Recent Progress in 2D-Nanomaterial-Based Triboelectric Nanogenerators

The integration of triboelectric nanogenerators (TENGs) and 2D nanomaterials brings about 2D-nanomaterial-based TENGs (2D-TENGs) that promote the rapid development of self-powered sensing systems and wearable electronics. Extraordinary physical, electronic, chemical, and optical properties of 2D nanomaterials endow 2D-TENGs with improved output performance. This review presents the state of the art of 2D-TENGs with respect to basic classifications, enhancement mechanisms, special advantages, output performances, and applications in energy harvesting and self-powered sensing. Furthermore, several challenges that can impede applications of 2D-TENGs are discussed.     

Robust Solid-Liquid Triboelectric Nanogenerators: Mechanisms,Strategies and Applications

Solid-liquid triboelectric nanogenerators (SL-TENGs) are a new technology that combines contact electrification (CE) and electrostatic induction to collect clean energy stored in natural water. Considering their unique advantages of high energy density, wide selection of materials and being suitable for large-scale promotion, they have attracted more and more attention in recent years, and numerous studies have shown their great potential in various applications. Many critical applications of SL-TENGs inevitably involve sustained and stable high electrical output. To achieve stable output performance and long cycle life in these applications, the adaptability of SL-TENGs to material selection, structural design, and working environment is necessary. Therefore, the construction of SL-TENGs matching different applications has become a critical research direction in TENGs. This review provides a historical summary of the development of SL-TENGs in the past few years and analyzes the key factors affecting their electrical output performance. The exciting achievements of different constructions of SL-TENGs for practical applications is also demonstrated such as energy harvesting, self-powered sensing, and self-powered cathodic protection. Finally, the development prospects of SL-TENGs and the significant challenges for their further development is discussed.     

Wearable and Implantable Triboelectric Nanogenerators

Triboelectric nanogenerators (TENGs) are a promising technology to convert mechanical energy to electrical energy based on coupled triboelectrification and electrostatic induction. With the rapid development of functional materials and manufacturing techniques, wearable and implantable TENGs have evolved into playing important roles in clinic and daily life from in vitro to in vivo. These flexible and light membrane‐like devices have the potential to be a new power supply or sensor element, to meet the special requirements for portable electronics, promoting innovation in electronic devices. In this review, the recent advances in wearable and implantable TENGs as sustainable power sources or self‐powered sensors are reviewed. In addition, the remaining challenges and future possible improvements of wearable and implantable TENG‐based self‐powered systems are discussed.     

Abrasion and Fracture Self-Healable Triboelectric Nanogenerator with Ultrahigh Stretchability and Long-Term Durability

Since triboelectrification results from materials in physical contact, the durability of triboelectric nanogenerators (TENGs) must be improved. However, it still poses challenges to continuously repair surface abrasion even when self-healable materials are introduced. In this study, an ultrastretchable TENG (US-TENG) that simultaneously heal the fracture and abrasion at room temperature is fabricated. By incorporating hydrogen bonds and dynamic metal-ligand coordination into polydimethylsiloxane chains, the synthesized elastomers possess ultrahigh stretchability (10 000%) and remarkable self-healing property (100% efficiency) at room temperature. Working in contact-separation mode, the electrical outputs with 2 × 2 cm² area can reach 140 V, 40 nC, 1.5 µA, respectively. Besides, the US-TENG shows a stretchability of 1800% with high electrical outputs. Even if it is stretched to break or scratched to wear out, it can recover its electrical outputs in 20 min and 2 h at room temperature, respectively. Finally, the application of the US-TENG as flexible power sources and self-powered pressure sensors is demonstrated. This designed strategy with ultrahigh stretchability and excellent self-healing properties can meet the wide applications of flexible and wearable electronics for long-term use.     

