Wearable Strain Sensors Based on a Porous Polydimethylsiloxane Hybrid with Carbon Nanotubes and Graphene

A Printable Deep Eutectic/Copper Conductive Colloid for Wearable Devices

The advancement of wearable electronics has placed higher demands on the comfort and convenience of flexible materials. In this work, a conductive pseudoplastic colloid was developed by utilizing the oxygen elements adsorbed on the surface of copper powder, which forms donor-acceptor interactions with the hydrogen bond donors in a deep eutectic solvent. The flakelike copper powder, serving as a conductive filler, provides more efficient spatial conductive pathways and further enhances the cross-linking ability between the copper powder and the deep eutectic solvent. The resulting deep eutectic/copper colloid not only exhibits low volume resistivity (1.19 × 10-3 (Ω·m)), high viscosity, and excellent thermal stability but also demonstrates outstanding strain-resistance characteristics. By printing onto a textile substrate, a flexible strain sensor with a wide linear strain range (5-90%) and ultrahigh sensitivity (gauge factor ≈ 1 × 105) was fabricated. This sensor can sensitively and stably detect human body movements such as joint and muscle motions. Furthermore, the sensor has been integrated into a portable glove for motion detection and human-machine interaction, showcasing its great potential as a high-performance wearable strain sensor.

High-Sensitivity ZnS:Cu Piezoresistive Sensor for Underwater Visual Sensing

In recent years, flexible wearable sensors have been used for human motion monitoring and human-computer interaction, but designing a sensitive, multifunctional composite sensor adaptable to complex scenarios remains challenging. In this work, we developed a multifunctional composite sensor in a multilayer design by combining highly conductive multiwalled carbon nanotubes (MWCNTs) and graphite (GP) with nanoluminescent materials ZnS:Cu to achieve visual feedback and signal detection. Through material optimization, we developed a sensor with stable luminescence (IML(300 Cycle) = 90%I0) and high sensitivity (GF = 13.65), enabling reliable signal detection in complex scenarios. Leveraging its strong luminescence and durability, a low-light gesture recognition system was built, effectively addressing recognition challenges and providing intuitive visual support. In addition, we apply the trained classifier model to unmanned vehicle control in low light, which further verifies the feasibility of the materials and algorithms. Finally, as a proof of concept, the multifunctional composite sensor is designed for real-time sensing in deep-sea exploration, rescue, and low-light underwater communication, demonstrating great potential for wearable devices.

Flexible, wearable mechano-acoustic sensors for body sound monitoring applications

Body sounds serve as a valuable source of health information, offering insights into systems such as the cardiovascular, pulmonary, and gastrointestinal systems. Additionally, body sound measurements are easily accessible, fast, and non-invasive, which has led to their widespread use in clinical auscultation for diagnosing health conditions. However, conventional devices like stethoscopes are constrained by rigid and bulky designs, limiting their potential for long-term monitoring and often leading to subjective diagnoses. Recently, flexible, wearable mechano-acoustic sensors have emerged as an innovative alternative for body sound auscultation, offering significant advantages over conventional rigid devices. This review explores these advanced sensors, delving into their sensing mechanisms, materials, configurations, and fabrication techniques. Furthermore, it highlights various health monitoring applications of flexible, wearable mechano-acoustic sensors based on body sound auscultation. Finally, the existing challenges and promising opportunities are addressed, providing a snapshot of the current picture and the strategies of future approaches in this rapidly evolving field.

Lignin-containing nanofiber-reinforced flexible strain sensors with excellent mechanical properties and ionic conductivity for human motion detection

A multifunctional hydrogel with outstanding mechanical properties and excellent ionic conductivity holds immense potential for applications in various fields, such as healthcare monitoring, and various devices, such as wearable devices and flexible electronics. However, developing hydrogels that combine high mechanical strength with efficient electrical conductivity remains a considerable challenge. Herein, an ion-conductive hydrogel with excellent mechanical properties and ionic conductivity is successfully created. This hydrogel integrates sensing capabilities, freeze tolerance, and long-term solvent retention through a synergistic combination of lignin-containing cellulose nanofibers (LCNF), polyvinyl alcohol chains, ethylene glycol, and aluminum chloride. The resulting hydrogel demonstrates exceptional mechanical performance in terms of various factors, including tensile strength (1.28 MPa), strain capacity (794.94 %), toughness (6.32 MJ/m3), and fatigue resistance. In addition, the incorporation of enhanced LCNF fillers harmonizes the mechanical properties and ionic conductivity of the ion-conductive hydrogel, effectively addressing the inherent trade-off between these two attributes-a common challenge associated with ionic hydrogels. Moreover, the ion-conductive hydrogel exhibits exceptional sensing stability (300 cycles at 80 % strain), ionic conductivity (0.82 S/m), and sensitivity along with near real-time response (300 ms), freeze tolerance (-24 °C), and prolonged solvent retention (180 d). This multifunctional ion-conductive hydrogel opens new pathways for designing advanced wearable sensors.

Advances in conducting nanocomposite hydrogels for wearable biomonitoring

Recent advancements in wearable biosensors and bioelectronics have led to innovative designs for personalized health management devices, with biocompatible conducting nanocomposite hydrogels emerging as a promising building block for soft electronics engineering. In this review, we provide a comprehensive framework for advancing biosensors using these engineered nanocomposite hydrogels, highlighting their unique properties such as high electrical conductivity, flexibility, self-healing, biocompatibility, biodegradability, and tunable architecture, broadening their biomedical applications. We summarize key properties of nanocomposite hydrogels for thermal, biomechanical, electrophysiological, and biochemical sensing applications on the human body, recent progress in nanocomposite hydrogel design and synthesis, and the latest technologies in developing flexible and wearable devices. This review covers various sensor types, including strain, physiological, and electrochemical sensors, and explores their potential applications in personalized healthcare, from daily activity monitoring to versatile electronic skin applications. Furthermore, we highlight the blueprints of design, working procedures, performance, detection limits, and sensitivity of these soft devices. Finally, we address challenges, prospects, and future outlook for advanced nanocomposite hydrogels in wearable sensors, aiming to provide a comprehensive overview of their current state and future potential in healthcare applications.

