Flexible sensors with zero Poisson's ratio.

Flexible sensors have been developed for the perception of various stimuli. However, complex deformation, usually resulting from forces or strains from multi-axes, can be challenging to measure due to the lack of independent perception of multiaxial stimuli. Herein, flexible sensors based on the metamaterial membrane with zero Poisson's ratio (ZPR) are proposed to achieve independent detection of biaxial stimuli. By deliberately designing the geometric dimensions and arrangement parameters of elements, the Poisson's ratio of an elastomer membrane can be modulated from negative to positive, and the ZPR membrane can maintain a constant transverse dimension under longitudinal stimuli. Due to the accurate monitoring of grasping force by ZPR sensors that are insensitive to curvatures of contact surfaces, rigid robotic manipulators can be guided to safely grasp deformable objects. Meanwhile, the ZPR sensor can also precisely distinguish different states of manipulators. When ZPR sensors are attached to a thermal-actuation soft robot, they can accurately detect the moving distance and direction. This work presents a new strategy for independent biaxial stimuli perception through the design of mechanical metamaterials, and may inspire the future development of advanced flexible sensors for healthcare, human-machine interfaces and robotic tactile sensing.

A Flexible Tribotronic Artificial Synapse with Bioinspired Neurosensory Behavior.

As key components of artificial afferent nervous systems, synaptic devices can mimic the physiological synaptic behaviors, which have attracted extensive attentions. Here, a flexible tribotronic artificial synapse (TAS) with bioinspired neurosensory behavior is developed. The triboelectric potential generated by the external contact electrification is used as the ion-gel-gate voltage of the organic thin film transistor, which can tune the carriers transport through the migration/accumulation of ions. The TAS successfully demonstrates a series of synaptic behaviors by external stimuli, such as excitatory postsynaptic current, paired-pulse facilitation, and the hierarchical memory process from sensory memory to short-term memory and long-term memory. Moreover, the synaptic behaviors remained stable under the strain condition with a bending radius of 20 mm, and the TAS still exhibits excellent durability after 1000 bending cycles. Finally, Pavlovian conditioning has been successfully mimicked by applying force and vibration as food and bell, respectively. This work demonstrates a bioinspired flexible artificial synapse that will help to facilitate the development of artificial afferent nervous systems, which is great significance to the practical application of artificial limbs, robotics, and bionics in future.

Ferromagnetic-Based Charge-Accumulation Triboelectric Nanogenerator With Ultrahigh Surface Charge Density.

An encouraging micro-energy harvesting technology, the triboelectric nanogenerator (TENG), has been proven to transfer ambient environmental micro-energy into electricity, but a low surface charge density results in low performance and limits the practical application of TENG. Here, a ferromagnetic-based charge-accumulation TENG (FC-TENG) is proposed with ultrahigh surface charge density and performances. The FC-TENG introduces a ferromagnetic media to enhance the output charge by magnetization effect. Meanwhile, the charge can also be continuously accumulated by the charge pump effects. Based on these two effects, an ultra-high surface charge density of 2.85 mC m-2 is obtained under ambient atmospheric conditions using an ultra-thin PET film (3 µm) and deposited Permalloy ferromagnetic electrodes. Meanwhile, the surface charge density of the FC-TENG can always maintain more than 1.5 mC m-2 , even if the relative humidity arrives at 90%. This work provides a prospective technical mode to enhance the surface charge density of TENG, which would shed a new insight and guidance on the high-performance TENG for various environmental conditions such as the ocean, industrial manufacturing, aerospace, and rail traffic.

Friction-Dominated Carrier Excitation and Transport Mechanism for GaN-Based Direct-Current Triboelectric Nanogenerators.

The semiconductor triboelectric nanogenerator (TENG) based on the tribovoltaic effect has the characteristics of direct current and high current density, but the energy transfer and conversion mechanism is not completely clear. Here, a series of gallium nitride (GaN)-based semiconductor direct-current TENGs (SDC-TENGs) are investigated for clarifying the carrier excitation and transport mechanism. During the friction process, the external output current always flows from GaN to silicon or aluminum, regardless of the direction of the built-in electric field, because of the semiconductor types. These results reveal that the carrier transport direction is dominated by the interfacial electric field formed by triboelectrification, which is also verified under different bias voltages. Moreover, the characteristics dependent on the frictional force have been systematically investigated under different normal forces and frictional modes. The open-circuit voltage and short-circuit current of SDC-TENG are both increased with a larger frictional force, which shows that the more severe friction results in both a larger interface electric field and more excited carriers. The maximum voltage can reach 25 V for lighting up a series of LEDs, which is enhanced by four times compared to the cutting-edge reported SDC-TENGs. This work has clarified the friction-dominated carrier excitation and transport mechanism for the tribovoltaic effect, which demonstrates the great potential of semiconductor materials for frictional energy recovery and utilization.