Textile‐Based Triboelectric Nanogenerators for Self‐Powered Wearable Electronics

Wearable smart electronic devices based on wireless systems use batteries as a power source. However, recent miniaturization and various functions have increased energy consumption, resulting in problems such as reduction of use time and frequent charging. These factors hinder the development of wearable electronic devices. In order to solve this energy problem, research studies on triboelectric nanogenerators (TENGs) are conducted based on the coupling of contact‐electrification and electrostatic induction effects for harvesting the vast amounts of biomechanical energy generated from wearer movement. The development of TENGs that use a variety of structures and materials based on the textile platform is reviewed, including the basic components of fibers, yarns, and fabrics made using various weaving and knitting techniques. These textile‐based TENGs are lightweight, flexible, highly stretchable, and wearable, so that they can effectively harvest biomechanical energy without interference with human motion, and can be used as activity sensors to monitor human motion. Also, the main application of wearable self‐powered systems is demonstrated and the directions of future development of textile‐based TENG for harvesting biomechanical energy presented.     

Bioinspired Self-healing Soft Electronics

Inspired by nature, various self-healing materials that can recover their physical properties after external damage have been developed. Recently, self-healing materials have been widely used in electronic devices for improving durability and protecting the devices from failure during operation. Moreover, self-healing materials can integrate many other intriguing properties of biological systems, such as stretchability, mechanical toughness, adhesion, and structural coloration, providing additional fascinating experiences. All of these inspirations have attracted extensive research on bioinspired self-healing soft electronics. This review presents a detailed discussion on bioinspired self-healing soft electronics. Firstly, two main healing mechanisms are introduced. Then, four categories of self-healing materials in soft electronics, including insulators, semiconductors, electronic conductors, and ionic conductors, are reviewed, and their functions, working principles, and applications are summarized. Finally, human-inspired self-healing materials and animal-inspired self-healing materials as well as their applications, such as organic field-effect transistors (OFETs), pressure sensors, strain sensors, chemical sensors, triboelectric nanogenerators (TENGs), and soft actuators, are introduced. This cutting-edge and promising field is believed to stimulate more excellent cross-discipline works in material science, flexible electronics, and novel sensors, accelerating the development of applications in human motion monitoring, environmental sensing, information transmission, etc.     

A Flexible Multifunctional Triboelectric Nanogenerator Based on MXene/PVA Hydrogel

Triboelectric nanogenerators (TENGs) represent an emerging technology in energy harvesting, medical treatment, and information technology. Flexible, portable, and self-powered electronic devices based on TENGs are much desired, whereas the complex preparation processes and high cost of traditional flexible electrodes hinder their practical applications. Here, an MXene/polyvinyl alcohol (PVA) hydrogel TENG (MH-TENG) is presented with simple fabrication, high output performance, and versatile applications. The doping of MXene nanosheets promotes the crosslinking of the PVA hydrogel and improves the stretchability of the composite hydrogel. The MXene nanosheets also form microchannels on surfaces, which not only enhances the conductivity of the hydrogel by improving the transport of ions but also generates an extra triboelectric output via a streaming vibration potential mechanism. The measured open-circuit voltage of the MH-TENG reaches up to 230 V even in a single-electrode mode. The MH-TENG can be stretched up to 200% of the original length and demonstrates a monotonical increasing relationship between the stretchable length and the short-circuit voltage. By utilizing the MH-TENG's outstanding stretchable property and ultrahigh sensitivity to mechanical stimuli, applications in wearable movement monitoring, high-precision written stroke recognition, and low-frequency mechanical energy harvesting are demonstrated.     

