexible interface bonding process introduces a self-soluble transition layer by direct-writing TPU bonding solution at the bonding interface, which enables the sensor to achieve signal fluctuations as low as 0.6% and a high interface strength of up to 1200 kPa. Moreover, it has been further validated for its capability of monitoring the physiological signals of athletes as well as the long-term reliable environmental resilience of the expansion pressure of the power cell. This work demonstrates that the proposed scheme sheds new light on the design of robust pressure sensors for harsh environments.

An ultrasensitive MXene-based electrochemical immunosensor for the detection and species identification of archaeological silk microtraces

The origin and dissemination of silk have been hotly debated in the field of archaeology, and the key to resolving this controversy lies in the detection and species identification of ancient silk microtraces. Herein, a taxonomically specific anti-fibroin monoclonal antibody was successfully prepared and a layer-by-layer self assembly electrochemical immunosensor was innovatively proposed for detecting silk traces based on flexible carbon cloth. The immunosensor possessed a broad linear range of 10-2-103 ng mL-1 and a detection limit of 2.15 pg mL-1 for the ultrasensitive detection of Bombyx mori silk traces. In addition, the elaborate immunosensor exhibited satisfactory high specificity, storage stability and reproducibility. In particular, the qualitative and quantitative performance of the immunosensor was excellent in the analysis of archaeological samples. Therefore, this work demonstrates that the proposed method not only provides a reliable analytical tool for exploring the origin and spread of archaeological silk, but also improves our understanding of how to use emerging materials like two-dimensional titanium carbide to creat innovative biosensors.

Machine Learning-Enhanced Biomass Pressure Sensor with Embedded Wrinkle Structures Created by Surface Buckling

Flexible piezoresistive sensors are core components of many wearable devices to detect deformation and motion. However, it is still a challenge to conveniently prepare high-precision sensors using natural materials and identify similar short vibration signals. In this study, inspired by microstructures of human skins, biomass flexible piezoresistive sensors were prepared by assembling two wrinkled surfaces of konjac glucomannan and k-carrageenan composite hydrogel. The wrinkle structures were conveniently created by hardness gradient-induced surface buckling and coated with MXene sheets to capture weak pressure signals. The sensor was applied to detect various slight body movements, and a machine learning method was used to enhance the identification of similar and short throat vibration signals. The results showed that the sensor exhibited a high sensitivity of 5.1 kPa-1 under low pressure (50 Pa), a fast response time (104 ms), and high stability over 100 cycles. The XGBoost machine learning model accurately distinguished short voice vibrations similar to those of individual English letters. Moreover, experiments and numerical simulations were carried out to reveal the mechanism of the wrinkle structure preparation and the excellent sensing performance. This biomass sensor preparation and the machine learning method will promote the optimization and application of wearable devices.

A dual-module co-regulated stable pressure sensor for human activity monitoring

Flexible pressure sensors have received significant attention for applications in wearable devices and human health monitoring in healthcare. Nevertheless, the preparation of pressure sensors with a wide operating range and high sensitivity remains a huge challenge. Herein, a piezoresistive sensor with a porous microstructure is prepared by using a sandpaper and salt dual-template method. Carbon black and carbon nanotubes are used as conductive fillers and the pressure sensor shows a high sensitivity (58.33 kPa-1) and extremely fast response and recovery times (3 and 10 ms, respectively). Furthermore, this CNT/PDMS/CB sensor shows a high detection limit of 400 kPa and excellent cycling stability of over 20 000 cycles. This high-performance pressure sensor is promising for many applications such as wearable electronics and health monitoring.

Ultra-Tough Waterborne Polyurethane-Based Graft-Copolymerized Piezoresistive Composite Designed for Rehabilitation Training Monitoring Pressure Sensors

