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Implantable sensors with the abilities of real-time healthcare monitoring and auxiliary training are important for exercise-induced or disease-induced muscle and ligament injuries. However, some of these implantable sensors have some shortcomings, such as requiring an external power supply or poor flexibility and stability. Herein, an organogel/silicone fiber-helical sensor based on a triboelectric nanogenerator (OFS-TENG) is developed for power-free and sutureable implantation ligament strain monitoring. The OFS-TENG with high stability and ultrastretchability is composed of an organogel fiber and a silicone fiber intertwined with a double helix structure. The organogel fiber possesses the merits of rapid preparation (15 s), good transparency (95%), high stretchability (600%), and favorable stability (over 6 months). The OFS-TENG is successfully implanted on the patellar ligament of the rabbit knee for the real-time monitoring of knee ligament stretch and muscle stress, which is expected to provide a solution for real-time diagnosis of muscle and ligament injuries. The prepared self-powered OFS-TENG can monitor data on human muscles and ligaments in real-time.

Sleep quality plays an essential role in human health and has become an index for assessing physical health. Self-powered, sensitive, noninvasive, comfortable, and low-cost sleep monitoring sensors for monitoring sleep behavior are still in high demand. Here, a pressure-sensitive, noninvasive, and comfortable smart pillow is developed based on a flexible and breathable triboelectric nanogenerator (FB-TENG) sensor array, which can monitor head movement in real time during sleep. The FB-TENG is based on flexible and breathable porous poly(dimethylsiloxane) (PDMS) with a fluorinated ethylene propylene (FEP) powder and exhibits pressure sensitivity and durability. The electrical output of the FB-TENG is further optimized by modifying the porous structure and the FEP powder. Combining the FB-TENG and the flexible printed circuit (FPC), a self-powered pressure sensor array is fabricated to realize touch sensing and motion track monitoring. The smart pillow is formed by laying the self-powered pressure sensor array on an ordinary pillow to realize real-time monitoring of the head position in a static state and head movement trajectory in a dynamic state during sleep. Additionally, the smart pillow also has an early warning function for falling out of bed. This work not only provides a viable sensing device for sleep monitoring but also could be extended to real-time monitoring of some diseases, such as brain diseases and cervical spondylosis, in the future. It is expected to introduce a practical strategy in the real-time mobile healthcare field for disease management.

By seamlessly integrating the wearing comfortability of textiles with the biomechanical energy harvesting function of a triboelectric nanogenerator (TENG), an emerging and advanced intelligent textile, i.e., smart textile TENG, is developed with remarkable abilities of autonomous power supply and self-powered sensing, which has great development prospects in the next-generation human-oriented wearable electronics. However, due to inadequate interface contact, insufficient electrification of materials, unavoidable air breakdown effect, output capacitance feature, and special textile structure, there are still several bottlenecks in the road towards the practical application of textile TENGs, including low output, high impedance, low integration, poor working durability, and so on. In this review, on the basis of mastering the existing theory of electricity generation mechanism of TENGs, some prospective strategies for improving the mechanical-to-electrical conversion performance of textile TENGs are systematically summarized and comprehensively discussed, including surface/interface physical treatments, atomic-scale chemical modification, structural optimization design, work environmental control, and integrated energy management. The advantages and disadvantages of each approach in output enhancement are further compared at the end of this review. It is hoped that this review can not only provide useful guidance for the research of textile TENGs to select optimization methods but also accelerate their large-scale practical process.

Respiration is a major vital sign, which can be used for early illness diagnosis and physiological monitoring. Wearable respiratory sensors present an exciting opportunity to monitor human respiratory behaviors in a real-time, noninvasive, and comfortable way. Among them, fiber-shaped triboelectric nanogenerators (FS-TENGs) are attractive for their comfort and high degree of freedom. However, the single-electrode FS-TENGs cannot respond to their own tensile strains, and the coaxial double-electrode FS-TENGs show low sensitivity to strain due to structural limitations. Here, a type of helical fiber strain sensor (HFSS) is developed, which can respond to tiny tensile strains. In addition, a smart wearable real-time respiratory monitoring system is developed based on the HFSSs, which can measure some key breathing parameters for disease prevention and medical diagnosis. An intelligent alarm can automatically call a preset mobile phone for help in response to respiratory behavior changes. This work provides an effective helical structure for fabricating highly sensitive strain sensors based on FS-TENGs and develops wearable self-powered real-time respiratory monitoring systems.

