Grating-structured freestanding triboelectric-layer nanogenerator for harvesting mechanical energy at 85% total conversion efficiency

Universal Analysis Method for Metamaterial-Based Wireless Power Transfer with Arbitrary Energy Source Waveforms: Application to Triboelectric Nanogenerators

Metamaterial-based wireless power transfer (MM-WPT) analysis has attracted substantial attention due to its great application potential. However, traditional MM-WPT analysis is constrained by frequency domain approaches which are suitable only for infinitely extended periodic signals or fixed-frequency sine waves but not suitable for complex waveforms of various energy sources. This paper presents an innovative time-domain system analysis method for MM-WPT systems tailored to evaluate energy sources with arbitrary waveforms. The foundation of the method is to use the unit impulse response. By convolving this impulse response with any type of excitation source, a temporal waveform of the voltage across the system's load can be obtained. It has demonstrated a high degree of correlation and agreement between theoretical calculations and experimental results for various input waveforms, affirming its validity, precision, and universality. Based on the framework, it is shown that triboelectric nanogenerators can efficiently self-powered transfer wireless energy through MM-WPT systems. Experiments reveal that the energy received is up to 59.6 times higher compared with that of WPT systems without metamaterials. When this system is applied in an implant, it demonstrates a remarkable energy transfer efficiency of 51% through biological tissues. These findings represent a significant breakthrough in optimizing WPT systems for compact and efficient self-powered energy applications.

Research Progress of Triboelectric Nanogenerators for Ocean Wave Energy Harvesting

The ocean wave energy is considered one of the most promising forms of marine blue energy due to its vast reserves and high energy density. However, traditional electromagnetic power generation technology suffers from drawbacks such as high maintenance costs, heavy structures, and low conversion efficiency, which restricts its application range. The triboelectric nanogenerator (TENG) uses Maxwell displacement current as its internal driving force, which can efficiently convert irregular, low-frequency, and dispersed mechanical energy into electrical energy. The generator utilizes the coupling effect between contact electrification and electrostatic induction, showing the significant advantages of light weight, high cost effectiveness, and easy expansion. Compared with traditional mechanical energy harvesting techniques such as electromagnetic generators, triboelectric nanogenerators exhibit higher efficiency and output performance in the low-frequency range. Thus, wave power generation technology based on triboelectric nanogenerators has emerged as a highly potential alternative in this field. Herein, recent progress to summarize the latest advancements in TENG-based ocean wave energy capture is reviewed. More importantly, the actual progress of TENG with different structures in wave energy harvesting is discussed, providing an overview of the current research status in this field for relevant researchers.

AI-Assisted Automated Identification of Human Activities Using Ficus religiosa Leaf-Based Triboelectric Nanogenerator

Movement monitoring and effective identification of different actions are the keys that help in fitness services, health status, clinical studies, etc. In this technological era, Internet of Things (IoT) technologies, including smart wireless devices and sensors, are very effectively used for monitoring human activities, but the demand for sustainable and green power sources is a crucial issue with these devices. Triboelectric nanogenerators (TENG) are proven to be promising applications in these devices because they harvest energy from the surrounding environment and eliminate the use of batteries as power sources. As a green energy source, this study emphasizes the fabrication of biodegradable materials-based TENGs, which are eco-friendly and are related to clean and green energy as per the UN's sustainable development goals SDG 7 (affordable and clean energy). In the present work, a natural Ficus religiosa leaf (FRL) of the F. religiosa tree is used in designing and fabricating a TENG (FRL-TENG). Also, an approach is discussed to compare the performance of FRL-TENG with TENGs fabricated from other waste biodegradable materials such as garlic tunic, onion tunic, and eggshell membrane (ESM). During the experimental study, it is observed that the FRL-based TENG produced maximum voltage in comparison to other material combinations selected in this study. The generated electric output from these TENG combinations is also used to power an array of tens of green-light-emitting diodes (LEDs). Furthermore, this paper also proposes the use of FRL-TENG as a wearable sensor to collect information and monitor the physical activities of the user, viz., walk, jump, and run. To recognize the movement status, the FRL-TENG sensor is integrated with an extra randomized tree-based machine learning model for accurately distinguishing the user's three activities with an accuracy of 96%. The work showcases an innovative approach to encourage customized uses of TENG sensors in human motion monitoring and permits the development of intelligent, self-powered systems for new applications.

From Friction to Function: A High-Voltage Sliding Triboelectric Nanogenerator for Highly Efficient Energy Autonomous IoTs and Self-Powered Actuation

An advanced energy autonomous system that simultaneously harnesses and stores energy on the same platform offers exciting opportunities for the near-future self-powered miniature electronics. However, achieving optimal synchronization between the power output of an energy harvester and the storage unit or integrating it seamlessly with real-time microelectronics to build a highly efficient energy autonomous system remains challenging. Herein, a unique bimetallic layered double hydroxide (LDH) based tribo-positive layer is introduced for a high-voltage sliding triboelectric nanogenerator (S-TENG) with an output voltage of ≈1485 V and power output of 250 µW, respectively. To demonstrate the potential of a self-charging power system, S-TENG is integrated with on-chip micro-supercapacitors (MSCs) as a storage unit. The MSC array effectively self-charged up to 4.8 V (within 220s), providing ample power to support micro-sensory systems. In addition, by utilizing the high-voltage output of the S-TENG, the efficient operation of electrostatic actuators and digital microfluidic (DMF) systems driven directly by simple mechanical motion is further demonstrated. Overall, this work can provide a solid foundation for the advancement of next-generation energy-autonomous systems.

Wearable Pendulum-Rotor-Separated Hybrid Generator for Smart Healthcare Monitoring

Human biomechanical energy, with features of fluctuating amplitudes and low frequency, has been considered as a potential sustainable power source for wearable healthcare monitoring devices. Developing an effective energy harvester to ensure robust energy harvesting efficiency remains highly desired. Herein, we propose a wearable pendulum-rotor-separated triboelectric-electromagnetic hybrid generator (PTEHG). The novel pendulum-rotor separation design can make the rotor propelled in one direction by the swinging pendulum, which can further facilitate a wearable hybrid energy harvester with stable energy harvesting, a broad operating bandwidth, and system reliability. By converting the biomechanical energy into electric power, the peak power density of 83.12 W/m3 is delivered by the PTEHG at a frequency of 1.6 Hz. A PTEHG-based healthcare monitoring system was also demonstrated for real-time motion tracking and fall detection. This work paves a new way for enhancing the efficiency of human biomechanical energy harvesting and presents a practical pathway for continuous healthcare monitoring.