An ultraweak mechanical stimuli actuated single electrode triboelectric nanogenerator with high energy conversion efficiency.

Triboelectric nanogenerator (TENG) as a new energy harvester has attracted significant attention due to its excellent output performance and high energy conversion efficiency at low-frequency, small-amplitude and weak-force compared with a traditional electromagnetic generator. Here, an ultraweak mechanical stimuli actuated single electrode triboelectric nanogenerator (UMA-TENG) has been studied with an atomic force microscope. The electrical output and force curve of UMA-TENG were studied at first, as well as the maximum output performance and highest energy conversion efficiency. Then the influence of the driving frequency, separation distance and motion amplitude was investigated, respectively. Moreover, by introducing an external switch to reach a cycle of maximized energy output, the maximum energy conversion efficiency of the UMA-TENG was up to 73.6% with an input mechanical energy of 48 pJ. This work demonstrates that the TENG shows excellent performance in ultraweak mechanical stimuli and could broaden the applications of the TENG in sensors, actuators, micro-robotics, micro-electro-mechanical-systems, and wearable electronics.

Raindrop energy-powered autonomous wireless hyetometer based on liquid-solid contact electrification.

Triboelectric nanogenerators (TENGs) can directly harvest energy via solid-liquid interface contact electrification, making them very suitable for harvesting raindrop energy and as active rainfall sensors. This technology is promising for realizing a fully self-powered system for autonomous rainfall monitoring combined with energy harvesting/sensing. Here, we report a raindrop energy-powered autonomous rainfall monitoring and wireless transmission system (R-RMS), in which a raindrop-TENG (R-TENG) array simultaneously serves as a raindrop energy harvester and rainfall sensor. At a rainfall intensity of 71 mm/min, the R-TENG array can generate an average short-circuit current, open-circuit voltage, and maximum output power of 15 µA, 1800 V, and 325 µW, respectively. The collected energy can be adjusted to act as a stable 2.5 V direct-current source for the whole system by a power management circuit. Meanwhile, the R-TENG array acts as a rainfall sensor, in which the output signal can be monitored and the measured data are wirelessly transmitted. Under a rainfall intensity of 71 mm/min, the R-RMS can be continuously powered and autonomously transmit rainfall data once every 4 min. This work has paved the way for raindrop energy-powered wireless hyetometers, which have exhibited broad prospects in unattended weather monitoring, field surveys, and the Internet of Things.

Multidimensional Force Sensors Based on Triboelectric Nanogenerators for Electronic Skin.

The ability to detect multidimensional forces is highly desired for electronic skin (E-skin) sensors. Here, based on single-electrode-mode triboelectric nanogenerators (S-TENGs), fully elastic E-skin that can simultaneously sense normal pressure and shear force has been proposed. With the hemispherical curve-structure design and further structural optimization, the pressure sensor exhibits a high linearity and sensitivity of 144.8 mV/kPa in the low-pressure region. By partitioning the lower tribolayer into two symmetric parts, a multidimensional force sensor has been fabricated in which the output voltage sum and ratio of the two S-TENGs can be used for normal pressure and shear force sensing, respectively. When the multidimensional force sensors are mounted at a two-fingered robotic manipulator, the change of the grabbing state can be recognized, indicating that the sensor may have great application potential in tactile sensing for robotic manipulation, human-robot interactions, environmental awareness, and object recognition.

Frequency Band Characteristics of a Triboelectric Nanogenerator and Ultra-Wide-Band Vibrational Energy Harvesting.

Micromechanical vibration, as one of the most prevalent forms of energy in an ambient environment, has surpassing application potentials as the power source for self-powered electronics. A triboelectric nanogenerator (TENG) can effectively convert vibrational energy to electricity, which has the unique benefit of a wide-band over a traditional vibration energy harvester due to the contact electrification mechanism. Herein, the frequency band characteristics of vibrational TENG (V-TENG) were systematically elaborated. The mechanical model of V-TENG was established to explore its working mechanism for wide-band vibrational energy harvesting. By simulation analysis and experimental validation, the bandwidth dependence of V-TENG on acceleration magnitude, proof mass, stiffness, and gap distance was investigated in detail. With optimized structural parameters, an ultra-wide-band vibration energy harvester (UVEH) was developed by a tandem spring-mass structure. Within the ultra-wide-band range from 3 to 45 Hz, the UVEH can invariably illuminate 36 serial light-emitting diodes (LEDs) and charge a 33 µF capacitor to 1.5 V within 35 s. This work has quantitatively studied frequency band characteristics of V-TENG and provided a promising strategy for wide-band vibrational energy harvesting from a machine, bridge, water wave, and human motion.