Prognosis of Cardiovascular Conditions Noninvasively Using Printable Elastomeric Electronic Skin

Lack of timely prognosis of cardiovascular condition (CVC) is resulting in increased mortality across the globe. Currently, available techniques are confined to medical facilities and need the intervention of specialists. Frequently, this impedes timely treatment, driven by socioeconomic factors. Consequently, the disease transcends toward incurable complications. In such a scenario, point-of-care diagnostic tools can help with prognosis at an early stage. Albeit there are such tools available, it is imperative to develop affordably in uncomplicated manufacturing techniques and should have simple readout and analysis modules for monitoring CVC. Accordingly, the solvent-free manufacturing of stencil printable liquid elastomer-carbon nanotube electronic skin-based strain sensor, capable of accurately detecting pulse (at different positions) and other parameters like augmentation index and stiffness index of artery related to the CVC, is reported. The Poincare plot, derived from the recorded data, measures heart rate variability, a key indicator linked to mortality. Thanks to the staggering linearity, gauge factor of 234.26, fast response time of 85 ms (measured from pulse data), and cyclic stability (over 500 cycles), assist in the ease of detection of vital parameters. Furthermore, the sensor patch demonstrates its capability to acquire pulse waves under different real-time artery conditions using cuff-based pressure applications.

Biointerface engineering of flexible and wearable electronics

Biointerface sensing is a cutting-edge interdisciplinary field that merges conceptual and practical aspects. Wearable bioelectronics enable efficient interaction and close contact with biological components such as tissues and organs, paving the way for a wide range of medical applications, including personal health monitoring and medical intervention. To be applicable in real-world settings, the patches must be stable and adhere to the skin without causing discomfort or allergies in both wet and dry conditions, as well as other desirable features such as being ultra-soft, thin, flexible, and stretchable. Biosensors have emerged as promising tools primarily used to directly detect biological and electrophysiological signals, enhancing the efficacy of personalized medical treatments and enabling accurate tracking of human well-being. This review highlights the engineering of skin-tissue surfaces/interfaces and their interactions with wearable patches, aiming for both a broad and in-depth understanding of the mechanical and physicochemical properties required for the advancement of flexible and wearable skin patches. Specifically, the advantages of flexible bioelectronics and sensors with optimized surface geometry for long-term diagnosis are discussed. This insight aims to guide the future development of functional materials that can interact with human tissue in a controlled manner. Finally, we provide perspectives on the challenges and potential applications of biointerface engineering in wearable devices.

Ultraflexible Sensor Development via 4D Printing: Enhanced Sensitivity to Strain, Temperature, and Magnetic Fields

This paper addresses the trade-off between sensitivity and sensing range in strain sensors, while introducing additional functionalities through an innovative 4D printing approach. The resulting ultraflexible sensor integrates carbon nanotubes/liquid metal hybrids and iron powders within an Ecoflex matrix. The optimization of this composition enables the creation of an uncured resin ideal for Direct Ink Writing (DIW) and a cured sensor with exceptional electromechanical, thermal, and magnetic performance. Notably, the sensor achieves a wide linear strain range of 350% and maintains a stable Gauge Factor of 19.8, offering an ultralow detection limit of 0.1% strain and a rapid 83-ms response time. Beyond superior strain sensing capabilities, the sensor exhibits outstanding thermal endurance for temperatures exceeding 300 °C, enhanced thermal conductivity, and a consistent resistance-temperature relationship, making it well-suited for high-temperature applications. Moreover, the inclusion of iron particles provides magnetic responsiveness, enabling synergistic applications in location and speed detection, particularly in home care. Leveraging DIW facilitates the creation of complex-shaped sensors with multiple functional materials, significantly broadening the sensor's capabilities. This convergence of additive manufacturing and multifunctional materials marks a transformative step in advancing the performance of next-generation sensors across diverse domains.

MXene-MWCNT Conductive Network for Long-Lasting Wearable Strain Sensors with Gesture Recognition Capabilities

In this work, a conductive composite film composed of multi-walled carbon nanotubes (MWCNTs) and multi-layer Ti3C2Tx MXene nanosheets is used to construct a strain sensor on sandpaper Ecoflex substrate. The composite material forms a sophisticated conductive network with exceptional electrical conductivity, resulting in sensors with broad detection ranges and high sensitivities. The findings indicate that the strain sensing range of the Ecoflex/Ti3C2Tx/MWCNT strain sensor, when the mass ratio is set to 5:2, extends to 240%, with a gauge factor (GF) of 933 within the strain interval from 180% to 240%. The strain sensor has demonstrated its robustness by enduring more than 33,000 prolonged stretch-and-release cycles at 20% cyclic tensile strain. Moreover, a fast response time of 200 ms and detection limit of 0.05% are achieved. During application, the sensor effectively enables the detection of diverse physiological signals in the human body. More importantly, its application in a data glove that is coupled with machine learning and uses the Support Vector Machine (SVM) model trained on the collected gesture data results in an impressive recognition accuracy of 93.6%.

Chemical-Physical Synergistic Assembly of MXene/CNT Nanocoatings in Silicone Foams for Reliable Piezoresistive Sensing in Harsh Environments

Owing to their high sensitivity across a wide stress range, mechanical reliability, and rapid response time, flexible polymer foam piezoresistive sensors have been extensively used in various fields. The reliable application of these sensors under harsh environments, however, is severely limited by structural devastation and poor interfacial bonding between polymers and conductive nanoparticles. To address the above issues, robust MXene/CNT nanocoatings on the foam surface, where the chemical assembly of MXene nanosheets and the physical anchoring of CNTs lead to strong interfacial bonding, are designed and described, which endows foams with structural reliability and unexpected multi-functionalities without compromising their instinct properties. The optimized foam nanocomposites thus maintain outstanding wide-temperature flexibility (-60-210 °C) and elasticity (≈3% residual strain after 1000 cycles). Moreover, the nanocomposites display good sensitivity at a relatively wide stress range of 0-70% and remarkable stability under acidic and alkaline settings. Furthermore, the foams with exceptional fire resistance (UL-94 V-0 rating) can provide stable sensing behavior (over 300 cycles) even after being exposed to flames for 5 s, making them one of the most reliable sensing materials so far. Clearly, this work widens applications of flexible piezoresistive sensors based on silicone foam nanocomposites for various harsh environments.

A Highly Sensitive Strain Sensor with Self-Assembled MXene/Multi-Walled Carbon Nanotube Sliding Networks for Gesture Recognition

Flexible electronics is pursuing a new generation of electronic skin and human-computer interaction. However, effectively detecting large dynamic ranges and highly sensitive human movements remains a challenge. In this study, flexible strain sensors with a self-assembled PDMS/MXene/MWCNT structure are fabricated, in which MXene particles are wrapped and bridged by dense MWCNTs, forming complex sliding conductive networks. Therefore, the strain sensor possesses an impressive sensitivity (gauge factor = 646) and 40% response range. Moreover, a fast response time of 280 ms and detection limit of 0.05% are achieved. The high performance enables good prospects in human detection, like human movement and pulse signals for healthcare. It is also applied to wearable smart data gloves, in which the CNN algorithm is utilized to identify 15 gestures, and the final recognition rate is up to 95%. This comprehensive performance strain sensor is designed for a wide array of human body detection applications and wearable intelligent systems.

