Ultraflexible Temperature-Strain Dual-Sensor Based on Chalcogenide Glass-Polymer Film for Human-Machine Interaction

Enhanced thermoelectric performance of (Ta1-xMox)4SiTe4/polyvinylidene fluoride (PVDF) organic-inorganic flexible thermoelectric composite films

Recently, one-dimensional van der Waals crystalline Ta4SiTe4 has been reported as a promising component to form flexible organic/inorganic thermoelectric films due to their unique structure and excellent electronic transport properties. However, Ta4SiTe4 based flexible composite films carefully tuned by elemental doping have not been studied yet. In this study, we systematically synthesized (Ta1-xMox)4SiTe4 whiskers, and a series of (Ta1-xMox)4SiTe4/PVDF composite films with varying Mo doping concentrations were also prepared. Upon doping Mo at the Ta-sites, the electrical conductivity was dramatically enhanced, while the Seebeck coefficient decreased with a higher doping content. As a result, (Ta0.995Mo0.005)4SiTe4/PVDF exhibited a maximum power factor of 547.5 μW m-1 K-2, which is among the highest for organic-inorganic composite films and is more than double that of the undoped Ta4SiTe4/PVDF composite film.

High-Performance Temperature Sensors for Early Warning Utilizing Flexible All-Inorganic Thermoelectric Films

The demand for highly sensitive temperature-response materials is critical for the advancement of intelligent temperature sensing and fire warning systems. Despite notable progress in thermoelectrical (TE) materials and devices, designing TE materials suitable for wide-range temperature monitoring across diverse scenarios remains a challenge. In this study, we introduce a TE temperature sensor for fire warnings and hot object recognition, utilizing an all-inorganic TE film composite of reduced graphene oxide (rGO)/Te nanowires (Te NWs). The resulting all-inorganic TE film, annealed at a high temperature, exhibits distinct response ratios to varying temperature changes, enabling consistently sensitive thermosensation. The robust linear relationship between open circuit voltage and temperature difference establishes it as an effective thermoreceptor for enhanced temperature alerts. Furthermore, we demonstrate that the assembled TE sensor provides rapid high-temperature warnings with adjustable threshold voltages (1-7 mV), achieving an ultrafast response time of approximately 4.8 s at 1 mV threshold voltage. Additionally, this TE sensor can be integrated with the gloves to monitor high-temperature objects in various scenarios, such as the brewed milk in daily life and heating reactors in industrial applications. These results offer perspectives for future innovations in intelligent temperature monitoring.

Muscle-Inspired Anisotropic Aramid Nanofibers Aerogel Exhibiting High-Efficiency Thermoelectric Conversion and Precise Temperature Monitoring for Firefighting Clothing

Enhancing the firefighting protective clothing with exceptional thermal barrier and temperature sensing functions to ensure high fire safety for firefighters has long been anticipated, but it remains a major challenge. Herein, inspired by the human muscle, an anisotropic fire safety aerogel (ACMCA) with precise self-actuated temperature monitoring performance is developed by combining aramid nanofibers with eicosane/MXene to form an anisotropically oriented conductive network. By combining the two synergies of the negative temperature-dependent thermal conductive eicosane, which induces a high-temperature differential, and directionally ordered MXene that establishes a conductive network along the directional freezing direction. The resultant ACMCA exhibited remarkable thermoelectric properties, with S values reaching 46.78 μV K-1 and κ values as low as 0.048 W m-1 K-1 at room temperature. Moreover, the prepared anisotropic aerogel ACMCA exhibited electrical responsiveness to temperature variations, facilitating its application in intelligent temperature monitoring systems. The designed anisotropic aerogel ACMCA could be incorporated into the firefighting clothing as a thermal barrier layer, demonstrating a wide temperature sensing range (50-400 °C) and a rapid response time for early high-temperature alerts (~ 1.43 s). This work provides novel insights into the design and application of temperature-sensitive anisotropic aramid nanofibers aerogel in firefighting clothing.