Advances in Triboelectric Nanogenerators for Self-powered Neuromodulation

Advances in implantable bioelectronics for the nervous system are reinventing the stimulation, inhibition, and sensing of neuronal activity. These efforts promise not just breakthrough treatments of several neurological and psychiatric conditions but also signal the beginning of a new era of computer-controlled human therapeutics. Batteries remain the major power source for all implanted electrical neuromodulation devices, which impairs miniaturization and necessitates replacement surgery when the battery is drained. Triboelectric nanogenerators (TENGs) have recently emerged as an innovative power solution for self-powered, closed loop electrical neurostimulation devices. TENGs can leverage the biomechanical activities of different body organs to sustainably generate electricity for electrical neurostimulation. This review features advances in TENGs as they pave the way for self-sustainable closed loop neurostimulation. A comprehensive review of TENG research for the neurostimulation of brain, autonomic, and somatic nervous systems is provided. The direction of growth of this field, publication trends, and modes of TENG in implantable bioelectronics are also discussed. Finally, an insightful outlook into challenges facing self-sustainable neuromodulators to reach clinical practice is provided, and solutions for neurological maladies are proposed.     

Cobalt-Nanoporous Carbon Functionalized Nanocomposite-Based Triboelectric Nanogenerator for Contactless and Sustainable Self-Powered Sensor Systems

Triboelectric nanogenerators (TENGs), which operate in contactless mode and avoid physical contact, are highly attractive for self-powered sensor systems aiming to achieve long-term reliable operation and reduce rubbing friction. Herein, an ultra-flexible and high-performance contactless double-layer TENG (CDL-TENG) is first designed and fabricated using a metal–organic framework-based cobalt nanoporous carbon (Co-NPC)/Ecoflex with MXene/Ecoflex nanocomposite layer for self-powered sensor applications. The porous structure of the Co-NPC provides a high-surface-area of the nanocomposite and the charge storage layer of the MXene/Ecoflex nanocomposite accumulates more negative charge to improve the functionality of the CDL-TENG two and three times, respectively. Compared with Ecoflex film-based TENGs, the fabricated CDL-TENG exhibits an eight-fold slower decay rate owing to charge trapping characteristics, which were confirmed by surface potential measurements. The CDL-TENG shows excellent humidity and acceleration sensitivity of about 0.3 V/% and 2.06 Vs² m⁻¹. The CDL-TENG also offers non-contact position detection performance in the 20 cm range. Furthermore, the CDL-TENG is successfully integrated with mobile-vehicles and an intelligent robot to perform obstacle and human-motion detection. Finally, a contactless door-lock password authentication system was demonstrated. These multifunctional benefits make it useful for numerous applications, including artificial intelligence, human-machine interfaces, and self-powered sensors.     

Multifunctional Mechanical Metamaterials with Embedded Triboelectric Nanogenerators

The flexibility of planar triboelectric nanogenerators (TENGs) enables them to be embedded into structures with complex geometries and to conform with any deformation of these structures. In return, the embedded TENGs function as either strain‐sensitive active sensors or energy harvesters while negligibly affecting the structure's original mechanical properties. This advantage inspires a new class of multifunctional materials where compliant TENGs are distributed into local operational units of mechanical metamaterial, dubbed TENG‐embedded mechanical metamaterials. This new class of metamaterial inherits the advantages of a traditional mechanical metamaterial, in that the deformation of the internal topology of material enables unusual mechanical properties. The concept is illustrated with experimental investigations and finite element simulations of prototypes based on two exemplar metamaterial geometries where functions of self‐powered sensing, energy harvesting, as well as the designated mechanical behavior are investigated. This work provides a new framework in producing multifunctional triboelectric devices.     

Progress in Triboelectric Materials: Toward High Performance and Widespread Applications

Triboelectric phenomena can be observed everywhere; however, they are consistently omitted from applications. Although almost all substances exhibit a triboelectrification effect in daily life, chemists as well as materials scientists have performed extensive investigations in both the aspects of basic science and practical applications to promote the development of triboelectric nanogenerators (TENGs). Here, a detailed survey of materials engineering for high triboelectric performance and multifunctional materials toward specific applications is summarized, including constructing micro/nanostructures, chemically modifying the frication surface, modulating bulk friction materials, the mechanism for improved performance, and preparing materials for implantable medical devices, bionic skin, and wearable electronic devices. Moreover, an in depth discussion of the current challenges and future efforts for strengthening the performance of TENGs is elaborated in detail, which will better guide new researchers toward a deeper understanding of and explorations about TENGs.     