Multiple Self-Powered Sensor-Integrated Mobile Manipulator for Intelligent Environment Detection

A multiple self-powered sensor-integrated mobile manipulator (MSIMM) system was proposed to address challenges in existing exploration devices, such as the need for a constant energy supply, limited variety of sensed information, and difficult human-computer interfaces. The MSIMM system integrates triboelectric nanogenerator (TENG)-based self-powered sensors, a bionic manipulator, and wireless gesture control, enhancing sensor data usability through machine learning. Specifically, the system includes a tracked vehicle platform carrying the manipulator and electronics, including a storage battery and a microcontroller unit (MCU). An integrated sensor glove and terminal application (APP) enable intuitive manipulator control, improving human-computer interaction. The system responds to and analyzes various environmental stimuli, including the droplet and fall height, temperature, pressure, material type, angles, angular velocity direction, and acceleration amplitude and direction. The manipulator, fabricated using 3D printing technology, integrates multiple sensors that generate electrical signals through the triboelectric effect of mechanical motion. These signals are classified using convolutional neural networks for accurate environmental monitoring. Our database shows signal recognition and classification accuracy exceeding 94%, with specific accuracies of 100% for pressure sensors, 99.55% for angle sensors, and 98.66, 95.91, 96.27, and 94.13% for material, droplet, temperature, and acceleration sensors, respectively.

A rolling-mode triboelectric nanogenerator with multi-tunnel grating electrodes and opposite-charge-enhancement for wave energy harvesting

In light of the crucial role of marine ecosystems and the escalating environmental conservation challenges, it is essential to conduct marine monitoring to help implement targeted environmental protection measures efficiently. Energy harvesting technologies, particularly triboelectric nanogenerators (TENGs), have great potential for prolonging the lifespan and enhancing the reliability of sensors in remote areas. However, the high internal resistance, low current, and friction-induced abrasion issues of TENGs limit their performance in practical applications. This work presents a rolling mode triboelectric nanogenerator that utilizes multi-tunnel grating electrodes and the opposite-charge-enhancement mechanism to harvest wave energy efficiently. The device achieves significant instantaneous and root mean square power density of 185.4 W/(m3·Hz) and 10.92 W/(m3·Hz), respectively. By utilizing stacked devices and an exclusively designed power management module, a self-powered ocean sensing system including computing and long-range wireless communication (0.8 km) capabilities was developed. Laboratory and in-situ ocean tests were conducted to assess and validate the system. This work offers a potential solution for the challenging deployment of marine self-powered sensing nodes.

Triboelectric Nanogenerator-Enabled Digital Twins in Civil Engineering Infrastructure 4.0: A Comprehensive Review

The emergence of digital twins has ushered in a new era in civil engineering with a focus on achieving sustainable energy supply, real-time sensing, and rapid warning systems. These key development goals mean the arrival of Civil Engineering 4.0.The advent of triboelectric nanogenerators (TENGs) demonstrates the feasibility of energy harvesting and self-powered sensing. This review aims to provide a comprehensive analysis of the fundamental elements comprising civil infrastructure, encompassing various structures such as buildings, pavements, rail tracks, bridges, tunnels, and ports. First, an elaboration is provided on smart engineering structures with digital twins. Following that, the paper examines the impact of using TENG-enabled strategies on smart civil infrastructure through the integration of materials and structures. The various infrastructures provided by TENGs have been analyzed to identify the key research interest. These areas encompass a wide range of civil infrastructure characteristics, including safety, efficiency, energy conservation, and other related themes. The challenges and future perspectives of TENG-enabled smart civil infrastructure are briefly discussed in the final section. In conclusion, it is conceivable that in the near future, there will be a proliferation of smart civil infrastructure accompanied by sustainable and comprehensive smart services.

Wearable All-Fabric Hybrid Energy Harvester to Simultaneously Harvest Radiofrequency and Triboelectric Energy

Distributed micro-energy harvesting devices offer the flexibility, sustainability, and multi-scenario applicability that will be critical to wearable electronic products in the Internet of Things. The radiofrequency and triboelectric (RF-TE) hybrid energy harvester (HEH) concept and prototype is presented for the first time, to simultaneously capture the energy from ambient electromagnetic waves and biological motions. The proposed hybrid energy harvesting system consists of a wearable rectenna, a triboelectric nanogenerator (TENG), and a power management circuit (PMC). Among them, the all-fabric rectenna exhibits good impedance matching characteristics in the ISM frequency. The flexible TENG unit can generate a maximum power density of 0.024 µW cm-2. The designed multifunctional fabric-based PMC can considerably enhance the controllability of harvested hybrid energy. Additionally, a normalizable fabric circuit board quasi surface mount technology (FCB-SMT) is proposed to integrate all modules on the same fabric substrate in one step, making the entire system superior mechanical robustness. The proposed wearable fabric-based RF-TE hybrid energy harvester is capable of successfully driving consumer electronics (such as sensors, watches, etc.). It provides a new energy solution strategy for self-powered wearable electronic devices and is anticipated to encourage the efficient utilization of renewable energy.

Fish Scale for Wearable, Self-Powered TENG

Flexible and wearable devices are attracting more and more attention. Herein, we propose a self-powered triboelectric nanogenerator based on the triboelectric effect of fish scales. As the pressure on the nanogenerator increases, the output voltage of the triboelectric nanogenerator increases. The nanogenerator can output a voltage of 7.4 V and a short-circuit current of 0.18 μA under a pressure of 50 N. The triboelectric effect of fish scales was argued to be related to the lamellar structure composed of collagen fiber bundles. The nanogenerator prepared by fish scales can sensitively perceive human activities such as walking, finger tapping, and elbow bending. Moreover, fish scales are a biomass material with good biocompatibility with the body. The fish-scale nanogenerator is a kind of flexible, wearable, and self-powered triboelectric nanogenerator showing great prospects in healthcare and body information monitoring.

Conventional and pulsed hybrid triboelectric nanogenerator with tunable output time and wider impedance matching range

Sliding grating-structured triboelectric nanogenerators (SG-TENGs) can multiply transferred charge, reduce open-circuit voltage, and increase short-circuit current, which have wide application prospects in self-powered systems. However, conventional SG-TENGs have an ultrahigh internal equivalent impedance, which reduces the output voltage and energy under low load resistances (<10 MΩ). The Pulsed SG-TENGs can reduce the equivalent impedance to near zero by introducing a synchronously triggered mechanical switch (STMS), but its limited output time causes the incomplete charge transfer under high load resistances (>1 GΩ). In this paper, a conventional and pulsed hybrid SG-TENG (CPH-SG-TENG) is developed through rational designing STMS with tunable width and output time. The matching relationship among grid electrode width, contactor width of STMS, sliding speed, and load resistance has been studied, which provides a feasible solution for simultaneous realization of high output energy under small load resistances and high output voltage under high load resistances. The impedance matching range is extended from zero to at least 10 GΩ. The output performance of CPH-SG-TENG under low and high load resistances are demonstrated by passive power management circuit and arc discharge, respectively. The general strategy using tunable STMS combines the advantages of conventional and pulsed TENGs, which has broad application prospects in the fields of TENGs and self-powered systems.