Triboelectric nanogenerators (TENGs) are useful for harvesting clean and widely distributed water droplet energy with high efficiency. However, the commonly used polymer films in TENGs for water droplet energy harvesting have the disadvantages of poor breathability, poor skin affinity, and irreparable hydrophobicity, which greatly hinder their wearable uses. Here, we report an all-fabric TENG (F-TENG), which not only has good air permeability and hydrophobic self-repairing properties but also shows effective energy conversion efficiency. The hydrophobic surface composed of SiO2 nanoparticles and poly(vinylidenefluoride-co-hexafluoropropylene)/perfluorodecyltrichlorosilane (PVDF-HFP/FDTS) exhibits a static contact angle of 157° and displays excellent acid and alkali resistance. Because of its low glass transition temperature, PVDF-HFP can facilitate the movement of FDTS molecules to the surface layer under heating conditions, realizing hydrophobic self-repairing performance. Furthermore, with the optimized compositions and structure, the water droplet F-TENG shows 7-fold enhancement of output voltage compared with the conventional single-electrode mode TENG, and a total energy conversion efficiency of 2.9% is achieved. Therefore, the proposed F-TENG can be used in multifunctional wearable devices for raindrop energy harvesting.

Energy-harvesting textiles based on triboelectric nanogenerators (TENGs) have attracted intense attention, due to their broad potential applications in wearable electronics. However, the bottlenecks of limited and alternating current electrical output have greatly hindered the development of textile TENGs. Here, by simply and easily coating two electrodes on the top side (breakdown electrode) and bottom side (friction electrode) of a polyester–cotton fabric, a light-weight, highly flexible and wearable fabric-based direct current TENG (FDC-TENG) with high power output is developed. Various structural parameters and environmental factors are thoroughly and systematically explored for comprehensive understanding of the FDC-TENG. The surface charges induced by triboelectrification can be unidirectionally and efficiently harvested through the conductive plasma channel caused by air breakdown, which endows the finger-sized FDC-TENG with the abilities to light up 99 bulbs and 1053 LEDs and drive watches and calculators easily and directly without rectifying or capacitor charging. This work may provide a paradigm shift for high output direct-current power fabrics and expand their scope for application in wearable electronics.

The development of elastic electronic technology has promoted the application of triboelectric nanogenerators (TENGs) in flexible wearable electronics. However, most of the flexible electronics cannot achieve the requirements of being extremely stretchable, transparent, and highly conductive at the same time. Herein, we report a TENG constructed using a double-network polymer ionic conductor sodium alginate/zinc sulfate/poly acrylic-acrylamide (SA-Zn) hydrogel, which exhibited outstanding stretchability (>10,000%), high transparency (>95%), and good conductivity (0.34 S·m-1). The SA-Zn hydrogel TENG (SH-TENG) could harvest energy from typical human movements, such as bending, stretching, and twisting, which could light up 234 green commercial LEDs easily. Additionally, the SH-TENG can be used to prepare a self-powered smart training band sensor for monitoring arm stretching motion. This work may provide an innovative platform for accessing the next generation of sustainable wearable and sports monitoring electronics.

Despite the great potential of triboelectric nanogenerators (TENGs) as a promising energy harvesting technology, their practical applications are still hindered by their pulsed outputs with a high crest factor. Here, through a simple and easy method involving electrode misalignment (EM) and circuit connection, an exceedingly stable direct current (DC) multi-phase TENG (MP-TENG) with a high average output power at a constant current is developed. The MP-TENG obtained by rectifying and superimposing TENG units with phase difference in parallel can realize an ultra-low crest factor of 1.05 and an average power increase of 40.1%, compared with conventional single-phase TENGs. Besides, when using rotator grids with different sizes from the electrodes and the EM method, the common materials in daily life such as wood and cloth fabrics have been applied to generate DC-like outputs with a crest factor of less than 1.1, which expands dramatically the selection range of TENG materials. Due to the excellent DC performance of the MP-TENG, 1000 LEDs and 54 bulbs can be easily lighted up without any flickers, and commercial electronics can be driven continuously to work stably. This work provides a paradigm shift to achieve a high-output constant direct current, which has widespread application prospects in the field of energy harvesting.