Three-dimensional flexible thermoelectric fabrics for smart wearables

Wearable thermoelectric devices, capable of converting body heat into electrical energy, provide the potential driving power for the Internet of Things, artificial intelligence, and soft robotics. However, critical parameters have long been overlooked for these practical applications. Here, we report a three-dimensional flexible thermoelectric device with a structure featuring an inner rigid and outer flexible woven design. Such a structure includes numerous small static air pockets that create a stable out-of-plane temperature difference, enabling precise temperature signal detection (accuracy up to 0.02 K). Particularly, this structure exhibits excellent multi-signal decoupling capability, excellent elasticity (>10,000 compression cycles), ultra-fast compression response (20 ms), stable output signal under 50% compressive strain, high breathability (1300 mm s-1), and washability. All these metrics achieve the highest values currently reported, fully meeting the requirements for body heat and moisture exchange, as demonstrated in our designed integrated smart mask and smart glove systems based on vector machine learning technology. This work shows that our three-dimensional flexible thermoelectric device has broad applicability in wearable electronics.

Revisiting the Interface Dynamics of MXene/Rubber Elastomers: Multiscale Mechanistic Insights into Collaborative Bonding for Robust Self-Healing Sensors

Sophisticated flexible strain sensors based on MXene/rubber with self-healing capabilities are poised to transform future deformable electronics by restoring impaired performance after repeated deformation. Despite their potential, integrating excellent self-healing properties with superior mechanical strength in a single system remains a significant challenge due to simplistic interface architectures with weak bonds and limited understanding of MXene/rubber interface dynamics. To address this, a novel metal coordination bonding scheme has been developed, synergizing with dynamic hydrogen bonding to enhance interface bonding strength, enabling both outstanding mechanical and self-healing properties. Using in situ synchrotron radiation techniques, a multiscale investigation of MXene/rubber interface dynamics provides valuable insights, linking bonding strength to mechanical performance. These findings not only deepen our understanding of interface evolution in deformable electrodes but also offer a promising path for designing advanced self-healable strain sensors with superior mechanical properties.

Wearable Resistive-Type Stretchable Strain Sensors: Materials and Applications

The rapid advancement of wearable electronics over recent decades has led to the development of stretchable strain sensors, which are essential for accurately detecting and monitoring mechanical deformations. These sensors have widespread applications, including movement detection, structural health monitoring, and human-machine interfaces. Resistive-type sensors have gained significant attention due to their simple design, ease of fabrication, and adaptability to different materials. Their performance, evaluated by metrics like stretchability and sensitivity, is influenced by the choice of strain-sensitive materials. This review offers a comprehensive comparison and evaluation of different materials used in resistive strain sensors, including metal and semiconductor films, low-dimensional materials, intrinsically conductive polymers, and gels. The review also highlights the latest applications of resistive strain sensors in motion detection, healthcare monitoring, and human-machine interfaces by examining device physics and material characteristics. This comparative analysis aims to support the selection, application, and development of resistive strain sensors tailored to specific applications.

Labyrinthine Wrinkle-Patterned Fiber Sensors Based on a 3D Stress Complementary Strategy for Machine Learning-Enabled Medical Monitoring and Action Recognition

Fiber strain sensors show good application potential in the field of wearable smart fabrics and equipment because of their characteristics of easy deformation and weaving. However, the integration of fiber strain sensors with sensitive response, good stretchability, and effective practical application remains a challenge. Herein, this paper proposes a new strategy based on 3D stress complementation through pre-stretching and swelling processes, and the polydimethylsiloxane (PDMS)/silver nanoparticle (AgNPs)/MXene/carbon nanotubes (CNTs) fiber sensor with the bilayer labyrinthian wrinkles conductive network on the PU fiber surface is fabricated. Benefiting from the wrinkled structure and the synergies of sensitive composite materials, the fiber sensor exhibits good stretchability (>150%), high sensitivity (maximum gauge factor is 57896), ultra-low detection limit (0.1%), fast response/recovery time (177/188 ms) and good long-term durability. It can be used as Morse code issuance and recognition to express the patient's symptoms and feelings. Further, the sensor enables comprehensive human movement monitoring and collects data of different characteristics with the assistance of machine learning, different letters/numbers are recognized and predicted with an accuracy of 99.17% and 99.33%. Therefore, this fiber sensor shows potential as a new generation of flexible strain sensors with applications in medical monitoring and human-computer interaction.

