Acoustically Enhanced Triboelectric Stethoscope for Ultrasensitive Cardiac Sounds Sensing and Disease Diagnosis.

Electronic stethoscope used to detect cardiac sounds that contain essential clinical information is a primary tool for diagnosis of various cardiac disorders. However, the linear electromechanical constitutive relation makes conventional piezoelectric sensors rather ineffective to detect low-intensity, low-frequency heart acoustic signal without the assistance of complex filtering and amplification circuits. Herein, it is found that triboelectric sensor features superior advantages over piezoelectric one for microquantity sensing originated from the fast saturated constitutive characteristic. As a result, the triboelectric sensor shows ultrahigh sensitivity (1215 mV Pa-1) than the piezoelectric counterpart (21 mV Pa-1) in the sound pressure range of 50-80 dB under the same testing condition. By designing a trumpet-shaped auscultatory cavity with a power function cross-section to achieve acoustic energy converging and impedance matching, triboelectric stethoscope delivers 36 dB signal-to-noise ratio for human test (2.3 times of that for piezoelectric one). Further combining with machine learning, five cardiac states can be diagnosed at 97% accuracy. In general, the triboelectric sensor is distinctly unique in basic mechanism, provides a novel design concept for sensing micromechanical quantities, and presents significant potential for application in cardiac sounds sensing and disease diagnosis.

Achieving Self-Reinforcing Triboelectric-Electromagnetic Hybrid Nanogenerator by Magnetocaloric and Magnetization Effects of Gadolinium.

Triboelectric-electromagnetic hybrid nanogenerator (TEHG) has emerged as a promising technology for distributed energy harvesting. However, currently reported hybrid generators are straightforward combinations of two functional components. Moreover, inevitable heat from friction intensifies material abrasion and degrades the performance of polymer-based triboelectric nanogenerators (TENGs). Here, a self-reinforcing TEHG (SR-TEHG) that harnesses the magnetocaloric and magnetization effects of gadolinium (Gd), is proposed. The synergy between TENG and electromagnetic generator (EMG) renders them an indivisible unit. Leveraging Gd's magnetocaloric effect, an efficient heat transfer mechanism is constructed to cool the tribolayer and strengthen the device's electrical stability. After 80 h of continuous operation, the optimized TENG occupies a charge decay rate of only 0.32% per hour, significantly outperforming most existing TENGs. Additionally, Gd's magnetization effect boosts the power of EMG by ≈80.84%. This work provides a universal solution in hybrid generators where internal components reinforce each other, achieving a synergistic effect of 1 + 1 > 2.

Spontaneous Wetting Induced by Contact-Electrification at Liquid-Solid Interface.

Wettability significantly influences various surface interactions and applications at the liquid-solid interface. However, the understanding is complicated by the intricate charge exchange occurring through contact electrification (CE) during this process. The understanding of the influence of triboelectric charge on wettability remains challenging, especially due to the complexities involved in concurrently measuring contact angles and interfacial electrical signals. Here, the relationship is investigated between surface charge density and change of contact angle of dielectric films after contact with water droplets. It is observed that the charge exchange when water spared lead to a spontaneous wetting phenomenon, which is termed as the contact electrification induced wetting (CEW). Notably, these results demonstrate a linear dependence between the change of contact angle (CA) of the materials and the density of surface charge on the solid surface. Continuous CEW tests show that not only the static CA but also the dynamics of wetting are influenced by the accumulation charges at the interface. The mechanism behind CEW involves the redistribution of surface charges on a solid surface and polar water molecules within liquid. This interaction results in a decrease in interface energy, leading to a reduction in the CA. Ab initio calculations suggest that the reduction in interface energy may stem from the enhanced surface charge on the substrate, which strengthens the hydrogen bond interaction between water and the substrate. These findings have the potential to advance the understanding of CE and wetting phenomena, with applications in energy harvesting, catalysis, and droplet manipulation at liquid-solid interfaces.

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.

Mechano-Driven Tribo-Electrophoresis Enabled Human-Droplet Interaction in 3D Space.

Electronically controlled droplet manipulation has widespread applications in biochemistry, life sciences, and industry. However, current technologies such as electrowetting, electrostatics, and surface charge printing rely on complex electrode arrays and external power supplies, leading to inefficient manipulation. In light of these limitations, a novel method is proposed, which leverages tribo-electrophoresis (TEP) to pipette in an oil medium, thereby enabling human-droplet interactions to be constructed with greater efficiency. The approach involves the rational design of a triboelectric nanogenerator-electrostatic tweezer that generates an electric field to charge the droplet and improves the maneuverability of the charged droplet, including aligned/non-aligned pipetting and stable transport in the clamped state, which can be accomplished solely by hand motion. The TEP method not only provides droplets with freedom to move in three dimensions but also offers a feasibility case for chemical reactions in the liquid phase and non-invasive sample extraction. This breakthrough establishes a cornerstone for human-droplet interactions capitalized on triboelectric nanogenerators, opening new avenues for research in droplet manipulation.