Conductive and superhydrophobic lignin/carbon nanotube coating with nest-like structure for deicing, oil absorption and wearable piezoresistive sensor

The development of multifunctional coatings is a trend. Here, a conductive and superhydrophobic coating with nest-like structure was prepared on the wood or polyurethane (PU) sponge by spraying or soaking methods. The coating contains lignin and carboxylated multi-wall carbon nanotubes (MWCNT) as the main materials, both methyl trimethoxysilane (MTMS) and polydimethylsiloxane (PDMS) as the modifiers. And benefiting from the protective effect of the nest-like structure, the coating exhibits excellent abrasion resistance (withstanding 43 abrasion cycles), stability, and UV resistance (little change in water contact angle after 240 h of ultraviolet (UV) irradiation) by optimizing the proportions. Additionally, the coating provides eminent deicing (complete removal after 142.7 s) and self-cleaning on the wood, as well as the superior sensing performance and oil absorption (15.0-49.6 g/g for various oils) on the PU sponge. When assembled into compressible piezoresistive sensor, it could clearly sense the signals of rapid, short, circulation, different speed and deformation, possessing a prosperous wearable device prospect. It is envisaged that the coating supplies a new platform for superhydrophobicity, wearable electronics and oil absorption.

Flexible Strain Sensor Based on Nickel Microparticles/Carbon Black Microspheres/Polydimethylsiloxane Conductive Composites for Human Motion Detection

Herein, we report a dual-functional flexible sensor (DFFS) using a magnetic conductive polymer composed of nickel (Ni), carbon black (CB), and polydimethylsiloxane (PDMS). The material selection for the DFFS utilizes the excellent elasticity of the PDMS matrix and the synergistic interaction between Ni and CB. The DFFS has a wide strain range of 0-170%, a high sensitivity of 74.13 (140-170%), and a low detection limit of 0.3% strain. The DFFS based on superior performance can accurately detect microstrain/microvibration, oncoming/contacting objects, and bicycle riding speed. Additionally, the DFFS can be used for comprehensive monitoring of human movements. Therefore, the DFFS of this work shows significant value for implementation in intelligent wearable devices and noncontact intelligent control.

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 smart wearable sensors for continuous human health monitoring

In recent years, the biochemical and biological research areas have shown great interest in a smart wearable sensor because of its increasing prevalence and high potential to monitor human health in a non-invasive manner by continuous screening of biomarkers dispersed throughout the biological analytes, as well as real-time diagnostic tools and time-sensitive information compared to conventional hospital-centered system. These smart wearable sensors offer an innovative option for evaluating and investigating human health by incorporating a portion of recent advances in technology and engineering that can enhance real-time point-of-care-testing capabilities. Smart wearable sensors have emerged progressively with a mixture of multiplexed biosensing, microfluidic sampling, and data acquisition systems incorporated with flexible substrate and bodily attachments for enhanced wearability, portability, and reliability. There is a good chance that smart wearable sensors will be relevant to the early detection and diagnosis of disease management and control. Therefore, pioneering smart wearable sensors into reality seems extremely promising despite possible challenges in this cutting-edge technology for a better future in the healthcare domain. This review presents critical viewpoints on recent developments in wearable sensors in the upcoming smart digital health monitoring in real-time scenarios. In addition, there have been proactive discussions in recent years on materials selection, design optimization, efficient fabrication tools, and data processing units, as well as their continuous monitoring and tracking strategy with system-level integration such as internet-of-things, cyber-physical systems, and machine learning algorithms.

Recent Advances in Laser-Induced Graphene-Based Materials for Energy Storage and Conversion

Laser-induced graphene (LIG) is a porous carbon nanomaterial that can be produced by irradiation of CO2 laser directly on the polymer substrate under ambient conditions. LIG has many merits over conventional graphene, such as simple and fast synthesis, tunable structure and composition, high surface area and porosity, excellent electrical and thermal conductivity, and good flexibility and stability. These properties make LIG a promising material for energy applications, such as supercapacitors, batteries, fuel cells, and solar cells. In this review, we highlight the recent advances of LIG in energy materials, covering the fabrication methods, performance enhancement strategies, and device integration of LIG-based electrodes and devices in the area of hydrogen evolution reaction, oxygen evolution reaction, oxygen reduction reaction, zinc-air batteries, and supercapacitors. This comprehensive review examines the potential of LIG for future sustainable and efficient energy material development, highlighting its versatility and multifunctionality in energy conversion.

Wearable sweat chloride sensors: materials, fabrication and their applications

Chloride is a crucial anion required for multiple functions in the human body including maintaining acid-base balance, fluid balance, electrical neutrality and supporting muscles and nerve cells. Low-chloride levels can cause nausea, diarrhoea, etc. Chloride levels are measured in different body fluids such as urine, serum, sweat and saliva. Sweat chloride measurements are used for multiple applications including disease diagnosis, sports monitoring, and geriatric care. For instance, a sweat chloride test is performed for cystic fibrosis screening. Further, sweat also offers continuous non-invasive access to body fluids for real-time monitoring of chloride that could be used for sports and geriatric care. This review focuses on wearable chloride sensors that are used for periodic and continuous chloride monitoring. The multiple sections in the paper discuss the clinical significance of chloride, detection methods, sensor fabrication methods and their application in cystic fibrosis screening, sports and geriatric care. Finally, the last section discusses the limitation of current sensors and future directions for wearable chloride sensors.

Highly Sensitive Flexible Sensors for Human Activity Monitoring and Personal Healthcare

Flexible sensors are capable of converting multiple human physiological signals into electrical signals for various applications in clinical diagnostics, athletics, and human-machine interaction. High-performance flexible strain sensors are particularly desirable for sensitive, reliable, and long-term monitoring, but current applications are still constrained due to high response threshold, low recoverability properties, and complex preparation methods. In this study, we present a stable and flexible strain sensor by a cost-effective self-assemble approach that demonstrates remarkable sensitivity (2169), ultrafast response and recovery time (112 ms), and wide dynamic response range (0-50%), as confirmed in human pulse and human-computer interaction. These excellent performances can be attributed to the design of a Polydimethylsiloxane (PDMS) substrate integrated with multiwalled carbon nanotubes (MWCNT) and graphene nanosheets (GNFs), which results in high electrical conductivity. The MWCNT serves as a bridge, connecting the GNFs to create an efficient conductive path even under a strain of 50%. We also demonstrate the strain sensor's capability in weak physiological signal pulse measurement and excellent resistance to mechanical fatigue. Moreover, the sensor shows diverse sensitivities in various tensile states with different signal patterns, making it highly suitable for full-range human monitoring and flexible wearable systems.