Harvesting Inertial Energy and Powering Wearable Devices: A Review

Amidst the swift progression of microelectronics and Internet of Things technology, wearable devices are gradually gaining ground in the domains of human health monitoring. Recently, human bioenergy harvesting has emerged as a plausible alternative to batteries. This paper delves into harvesting human inertial energy that stimulates inertial masses through human motion and then transmutes the motion of the inertial masses into electrical energy. The inertial energy harvester is better suited for low-frequency and irregular human motion. This review first identifies the sources of human motion excitation that are compatible with inertial energy harvesters and then provides a summary of the operating principles and the comparisons of the commonly used energy conversion mechanisms, including electromagnetic, piezoelectric, and triboelectric transducers. The review thoroughly summarizes the latest advancements in human inertial energy-harvesting technology that are categorized and grouped based on their excitation sources and mechanical modulation methods. In addition, the review outlines the applications of inertial energy harvesters in powering wearable devices, medical health monitoring, and as mobile power sources. Finally, the challenges faced by inertial energy-harvesting technologies are discussed, and the review provides a perspective on the potential developments in the field.

A Self-Powered Flexible Displacement Sensor Based on Triboelectric Effect for Linear Feed System

The detection and feedback of displacement and velocity significantly impact the control accuracy of the linear feed system. In this study, we propose a flexible and self-powered displacement sensor based on the triboelectric effect, designed for seamless integration into linear feed systems. The displacement sensor comprises two parts, the mover and stator, operating in a sliding mode. This sensor can precisely detect the displacement of the linear feed system with a large detection range. Additionally, the sensor is capable of real-time velocity detection of linear feed systems, with an error rate below 0.5%. It also offers advantages, such as excellent flexibility, compact size, stability, easy fabrication, and seamless integration, with linear feed systems. These results highlight the potential of the self-powered displacement sensor for various applications in linear feed systems.

Standardized Volume Power Density Boost in Frequency-Up Converted Contact-Separation Mode Triboelectric Nanogenerators

The influence of a mechanical structure's volume increment on the volume power density (VPD) of triboelectric nanogenerators (TENGs) is often neglected when considering surface charge density and surface power density. This paper aims to address this gap by introducing a standardized VPD metric for a more comprehensive evaluation of TENG performance. The study specifically focuses on 2 frequency-up mechanisms, namely, the integration of planetary gears (PG-TENG) and the implementation of a double-cantilever structure (DC-TENG), to investigate their impact on VPD. The study reveals that the PG-TENG achieves the highest volume average power density, measuring at 0.92 W/m3. This value surpasses the DC-TENG by 1.26 times and the counterpart TENG by a magnitude of 69.9 times. Additionally, the PG-TENG demonstrates superior average power output. These findings introduce a new approach for enhancing TENGs by incorporating frequency-up mechanisms, and highlight the importance of VPD as a key performance metric for evaluating TENGs.

Single-Line Multi-Channel Flexible Stress Sensor Arrays

Flexible stress sensor arrays, comprising multiple flexible stress sensor units, enable accurate quantification and analysis of spatial stress distribution. Nevertheless, the current implementation of flexible stress sensor arrays faces the challenge of excessive signal wires, resulting in reduced deformability, stability, reliability, and increased costs. The primary obstacle lies in the electric amplitude modulation nature of the sensor unit's signal (e.g., resistance and capacitance), allowing only one signal per wire. To overcome this challenge, the single-line multi-channel signal (SLMC) measurement has been developed, enabling simultaneous detection of multiple sensor signals through one or two signal wires, which effectively reduces the number of signal wires, thereby enhancing stability, deformability, and reliability. This review offers a general knowledge of SLMC measurement beginning with flexible stress sensors and their piezoresistive, capacitive, piezoelectric, and triboelectric sensing mechanisms. A further discussion is given on different arraying methods and their corresponding advantages and disadvantages. Finally, this review categorizes existing SLMC measurement methods into RLC series resonant sensing, transmission line sensing, ionic conductor sensing, triboelectric sensing, piezoresistive sensing, and distributed fiber optic sensing based on their mechanisms, describes the mechanisms and characteristics of each method and summarizes the research status of SLMC measurement.

Unique Contact Point Modification Technique for Boosting the Performance of a Triboelectric Nanogenerator and Its Application in Road Safety Sensing and Detection

A triboelectric nanogenerator (TENG) is a potential technique that can convert waste kinetic energy to electrical energy by contact separation followed by electrostatic induction. Herein, a unique contact point modification technique has been reviewed carefully via the enlargement of the effective surface area of the tribo layer by using a simple and scalable printing method. In this study, the zinc sulfide (ZnS) nanostructure morphology has been introduced directly on an aluminum electrode (Al) as a tribo positive layer by a modified hydrothermal method and different line patterns directly printed on overhead projector (OHP) transparent sheets by a monochrome laser printer as a tribo negative layer to increase the effective contact area and work-function difference between two tribo layers. This dual parameter results in ∼11 times increment in the open-circuit output voltage (∼420 V) and ∼17 times increment in the short-circuit current density (∼83.33 mA m^-2) compared to the normal one. Furthermore, with the proposed surface modification technique, an ultrahigh instantaneous output power density of ∼3.9 W m^-2 at a load resistance of 2 MΩ was easily achieved. The direct energy conversion efficiency reached up to 66.67% at 2 MΩ load, which is very high compared to other traditional TENGs. Further, the fabricated TENG demonstrated efficacy in novel road safety sensing applications in hilly areas to control vehicle movement. Therefore, the current idea of surface engineering using a laser printer will be helpful for energy-harvesting enthusiasts to develop more efficient nanogenerators for higher energy conversions.

High-Precision Wearable Displacement Sensing System for Clinical Diagnosis of Anterior Cruciate Ligament Tears

An anterior cruciate ligament (ACL) tear is a common musculoskeletal injury with a high incidence. Traditional diagnosis employs magnetic response imaging (MRI), physical testing, or other clinical examination, which relies on complex and expensive medical instruments, or individual doctoral experience. Herein, we propose a wearable displacement sensing system based on a grating-structured triboelectric stretch sensor to diagnose the ACL injuries. The stretch sensor exhibits a high resolution (0.2 mm) and outstanding robustness (over 1,000,000 continuous operation cycles). This system is employed in clinical trial to diagnose ACL injuries. It measures the displacement difference between the affected leg and the healthy leg during Lachman test. And when such a difference is greater than 3 mm, the ACL is considered to be at risk for injury or tear. Compared with the gold standard of arthroscopy, the consistency rate of this wearable diagnostic system reached about 85.7%, which is higher than that of the Kneelax3 arthrometer (78.6%) with a large volume. This shows that the wearable system possesses the feasibility to supplement and improve existing arthrometers for facile diagnosing ACL injuries. It may take a promising step for wearable healthcare.

Large Harvested Energy by Self-Excited Liquid Suspension Triboelectric Nanogenerator with Optimized Charge Transportation Behavior

To enhance the durability of triboelectric nanogenerator (TENG), liquid lubrication has been used to reduce mechanical abrasion. However, as the charge transportation behavior in dielectric liquid is not clearly understood, the output energy is still low although some improvements have been reported. Herein, the charge transportation behaviors in dielectric liquid by self-excited liquid suspension triboelectric nanogenerator (LS-TENG) are systematically investigated. The important role of solid-liquid triboelectrification effect, charge-liquid transmission and dissipation effect, and the homogeneous dielectric induction effect in promoting its output performance is found. The LS-TENG with a dual dielectric tribolayer has advantages of slight driving force and long lifetime for harvesting micro energy. The output of LS-TENG remains almost constant for more than 234 k operating cycles. A high charge density of 704 µC m^-2 is obtained, 2.7 times as much as that of the current highest record in non-contact TENG. Additionally, the rotary LS-TENG lights up 4200 LEDs and continuously powers a variety of wireless sensors by harvesting wind energy at low wind speed. This work provides an important insight toward the charge transportation mechanism in dielectric liquid, and a prospective strategy for achieving highly robust TENG in micro energy harvesting for practical applications.