The seamless integration of emerging triboelectric nanogenerator (TENG) technology with traditional wearable textile materials has given birth to the next-generation smart textiles, i.e., textile TENGs, which will play a vital role in the era of Internet of Things and artificial intelligences. However, low output power and inferior sensing ability have largely limited the development of textile TENGs. Among various approaches to improve the output and sensing performance, such as material modification, structural design, and environmental management, a 3D fabric structural scheme is a facile, efficient, controllable, and scalable strategy to increase the effective contact area for contact electrification of textile TENGs without cumbersome material processing and service area restrictions. Herein, the recent advances of the current reported textile TENGs with 3D fabric structures are comprehensively summarized and systematically analyzed in order to clarify their superiorities over 1D fiber and 2D fabric structures in terms of power output and pressure sensing. The forward-looking integration abilities of the 3D fabrics are also discussed at the end. It is believed that the overview and analysis of textile TENGs with distinctive 3D fabric structures will contribute to the development and realization of high-power output micro/nanowearable power sources and high-quality self-powered wearable sensors.

Textile-based triboelectric nanogenerators (T-TENGs), combining the functions of energy harvesting and self-powered sensing with advantages of breathability and flexibility, have received intensive attention, which is vital to the rapid advancements in smart textiles. However, there exists few reports of T-TENGs applied to fires under the intelligent era of high requirements for devices with versatility and multiscenario practicability. Here, in combination with flame-retardant conductive cotton fabric, polytetrafluoroethylene-coated cotton fabric, and a divider, a low-cost and environmentally friendly flame-retardant textile-based triboelectric nanogenerator (FT-TENG) is developed, which is endowed with excellent fire resistance and outstanding energy harvesting capabilities. The cotton fabrics treated with a layer-by-layer self-assembly method show great self-extinguishing performance. Besides, the maximum peak power density of the FT-TENG can reach 343.19 mW/m2 under the tapping frequency of 3 Hz. Furthermore, the FT-TENG still keeps 49.2% of the initial electrical output even after being burned at 17 different positions; 34.48% of the electrical output is also retained when the FT-TENG is exposed to 220 °C. Moreover, the FT-TENGs are successfully applied as energy harvesters for firefighters and self-powered sensors for forest self-rescue and fire alarm systems. This work may provide a promising potential for multifunctional smart textiles in energy harvesting, self-powered sensing, and life or property security.

A breathable, biodegradable, antibacterial, and self-powered skin is developed., Mimicking the comprehensive functions of human sensing via electronic skins (e-skins) is highly interesting for the development of human-machine interactions and artificial intelligences. Some e-skins with high sensitivity and stability were developed; however, little attention is paid to their comfortability, environmental friendliness, and antibacterial activity. Here, we report a breathable, biodegradable, and antibacterial e-skin based on all-nanofiber triboelectric nanogenerators, which is fabricated by sandwiching silver nanowire (Ag NW) between polylactic-co-glycolic acid (PLGA) and polyvinyl alcohol (PVA). With micro-to-nano hierarchical porous structure, the e-skin has high specific surface area for contact electrification and numerous capillary channels for thermal-moisture transfer. Through adjusting the concentration of Ag NW and the selection of PVA and PLGA, the antibacterial and biodegradable capability of e-skins can be tuned, respectively. Our e-skin can achieve real-time and self-powered monitoring of whole-body physiological signal and joint movement. This work provides a previously unexplored strategy for multifunctional e-skins with excellent practicability.

Textiles are considered as the second skin of human beings, and one of its essential tasks is thermal management. Nowadays, textile-based wearable pressure sensing technology represents a fascinating developing trend due to its predominant breathability, moisture permeability, and excellent comfortability. At the same time, thermal-regulating textiles have also received extensive attention in academia and industrial fields, because they can provide personal thermal comfort for the human body. However, there are few textiles that have the dual functions of pressure sensing and thermal regulation. Herein, a novel and multifunctional smart fiber is fabricated by using a hollow silicone rubber fiber filled with a liquid metal (LM) electrode. It can automatically switch between heating mode and cooling mode to adjust microenvironment temperature around human skin through solid-liquid phase transformation of phase change materials (PCMs). In addition, as a self-powered pressure sensor, the fiber can respond to versatile mechanical stimuli, such as human walking, running, wrist movements, and joint bending at different angles. This work provides a feasible way for constructing multifunctional smart fibers, which show great application prospects in personal motion monitoring and real-time thermal regulation.