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.

Network Topology Optimization of Triboelectric Nanogenerators for Effectively Harvesting Ocean Wave Energy.

The emerging triboelectric nanogenerator (TENG) network shows great potential in harvesting the ocean wave energy, which can help to achieve large-scale clean wave power generation. However, due to the lack of an effective networking strategy and theoretical guidance, the practicability of the TENG network is heavily restricted. In this paper, based on the typical spherical TENG, we investigated the networking design of TENGs. Four fundamental forms of electrical networking topology are proposed for large-scale TENG networks, and the influences of cable resistance and output phase asynchrony of each unit to the network output were systematically investigated. The research results show that the forms of electrical networking topology can produce an important influence on the output power of large-scale TENG networks. This is the first strategy analysis for the TENG network, which provides a theoretical basis and a universal method for the optimization design of large-scale power networks.

Overview of micro/nano-wind energy harvesters and sensors.

Wind energy is a kind of renewable and widely distributed energy and has attracted more and more attention from researchers in both energy harvesting and sensing fields. Here, micro/nano-wind energy harvesters and sensors have been systematically reviewed. Based on the fundamental wind energy harvesting principle, the windmill-based and aeroelastic harvesters are analyzed at first. On this basis, four kinds of energy harvesters for converting wind energy of different regions and scales have been compared and summarized. An electromagnetic energy harvester is exploited to scavenge macro-scale wind energy, while piezoelectric, electrostatic and triboelectric energy harvesters are applied to collect micro-scale wind energy. In addition, several micro/nano-wind sensors have also been surveyed. Passive wind sensors are exploited and improved to realize high precision and multi-functionality, while active wind sensors and self-powered sensing systems are used for wireless and intelligent wind information monitoring. Finally, the existing challenges and future perspectives in both micro/nano-wind energy harvesters and sensors have been discussed.

Intrinsically Stretchable Organic-Tribotronic-Transistor for Tactile Sensing.

Stretchable electronics are of great significance for the development of the next-generation smart interactive systems. Here, we propose an intrinsically stretchable organic tribotronic transistor (SOTT) without a top gate electrode, which is composed of a stretchable substrate, silver nanowire electrodes, semiconductor blends, and a nonpolar elastomer dielectric. The drain-source current of the SOTT can be modulated by external contact electrification with the dielectric layer. Under 0-50% stretching both parallel and perpendicular to the channel directions, the SOTT retains great output performance. After being stretched to 50% for thousands of cycles, the SOTT can survive with excellent stability. Moreover, the SOTT can be conformably attached to the human hand, which can be used for tactile signal perception in human-machine interaction and for controlling smart home devices and robots. This work has realized a stretchable tribotronic transistor as the tactile sensor for smart interaction, which has extended the application of tribotronics in the human-machine interface, wearable electronics, and robotics.

Nanoscale triboelectrification gated transistor.

Tribotronics has attracted great attention owing to the demonstrated triboelectrification-controlled electronics and established direct modulation mechanism by external mechanical stimuli. Here, a nanoscale triboelectrification-gated transistor has been studied with contact-mode atomic force microscopy and scanning Kevin probe microscopy. The detailed working principle was analyzed at first, in which the nanoscale triboelectrification can tune the carrier transport in the transistor. Then with the manipulated nanoscale triboelectrification, the effects of contact force, scan speed, contact cycles, contact region and charge diffusion on the transistor were investigated, respectively. Moreover, the manipulated nanoscale triboelectrification serving as a rewritable floating gate has demonstrated different modulation effects by an applied tip voltage. This work has realized the nanoscale triboelectric modulation on electronics, which could provide a deep understanding for the theoretical mechanism of tribotronics and may have great applications in nanoscale transistor, micro/nano-electronic circuit and nano-electromechanical system.

Triboelectric Effect-Driven Liquid Metal Actuators.