Waterproof, Light Responsive, and Highly Sensitive Fabric Strain Sensor for Flexible Electronics

Corrosion resistant, durable, and lightweight flexible strain sensor with multiple functionalities is an urgent demand for modern flexible wearable devices. However, currently developed wearable devices are still limited by poor environmental adaptability and functional singleness. In this work, a conductive fabric with multifunctionality in addition to sensing was successfully prepared by assembling zero dimensional silver nanoparticles (AgNPs) and one-dimensional carbon nanotubes (CNTs) layer by layer on the surface of the elastic polypropylene nonwoven fabric (named PACS fabric). Polystyrene-block-poly(ethylene-co-butylene)-block-polystyrene (SEBS) added as binder materials favored strong interaction between conductive fillers and the fabric. Benefiting from the synergistic interaction among the conductive fillers with different dimensions and the fabric, the strain sensor based on the conductive fabric showed high sensitivity (GF up to 8064), wide detection range (0-200%), and excellent stability and durability (more than 10000 stretch-release cycles). Besides, the prepared conductive fabric showed superhydrophobicity (water contact angle = 154°) with excellent durability. This ensured the performance stability of the fabric sensor in harsh environments. At the same time, the fabric also showed excellent photothermal conversion performance (90 °C at a power density of 0.2 W/cm2 within 20 s). The PACS fabric strain sensor proved excellent performance and environmental adaptability, revealing great potential to be applied in human motion monitoring, self-cleaning, biomedicine, and other fields.

Integration of Capacitive Pressure Sensor-on-Chip with Lead-Free Perovskite Solar Cells for Continuous Health Monitoring

The increasing prevalence of hypertension necessitates continuous blood pressure monitoring. This can be safely and painlessly achieved using non-invasive wearable electronic devices. However, the integration of analog, digital, and power electronics into a single system poses significant challenges. Therefore, we demonstrated a comprehensive multi-scale simulation of a sensor-on-chip that was based on a capacitive pressure sensor. Two analog interfacing circuits were proposed for a full-scale operation ranging from 0 V to 5 V, enabling efficient digital data processing. We also demonstrated the integration of lead-free perovskite solar cells as a mechanism for self-powering the sensor. The proposed system exhibits varying sensitivity from 1.4 × 10-3 to 0.095 (kPa)-1, depending on the pressure range of measurement. In the most optimal configuration, the system consumed 50.5 mW, encompassing a 6.487 mm2 area for the perovskite cell and a CMOS layout area of 1.78 × 1.232 mm2. These results underline the potential for such sensor-on-chip designs in future wearable health-monitoring technologies. Overall, this paper contributes to the field of wearable health-monitoring technologies by presenting a novel approach to self-powered blood pressure monitoring through the integration of capacitive pressure sensors, analog interfacing circuits, and lead-free perovskite solar cells.

Efficient Fabrication of TPU/MXene/Tungsten Disulfide Fibers with Ultra-Fast Response for Human Respiratory Pattern Recognition and Disease Diagnosis via Deep Learning

Flexible wearable pressure sensors have received increasing attention as the potential application of flexible wearable devices in human health monitoring and artificial intelligence. However, the complex and expensive process of the conductive filler has limited its practical production and application on a large scale to a certain extent. This study presents a kind of piezoresistive sensor by sinking nonwoven fabrics (NWFs) into tungsten disulfide (WS2) and Ti3C2Tx MXene solutions. With the advantages of a simple production process and practicality, it is conducive to the realization of large-scale production. The assembled flexible pressure sensor exhibits high sensitivity (45.81 kPa-1), wide detection range (0-410 kPa), fast response/recovery time (18/36 ms), and excellent stability and long-term durability (up to 5000 test cycles). Because of the high elastic modulus of MXene and the synergistic effect between WS2 and MXene, the detection range and sensitivity of the piezoresistive pressure sensor are greatly improved, realizing the stable detection of human motion status in all directions. Meanwhile, its high sensitivity at low pressure allows the sensor to accurately detect weak signals such as weak airflow and wrist pulses. In addition, combining the sensor with deep-learning makes it easy to recognize human respiratory patterns with high accuracy, demonstrating its potential impact in the fields of ergonomics and low-cost flexible electronics.

Wearable Smart Fabric Based on Hybrid E-Fiber Sensor for Real-Time Finger Motion Detection

Wearable electronic sensors have attracted considerable interest in hand motion monitoring because of their small size, flexibility, and biocompatibility. However, the range of motion and sensitivity of many sensors are inadequate for complex and precise finger motion capture. Here, organic and inorganic materials were incorporated to fabricate a hybrid electronic sensor and optimized and woven into fabric for hand motion detection. The sensor was made from flexible porous polydimethylsiloxane (PDMS) filled with multiwalled carbon nanotubes (MWCNTs). The weight ratios of MWCNTs and geometric characteristics were optimized to improve the hybrid electronic sensor, which showed a high elongation at the breaking point (i.e., more than 100%) and a good sensitivity of 1.44. The strain-related deformation of the PDMS/MWCNT composite network resulted in a variation in the sensor resistance; thus, the strain level that corresponds to different finger motions is be calculated. Finally, the fabricated and optimized electronic sensor in filiform structure with a 6% MWCNT ratio was integrated with smart fabric to create a finger sleeve for real-time motion capture. In conclusion, a novel hybrid E-fiber sensor based on PDMS and MWCNTs was successfully fabricated in the current study with an optimal M/P ratio and structure, and textile techniques were adopted as new packaging approaches for such soft electronic sensors to create smart fabric for wearable and precise detection with highly enhanced sensing performance. The successful results in the current study demonstrate the great potential of such hybrid soft sensors in smart wearable healthcare management, including motion detection.