Asymmetric-Internal-Capacitance-Induced Charge Aggregation for the Hot-Surface Triboelectric Nanogenerator

Surface charge density (σ_SC) is essential to the output of the triboelectric nanogenerator (TENG). Massive efforts have been made to improve it, which can be totally categorized into four types. Two of them are utilized to optimize the basic electrification of the TENG, and the other two are for the device configuration and following circuits. However, the basic electrification of the 100 μm-thick film under ambient conditions still stays below 200 μC m^-2. Herein, we proposed a brand-new technical route, by designing an asymmetric-internal-capacitance configuration, which forms a "hot surface" rich in free electrons at the electrification interface and finally promotes σ_SC to 550 μC m^-2. Specifically, σ_SC of Cu is improved by 35 times, reaching 9.48 times that of nylon that is reported to be a strong positive triboelectric material. Furthermore, the hot surface improves the output of the TENG by 12.8 times and drives multiple devices floating in water to work stably, showing great potential in harvesting water wave energy (blue energy).

A Flexible Triboelectric Nanogenerator Based on Multilayer MXene/Cellulose Nanofibril Composite Film for Patterned Electroluminescence Display

The flexible self-powered display system integrating a flexible triboelectric nanogenerator (TENG) and flexible alternating current electroluminescence (ACEL) has attracted increasing attention for its promising potential in human-machine interaction applications. In this work, a performance-enhanced MXene/cellulose nanofibril (CNF)/MXene-based TENG (MCM-TENG) is reported for powering a flexible patterned ACEL device in order to realize self-powered display. The MCM multilayer composite film was self-assembled through the layer-by-layer method. The MCM film concurrently acted as a triboelectric layer and electrode layer due to its high conductivity and strength. Moreover, the effect of CNF concentration and number of layers on the output performance of TENG was investigated. It was found that the MCM-TENG realized the optimum output performance. Finally, a flexible self-powered display device was realized by integrating the flexible TENG and ACEL. The MCM-TENG with an output voltage of ≈90 V at a frequency of 2 Hz was found to be efficient enough to power the ACEL device. Therefore, the as-fabricated flexible TENG demonstrates a promising potential in terms of self-powered displays and human-machine interaction.

High Output Performance and Ultra-Durable DC Output for Triboelectric Nanogenerator Inspired by Primary Cell

Triboelectric nanogenerator (TENG) is regarded as an effective strategy to convert environment mechanical energy into electricity to meet the distributed energy demand of large number of sensors in the Internet of Things (IoTs). Although TENG based on the coupling of triboelectrification and air-breakdown achieves a large direct current (DC) output, material abrasion is a bottleneck for its applications. Here, inspired by primary cell and its DC signal output characteristics, we propose a novel primary cell structure TENG (PC-TENG) based on contact electrification and electrostatic induction, which has multiple working modes, including contact separation mode, freestanding mode and rotation mode. The PC-TENG produces DC output and operates at low surface contact force. It has an ideal effective charge density (1.02 mC m-2). Meanwhile, the PC-TENG shows a superior durability with 99% initial output after 100,000 operating cycles. Due to its excellent output performance and durability, a variety of commercial electronic devices are powered by PC-TENG via harvesting wind energy. This work offers a facile and ideal scheme for enhancing the electrical output performance of DC-TENG at low surface contact force and shows a great potential for the energy harvesting applications in IoTs.

Triboelectric Nanogenerators for Harvesting Diverse Water Kinetic Energy

The water covering the Earth’s surface not only supports life but also contains a tremendous amount of energy. Water energy is the most important and widely used renewable energy source in the environment, and the ability to extract the mechanical energy of water is of particular interest since moving water is ubiquitous and abundant, from flowing rivers to falling rain drops. In recent years, triboelectric nanogenerators (TENGs) have been promising for applications in harvesting kinetic energy from water due to their merits of low cost, light weight, simple structure, and abundant choice of materials. Furthermore, TENGs can also be utilized as self-powered active sensors for monitoring water environments, which relies on the output signals of the TENGs caused by the movement and composition of water. Here, TENGs targeting the harvest of different water energy sources have been systematically summarized and analyzed. The TENGs for harvesting different forms of water energy are introduced and divided on the basis of their basic working principles and modes, i.e., in the cases of solid–solid and solid–liquid. A detailed review of recent important progress in TENG-based water energy harvesting is presented. At last, based on recent progresses, the existing challenges and future prospects for TENG-based water energy harvesting are also discussed.

Recent Development of Morphology-Controlled Hybrid Nanomaterials for Triboelectric Nanogenerator: A Review

Being cognizant of modern electronic devices, the scientists are continuing to investigate renewable green-energy resources for a decade. Amid different energy harvesting systems, the triboelectric nanogenerators (TENGs) have been found to be the most promising mechanical harvesting technology and have drawn attention to generate electrical energy. Thanks to its instant output power, choice to opt for wide-ranging materials, low maintenance cost, easy fabrication process and environmentally friendly nature. Due to numerous working modes of TENGs, it is dedicated to desired application at ambient conditions. In this review, an advance correlation of TENGs have been explained based on the variety of nanostructures, including 0D, 1D, 2D, 3D, metal organic frameworks (MOFs), coordination polymers (CPs), covalent organic frameworks (COFs), and perovskite materials. Moreover, an overview of previous and current perspectives of various nanomaterials, synthesis, fabrication and their applications in potential fields have been discussed in detail.

Wearable devices for continuous monitoring of biosignals: Challenges and opportunities

The ability for wearable devices to collect high-fidelity biosignals continuously over weeks and months at a time has become an increasingly sought-after characteristic to provide advanced diagnostic and therapeutic capabilities. Wearable devices for this purpose face a multitude of challenges such as formfactors with long-term user acceptance and power supplies that enable continuous operation without requiring extensive user interaction. This review summarizes design considerations associated with these attributes and summarizes recent advances toward continuous operation with high-fidelity biosignal recording abilities. The review also provides insight into systematic barriers for these device archetypes and outlines most promising technological approaches to expand capabilities. We conclude with a summary of current developments of hardware and approaches for embedded artificial intelligence in this wearable device class, which is pivotal for next generation autonomous diagnostic, therapeutic, and assistive health tools.