Structure-Crack Detection and Digital Twin Demonstration Based on Triboelectric Nanogenerator for Intelligent Maintenance.

The accomplishment of condition monitoring and intelligent maintenance for cantilever structure-based energy harvesting devices remains a challenge. Here, to tackle the problems, a novel cantilever-structure freestanding triboelectric nanogenerator (CSF-TENG) is proposed, which can capture ambient energy or transmit sensory information. First, with and without a crack in cantilevers, the simulations are carried out. According to simulation results, the maximum change ratios of natural frequency and amplitude are 1.1% and 2.2%, causing difficulties in identifying defects by these variations. Thus, based on Gramian angular field and convolutional neural network, a defect detection model is established to achieve the condition monitoring of the CSF-TENG, and the experimental result manifests that the accuracy of the model is 99.2%. Besides, the relation between the deflection of cantilevers and the output voltages of the CSF-TENG is first built, and then the defect identification digital twin system is successfully created. Consequently, the system is capable of duplicating the operation of the CSF-TENG in a real environment, and displaying defect recognition results, so the intelligent maintenance of the CSF-TENG can be realized.

A Magnetic-Multiplier-Enabled Hybrid Generator with Frequency Division Operation and High Energy Utilization Efficiency.

The hybrid electromagnetic-triboelectric generator (HETG) is a prevalent device for mechanical energy harvesting. However, the energy utilization efficiency of the electromagnetic generator (EMG) is inferior to that of the triboelectric nanogenerator (TENG) at low driving frequencies, which limits the overall efficacy of the HETG. To tackle this issue, a layered hybrid generator consisting of a rotating disk TENG, a magnetic multiplier, and a coil panel is proposed. The magnetic multiplier not only forms the EMG part with its high-speed rotor and the coil panel but also facilitates the EMG to operate at a higher frequency than the TENG through frequency division operation. The systematic parameter optimization of the hybrid generator reveals that the energy utilization efficiency of EMG can be elevated to that of rotating disk TENG. Incorporating a power management circuit, the HETG assumes the responsibility for monitoring the water quality and fishing conditions by collecting low-frequency mechanical energy. The magnetic- multiplier-enabled hybrid generator demonstrated in this work offers a universal frequency division approach to improve the overall outputs of any hybrid generator that collects rotational energy, expanding its practical applications in diverse multifunctional self-powered systems.

Solid-Liquid Triboelectric Nanogenerator Based on Vortex-Induced Resonance.

Energy converters based on vortex-induced vibrations (VIV) have shown great potential for harvesting energy from low-velocity flows, which constitute a significant portion of ocean energy. However, solid-solid triboelectric nanogenerators (TENG) are not wear-resistant in corrosive environments. Therefore, to effectively harvest ocean energy over the long term, a novel solid-liquid triboelectric nanogenerator based on vortex-induced resonance (VIV-SL-TENG) is presented. The energy is harvested through the resonance between VIV of a cylinder and the relative motions of solid-liquid friction pairs inside the cylinder. The factors that affect the output performance of the system, including the liquid mass ratio and the deflection angle of the friction plates, are studied and optimized by establishing mathematical models and conducting computational fluid dynamics simulations. Furthermore, an experimental platform for the VIV-SL-TENG system is constructed to test and validate the performance of the harvester under different conditions. The experiments demonstrate that the energy harvester can successfully convert VIV energy into electrical energy and reach maximum output voltage in the resonance state. As a new type of energy harvester, the presented design shows a promising potential in the field of 'blue energy' harvesting.

An Advanced Strategy to Enhance TENG Output: Reducing Triboelectric Charge Decay.

The Internet of Things (IoT) is poised to accelerate the construction of smart cities. However, it requires more than 30 billion sensors to realize the IoT vision, posing great challenges and opportunities for industries of self-powered sensors. Triboelectric nanogenerator (TENG), an emerging new technology, is capable of easily converting energy from surrounding environment into electricity, thus TENG has tremendous application potential in self-powered IoT sensors. At present, TENG encounters a bottleneck to boost output for large-scale commercial use if just by promoting triboelectric charge generation, because the output is decided by the triboelectric charge dynamic equilibrium between generation and decay. To break this bottleneck, the strategy of reducing triboelectric charge decay to enhance TENG output is focused. First, multiple mechanisms of triboelectric charge decay are summarized in detail with basic theoretical principles for future research. Furthermore, recent advances in reducing triboelectric charge decay are thoroughly reviewed and outlined in three aspects: inhibition and application of air breakdown, simultaneous inhibition of air breakdown and triboelectric charge drift/diffusion, and inhibition of triboelectric charge drift/diffusion. Finally, challenges and future research focus are proposed. This review provides reference and guidance for enhancing TENG output.