Wearable sensors for telehealth based on emerging materials and nanoarchitectonics

Wearable sensors have made significant progress in sensing physiological and biochemical markers for telehealth. By monitoring vital signs like body temperature, arterial oxygen saturation, and breath rate, wearable sensors provide enormous potential for the early detection of diseases. In recent years, significant advancements have been achieved in the development of wearable sensors based on two-dimensional (2D) materials with flexibility, excellent mechanical stability, high sensitivity, and accuracy introducing a new approach to remote and real-time health monitoring. In this review, we outline 2D materials-based wearable sensors and biosensors for a remote health monitoring system. The review focused on five types of wearable sensors, which were classified according to their sensing mechanism, such as pressure, strain, electrochemical, optoelectronic, and temperature sensors. 2D material capabilities and their impact on the performance and operation of the wearable sensor are outlined. The fundamental sensing principles and mechanism of wearable sensors, as well as their applications are explored. This review concludes by discussing the remaining obstacles and future opportunities for this emerging telehealth field. We hope that this report will be useful to individuals who want to design new wearable sensors based on 2D materials and it will generate new ideas.

Research status of polysiloxane-based piezoresistive flexible human electronic sensors

Flexible human body electronic sensor is a multifunctional electronic device with flexibility, extensibility, and responsiveness. Piezoresistive flexible human body electronic sensor has attracted the extensive attention of researchers because of its simple preparation process, high detection sensitivity, wide detection range, and low power consumption. However, the wearability and affinity to the human body of traditional flexible human electronic sensors are poor, while polysiloxane materials can be mixed with other electronic materials and have good affinity toward the human body. Therefore, polysiloxane materials have become the first choice of flexible matrixes. In this study, the research progress and preparation methods of piezoresistive flexible human electronic sensors based on polysiloxane materials in recent years are summarized, the challenges faced in the development of piezoresistive flexible human electronic sensors are analyzed, and the future research directions are prospected.

Phosphonium Ionic Liquid-Activated Sulfur Vulcanization: A Way Forward to Reduce Zinc Oxide Levels in Industrial Rubber Formulations

Extensive use of zinc oxide and accelerators such as diphenyl guanidine (DPG) in the vulcanization of rubber composites entail potential environmental risks. These are pervasive contaminants of roadway runoff originating from tire wear particles (TWPs). Herein, the effect of phosphonium ionic liquids (PILs) in styrene-butadiene rubber compounds was demonstrated with reduced ZnO loading and no DPG to minimize the environmental footprint of the vulcanization process. The structure and chemistry of PILs were found to be the influencing parameters impelling the cross-linking kinetics, enabling shorter induction times. The generation of active Zn²⁺ sites by PILs was examined through FTIR spectroscopy, calorimetry, and molecular dynamics simulations. From a tire application perspective, the PILs not only enhanced the cure kinetics but also improved the dynamic-mechanical behavior of the rubber composites. Consequently, the harm caused by TWPs to the atmosphere, fuel intake, and CO₂ emissions was minimal, thereby confirming the potential use of PILs in the tire industry.

Flexible Room-Temperature Ammonia Gas Sensors Based on PANI-MWCNTs/PDMS Film for Breathing Analysis and Food Safety

Gas sensors have played a critical role in healthcare, atmospheric environmental monitoring, military applications and so on. In particular, flexible sensing devices are of great interest, benefitting from flexibility and wearability. However, developing flexible gas sensors with a high sensitivity, great stability and workability is still challenging. In this work, multi-walled carbon nanotubes (MWCNTs) were grown on polydimethylsiloxane (PDMS) films, which were further modified with polyaniline (PANI) using a simple chemical oxidation synthesis. The superior flexibility of the PANI-MWCNTs/PDMS film enabled a stable initial resistance value, even under bending conditions. The flexible sensor showed excellent NH₃ sensing performances, including a high response (11.8 ± 0.2 for 40 ppm of NH₃) and a low limit of detection (10 ppb) at room temperature. Moreover, the effect of a humid environment on the NH₃ sensing performances was investigated. The results show that the response of the sensor is enhanced under high humidity conditions because water molecules can promote the adsorption of NH₃ on the PANI-MWCNTs/PDMS films. In addition, the PANI-MWCNTs/PDMS film sensor had the abilities of detecting NH₃ in the simulated breath of patients with kidney disease and the freshness of shrimp. These above results reveal the potential application of the PANI-MWCNTs/PDMS sensor for monitoring NH₃ in human breath and food.

A Pressure and Temperature Dual-Parameter Sensor Based on a Composite Material for Electronic Wearable Devices

Wearable sensors integrating multiple functionalities are highly desirable in artificial wearable devices, which are of great significance in the field of biomedical research and for human-computer interactions. However, it is still a great challenge to simultaneously perceive multiple external stimuli such as pressure and temperature with one single sensor. Combining the piezoresistive effect with the negative temperature coefficient of resistance, in this paper, we report on a pressure-temperature dual-parameter sensor composed of a polydimethylsiloxane film, carbon nanotube sponge, and poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate). The proposed multifunctional sensor can stably monitor pressure signals with a high sensitivity of 16 kPa^-1, has a range of up to 2.5 kPa, and also has a fast response time. Meanwhile, the sensor can also respond to temperature changes with an ultrahigh sensitivity rate of 0.84% °C^-1 in the range of 20 °C to 80 °C. To validate the applicability of our sensor in practical environments, we conducted real-scene tests, which revealed its capability for monitoring = human motion signals while simultaneously sensing external temperature stimuli, reflecting its great application prospects for electronic wearable devices.

Printing Formation of Flexible (001)-Oriented PZT Films on Plastic Substrates

High-quality, uniaxially oriented, and flexible PbZr_0.52Ti_0.48O₃ (PZT) films were fabricated on flexible RbLaNb₂O₇/BaTiO₃ (RLNO/BTO)-coated polyimide (PI) substrates. All layers were fabricated by a photo-assisted chemical solution deposition (PCSD) process using KrF laser irradiation for photocrystallization of the printed precursors. The Dion-Jacobson perovskite RLNO thin films on flexible PI sheets were employed as seed layers for the uniaxially oriented growth of PZT films. To obtain the uniaxially oriented RLNO seed layer, a BTO nanoparticle-dispersion interlayer was fabricated to avoid PI substrate surface damage under excess photothermal heating, and the RLNO has been orientedly grown only at around 40 mJ·cm^-2 at 300 °C. The prepared RLNO seed layer on the BTO/PI substrate showed very high (010)-oriented growth with a very high Lotgering factor (F(010) = 1.0). By using the flexible (010)-oriented RLNO film on BTO/PI, PZT film crystal growth was possible via KrF laser irradiation of a sol-gel-derived precursor film at 50 mJ·cm^-2 at 300 °C. The obtained PZT film showed highly (001)-oriented growth on the flexible plastic substrates with F(001) = 0.92 without any micro-cracks. The RLNO was only uniaxial-oriented grown at the top part of the RLNO amorphous precursor layer. The oriented grown and amorphous phases of RLNO would have two important roles for this multilayered film formation: (1) triggering orientation growth of the PZT film at the top and (2) the stress relaxation of the underneath BTO layer to suppress the micro-crack formation. This is the first time that PZT films have been crystallized directly on flexible substrates. The combined processes of photocrystallization and chemical solution deposition are a cost-effective and highly on-demand process for the fabrication of flexible devices.