A Tuning-Fork Triboelectric Nanogenerator with Frequency Multiplication for Efficient Mechanical Energy Harvesting

As a new technology for high-entropy energy harvesting, a triboelectric nanogenerator (TENG) has broad applications in sensor networks and internet of things as a power source, but its average power density is limited by the fixed low-frequency output. Here, a frequency-multiplication TENG based on intrinsic high frequency of tuning fork is proposed which enables converting low-frequency mechanical energy into high-frequency electric energy. A tuning-fork TENG is used to systematically study the effects of intrinsic frequency, dielectric's thickness, and gap distance on its electric performance, and a total transferred charges of 4.3 µC and an average power density of 9.42 mW m^-2 are realized at the triggering frequency of 0.2 Hz, which are 71 times and 5.7 times than that of the single-cycle output of conventional contact-separation TENG, respectively. Moreover, the crest factor also decreases from 3.5 to around 1.5. Then, a homemade tuning fork-like TENG is reasonably designed for harvesting ambient wind energy, achieving an average power density of 20.02 mW m^-2 at a wind speed of 7 m s^-1 . Specially, its impedance resistance is independent of the mechanical triggering frequency, simplifying the back-end power management circuit design. Therefore, the frequency-multiplication TENG shows a great potential for efficient distributed energy harvesting.

Improving Wastewater Treatment by Triboelectric-Photo/Electric Coupling Effect

The ability to meet higher effluent quality requirements and the reduction of energy consumption are the biggest challenges in wastewater treatment worldwide. A large proportion of the energy generated during wastewater treatment processes is neglected and lost in traditional wastewater treatment plants. As a type of energy harvesting system, triboelectric nanogenerators (TENGs) can extensively harvest the microscale energies generated from wastewater treatment procedures and auxiliary devices. This harvested energy can be utilized to improve the removal efficiency of pollutants through photo/electric catalysis, which has considerable potential application value in wastewater treatment plants. This paper gives an overall review of the generated potential energies (e.g., water wave energy, wind energy, and acoustic energy) that can be harvested at various stages of the wastewater treatment process and introduces the application of TENG devices for the collection of these neglected energies during wastewater treatment. Furthermore, the mechanisms and catalytic performances of TENGs coupled with photo/electric catalysis (e.g., electrocatalysis, photoelectric catalysis) are discussed to realize higher pollutant removal efficiencies and lower energy consumption. Then, a thorough, detailed investigation of TENG devices, electrode materials, and their coupled applications is summarized. Finally, the intimate coupling of self-powered photoelectric catalysis and biodegradation is proposed to further improve removal efficiencies in wastewater treatment. This concept is conducive to improving knowledge about the underlying mechanisms and extending applications of TENGs in wastewater treatment to better solve the problems of energy demand in the future.

A New Self-Healing Triboelectric Nanogenerator Based on Polyurethane Coating and Its Application for Self-Powered Cathodic Protection

With the increasing demand for carbon neutrality, the development of renewable and recycle green energy has attracted wide attention from researchers. A novel self-healing triboelectric nanogenerator (TENG) was constructed by applying a linear silicone-modified polyurethane (PU) coating as a triboelectric layer, which was obtained by reacting hydroxypropyl silicone oil and hexamethylene diisocyanate under the catalysis of Sn. The linear self-healing coating as the friction electrode could effectively alleviate the damages of TENG devices during long-term energy harvesting. When the triboelectric layer of the TENG device shows abrasion, the broken silicone-modified polyurethane polymer chains would gradually be cross-linked again through hydrogen bonding to achieve a self-healing effect. The entire self-healing process of the friction coating could be completed in 30 min at room temperature. The PU-based self-healing TENG exhibits an evident and stable output performance with a short-circuit current of 31.9 μA and output voltage of 517.5 V after multiple cutting-healing cycles, which could light 480 commercial LEDs. Besides, a self-powered cathodic protection system supplied by the self-healing TENG was constructed, which could transfer negative triboelectric charges to the protected metal surface to achieve an anti-corrosion effect by harvesting mechanical energy. Due to the self-healing characteristics of the TENG device as the power supply part, this intelligent system possesses great application potential in the long-term corrosion protection of multiple metal application industries, such as the marine industry.

Modeling the Triboelectric Behaviors of Elastomeric Nonwoven Fabrics

Theoretical modeling of triboelectric nanogenerators (TENGs) is fundamental to their performance optimization, since it can provide useful guidance on the material selection, structure design, and parameter control of relevant systems. Built on the theoretical model of film-based TENGs, here, an analytical model is introduced for conductor-to-dielectric contact-mode nonwoven-based TENGs, which copes with the unique hierarchical structure of nonwovens and details the correlation between the triboelectric output (maximum transferred charge density) and nonwoven structural parameters (thickness, solidity, and average fiber diameter). A series of styrene-ethylene-butylene-styrene nonwoven samples are fabricated through a melt-blowing process to map nonwoven structural features within certain ranges, while an ion-injection protocol is adopted to quantify the triboelectric output with superior consistency and reproducibility. With a database containing structural features and triboelectric output of 43 nonwoven samples, a good model fitting is achieved via nonlinear regression analysis in Python, which also shows good predictive power and suggests the existing of tribo-output maxima at a specific thickness, solidity, or average fiber diameter when other structural parameters are fixed. The model is also successfully applied to a group of polypropylene meltblown nonwovens, which verifies its universality on meltblown-nonwoven-based TENGs.

An Ultrarobust and High-Performance Rotational Hydrodynamic Triboelectric Nanogenerator Enabled by Automatic Mode Switching and Charge Excitation

The triboelectric nanogenerator (TENG) is an emerging technology for ambient mechanical energy harvesting, which provides a possibility to realize wild environment monitoring by self-powered sensing systems. However, TENGs are limited in some practical applications as a result of their low output performance (low charge density) and mechanical durability (material abrasion). Herein, an ultrarobust and high-performance rotational TENG enabled by automatic mode switching (contact mode at low speed and noncontact at high speed) and charge excitation is proposed. It displays excellent stability, maintaining 94% electrical output after 72 000 cycles, much higher than that of the normal contact-mode TENG (30%). Due to its high electrical stability and large electrical output, this TENG powers 944 green light-emitting diodes to brightness in series. Furthermore, by harvesting water-flow energy, various commercial capacitors can be charged quickly, and a self-powered fire alarm and self-powered temperature and humidity detection are realized. This work provides an ideal scheme for enhancing the mechanical durability, broadening the range of working frequency, and improving the electrical output of TENGs. In addition, the high-performance hydrodynamic TENG demonstrated in this work will have great applications for Internet of Things in remote areas.

A Stretchable and Self-Healing Hybrid Nano-Generator for Human Motion Monitoring

Transparent stretchable wearable hybrid nano-generators present great opportunities in motion sensing, motion monitoring, and human-computer interaction. Herein, we report a piezoelectric-triboelectric sport sensor (PTSS) which is composed of TENG, PENG, and a flexible transparent stretchable self-healing hydrogel electrode. The piezoelectric effect and the triboelectric effect are coupled by a contact separation mode. According to this effect, the PTSS shows a wide monitoring range. It can be used to monitor human multi-dimensional motions such as bend, twist, and rotate motions, including the screw pull motion of table tennis and the 301C skill of diving. In addition, the flexible transparent stretchable self-healing hydrogel is used as the electrode, which can meet most of the motion and sensing requirements and presents the characteristics of high flexibility, high transparency, high stretchability, and self-healing behavior. The whole sensing system can transmit signals through Bluetooth devices. The flexible, transparent, and stretchable wearable hybrid nanogenerator can be used as a wearable motion monitoring sensor, which provides a new strategy for the sports field, motion monitoring, and human-computer interaction.