Effective training is crucial for patients who need rehabilitation for achieving optimal recovery and reducing complications. Herein, a wireless rehabilitation training monitoring band with a highly sensitive pressure sensor is proposed and designed. It utilizes polyaniline@waterborne polyurethane (PANI@WPU) as a piezoresistive composite material, which is prepared via the in situ grafting polymerization of PANI on the WPU surface. WPU is designed and synthesized with tunable glass transition temperatures ranging from -60 to 0 °C. Dipentaerythritol (Di-PE) and ureidopyrimidinone (UPy) groups are introduced, endowing the material with good tensile strength (14.2 MPa), toughness (62 MJ-1 m-3 ), and great elasticity (low permanent deformation: 2%). Di-PE and UPy enhance the mechanical properties of WPU by increasing the cross-linking density and crystallinity. Combining the toughness of WPU and the high-density microstructure derived by hot embossing technology, the pressure sensor exhibits high sensitivity (168.1 kPa-1 ), fast response time (32 ms), and excellent stability (10 000 cycles with 3.5% decay). In addition, the rehabilitation training monitoring band is equipped with a wireless Bluetooth module, which can be easily applied to monitor the rehabilitation training effect of patients using an applet. Therefore, this work has the potential to significantly broaden the application of WPU-based pressure sensors for rehabilitation monitoring.

Flexible tactile sensors with biomimetic microstructures: Mechanisms, fabrication, and applications

In recent years, flexible devices have gained rapid development with great potential in daily life. As the core component of wearable devices, flexible tactile sensors are prized for their excellent properties such as lightweight, stretchable and foldable. Consequently, numerous high-performance sensors have been developed, along with an array of innovative fabrication processes. It has been recognized that the improvement of the single performance index for flexible tactile sensors is not enough for practical sensing applications. Therefore, balancing and optimization of overall performance of the sensor are extensively anticipated. Furthermore, new functional characteristics are required for practical applications, such as freeze resistance, corrosion resistance, self-cleaning, and degradability. From a bionic perspective, the overall performance of a sensor can be optimized by constructing bionic microstructures which can deliver additional functional features. This review briefly summarizes the latest developments in bionic microstructures for different types of tactile sensors and critically analyzes the sensing performance of fabricated flexible tactile sensors. Based on this, the application prospects of bionic microstructure-based tactile sensors in human detection and human-machine interaction devices are introduced.

Reliable and Scalable Piezoresistive Sensors with an MXene/MoS2 Hierarchical Nanostructure for Health Signals Monitoring

The increased popularity of wearable electronic devices has led to a greater need for advanced sensors. However, fabricating pressure sensors that are flexible, highly sensitive, robust, and compatible with large-scale fabrication technology is challenging. This work investigates a piezoresistive sensor constructed from an MXene/MoS2 hierarchical nanostructure, which is obtained through an easy and inexpensive fabrication process. The sensor exhibits a high sensitivity of 0.42 kPa-1 (0-1.5 kPa), rapid response (∼36 ms), and remarkable mechanical durability (∼10,000 cycles at 13 kPa). The sensor has been demonstrated to be successful in detecting human motion, speech recognition, and physiological signals, particularly in analyzing human pulse. These data can be used to alert and identify irregularities in human health. Additionally, the sensing units are able to construct sensor arrays of various sizes and configurations, enabling pressure distribution imaging in a variety of application scenarios. This research proposes a cost-effective and scalable approach to fabricating piezoresistive sensors and sensor arrays, which can be utilized for monitoring human health and for use in human-machine interfaces.

Highly sensitive piezoresistive and thermally responsive fibrous networks from the in situ growth of PEDOT on MWCNT-decorated electrospun PU fibers for pressure and temperature sensing

Flexible electronics have demonstrated various strategies to enhance the sensory ability for tactile perception and wearable physiological monitoring. Fibrous microstructures have attracted much interest because of their excellent mechanical properties and fabricability. Herein, a structurally robust fibrous mat was first fabricated by electrospinning, followed by a sequential process of functionalization utilizing ultrasonication treatment and in situ polymerization growth. Electrospun polyurethane (PU) microfibers were anchored with multi-walled carbon nanotubes (MWCNTs) to form conductive paths along each fiber by a scalable ultrasonic cavitation treatment in an MWCNT suspension. After, a layer of poly(3,4-ethylene dioxythiophene) (PEDOT) was grown on the surface of PU fibers decorated with MWCNTs to enhance the conductive conjunctions of MWCNTs. Due to the superior electromechanical behaviors and mechanical reinforcement of PEDOT, the PEDOT/MWCNT@PU mat-based device exhibits a wide working range (0-70 kPa), high sensitivity (1.6 kPa-1), and good mechanical robustness (over 18,000 cycles). The PEDOT/MWCNT@PU mat-based sensor also demonstrates a good linear response to different temperature variations because of the thermoelectricity of the PEDOT/MWCNT composite. This novel strategy for the fabrication of multifunctional fibrous mats provides a promising opportunity for future applications for high-performance wearable devices.