Characterization of Conductive Carbon Nanotubes/Polymer Composites for Stretchable Sensors and Transducers

The increasing interest in stretchable conductive composite materials, that can be versatile and suitable for wide-ranging application, has sparked a growing demand for studies of scalable fabrication techniques and specifically tailored geometries. Thanks to the combination of the conductivity and robustness of carbon nanotube (CNT) materials with the viscoelastic properties of polymer films, in particular their stretchability, "surface composites" made of a CNT on polymeric films are a promising way to obtain a low-cost, conductive, elastic, moldable, and patternable material. The use of polymers selected for specific applications, however, requires targeted studies to deeply understand the interface interactions between a CNT and the surface of such polymer films, and in particular the stability and durability of a CNT grafting onto the polymer itself. Here, we present an investigation of the interface properties for a selected group of polymer film substrates with different viscoelastic properties by means of a series of different and complementary experimental techniques. Specifically, we studied the interaction of a single-wall carbon nanotube (SWCNT) deposited on two couples of different polymeric substrates, each one chosen as representative of thermoplastic polymers (i.e., low-density polyethylene (LDPE) and polypropylene (PP)) and thermosetting elastomers (i.e., polyisoprene (PI) and polydimethylsiloxane (PDMS)), respectively. Our results demonstrate that the characteristics of the interface significantly differ for the two classes of polymers with a deeper penetration (up to about 100 μm) into the polymer bulk for the thermosetting substrates. Consequently, the resistance per unit length varies in different ranges, from 1-10 kΩ/cm for typical thermoplastic composite devices (30 μm thick and 2 mm wide) to 0.5-3 MΩ/cm for typical thermosetting elastomer devices (150 μm thick and 2 mm wide). For these reasons, the composites show the different mechanical and electrical responses, therefore suggesting different areas of application of the devices based on such materials.

Performance Deficiency Improvement of CNT-Based Strain Sensors by Magnetic-Induced Patterning

As one of the most promising candidates, ubiquitous cycling degradation seriously affects the accuracy of carbon nanotube (CNT)-based sensors, and the reason for which is still unclear. Herein, the cycling degradation mechanism of CNT-based strain sensors has been detected by comparatively investigating the difference between the sensing behavior of CNT- and silver nanowire (Ag-NW)-based sensors, from which the microcrack-disconnection and unfolding-tunneling effects have been clarified as the sensing mechanism for Ag-NWs and CNT-based strain sensors, respectively. Furthermore, sliding and unfolding behaviors resulting from the weak interaction between CNTs have been proven to cause degradation. Correspondingly, a creative magnetically induced patterning method is proposed by utilizing magnetic nanoparticles as obstacles to prevent the CNTs from relative sliding. Benefiting from the advantageous factor, the performance deficiency of the CNT-based sensor has been overcome, and the sensitivity was significantly improved up to 5.2 times with accurate human activity detection. The competitive sensing performance of the CNTs demonstrates the reference value of the deficiency mechanism and solution scheme obtained in this study.

Comparison of a Wearable Accelerometer/Gyroscopic, Portable Gait Analysis System (LEGSYS+TM) to the Laboratory Standard of Static Motion Capture Camera Analysis

Examination of gait patterns has been used to determine severity, intervention triage and prognostic measures for many health conditions. Methods that generate detailed gait data for clinical use are typically logistically constrained to a formal gait laboratory setting. This has led to an interest in portable analysis systems for near clinical or community-based assessments. The following study assessed with the wearable accelerometer/gyroscopic, gait analysis system (LEGSYS+^TM) and the standard of static motion capture camera (MOCAP) analysis during a treadmill walk at three different walking speeds in healthy participants (n = 15). To compare each speed, 20 strides were selected from the MOCAP data and compared with the LEGSYS+ strides at the same time point. Both scatter and bland-Altman plots with accompanying linear regression analysis for each of the parameters. Each stride parameter showed minimal or a consistent difference between the LEGSYS+ and MOCAP, with the phase parameters showing inconsistencies between the systems. Overall, LEGSYS+ stride parameters can be used in the clinical setting, with the utility of phase parameters needing to be taken with caution.

Bioinspired Dual-Mode Stretchable Strain Sensor Based on Magnetic Nanocomposites for Strain/Magnetic Discrimination

Recently, flexible stretchable sensors have been gaining attention for their excellent adaptability for electronic skin applications. However, the preparation of stretchable strain sensors that achieve dual-mode sensing while still retaining ultra-low detection limit of strain, high sensitivity, and low cost is a pressing task. Herein, a high-performance dual-mode stretchable strain sensor (DMSSS) based on biomimetic scorpion foot slit microstructures and multi-walled carbon nanotubes (MWCNTs)/graphene (GR)/silicone rubber (SR)/Fe₃ O₄ nanocomposites is proposed, which can accurately sense strain and magnetic stimuli. The DMSSS exhibits a large strain detection range (≈160%), sensitivity up to 100.56 (130-160%), an ultra-low detection limit of strain (0.16% strain), and superior durability (9000 cycles of stretch/release). The sensor can accurately recognize sign language movement, as well as realize object proximity information perception and whole process information monitoring. Furthermore, human joint movements and micro-expressions can be monitored in real-time. Therefore, the DMSSS of this work opens up promising prospects for applications in sign language pose recognition, non-contact sensing, human-computer interaction, and electronic skin.

PDMS/Ag/Mxene/Polyurethane Conductive Yarn as a Highly Reliable and Stretchable Strain Sensor for Human Motion Monitoring

The conductivity and sensing stability of yarn-based strain sensors are still challenges when it comes to practical applications. To address these challenges, surface engineering of polyurethane (PU) yarn was introduced to improve its surface hydrophilicity for better deposition of MXene nanosheets in its dispersion. The introduction of Ag nanoparticles via magnetron sputtering greatly improved the surface conductivity; meanwhile, the encapsulation of the PDMS protective layer effectively enhanced the sensing stability over 15,000 cycling process, as well as the working range with a gauge factor value over 700 under a strain range of 150-300%. Moreover, the exploration of its applications in human motion monitoring indicate that the prepared strain-sensing yarn shows great potential in detecting both tiny motions or large-scale movements of the human body, which will be suitable for further development into multifunctional smart wearable sensors or metaverse applications in the future.