Hybrid Triboelectric-Electromagnetic Nanogenerators for Mechanical Energy Harvesting: A Review

Motion-driven electromagnetic-triboelectric energy generators (E-TENGs) hold a great potential to provide higher voltages, higher currents and wider operating bandwidths than both electromagnetic and triboelectric generators standing alone. Therefore, they are promising solutions to autonomously supply a broad range of highly sophisticated devices. This paper provides a thorough review focused on major recent breakthroughs in the area of electromagnetic-triboelectric vibrational energy harvesting. A detailed analysis was conducted on various architectures including rotational, pendulum, linear, sliding, cantilever, flexible blade, multidimensional and magnetoelectric, and the following hybrid technologies. They enable highly efficient ways to harvest electric energy from many forms of vibrational, rotational, biomechanical, wave, wind and thermal sources, among others. Open-circuit voltages up to 75 V, short-circuit currents up to 60 mA and instantaneous power up to 144 mW were already achieved by these nanogenerators. Their transduction mechanisms, including proposed models to make intelligible the involved physical phenomena, are also overviewed here. A comprehensive analysis was performed to compare their respective construction designs, external excitations and electric outputs. The results highlight the potential of hybrid E-TENGs to convert unused mechanical motion into electric energy for both large- and small-scale applications. Finally, this paper proposes future research directions toward optimization of energy conversion efficiency, power management, durability and stability, packaging, energy storage, operation input, research of transduction mechanisms, quantitative standardization, system integration, miniaturization and multi-energy hybrid cells.

Triboelectric Nanogenerators for Energy Harvesting in Ocean: A Review on Application and Hybridization

With recent advancements in technology, energy storage for gadgets and sensors has become a challenging task. Among several alternatives, the triboelectric nanogenerators (TENG) have been recognized as one of the most reliable methods to cure conventional battery innovation’s inadequacies. A TENG transfers mechanical energy from the surrounding environment into power. Natural energy resources can empower TENGs to create a clean and conveyed energy network, which can finally facilitate the development of different remote gadgets. In this review paper, TENGs targeting various environmental energy resources are systematically summarized. First, a brief introduction is given to the ocean waves’ principles, as well as the conventional energy harvesting devices. Next, different TENG systems are discussed in details. Furthermore, hybridization of TENGs with other energy innovations such as solar cells, electromagnetic generators, piezoelectric nanogenerators and magnetic intensity are investigated as an efficient technique to improve their performance. Advantages and disadvantages of different TENG structures are explored. A high level overview is provided on the connection of TENGs with structural health monitoring, artificial intelligence and the path forward.

Recent Advances of Energy Solutions for Implantable Bioelectronics

The emerging field of implantable bioelectronics has attracted widespread attention in modern society because it can improve treatment outcomes, reduce healthcare costs, and lead to an improvement in the quality of life. However, their continuous operation is often limited by conventional bulky and rigid batteries with a limited lifespan, which must be surgically removed after completing their missions and/or replaced after being exhausted. Herein, this paper gives a comprehensive review of recent advances in nonconventional energy solutions for implantable bioelectronics, emphasizing the miniaturized, flexible, biocompatible, and biodegradable power devices. According to their source of energy, the promising alternative energy solutions are sorted into three main categories, including energy storage devices (batteries and supercapacitors), internal energy-harvesting devices (including biofuel cells, piezoelectric/triboelectric energy harvesters, thermoelectric and biopotential power generators), and external wireless power transmission technologies (including inductive coupling/radiofrequency, ultrasound-induced, and photovoltaic devices). Their fundamentals, materials strategies, structural design, output performances, animal experiments, and typical biomedical applications are also discussed. It is expected to offer complementary power sources to extend the battery lifetime of bioelectronics while acting as an independent power supply. Thereafter, the existing challenges and perspectives associated with these powering devices are also outlined, with a focus on implantable bioelectronics.

High performance floating self-excited sliding triboelectric nanogenerator for micro mechanical energy harvesting

Non-contact triboelectric nanogenerator (TENG) enabled for both high conversion efficiency and durability is appropriate to harvest random micro energy owing to the advantage of low driving force. However, the low output (<10 μC m^-2) of non-contact TENG caused by the drastic charge decay limits its application. Here, we propose a floating self-excited sliding TENG (FSS-TENG) by a self-excited amplification between rotator and stator to achieve self-increased charge density, and the air breakdown model of non-contact TENG is given for a maximum charge density. The charge density up to 71.53 μC m^-2 is achieved, 5.46 times as that of the traditional floating TENG. Besides, the high output enables it to continuously power small electronics at 3 m s^-1 weak wind. This work provides an effective strategy to address the low output of floating sliding TENG, and can be easily adapted to capture the varied micro mechanical energies anywhere.

Structural and Chemical Modifications Towards High-Performance of Triboelectric Nanogenerators

Harvesting abundant mechanical energy has been considered one of the promising technologies for developing autonomous self-powered active sensors, power units, and Internet-of-Things devices. Among various energy harvesting technologies, the triboelectric harvesters based on contact electrification have recently attracted much attention because of their advantages such as high performance, light weight, and simple design. Since the first triboelectric energy-harvesting device was reported, the continuous investigations for improving the output power have been carried out. This review article covers various methods proposed for the performance enhancement of triboelectric nanogenerators (TENGs), such as a triboelectric material selection, surface modification through the introduction of micro-/nano-patterns, and surface chemical functionalization, injecting charges, and their trapping. The main purpose of this work is to highlight and summarize recent advancements towards enhancing the TENG technology performance through implementing different approaches along with their potential applications. This paper presents a comprehensive review of the TENG technology and its factors affecting the output power as material selection, surface physical and chemical modification, charge injection, and trapping techniques.

Tribo-charge enhanced hybrid air filter masks for efficient particulate matter capture with greatly extended service life

Face masks have been an effective and indispensable personal protective measure against particulate matter pollutants and respiratory diseases, especially the novel Coronavirus disease recently. However, disposable surgical face masks suffer from low filtration efficiency for particles ranging from nano- to micro-size, and the limited service life of ~ 4 h. Here, a nano/micro fibrous hybrid air filter mask composing of electrospun nanofibrous network and poly(3,4-ethylenedioxythiophene:poly(styrenesulfonate) coated polypropylene (PP) is proposed. Furthermore, the resultant filter is supplied with tribo-charges by a freestanding sliding triboelectric nanogenerator. Through the enhanced synergistic effect of mechanical interception and electrostatic forces, the hybrid air filter demonstrates high filtration efficiency for particle size of 11.5 nm to 2.5 µm, with a 9.3-34.68% enhancement for particles of 0.3-2.5 µm compared to pristine PP, and 48-h stable filtration efficiency of 94% (0.3-0.4 µm) and 99% (1-2.5 µm) with a low pressure drop of ~110 Pa. In addition, sterilization ability of the tribo-charge enhanced air filter is demonstrated. This work provides a facile and cost-effective approach for state-of-the-art face masks toward high filtration performance of nano- to micro- particles with greatly extended service life.