Flexible electronics for cardiovascular healthcare monitoring

Cardiovascular diseases (CVDs) are one of the most urgent threats to humans worldwide, which are responsible for almost one-third of global mortality. Over the last decade, research on flexible electronics for monitoring and treatment of CVDs has attracted tremendous attention. In contrast to conventional medical instruments in hospitals that are usually bulky, hard to move, monofunctional, and time-consuming, flexible electronics are capable of continuous, noninvasive, real-time, and portable monitoring. Notable progress has been made in this emerging field, and thus a number of significant achievements and concomitant research prospects deserve attention for practical implementation. Here, we comprehensively review the latest progress of flexible electronics for CVDs, focusing on new functions provided by flexible electronics. First, the characteristics of CVDs and flexible electronics and the foundation of their combination are briefly reviewed. Then, four representative applications of flexible electronics for CVDs are elaborated: blood pressure (BP) monitoring, electrocardiogram (ECG) monitoring, echocardiogram monitoring, and direct epicardium monitoring. Their operational principles, progress, merits and demerits, and future efforts are discussed. Finally, the remaining challenges and opportunities for flexible electronics for cardiovascular healthcare are outlined.

Ti3C2Tx MXene-Based Multifunctional Tactile Sensors for Precisely Detecting and Distinguishing Temperature and Pressure Stimuli

Although skin-like sensors that can simultaneously detect various physical stimuli are of fair importance in cutting-edge human-machine interaction, robotic, and healthcare applications, they still face challenges in facile, scalable, and cost-effective production using conventional active materials. The emerging two-dimensional transition metal carbide, Ti3C2Tx MXene, integrated with favorable thermoelectric properties, metallic-like conductivity, and a hydrophilic surface, is promising for solving these problems. Herein, skin-like multifunctional sensors are designed to precisely detect and distinguish temperature and pressure stimuli without cross-talk by decorating elastic and porous substrates with MXene sheets. Because the combination of the thermoelectric and conductive MXene with the thermally insulating, elastic, and porous substrate integrates efficient Seebeck and piezoresistive effects, the resultant sensor exhibits not only an ultralow detection limit (0.05 K), high signal-to-noise ratio, and excellent cycling stability for temperature detection but also high sensitivity, fast response time, and outstanding durability for pressure detection. Based on the impressive dual-mode sensing properties and independent temperature and pressure detections, a multimode input terminal and an electronic skin are created, exhibiting great potential in robotic and human-machine interaction applications. This work provides a scalable fabrication of multifunctional tactile sensors for precisely detecting and distinguishing temperature and pressure stimuli.

Free-standing conductive nickel metal-organic framework nanowires as bifunctional electrodes for wearable pressure sensors and Ni-Zn batteries

Free-standing metal-organic frameworks (MOFs) with controllable structure and good stability are emerging as promising materials for applications in flexible pressure sensors and energy-storage devices. However, the inherent low electrical conductivity of MOF-based materials requires complex preparation processes that involve high-temperature carbonization. This work presents a simple method to grow conductive nickel MOF nanowire arrays on carbon cloth (Ni-CAT@CC) and use Ni-CAT@CC as the functional electrodes for flexible piezoresistive sensor. The resulting sensor is able to monitor human activity, including elbow bending, knee bending, and wrist bending. Besides, the soft-packaged aqueous Ni-Zn battery is assembled with Ni-CAT@CC, a piece of glass microfiber filters, and Zn foil acting as cathode, separator, and anode, respectively. The Ni-Zn battery can be used as a power source for finger pressure monitoring. This work demonstrates free-standing MOF-based nanowires as bifunctional fabric electrodes for wearable electronics.

MXene Derivatives for Energy Storage and Conversions

Associated with the rapid development of 2D transition metal carbides, nitrides, and carbonitrides (MXenes), MXene derivatives have been recently exploited and exhibited unique physical/chemical properties, holding promising applications in the areas of energy storage and conversions. This review provides a comprehensive summarization of the latest research and progress on MXene derivatives, including termination-tailored MXenes, single-atom implanted MXenes, intercalated MXenes, van der Waals atomic layers, and non-van der Waals heterostructures. The intrinsic relationship between structure, properties, and corresponding applications for MXene derivatives are then emphasized. Finally, the essential challenges are addressed and perspectives for the MXene derivatives are also discussed.