Carbon nanotube-graphene hybrids for soft electronics, sensors, and actuators

Soft devices that are mechanically flexible and stretchable are considered as the building blocks for various applications ranging from wearable devices to robotics. Among the many candidate materials for constructing soft devices, carbon nanomaterials such as carbon nanotubes (CNTs) and graphene have been actively investigated owing to their outstanding characteristics, including their intrinsic flexibility, tunable conductivity, and potential for large-area processing. In particular, hybrids of CNTs and graphene can improve the performance of soft devices and provide them with novel capabilities. In this review, the advances in CNT-graphene hybrid-based soft electrodes, transistors, pressure and strain sensors, and actuators are discussed, highlighting the performance improvements of these devices originating from the synergistic effects of the hybrids of CNT and graphene. The integration of multidimensional heterogeneous carbon nanomaterials is expected to be a promising approach for accelerating the development of high-performance soft devices. Finally, current challenges and future opportunities are summarized, from the processing of hybrid materials to the system-level integration of multiple components.

High-Sensitive Wearable Strain Sensors Based on the Carbon Nanotubes@Porous Soft Silicone Elastomer with Excellent Stretchability, Durability, and Biocompatibility

Wearable strain sensors can transfer human physical motions into digital features and connect the real world to the virtual world. However, there is still a huge challenge to prepare breathable strain sensors with good sensitivity, stretchability, softness, durability, and biocompatibility, simultaneously. Herein, we employ the soft silicone elastomer as a highly stretchable substrate and propose a new strain sensor based on the carbon nanotubes@porous soft silicone elastomer (CNTs@PSSE) by salt-template-assisted and dip-coating methods. The CNTs (conductive fillers) are firmly embedded in the PSSE. The obtained sensors exhibit excellent sensitivity up to 2845.1 and a large sensing strain range of 186%. Notably, the CNTs@PSSE sensors also possess strong robustness, which can resist ultrasonic deterioration and carry out more than 10,000 high-frequency stretch-relax cycles in the presence of an obvious notch caused by the scissor. Moreover, the excellent biocompatibility indicates that the sensors can be safely attached to human skin for precisely detecting full-range human motions and being configured on smart wireless gloves for synchronous control of the bionic hand robot.

Highly stretchable strain sensors with improved sensitivity enabled by a hybrid of carbon nanotube and graphene

The development of high-performance strain sensors has attracted significant attention in the field of smart wearable devices. However, stretchable strain sensors usually suffer from a trade-off between sensitivity and sensing range. In this study, we investigate a highly sensitive and stretchable piezoresistive strain sensor composed of a hybrid film of 1D multi-walled carbon nanotube (MWCNT) and 2D graphene that forms a percolation network on Ecoflex substrate by spray coating. The mass of spray-coated MWCNT and graphene and their mass ratio are modulated to overcome the trade-off between strain sensitivity and sensing range. We experimentally found that a stable percolation network is formed by 0.18 mg of MWCNTs (coating area of 200 mm2), with a maximum gauge factor (GF) of 1,935.6 and stretchability of 814.2%. By incorporating the 0.36 mg of graphene into the MWCNT film (i.e., a mass ratio of 1:2 between MWCNT and graphene), the GF is further improved to 12,144.7 in a strain range of 650–700%. This high GF is caused by the easy separation of the graphene network under the applied strain due to its two-dimensional (2D) shape. High stretchability originates from the high aspect ratio of MWCNTs that bridges the randomly distributed graphenes, maintaining a conductive network even under sizeable tensile strain. Furthermore, a small difference in work function between MWCNT and graphene and their stable percolation network enables sensitive UV light detection even under a significant strain of 300% that cannot be achieved by sensors composed of MWCNT- or graphene-only. The hybrids of MWCNT and graphene provide an opportunity to achieve high-performance stretchable devices.

Electromechanical Performance of Strain Sensors Based on Viscoelastic Conductive Composite Polymer Fibers

Flexible conductive polymer composite (CPC) fibers that show large changes in resistance with deformation have recently gained much attention as strain-sensing components for future wearable electronics. However, the electrical resistance of these materials decays with time during dynamic cyclic loading, a deformation performed to simulate their real application as strain sensors. Despite the extensive research on CPC fibers, the mechanism leading to this decay in the electromechanical response under repetitive cycles remains unreported. Herein, this behavior is investigated using fiber-based strain sensors wet spun from thermoplastic polyurethane (TPU) consisting of a carbonaceous hybrid conductive filler system of carbon black (CB) and carbon nanotubes (CNTs). We found electrical viscosity to predict the observed electromechanical resistance decay. This implies that cycling these materials enables the relaxation of both the polymer chains and the conductive network. In addition, the resulting piezoresistive fibers are sensitive to deformation in the region of low strain (gauge factor of 6.0 within 3.0% strain), remain conductive under 280.5% deformation, and are stable for more than 2000 cycles. Finally, we demonstrate the potential of TPU/CB-CNT fibers as strain sensors for monitoring human motion.

An ultrasensitive and stretchable strain sensor based on a microcrack structure for motion monitoring

Flexible strain sensors are promising candidates for intelligent wearable devices. Among previous studies, although crack-based sensors have attracted a lot of attention due to their ultrahigh sensitivity, large strain usually causes fractures in the conductive paths. Because of the unstable crack structure, the tradeoff between sensitivity and workable strain range is still a challenge. As carbon nanotubes (CNTs) and silver nanowires (AgNWs) can form a strong interface with the thermoplastic substrate and strengthen the conductive network by capillary force during water evaporation, CNTs and AgNWs were deposited on electrospun TPU fiber mats via vacuum-assisted filtration in this work. The prestretching treatment constructed a microcrack structure that endowed the sensor with the combined characteristics of a wide working range (0~171% strain), ultrahigh sensitivity (a gauge factor of 691 within 0~102% strain, ~2 × 10⁴ within 102~135% strain, and >11 × 10⁴ within 135~171% strain), a fast response time (~65 ms), small hysteresis, and superior durability (>2000 cycles). Subsequently, the sensing mechanism of the sensor was studied. Distributed microcrack propagation based on the "island-bridge" structure was explained in detail, and its influence on the strain-sensing behavior of the sensor was analyzed. Finally, the sensor was assembled to monitor various vibration signals and human motions, demonstrating its potential applications in the fields of electronic skin and human health monitoring.