Role of MoOx Surficial Modification in Enhancing the OER Performance of Ru-Pyrochlore.

Pyrochlore ruthenate (Y2 Ru2 O7-δ ) is highlighted as a promising oxygen evolution reaction (OER) catalyst for water splitting in polymer electrolyte membrane electrolyzers. However, an efficient electronic modulation strategy for Y2 Ru2 O7-δ is required to overcome its electrochemical inertness. Herein, a surface manipulation strategy involving implanting MoOx moieties on nano Y2 Ru2 O7-δ (Mo-YRO) using wet chemical peroxone method is demonstrated. In contrast to electronic structure regulation by intramolecular charge transfer (i.e., substitutional strategies), the heterogeneous Mo-O-Ru micro-interfaces facilitate efficient intermolecular electron transfer from [RuO6 ] to MoOx . This eliminates the bandgap by inducing Ru 4d delocalization and band alignment rearrangement. The MoOx modifiers also alleviate distortion of [RuO6 ] by shortening Ru-O bond and enlarging Ru-O-Ru bond angle. This electronic and geometric structure tailoring enhances the OER performance, showing a small overpotential of 240 mV at 10 mA cm-2 . Moreover, the electron-accepting MoOx moieties provide more electronegative surfaces, which serve as a protective "fence" to inhibit the dissolution of metal ions, thereby stabilizing the electrochemical activity. This study offers fresh insights into the design of new-based pyrochlore electrocatalysts, and also highlights the versatility of surface engineering as a way of optimizing electronic structure and catalytic performance of other related materials.

A Self-Powered, Highly Embedded and Sensitive Tribo-Label-Sensor for the Fast and Stable Label Printer.

Label-sensor is an essential component of the label printer which is becoming a most significant tool for the development of Internet of Things (IoT). However, some drawbacks of the traditional infrared label-sensor make the printer fail to realize the high-speed recognition of labels as well as stable printing. Herein, we propose a self-powered and highly sensitive tribo-label-sensor (TLS) for accurate label identification, positioning and counting by embedding triboelectric nanogenerator into the indispensable roller structure of a label printer. The sensing mechanism, device parameters and deep comparison with infrared sensor are systematically studied both in theory and experiment. As the results, TLS delivers 6 times higher signal magnitude than traditional one. Moreover, TLS is immune to label jitter and temperature variation during fast printing and can also be used for transparent label directly and shows long-term robustness. This work may provide an alternative toolkit with outstanding advantages to improve current label printer and further promote the development of IoT.

A cantilever-structure triboelectric nanogenerator for energy harvesting and defect detection via long short-term memory network.

The electric signals of cantilever energy harvesting devices with/without a crack were mainly obtained by external sensors, so detecting device damage on a large scale is difficult. To tackle the issue, a cantilever-structure freestanding triboelectric nanogenerator (CSF-TENG) device was proposed, which can scavenge ambient energy and act as a self-powered sensor. Firstly, the relation between the peak-to-peak voltage and amplitude of the CSF-TENG was established. Next, the output performance of the CSF-TENG was measured. Then, depending on electric signals output by the CSF-TENG, a cantilever defect identification model was built by using the wavelet packet and long short-term memory (LSTM) algorithms. The experimental results manifest that the accuracy of the model is about 98.6%. Thus, the CSF-TENG with a crack can be detected timely due to its self-monitoring ability, which is of great significance for the development of self-powered sensor networks.

A Mutual Boosting Self-Excitation Hybrid Cell for Harvesting High Entropy Energy at 32% Efficiency.

Triboelectric nanogenerators (TENGs) and dielectric elastomer generators (DEGs) are potentially promising energy conversion technologies, but they still have limitations due to their own intrinsic characteristics, including the low energy output of TENGs caused by the air breakdown effect, and external polarization voltage requirement for DEGs, which severely limit their practical applications. Herein, coupling TENG with DEG is proposed to build a mutual beneficial self-excitation hybrid generator (named TDHG) for harvesting distributed and low-quality mechanical energy (high entropy energy). Experimental results demonstrate that the output charges of this TDHG are enhanced by fivefold of that of the conventional charge-excitation TENG, and continuous operation of DEG is also realized by simple mechanical triggering. More importantly, owing to the high peak power contributed by TENG and the long output pulse duration guaranteed by DEG, the TDHG realizes a much higher energy conversion efficiency of 32% in comparison to either the TENG (3.6%) or DEG (13.2%). This work proposes a new design concept for hybridized energy harvester toward highly efficient mechanical energy harvesting.