High-Quality SnSe Thin Films for Self-Powered Devices and Multilevel Information Encryption

As semiconductor technology advances toward miniaturization and portability, thin films with excellent thermoelectric performance have garnered increasing attention, particularly for applications in self-powered devices and temperature-responsive sensors. The high Seebeck coefficient of SnSe thin films makes them promising for temperature sensing, but their poor electrical conductivity limits their potential as thermoelectric generators. In this work, high-quality a-axis oriented SnSe thin films were deposited on quartz substrates by using magnetron sputtering. The substrate temperature was optimized to improve the crystallinity of the SnSe thin film, resulting in larger grain sizes, which subsequently contributes to the improved carrier mobility. The Seebeck coefficient is enhanced while optimizing the electrical conductivity, enabling the SnSe thin film to achieve both excellent sensing and power generation performance. The SnSe film deposited at 673 K exhibits a high power factor of approximately 346 μW m-1 K-2 at 620 K. A temperature-responsive sensing array was developed for multilevel information encryption, showing significant potential for applications in password encryption. The maximum output power density of the optimized thermoelectric generator with six SnSe legs is about 9 W m-2 at a temperature difference of 50 K.

Flexible and Stable GaN Piezoelectric Sensor for Motion Monitoring and Fall Warning

Wearable devices have potential applications in health monitoring and personalized healthcare due to their portability, conformability, and excellent mechanical flexibility. However, their performance is often limited by instability in acidic or basic environments. In this study, a flexible sensor with excellent stability based on a GaN nanoplate was developed through a simple and controllable fabrication process, where the linearity and stability remained at almost 99% of the original performance for 40 days in an air atmosphere. Moreover, perfect stability was also demonstrated in acid-base environments, with pH values ranging from 1 to 13. Based on its excellent stability and piezotronic performance, a flexible device for motion monitoring was developed, capable of detecting motions such as finger, knee, and wrist bending, as well as swallowing. Furthermore, gesture recognition and intelligent fall monitoring were explored based on the bending properties. In addition, an intelligent fall warning system was proposed for the personalized healthcare application of elders by applying machine learning to analyze data collected from typical activities. Our research provides a path for stable and flexible electronics and personalized healthcare applications.

Fiber-Shaped Temperature Sensor with High Seebeck Coefficient for Human Health and Safety Monitoring

Wearable thermoelectric (TE)-based temperature sensors capable of detecting and transmitting temperature data from the human body and environment show promise in intelligent medical systems, human-machine interfaces, and electronic skins. However, it has remained a challenge to fabricate the flexible temperature sensors with superior sensing performance, primarily due to the low Seebeck coefficient of the TE materials. Here, we report an inorganic amorphous TE material, Ge5As55Te40, with a high Seebeck coefficient of 1050 μV/K, which is around 3 times higher than the organic TE materials and 2 times higher than the inorganic crystal TE materials. Due to the strong anticrystallization ability, the amorphous state of Ge5As55Te40 can be well maintained during the thermal fiber-drawing process. The resulting TE fibers demonstrate superior temperature sensing properties, encompassing a broad working range (25-115 °C), a precise temperature resolution of 0.1 K, and a rapid response time of 5 s. Importantly, the TE properties of the fiber show high stability after repeated temperature variations between 5 and 10 K. Moreover, the fibers can be integrated into a mask and a wearable fabric for monitoring human respiratory rate and providing early warning for fire source proximity. These results highlight that the TE fiber with both natural flexibility and superior temperature sensing performance may find potential applications in wearable systems.

Strong and Robust Core-Shell Ceramic Fibers Composed of Highly Compacted Nanoparticles for Multifunctional Electronic Skin

Functional fibers composed of textiles are considered a promising platform for constructing electronic skin (e-skin). However, developing robust electronic fibers with integrated multiple functions remains a formidable task especially when a complex service environment is concerned. In this work, a continuous and controllable strategy is demonstrated to prepare e-skin-oriented ceramic fibers via coaxial wet spinning followed by cold isostatic pressing. The resulting core-shell structured fiber with tightly compacted Al-doped ZnO nanoparticles in the core and highly ordered aramid nanofibers in the shell exhibit excellent tensile strength (316 MPa) with ultra-high elongation (33%). Benefiting from the susceptible contacts between conducting ceramic nanoparticles, the ceramic fiber shows both ultrahigh sensitivity (gauge factor = 2141) as a strain sensor and a broad working range up to 70 °C as a temperature sensor. Furthermore, the tunable core-shell structure of the fiber enables the optimization of impedance matching and attenuation of electromagnetic waves for the corresponding textile, resulting in a minimum reflection loss of -39.1 dB and an effective absorption bandwidth covering the whole X-band. Therefore, the versatile core-shell ceramic fiber-derived textile can serve as a stealth e-skin for monitoring the motion and temperature of robots under harsh conditions.