All-Nanofibrous Ionic Capacitive Pressure Sensor for Wearable Applications

Currently, with the development of electronic skins (e-skins), wearable pressure sensors with low energy consumption and excellent wearability for long-term physiological signal monitoring are urgently desired but remain a challenge. Capacitive-type devices are desirable candidates for wearable applications, but traditional capacitive pressure sensors are limited by low capacitance and sensitivity. In this study, an all-nanofibrous ionic pressure sensor (IPS) is developed, and the formation of an electrical double layer at the electrode/electrolyte contact interface significantly enhances the capacitance and sensing properties. The IPS is fabricated by sandwiching a nanofibrous ionic gel sensing layer between two thermoplastic polyurethane nanofibrous membranes with graphene electrodes. The IPS has a high sensitivity of 217.5 kPa^-1 in the pressure range of 0-5 kPa, which is much higher than that of conventional capacitive pressure sensors. Combined with the rapid response and recovery speed (30 and 60 ms), the IPS is suitable for real-time monitoring of multiple physiological signals. Moreover, the nanofiber network endows the IPS with excellent air permeability and heat dissipation, which guarantees comfort during long-term wearing. This work provides a viable strategy to improve the wearability of wearable sensors, which can promote healthcare and human-machine interaction applications.

Wearable RGO/MXene Piezoresistive Pressure Sensors with Hierarchical Microspines for Detecting Human Motion

Flexible piezoresistive pressure sensors may exhibit excellent sensing performances to be applied in wearable electronics, medical diagnosis, and electronic skin. Herein, we report a multi-layer and phased-responsive reduced graphene oxide/MXene-based piezoresistive pressure sensor with hierarchical microspines constructed by sandpaper as the template. Thanks to the multi-level and multi-layer structure, the obtained sensor realized phased response and showed wide detection range (up to 70 kPa), fast response (response/recovery time of 40/80 ms), and excellent working stability (1000 fatigue cycles). Furthermore, the sensor was successfully applied for detecting various human motions including pulse beats, cheek bulging, nodding, finger bending, speech recognition, handwriting, and other pressure signals. Besides, a 6 × 6 sensing matrix integrated by the sensors was able to sensitively perceive the distribution of plane pressure. The findings in this work conceivably stand out as a new strategy to fabricate high-performance piezoresistive pressure sensors in the fields of intelligent healthcare and medical diagnosis, wearable electronic devices, electronic skin, and human-machine interaction.

Flexible Full-Surface Conformal Encapsulation for Each Fiber in Graphene Glass Fiber Fabric against Thermal Oxidation

Encapsulation for carbon-based electronic devices against oxidation can enhance their long-term working stability. Graphene glass fiber fabric (GGFF), as an advanced flexible electrothermal material, also struggles with graphene oxidation. The flexible, full-surface, conformal encapsulation for each fiber in the large-area fabric puts forward high requirements for encapsulating materials and techniques. Herein, the nanometer-thick h-BN layer was in situ grown on cambered surfaces of each fiber in GGFF with the chemical vapor deposition method. Stable heating duration (500 °C) of h-BN-encapsulated GGFF (h-BN/GGFF) was increased by 1 order of magnitude without compromising the electrothermal performances and flexibility. Theoretical simulations revealed that the enhanced oxidation resistance of h-BN/GGFF was attributed to the decreased interaction and adsorption life of oxygen. The proposed flexible, full-surface, conformal encapsulation technique targeting the fiber-shaped graphene electrothermal device is scalable and can be extended to the other carbon materials, even devices with intricate shapes, which will promote the development of flexible electronics.

Atomic Simulations of (8,0)CNT-Graphene by SCC-DFTB Algorithm

Self-consistent density functional tight binding (SCC-DFTB) approaches were used to study optimized structures, energy, differential charge density, and Mülliken populations for the (8,0) carbon nanotubes (CNTs) connected to the graphene having different topology defects. Based on the calculations, nine seamless (8,0)CNT-graphenes were selected. For these connected systems, geometric configurations of the graphene and nanotubes were characterized, and the nearest neighbor length of C-C atoms and average length were obtained. The intrinsic energy, energy gap, and chemical potential were analyzed, and they presented apparent differences for different connection modes. Differential charge densities of these connection modes were analyzed to present covalent bonds between the atoms. We have also thoroughly analyzed the Mülliken charge transfer among the C atoms at the junctions.

Highly Sensitive and Stretchable MXene/CNTs/TPU Composite Strain Sensor with Bilayer Conductive Structure for Human Motion Detection

The universal application of wearable strain sensors in various situations for human-activity monitoring is considerably limited by the contradiction between high sensitivity and broad working range. There still remains a huge challenge to design sensors featuring simultaneous broad working range and high sensitivity. Herein, a typical bilayer-conductive structure Ti₃C₂T_x MXene/carbon nanotubes (CNTs)/thermoplastic polyurethane (TPU) composite film was developed by a simple and scalable vacuum filtration process utilizing a porous electrospun thermoplastic polyurethane (TPU) mat as a skeleton. The MXene/CNTs/TPU strain sensor is composed of two parts: a brittle densely stacked MXene upper lamella and a flexible MXene/CNT-decorated fibrous network lower layer. Benefiting from the synergetic effect of the two parts along with hydrogen-bonding interactions between the porous TPU fiber mat and MXene sheets, the MXene/CNTs/TPU strain sensor possesses both a broad working range (up to 330%) and high sensitivity (maximum gauge factor of 2911) as well as superb long-term durability (2600 cycles under the strain of 50%). Finally, the sensor can be successfully employed for human movement monitoring, from tiny facial expressions, respiration, and pulse beat to large-scale finger and elbow bending, demonstrating a promising and attractive application for wearable devices and human-machine interaction.

Reconfigurable, Stretchable Strain Sensor with the Localized Controlling of Substrate Modulus by Two-Phase Liquid Metal Cells

Strain modulation based on the heterogeneous design of soft substrates is an effective method to improve the sensitivity of stretchable resistive strain sensors. In this study, a novel design for reconfigurable strain modulation in the soft substrate with two-phase liquid cells is proposed. The modulatory strain distribution induced by the reversible phase transition of the liquid metal provides reconfigurable strain sensing capabilities with multiple combinations of operating range and sensitivity. The effectiveness of our strategy is validated by theoretical simulations and experiments on a hybrid carbonous film-based resistive strain sensor. The strain sensor can be gradually switched between a highly sensitive one and a wide-range one by selectively controlling the phases of liquid metal in the cell array with a external heating source. The relative change of sensitivity and operating range reaches a maximum of 59% and 44%, respectively. This reversible heterogeneous design shows great potential to facilitate the fabrication of strain sensors and might play a promising role in the future applications of stretchable strain sensors.