Editorial for Special Issue: Advanced Materials and Technologies in Nanogenerators.

Nanogenerators, based on Maxwell's displacement current as the driving force, have inspired a new and developing field since their invention in 2006 [...].

Ultrahigh Performance Triboelectric Nanogenerator Enabled by Charge Transmission in Interfacial Lubrication and Potential Decentralization Design.

Triboelectric nanogenerator (TENG) is a promising strategy for harvesting low frequency mechanical energy. However, the bottlenecks of limited electric output by air/dielectric breakdown and poor durability by material abrasion seriously restrict its further improvement. Herein, we propose a liquid lubrication promoted sliding mode TENG to address both issues. Liquid lubrication greatly reduces interface material abrasion, and its high breakdown strength and charge transmission effect further enhance device charge density. Besides, the potential decentralization design by the voltage balance bar effectively suppresses the dielectric breakdown. In this way, the average power density up to 87.26 W·m-2·Hz-1, energy conversion efficiency of 48%, and retention output of 90% after 500,000 operation cycles are achieved, which is the highest average power density and durability currently. Finally, a cell phone is charged to turn on by a palm-sized TENG device at 2 Hz within 25 s. This work has a significance for the commercialization of TENG-based self-powered systems.

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.

Interface Defect Detection and Identification of Triboelectric Nanogenerators via Voltage Waveforms and Artificial Neural Network.

To provide a robust working environment for TENGs, most TENGs are designed as sealed structures that isolate TENGs from the external environment, and thus their operating conditions cannot be directly monitored. Here, for the first time, we propose an artificial neural network for interface defect detection and identification of triboelectric nanogenerators via training voltage waveforms. First, interface defects of TENGs are classified and their causes are discussed in detail. Then we build a lightweight artificial neural network model which shows high sensitivity to voltage waveforms and low time complexity. The model takes 2.1 s for training one epoch, and the recognition rate of defect detection is 98.9% after 100 epochs. Meanwhile, the model successfully demonstrates the learning ability for low-resolution samples (100 × 75 pixels), which can identify six types of TENG defects, such as edge fracture, adhesion, and abnormal vibration, with a high recognition rate of 93.6%. This work provides a new strategy for the fault diagnosis and intelligent application of TENGs.

Achieving Remarkable Charge Density via Self-Polarization of Polar High-k Material in a Charge-Excitation Triboelectric Nanogenerator.

Boosting output charge density is top priority for achieving high-performance triboelectric nanogenerators (TENGs). The charge-excitation strategy is demonstrated to be a superior approach to acquire high output charge density. Meanwhile, the molecular charge behaviors in the dielectric under a strong electric field from high charge density bring new physics that are worth exploring. Here, a rapid self-polarization effect of a polar dielectric material by the superhigh electric field in a charge-excitation TENG is reported, by which the permittivity of the polar dielectric material realizes self-increase to a saturation, and thus enhances the output charge density. Consequently, an ultrahigh charge density of 3.53 mC m-2 is obtained with 7 µm homemade lead zirconate titanate-poly(vinylidene fluoride) composite film in the atmosphere with 5% relative humidity, which is the highest charge density for TENGs with high durability currently. This work provides new guidance for dielectric material optimization under charge excitation to boost the output performance of TENGs toward practical applications.

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.

A Mobile and Self-Powered Micro-Flow Pump Based on Triboelectricity Driven Electroosmosis.

Electroosmotic pumps have been widely used in microfluidic systems. However, traditional high-voltage (HV)-sources are bulky in size and induce numerous accessional reactions, which largely reduce the system's portability and efficiency. Herein, a motion-controlled, highly efficient micro-flow pump based on triboelectricity driven electroosmosis is reported. Utilizing the triboelectric nanogenerator (TENG), a strong electric field can be formed between two electrodes in the microfluidic channel with an electric double layer, thus driving the controllable electroosmotic flow by biomechanical movements. The performance and operation mechanism of this triboelectric electroosmotic pump (TEOP) is systematically studied and analyzed using a basic free-standing mode TENG. The TEOP produces ≈600 nL min-1 micro-flow with a Joule heat down to 1.76 J cm-3 nL-1 compared with ≈50 nL min-1 and 8.12 J cm-3 nL-1 for an HV-source. The advantages of economy, efficiency, portability, and safety render the TEOP a more conducive option to achieve wider applications in motion-activated micro/nanofluidic transportation and manipulation.