Advancing flexible thermoelectrics for integrated electronics

With the increasing demand for energy and the climate challenges caused by the consumption of traditional fuels, there is an urgent need to accelerate the adoption of green and sustainable energy conversion and storage technologies. The integration of flexible thermoelectrics with other various energy conversion technologies plays a crucial role, enabling the conversion of multiple forms of energy such as temperature differentials, solar energy, mechanical force, and humidity into electricity. The development of these technologies lays the foundation for sustainable power solutions and promotes research progress in energy conversion. Given the complexity and rapid development of this field, this review provides a detailed overview of the progress of multifunctional integrated energy conversion and storage technologies based on thermoelectric conversion. The focus is on improving material performance, optimizing the design of integrated device structures, and achieving device flexibility to expand their application scenarios, particularly the integration and multi-functionalization of wearable energy conversion technologies. Additionally, we discuss the current development bottlenecks and future directions to facilitate the continuous advancement of this field.

Quasi-1D Conductive Network Composites for Ultra-Sensitive Strain Sensing

Highly performance flexible strain sensor is a crucial component for wearable devices, human-machine interfaces, and e-skins. However, the sensitivity of the strain sensor is highly limited by the strain range for large destruction of the conductive network. Here the quasi-1D conductive network (QCN) is proposed for the design of an ultra-sensitive strain sensor. The orientation of the conductive particles can effectively reduce the number of redundant percolative pathways in the conductive composites. The maximum sensitivity will reach the upper limit when the whole composite remains only "one" percolation pathway. Besides, the QCN structure can also confine the tunnel electron spread through the rigid inclusions which significantly enlarges the strain-resistance effect along the tensile direction. The strain sensor exhibits state-of-art performance including large gauge factor (862227), fast response time (24 ms), good durability (cycled 1000 times), and multi-mechanical sensing ability (compression, bending, shearing, air flow vibration, etc.). Finally, the QCN sensor can be exploited to realize the human-machine interface (HMI) application of acoustic signal recognition (instrument calibration) and spectrum restoration (voice parsing).

Strain-Temperature Dual Sensor Based on Deep Learning Strategy for Human-Computer Interaction Systems

Thermoelectric (TE) hydrogels, mimicking human skin, possessing temperature and strain sensing capabilities, are well-suited for human-machine interaction interfaces and wearable devices. In this study, a TE hydrogel with high toughness and temperature responsiveness was created using the Hofmeister effect and TE current effect, achieved through the cross-linking of PVA/PAA/carboxymethyl cellulose triple networks. The Hofmeister effect, facilitated by Na+ and SO42- ions coordination, notably increased the hydrogel's tensile strength (800 kPa). Introduction of Fe2+/Fe3+ as redox pairs conferred a high Seebeck coefficient (2.3 mV K-1), thereby enhancing temperature responsiveness. Using this dual-responsive sensor, successful demonstration of a feedback mechanism combining deep learning with a robotic hand was accomplished (with a recognition accuracy of 95.30%), alongside temperature warnings at various levels. It is expected to replace manual work through the control of the manipulator in some high-temperature and high-risk scenarios, thereby improving the safety factor, underscoring the vast potential of TE hydrogel sensors in motion monitoring and human-machine interaction applications.

Manipulating Hetero-Nanowire Films for Flexible and Multifunctional Thermoelectric Devices

Flexible thermoelectric devices hold significant promise in wearable electronics owing to their capacity for green energy generation, temperature sensing, and comfortable wear. However, the simultaneous achievement of excellent multifunctional sensing and power generation poses a challenge in these devices. Here, ordered tellurium-based hetero-nanowire films are designed for flexible and multifunctional thermoelectric devices by optimizing the Seebeck coefficient and power factor. The obtained devices can efficiently detect both object and environment temperature, thermal conductivity, heat proximity, and airflow. In addition, combining the thermoelectric units with radiative cooling materials exhibits remarkable thermal management capabilities, preventing device overheating and avoiding degradation in power generation. Impressively, this multifunctional electronics exhibits excellent resistance in extreme low earth orbit environments. The fabrication of such thermoelectric devices provides innovative insights into multimodal sensing and energy harvesting.