Integration of advanced nanogenerator technology with conventional textile processes fosters the emergence of textile-based nanogenerators (NGs), which will inevitably promote the rapid development and widespread applications of next-generation wearable electronics and multifaceted artificial intelligence systems. NGs endow smart textiles with mechanical energy harvesting and multifunctional self-powered sensing capabilities, while textiles provide a versatile flexible design carrier and extensive wearable application platform for their development. However, due to the lack of an effective interactive platform and communication channel between researchers specializing in NGs and those good at textiles, it is rather difficult to achieve fiber/fabric-based NGs with both excellent electrical output properties and outstanding textile-related performances. To this end, a critical review is presented on the current state of the arts of wearable fiber/fabric-based piezoelectric nanogenerators and triboelectric nanogenerators with respect to basic classifications, material selections, fabrication techniques, structural designs, and working principles, as well as potential applications. Furthermore, the potential difficulties and tough challenges that can impede their large-scale commercial applications are summarized and discussed. It is hoped that this review will not only deepen the ties between smart textiles and wearable NGs, but also push forward further research and applications of future wearable fiber/fabric-based NGs.

Fiber/Fabric-Based Piezoelectric and Triboelectric Nanogenerators for Flexible/Stretchable and Wearable Electronics and Artificial Intelligence.

Recent advances in soft materials and system integration technologies have provided a unique opportunity to design various types of wearable flexible hybrid electronics (WFHE) for advanced human healthcare and human-machine interfaces. The hybrid integration of soft and biocompatible materials with miniaturized wireless wearable systems is undoubtedly an attractive prospect in the sense that the successful device performance requires high degrees of mechanical flexibility, sensing capability, and user-friendly simplicity. Here, the most up-to-date materials, sensors, and system-packaging technologies to develop advanced WFHE are provided. Details of mechanical, electrical, physicochemical, and biocompatible properties are discussed with integrated sensor applications in healthcare, energy, and environment. In addition, limitations of the current materials are discussed, as well as key challenges and the future direction of WFHE. Collectively, an all-inclusive review of the newly developed WFHE along with a summary of imperative requirements of material properties, sensor capabilities, electronics performance, and skin integrations is provided.

Advanced Soft Materials, Sensor Integrations, and Applications of Wearable Flexible Hybrid Electronics in Healthcare, Energy, and Environment.

Substantial effort has been devoted to both scientific and technological developments of wearable, flexible, semitransparent, and sensing electronics (e.g., organic/perovskite photovoltaics, organic thin-film transistors, and medical sensors) in the past decade. The key to realizing those functionalities is essentially the fabrication of conductive electrodes with desirable mechanical properties. Conductive polymers (CPs) of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) have emerged to be the most promising flexible electrode materials over rigid metallic oxides and play a critical role in these unprecedented devices as transparent electrodes, hole transport layers, interconnectors, electroactive layers, or motion-sensing conductors. Here, the current status of research on PEDOT:PSS is summarized including various approaches to boosting the electrical conductivity and mechanical compliance and stability, directly linked to the underlying mechanism of the performance enhancements. Along with the basic principles, the most cutting edge-progresses in devices with PEDOT:PSS are highlighted. Meanwhile, the advantages and plausible problems of the CPs and as-fabricated devices are pointed out. Finally, new perspectives are given for CP modifications and device fabrications. This work stresses the importance of developing CP films and reveals their critical role in the evolution of these next-generation devices featuring wearable, deformable, printable, ultrathin, and see-through characteristics.

https://onlinelibrary.wiley.com/doi/epdf/10.1002/advs.201900813

PEDOT:PSS for Flexible and Stretchable Electronics: Modifications, Strategies, and Applications

Wireless wearable sweat biosensors have gained huge traction due to their potential for noninvasive health monitoring. As high energy consumption is a crucial challenge in this field, efficient energy harvesting from human motion represents an attractive approach to sustainably power future wearables. Despite intensive research activities, most wearable energy harvesters suffer from complex fabrication procedures, poor robustness, and low power density, making them unsuitable for continuous biosensing. Here, we propose a highly robust, mass-producible, and battery-free wearable platform that efficiently extracts power from body motion through a flexible printed circuit board (FPCB)-based freestanding triboelectric nanogenerator (FTENG). The judiciously engineered FTENG displays a high power output of ~416 mW m-2 Through seamless system integration and efficient power management, we demonstrate a battery-free triboelectrically driven system that is able to power multiplexed sweat biosensors and wirelessly transmit data to the user interfaces through Bluetooth during on-body human trials.

Wireless battery-free wearable sweat sensor powered by human motion

Considerable progress in materials development and device integration for mechanically bendable and stretchable optoelectronics will broaden the application of "Internet-of-Things" concepts to a myriad of new applications. When addressing the needs associated with the human body, such as the detection of mechanical functions, monitoring of health parameters, and integration with human tissues, optoelectronic devices, interconnects/circuits enabling their functions, and the core passive components from which the whole system is built must sustain different degrees of mechanical stresses. Herein, the basic characteristics and performance of several of these devices are reported, particularly focusing on the conducting element constituting them. Among these devices, strain sensors of different types, energy storage elements, and power/energy storage and generators are included. Specifically, the advances during the past 3 years are reported, wherein mechanically flexible conducting elements are fabricated from (0D, 1D, and 2D) conducting nanomaterials from metals (e.g., Au nanoparticles, Ag flakes, Cu nanowires), carbon nanotubes/nanofibers, 2D conductors (e.g., graphene, MoS2 ), metal oxides (e.g., Zn nanorods), and conducting polymers (e.g., poly(3,4-ethylenedioxythiophene):poly(4-styrene sulfonate), polyaniline) in combination with passive fibrotic and elastomeric materials enabling, after integration, the so-called electronic skins and electronic textiles.