Triboelectric Nanogenerators for Therapeutic Electrical Stimulation

Current solutions developed for the purpose of in and on body (IOB) electrical stimulation (ES) lack autonomous qualities necessary for comfortable, practical, and self-dependent use. Consequently, recent focus has been placed on developing self-powered IOB therapeutic devices capable of generating therapeutic ES for human use. With the recent invention of the triboelectric nanogenerator (TENG), harnessing passive human biomechanical energy to develop self-powered systems has allowed for the introduction of novel therapeutic ES solutions. TENGs are especially effective at providing ES for IOB therapeutic systems given their bioconformability, low cost, simple manufacturability, and self-powering capabilities. Due to the key role of naturally induced electrical signals in many physiological functions, TENG-induced ES holds promise to provide a novel paradigm in therapeutic interventions. The aim here is to detail research on IOB TENG devices applied for ES-based therapy in the fields of regenerative medicine, neurology, rehabilitation, and pharmaceutical engineering. Furthermore, considering TENG-produced ES can be measured for sensing applications, this technology is paving the way to provide a fully autonomous personalized healthcare system, capable of IOB energy generation, sensing, and therapeutic intervention. Considering these grounds, it seems highly relevant to review TENG-ES research and applications, as they could constitute the foundation and future of personalized healthcare.

A composite triboelectric nanogenerator based on flexible and transparent film impregnated with ZIF-8 nanocrystals

The triboelectric nanogenerator (TENG), based on the triboelectrification coupled with electrostatic induction, can directly convert ambient mechanical energy into electric energy. However, the output performance of TENG is still low and demands further improvement to speed up the commercial application. In this work, we demonstrate a TENG based on a flexible and transparent composite film made of PDMS and ZIF-8. When the amount of the ZIF-8 is 4 wt%, the generated output current and voltage of the TENG are gradually increased up to 16.3μA and 176 V, which are 210% and 230% higher than that of TENG without ZIF-8, respectively. Impregnated ZIF-8 which exhibits a positive polarity lowers the effective work function of the PDMS and enhance the surface charge density, verified by Kelvin probe force microscope measurement.

Moderately Transparent Chitosan-PVA Blended Membrane for Strong Mechanical Stiffness and as a Robust Bio-Material Energy Harvester Through Contact-Separation Mode TENG

The detection of sustainable materials from naturally available resources using a simple fabrication process is highly important for novel research. Here, we used chitosan-PVA (Chs-PVA) blend films via layer-by-layer casting technologies for generating power through mechanical induction through triboelectric nanogenerators. The proposed Chs-PVA biodegradable film (i.e., thickness of 60 ± 5 μm) is facile, ecofriendly, highly flexible, mechanically strong, cost-effective, and easy to scale up. FT-IR analysis of the ChS-PVA blend membrane showed the strong interactions between the amines of ChS and hydroxyl groups of PVA through chemical cross-linking by hydrogen bonding. More importantly, the triboelectric nanogenerators (TENG) values of ChS-PVA films were 3–4 orders of magnitude lower than chitosan films reported before. Layer-on-layer cast films in particular exhibited high tensile strength (15.8 ± 1 MPa) and were more than three times stronger than other polyelectrolyte multilayer films. Both types of films remained stable in an acidic environment. Furthermore, the layer-on-layer-assembled films presented greater open circuit voltage (Voc) and short circuit current (Isc) values compared to pure ChS and PVA films. The ChS-PVA membrane can be used as a functional layer to produce charges by collecting get-up-and-go through vertical contact and separation mode TENG counters to the PVDF membrane. The enhancement of Voc and Isc of ChS-PVA TENG were 244 and 1,080% from ChS TENG. Where in the case of PVA TENG, the enhancement of Voc and Isc were increased by 633 and 2,888%, respectively due to the availability of free loan pair on the -NH2 and -OH functional groups. The novel ChS-PVA TENG is a potential candidate for satisfying the tight requirement of an optimized energy harvesting device as an alternate bio-material option for contact-separation mode TENGs.

Comparison of applied torque and energy conversion efficiency between rotational triboelectric nanogenerator and electromagnetic generator

Triboelectric nanogenerator (TENG) is regarded as an equally important mechanical energy harvesting technology as electromagnetic generator (EMG). Here, the input mechanical torques and energy conversion efficiencies of the rotating EMG and TENG are systematically measured, respectively. At constant rotation rates, the input mechanical torque of EMG is balanced by the friction resisting torque and electromagnetic resisting torque, which increases with the increasing rotation rate due to Ampere force. While the input mechanical torque of TENG is balanced by the friction resisting torque and electrostatic resisting torque, which is nearly constant at different rotation rates. The energy conversion efficiency of EMG increases with the increasing input mechanical power, while that of the TENG remains nearly constant. Compared with the EMG, the TENG has a higher conversion efficiency at a low input mechanical power, which demonstrates a remarkable merit of the TENG for efficiently harvesting weak ambient mechanical energy.

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The applied torque of the rotating EMG and TENG are systematically measured

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The energy conversion efficiencies of both generators are quantified and compared

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This work has demonstrated a remarkable merit of the TENG under a gentle-triggering

A self-sustainable wearable multi-modular E-textile bioenergy microgrid system

Despite the fast development of various energy harvesting and storage devices, their judicious integration into efficient, autonomous, and sustainable wearable systems has not been widely explored. Here, we introduce the concept and design principles of e-textile microgrids by demonstrating a multi-module bioenergy microgrid system. Unlike earlier hybrid wearable systems, the presented e-textile microgrid relies solely on human activity to work synergistically, harvesting biochemical and biomechanical energy using sweat-based biofuel cells and triboelectric generators, and regulating the harvested energy via supercapacitors for high-power output. Through energy budgeting, the e-textile system can efficiently power liquid crystal displays continuously or a sweat sensor-electrochromic display system in pulsed sessions, with half the booting time and triple the runtime in a 10-min exercise session. Implementing "compatible form factors, commensurate performance, and complementary functionality" design principles, the flexible, textile-based bioenergy microgrid offers attractive prospects for the design and operation of efficient, sustainable, and autonomous wearable systems.

A Self-Powered Vector Angle/Displacement Sensor Based on Triboelectric Nanogenerator

Recently, grating-structured triboelectric nanogenerators (TENG) operating in freestanding mode have been the subject of intensive research. However, standard TENGs based on interdigital electrode structures are unable to realize real-time sensing of the direction of the freestanding electrode movement. Here, a newly designed TENG, consisting of one group of grating freestanding electrodes and three groups of interdigitated induction electrodes with the identical period, has been demonstrated as a self-powered vector angle/displacement sensor (SPVS), capable of distinguishing the real-time direction of the freestanding electrode displacement. Thanks to the unique coupling effect between triboelectrification and electrostatic induction, periodic alternating voltage signals are generated in response to the rotation/sliding movement of the top freestanding electrodes on the bottom electrodes. The output peak-to-peak voltage of the SPVS can reach as high as 300 V at the rotation rate of 48 rpm and at the sliding velocity of 0.1 m/s, respectively. The resolution of the sensor reaches 8°/5 mm and can be further enhanced by decreasing the width of the electrodes. This present work not only demonstrates a novel method for angle/displacement detection but also greatly expands the applicability of TENG as self-powered vector sensors.