A Self-Healing Triboelectric Nanogenerator Based on Feathers for Sensing and Energy Harvesting

A useful direction to solve the energy problem is the effective repeated use of biomaterial for mechanical energy collection and sensing applications. Here, a feather-based single-electrode triboelectric nanogenerator (F-STENG) by only sputtering copper atoms on the feathers are presented. The feather of F-STENG, as a natural material, has environmental friendliness, which is different from the polymer materials of other triboelectric nanogenerators. F-STENG has super durability due to its feather structure self-healing property. The device has a high output voltage of 90 V and an output current of 3.5 µA. After breaking and self-healing lots of times, the output performance is also 80% of the original. F-STENG has a high sensitivity to temperature, humidity, and wind speed, and the sensitivity is 0.50 V °C⁻¹, -0.98 V RH⁻¹, and 1.67 μA m⁻¹ s⁻¹. The output power of F-STENG is 0.62 mW g⁻¹, which can realize global positioning and photographing to solve the module energy consumption problem. F-STENG provides an effective way for the application of self-powered sensors and equipment in military, industrial, transportation, and daily life.     

3D Printing-Directed Synergistic Design of High-Performance Zinc-Ion Hybrid Capacitors and Nanogenerators for All-In-One Self-Powered Energy Wristband

Advanced wearable self-powered energy systems that simultaneously achieve energy harvesting and energy storage offer exciting opportunities for flexible electronics, information communication, and even intelligent environmental monitoring. However, building and integrating synergistic energy storage from energy harvester unit into a single power source is highly challenging. Herein, a unique 3D printing-directed synergistic design of high-performance zinc-ion hybrid capacitors (ZIHCs) and triboelectric nanogenerators (TENGs) is proposed for the all-in-one self-powered wearable energy wristband. With advanced ink design, high-performance flexible ZIHCs are built up as the excellent energy storage unit with remarkable electrochemical behaviors and synergistic matching from TENGs. An exceptional device capacitance of 239.0 mF cm⁻², moderate potential window, high-rate capability, robust cycling stability, and excellent flexibility are achieved. Intrinsic charge storage process is also revealed, further demonstrating the outstanding electrochemical stability of the in-plane flexible ZIHCs. Moreover, using 3D printing-directed synergistic design, an advanced all-in-one self-powered energy wristband is developed, where an efficient harvesting of body vibration/movement energy and a reliable storage of harvested energy are simultaneously realized, representing a substantial step toward future practical applications in portable and wearable electronics.     

Segmented Swing-Structured Fur-Based Triboelectric Nanogenerator for Harvesting Blue Energy toward Marine Environmental Applications

Reducing carbon emissions to realize carbon neutrality is crucial to the environmental protection, and developing clean and renewable energy sources is an effective means to achieve this goal. Triboelectric nanogenerators (TENGs) provide a promising energy technology for converting the abundant renewable ocean wave energy on the earth surface. In this work, a segmented swing-structured fur-based TENG (SSF-TENG) is designed and fabricated to harvest low frequency water wave energy. The introduction of soft and dense rabbit furs reduces the frictional resistance and material wear, and the design and optimization of segmented structures further enhance the output performance of TENG. The use of ultra-lubricated bearings makes the SSF-TENG achieve an extended period of energy harvesting of more than 5 min after one triggering, with a total energy conversion efficiency of up to 23.6%. Under the real water wave triggering, the SSF-TENG can deliver a maximum peak power of 6.2 mW and an average power of 0.74 mW. Furthermore, through effective water wave energy harvesting by the SSF-TENG or array, self-powered marine environmental applications are successfully demonstrated, which establishes a solid foundation for large-scale blue energy harvesting and realization of smart oceans.     