Mechanically Flexible Conductors for Stretchable and Wearable E-Skin and E-Textile Devices

A Wrinkled PEDOT:PSS Film Based Stretchable and Transparent Triboelectric Nanogenerator for Wearable Energy Harvesters and Active Motion Sensors

As a promising energy converter, the requirement for miniaturization and high-accuracy of triboelectric nanogenerators always remains urgent. In this work, a micro triboelectric ultrasonic device was developed by integrating a triboelectric nanogenerator and micro-electro-mechanical systems technology. To date, it sets a world record for the smallest triboelectric device, with a 50 µm-sized diaphragm, and enables the working frequency to be brought to megahertz. This dramatically improves the miniaturization and chip integration of the triboelectric nanogenerator. With 63 kPa@1 MHz ultrasound input, the micro triboelectric ultrasonic device can generate the voltage signal of 16.8 mV and 12.7 mV through oil and sound-attenuation medium, respectively. It also achieved the signal-to-ratio of 20.54 dB and exhibited the practical potential for signal communication by modulating the incident ultrasound. Finally, detailed optimization approaches have also been proposed to further improve the output power of the micro triboelectric ultrasonic device. Miniaturizing efficient triboelectric nanogenerators remains a challenge. Here, the authors propose a MEMS-based micro triboelectric device for acoustic energy transfer and signal communication which is capable of generating the voltage signal of 16.8 mV and 12.7 mV through oil and sound-attenuation medium respectively with 63kPa at 1 MHz ultrasound input.

/pdf/micro-triboelectric-ultrasonic-device-for-acoustic-energy-21to8fvxcj.pdf

Micro triboelectric ultrasonic device for acoustic energy transfer and signal communication.

Continuous deforming always leads to the performance degradation of a flexible triboelectric nanogenerator due to the Young’s modulus mismatch of different functional layers. In this work, we fabricated a fiber-shaped stretchable and tailorable triboelectric nanogenerator (FST–TENG) based on the geometric construction of a steel wire as electrode and ingenious selection of silicone rubber as triboelectric layer. Owing to the great robustness and continuous conductivity, the FST–TENGs demonstrate high stability, stretchability, and even tailorability. For a single device with ~ 6 cm in length and ~ 3 mm in diameter, the open-circuit voltage of ~ 59.7 V, transferred charge of ~ 23.7 nC, short-circuit current of ~ 2.67 μA and average power of ~ 2.13 μW can be obtained at 2.5 Hz. By knitting several FST–TENGs to be a fabric or a bracelet, it enables to harvest human motion energy and then to drive a wearable electronic device. Finally, it can also be woven on dorsum of glove to monitor the movements of gesture, which can recognize every single finger, different bending angle, and numbers of bent finger by analyzing voltage signals.
Highlights:
1 Owing to the great robustness, continuous conductivity, and geometric construction of a steel wire electrode, the FST–TENGs demonstrate high stability, stretchability, and even tailorability.2 By knitting several FST–TENGs to be a fabric or a bracelet worn on the human body, it enables to harvest human motion energy.3 The FST–TENGs can also be woven on dorsum of glove to monitor the movements of gesture.

/pdf/spiral-steel-wire-based-fiber-shaped-stretchable-and-4iidj8soj4.pdf

Spiral Steel Wire Based Fiber-Shaped Stretchable and Tailorable Triboelectric Nanogenerator for Wearable Power Source and Active Gesture Sensor

Stretchable triboelectric nanogenerators (TENGs) have been made available for a large amplitude of human body movement as an effective wearable power source However, the elasticity and performance of such a kind of device seriously decline in a harsh environment, especially below sub-zero temperature In this work, an anti-freezing hydrogel was synthesized by one-step radical polymerization of acrylamide monomer in hydroxyethyl cellulose aqueous solution After adding LiCl into the hydrogel, it can be cooled to temperatures as low as −69 °C without freezing The hydroxyethyl cellulose not only enhances the mechanical properties as a physical crosslinking agent but also provides water retention properties With a constant elongation of ∼150%, the anti-freezing hydrogel based TENG (AH-TENG) has been successfully demonstrated to harvest human biomechanical energy to drive wearable electronic devices, even in a harsh ice and snow environment At a fixed frequency of 25 Hz, an AH-TENG with a 3 × 3 cm2 area achieved an output of 285 V, 155 μA, 90 nC and an instantaneous peak power density of 626 mW m−2, respectively This work is anticipated to provide a promising approach for the advancement of flexible energy sources under harsh conditions

Jihong Shi

Papers

A Wrinkled PEDOT:PSS Film Based Stretchable and Transparent Triboelectric Nanogenerator for Wearable Energy Harvesters and Active Motion Sensors

Micro triboelectric ultrasonic device for acoustic energy transfer and signal communication.

Spiral Steel Wire Based Fiber-Shaped Stretchable and Tailorable Triboelectric Nanogenerator for Wearable Power Source and Active Gesture Sensor

An anti-freezing hydrogel based stretchable triboelectric nanogenerator for biomechanical energy harvesting at sub-zero temperature

Charge-trapping-blocking layer for enhanced triboelectric nanogenerators