Owing to the perfect combination of metallic conductivity, liquid fluidity, and mechanical stability, liquid metals (LMs) are considered the potential materials of the electromechanical system and the basic components of liquid robot. Here, triboelectric effect-driven liquid metal actuators (TLMAs) have been demonstrated, in which the motions of LMs could be driven and accurately controlled by triboelectric nanogenerators (TENGs). By the coulomb forces of transferred triboelectric charges in interdigital electrodes and inertia effects, the LM could be driven for continuous linear and circular motions, respectively. As an alternative structure, the motion of LMs could also be accurately controlled, in which the reciprocating and merging of LMs have been demonstrated. Moreover, the TLMA could serve as a control switch to modulate on/off of an LED by integrating into a circuit, and a transport system based on TLMA has also been demonstrated. In this work, the TENG is used as a medium to first drive and accurately control LMs by external mechanical force, which may have great applications in Micro-Electro-Mechanical System (MEMS), liquid robot, and intelligent microcontroller.

Triboelectric micromotors actuated by ultralow frequency mechanical stimuli.

A high-speed micromotor is usually actuated by a power source with high voltage and frequency. Here we report a triboelectric micromotor by coupling a micromotor and a triboelectric nanogenerator, in which the micromotor can be actuated by ultralow-frequency mechanical stimuli. The performances of the triboelectric micromotor are exhibited at various structural parameters of the micromotor, as well as at different mechanical stimuli of the triboelectric nanogenerator. With a sliding range of 50 mm at 0.1 Hz, the micromotor can start to rotate and reach over 1000 r min-1 at 0.8 Hz. The maximum operation efficiency of the triboelectric micromotor can reach 41%. Additionally, the micromotor is demonstrated in two scanning systems for information recognition. This work has realized a high-speed micromotor actuated by ultralow frequency mechanical stimuli without an external power supply, which has extended the application of triboelectric nanogenerator in micro/nano electromechanical systems, intelligent robots and autonomous driving.

Stretchable Triboelectric-Photonic Smart Skin for Tactile and Gesture Sensing.

Smart skin is expected to be stretchable and tactile for bionic robots as the medium with the ambient environment. Here, a stretchable triboelectric-photonic smart skin (STPS) is reported that enables multidimensional tactile and gesture sensing for a robotic hand. With a grating-structured metal film as the bioinspired skin stripe, the STPS exhibits a tunable aggregation-induced emission in a lateral tensile range of 0-160%. Moreover, the STPS can be used as a triboelectric nanogenerator for vertical pressure sensing with a maximum sensitivity of 34 mV Pa-1 . The pressure sensing characteristics can remain stable in different stretching conditions, which demonstrates a synchronous and independent sensing property for external stimuli with great durability. By integrating on a robotic hand as a conformal covering, the STPS shows multidimensional mechanical sensing abilities for external touch and different gestures with joints bending. This work has first demonstrated a triboelectric-photonic coupled multifunctional sensing terminal, which may have great applications in human-machine interaction, soft robots, and artificial intelligence.

Self-Powered Electrostatic Adsorption Face Mask Based on a Triboelectric Nanogenerator.

The physical filtration mechanism of a traditional face mask has a low removal efficiency of ultrafine particulates in the size range of 10-1000 nm, which are badly harmful to human health. Herein, a novel self-powered electrostatic adsorption face mask (SEA-FM) based on the poly(vinylidene fluoride) electrospun nanofiber film (PVDF-ESNF) and a triboelectric nanogenerator (TENG) driven by respiration (R-TENG) is developed. The ultrafine particulates are electrostatically adsorbed by the PVDF-ESNF, and the R-TENG can continually provide electrostatic charges in this adsorption process by respiration. On the basis of the R-TENG, the SEA-FM shows that the removal efficiency of coarse and fine particulates is higher than 99.2 wt % and the removal efficiency of ultrafine particulates is still as high as 86.9 wt % after continually wearing for 240 min and a 30-day interval. This work has proposed as a new method of wearable air filtration and may have great prospects in human health, self-powered electronics, and wearable devices.

Tribotronic bipolar junction transistor for mechanical frequency monitoring and use as touch switch.

Tribotronics, a new field that involves the coupling of triboelectricity and semiconductors, has attracted great interest in the nanoenergy and nanoelectronics domains. This paper proposes a tribotronic bipolar junction transistor (TBJT) that incorporates a bipolar junction transistor and a triboelectric nanogenerator (TENG) in the single-electrode mode. When the mobile triboelectric layer slides on the device surface for electrification, a bias voltage is created and applied to the emitter junction, and then the base current from the TENG is amplified. Based on the fabricated TBJT, a mechanical frequency monitoring sensor with high sensitivity and excellent stability and a finger-triggered touch switch were developed. This work demonstrated for the first time a tribotronic device with simultaneously controlled voltage and current voltage/current simultaneously controlled tribotronic device, which has promising potential applications in micro/nano-sensors, human-machine interactions, intelligent instrumentation, wearable electronics, and other applications.