Bioinspired Triboelectric Nanogenerators as Self‐Powered Electronic Skin for Robotic Tactile Sensing

Electronic skin (e‐skin) has been under the spotlight due to great potential for applications in robotics, human–machine interfaces, and healthcare. Meanwhile, triboelectric nanogenerators (TENGs) have been emerging as an effective approach to realize self‐powered e‐skin sensors. In this work, bioinspired TENGs as self‐powered e‐skin sensors are developed and their applications for robotic tactile sensing are also demonstrated. Through the facile replication of the surface morphology of natural plants, the interlocking microstructures are generated on tribo‐layers to enhance triboelectric effects. Along with the adoption of polytetrafluoroethylene (PTFE) tinny burrs on the microstructured tribo‐surface, the sensitivity for pressure measurement is boosted with a 14‐fold increase. The tactile sensing capability of the TENG e‐skin sensors are demonstrated through the characterizations of handshaking pressure and bending angles of each finger of a bionic hand during handshaking with human. The TENG e‐skin sensors can also be utilized for tactile object recognition to measure surface roughness and discern hardness. The facile fabrication scheme of the self‐powered TENG e‐skin sensors enables their great potential for applications in robotic dexterous manipulation, prosthetics, human–machine interfaces, etc.     

Metal-Organic Framework Reinforced Highly Stretchable and Durable Conductive Hydrogel-Based Triboelectric Nanogenerator for Biomotion Sensing and Wearable Human-Machine Interfaces

Flexible triboelectric nanogenerators (TENGs) with multifunctional sensing capabilities offer an elegant solution to address the growing energy supply challenges for wearable smart electronics. Herein, a highly stretchable and durable electrode for wearable TENG is developed using ZIF-8 as a reinforcing nanofiller in a hydrogel with LiCl electrolyte. ZIF-8 nanocrystals improve the hydrogel's mechanical properties by forming hydrogen bonds with copolymer chains, resulting in 2.7 times greater stretchability than pure hydrogel. The hydrogel electrode is encapsulated by microstructured silicone layers that act as triboelectric materials and prevent water loss from the hydrogel. Optimized ZIF-8-based hydrogel electrodes enhance the output performance of TENG through the dynamic balance of electric double layers (EDLs) during contact electrification. Thus, the as-fabricated TENG delivers an excellent power density of 3.47 Wm^–2, which is 3.2 times higher than pure hydrogel-based TENG. The developed TENG can scavenge biomechanical energy even at subzero temperatures to power small electronics and serve as excellent self-powered pressure sensors for human-machine interfaces (HMIs). The nanocomposite hydrogel-based TENG can also function as a wearable biomotion sensor, detecting body movements with high sensitivity. This study demonstrates the significant potential of utilizing ZIF-8 reinforced hydrogel as an electrode for wearable TENGs in energy harvesting and sensor technology.     

Analysis and Experiment of Self-Powered,Pulse-Based Energy Harvester Using 400 V FEP-Based Segmented Triboelectric Nanogenerators and 98.2% Tracking Efficient Power Management IC for Multi-Functional IoT Applications

A self-powered system for the Internet of Things (IoT) is demonstrated for efficient energy harvesting of naturally available mechanical energy. In this system, new contact-separation mode triboelectric nanogenerators (TENGs), based on fluorinated ethylene propylene, are investigated using the segmented multi-TENG configuration to reduce the effect of parasitic capacitance. The TENG extraction is optimized using a unit step excitation involved with the Dawson function to achieve a high voltage (400 V) and a high current (26.6 µA). To fully extract the power of the TENGs, the power management integrated circuit (PMIC) specially designed for adaptively controlled, high-voltage (HV) maximum power point tracking (MPPT) is proposed. The PMIC implemented in a bipolar CMOS-DMOS 180 nm process can handle a wide input range (5–70 V) by consuming 420 nW. The MPPT control allows a wide range of impedance matching from 10 to 300 MΩ, achieving a tracking efficiency of up to 98.2%. The end-to-end efficiency of 88% demonstrates state-of-the-art performance. To supply a higher instantaneous power than that available from the TENGs, a duty-cycling technique is successfully demonstrated. The proposed energy harvesting system provides a promising approach to realizing sustainable and autonomous energy sources for various IoT applications.     