Triboelectric Nanogenerator: Structure, Mechanism, and Applications

With the rapid development of the Internet of Things (IoT), the number of sensors utilized for the IoT is expected to exceed 200 billion by 2025. Thus, sustainable energy supplies without the recharging and replacement of the charge storage device have become increasingly important. Among various energy harvesters, the triboelectric nanogenerator (TENG) has attracted considerable attention due to its high instantaneous output power, broad selection of available materials, eco-friendly and inexpensive fabrication process, and various working modes customized for target applications. The TENG harvests electrical energy from wasted mechanical energy in the ambient environment. Three types of operational modes based on contact-separation, sliding, and freestanding are reviewed for two different configurations with a double-electrode and a single-electrode structure in the TENGs. Various charge transfer mechanisms to explain the operational principles of TENGs during triboelectrification are also reviewed for electron, ion, and material transfers. Thereafter, diverse methodologies to enhance the output power considering the energy harvesting efficiency and energy transferring efficiency are surveyed. Moreover, approaches involving not only energy harvesting by a TENG but also energy storage by a charge storage device are also reviewed. Finally, a variety of applications with TENGs are introduced. This review can help to advance TENGs for use in self-powered sensors, energy harvesters, and other systems. It can also contribute to assisting with more comprehensive and rational designs of TENGs for various applications.

Advances in triboelectric nanogenerators for biomedical sensing

Biomedical sensors have been essential in improving healthcare outcomes over the past 30 years, though limited power source access and user wearability restraints have prevented them from taking a constant and active biomedical sensing role in our daily lives. Triboelectric nanogenerators (TENGs) have demonstrated exceptional capabilities and versatility in delivering self-powered and wear-optimized biomedical sensors, and are paving the way for a novel platform technology able to fully integrate into the developing 5G/Internet-of-Things ecosystem. This novel paradigm of TENG-based biomedical sensors aspires to provide ubiquitous and omnipresent real-time biomedical sensing for us all. In this review, we cover the remarkable developments in TENG-based biomedical sensing which have arisen in the last octennium, focusing on both in-body and on-body biomedical sensing solutions. We begin by covering TENG as biomedical sensors in the most relevant, mortality-associated clinical fields of pneumology and cardiology, as well as other organ-related biomedical sensing abilities including ambulation. We also include an overview of ambient biomedical sensing as a field of growing interest in occupational health monitoring. Finally, we explore TENGs as power sources for third party biomedical sensors in a number of fields, and conclude our review by focusing on the future perspectives of TENG biomedical sensors, highlighting key areas of attention to fully translate TENG-based biomedical sensors into clinically and commercially viable digital and wireless consumer and health products.

A Review and Perspective for the Development of Triboelectric Nanogenerator (TENG)-Based Self-Powered Neuroprosthetics

Neuroprosthetics have become a powerful toolkit for clinical interventions of various diseases that affect the central nervous or peripheral nervous systems, such as deep brain stimulation (DBS), functional electrical stimulation (FES), and vagus nerve stimulation (VNS), by electrically stimulating different neuronal structures. To prolong the lifetime of implanted devices, researchers have developed power sources with different approaches. Among them, the triboelectric nanogenerator (TENG) is the only one to achieve direct nerve stimulations, showing great potential in the realization of a self-powered neuroprosthetic system in the future. In this review, the current development and progress of the TENG-based stimulation of various kinds of nervous systems are systematically summarized. Then, based on the requirements of the neuroprosthetic system in a real application and the development of current techniques, a perspective of a more sophisticated neuroprosthetic system is proposed, which includes components of a thin-film TENG device with a biocompatible package, an amplification circuit to enhance the output, and a self-powered high-frequency switch to generate high-frequency current pulses for nerve stimulations. Then, we review and evaluate the recent development and progress of each part.

Respiration-driven triboelectric nanogenerators for biomedical applications

As a fundamental and ubiquitous body motion, respiration offers a large amount of biomechanical energy with an average power up to the Watt level through movements of multiple muscles. The energy from respiration featured with excellent stability, accessibility and continuality inspires the design and engineering of biomechanical energy harvesting devices, such as triboelectric nanogenerators (TENGs), to realize human-powered electronics. This review article is thus dedicated to the emerging respiration-driven TENG technology, covering fundamentals, applications, and perspectives. Specifically, the human breathing mechanics are first introduced serving as the base for the developments of TENG devices with different configurations. Biomedical applications including electrical energy generation, healthcare monitoring, air filtration, gas sensing, electrostimulation, and powering implantable medical devices are then analyzed focusing on the design-application relationships. At last, current developments are summarized and critical challenges for driving these intriguing developments toward practical applications are discussed together with promising solutions.

Dripping Channel Based Liquid Triboelectric Nanogenerators for Energy Harvesting and Sensing

Dripping liquid is one of the most common practices in chemistry, but one rarely thinks that the contact of liquid with air could introduce charges in the liquid, which may affect the chemical reaction. Here, we propose a functional dripping channel based on a liquid triboelectric nanogenerator (L-TENG) to effectively harvest energy from liquid droplets and sense their motion. The L-TENG is a hybrid of a grating-electrode L-TENG and a single-electrode L-TENG, which are for energy harvesting and sensing, respectively. When dripping from a funnel, the energy of the droplets can be successively harvested and stored. Meanwhile, the single-electrode L-TENG can identify the time interval for the liquid flow and the number of droplets, providing information on the chemical process. The device with enhanced energy harvesting and sensing functions should have great application prospects in intelligent laboratory systems and can also contribute to the optimization of the L-TENGs for harvesting liquid droplet energy.

Ternary Electrification Layered Architecture for High-Performance Triboelectric Nanogenerators

The triboelectric nanogenerator (TENG) has been proved to be a green and efficient energy harnessing technology for electricity generation from ambient mechanical motions based on its ability to leverage the triboelectrification process. Enhancing TENG output performance through rational structural design still triggers increasing research interest. Here, we report a ternary electrification layered architecture beyond the current binary TENG systems, with improved performance for mechanical energy harvesting. Introducing a ternary Kapton layer into the traditional binary electrification layered architecture of TENGs consisting of copper and fluorinated ethylene propylene, yields a 2.5 times enhancement of peak power output, representing a 6.29-fold increase compared to the TENG composed of copper and Kapton. A wide-range of material configurations were systematically tested using this ternary electrification layered architecture to prove its practical effectiveness. The ternary electrification layered architecture invented in this work provides an alternative strategy to enhance TENG output performance, which represents a solid step for TENGs application in high-performance mechanical energy harvesting.