Advances in Triboelectric Nanogenerators for Self-Powered Regenerative Medicine

Over the last decade, in pursuit to provide suitable alternatives for power supplies of medical devices in regenerative medicine, extensive research on nanogenerators has been developed. Such devices can overcome current commercial battery challenges, including intense heat-on-body complications due to the electrical current during therapeutic usage, leading to protein denaturation, cell structure destruction, and even cell necrosis. In addition, these traditional batteries contain a bulky and heavy structure that prevents them from providing sustainable on body biomedical therapeutic intervention. Furthermore, advantages such as wide-range biocompatible and biodegradable materials, lightweight, and sufficient stretchability for device construction can minimize the side effects of implantable devices, including inflammation or toxicity, as well as eliminate secondary surgery to replace or remove batteries. Triboelectric nanogenerators (TENGs) are associated with harvesting mechanical energy in various forms, among which human body motions can serve as a renewable power source for healthcare systems. This review is written to emphasize the importance of TENG's applications in regenerative medicine and modulation purposes, particularly for the nervous system. Some crucial parameters for implantable consideration are discussed. In the concluding remarks, features for clinical utilization including output efficiency, encapsulation, stability, and miniaturization are suggested as challenges and prospects.     

Multiscale Structural Nanocellulosic Triboelectric Aerogels Induced by Hofmeister Effect

The advent of self-powered wearable electronics will revolutionize the fields of smart healthcare and sports monitoring. This technological advancement necessitates more stringent design requirements for triboelectric materials. The triboelectric aerogels must enhance their mechanical properties to address the issue of structural collapse in real-world applications. This study fabricates stiff nanocellulosic triboelectric aerogels with multiscale structures induced by the Hofmeister effect. The aggregation and crystallization of polymer molecular chains are enhanced by the Hofmeister effect, while ice crystal growth imparts a porous structure to the aerogel at the micron scale. Therefore, the triboelectric aerogel exhibits exceptional stiffness, boasting a Young's modulus of up to 142.9 MPa and a specific modulus of up to 340.6 kN m kg^–1, while remaining undeformed even after supporting 6600 times its weight. Even after withstanding an impact of 343 kPa, highly robust wearable self-powered sensors fabricated with triboelectric aerogels remain operational. Additionally, the self-powered sensor is capable of accurately detecting human movements, particularly in abnormal fall postures detection. This study provides considerable research and practical value for promoting material design and broadening application scenarios for self-powered wearable electronics.     

Multifunctional and Physically Transient Supercapacitors,Triboelectric Nanogenerators,and Capacitive Sensors

Electronic waste (e-waste) grows in parallel with the increasing need for consumer electronics. This, unfortunately, is leading to pollution and massive ecological problems worldwide. A solution to this problem is the use of transient electronics. While transiency of a few components such as transistors and batteries have been proposed already, it is crucial to have all components in electronic devices to be transient. Therefore, the transiency of more electronic components should be demonstrated to alleviate the e-waste problem. Herein, multifunctional nanocomposite electrodes are fabricated using poly(vinyl alcohol), carbon black, and activated carbon. These simple electrodes are then used to fabricate physically transient supercapacitors, triboelectric nanogenerators, and capacitive sensors. Transient supercapacitors are used numerous times with excellent supercapacitive behavior before being discarded, which show promise as an energy storage component for transient systems. The fabricated transient triboelectric nanogenerators are used to harvest mechanical energy, eliminated the need for an external power supply, paving the way to self-powered devices, such as a touchpad as demonstrated herein. The fabricated transient capacitive sensors, on the other hand, have shown long linear sensitivities and offered waste-free monitoring of physiological signals and body motions.