As the world is marching into the era of the internet of things (IoTs) and artificial intelligence, the most vital development for hardware is a multifunctional array of sensing systems, which forms the foundation of the fourth industrial revolution toward an intelligent world. Given the need for mobility of these multitudes of sensors, the success of the IoTs calls for distributed energy sources, which can be provided by solar, thermal, wind, and mechanical triggering/vibrations. The triboelectric nanogenerator (TENG) for mechanical energy harvesting developed by Z.L. Wang's group is one of the best choices for this energy for the new era, since triboelectrification is a universal and ubiquitous effect with an abundant choice of materials. The development of self‐powered active sensors enabled by TENGs is revolutionary compared to externally powered passive sensors, similar to the advance from wired to wireless communication. In this paper, the fundamental theory, experiments, and applications of TENGs are reviewed as a foundation of the energy for the new era with four major application fields: micro/nano power sources, self‐powered sensors, large‐scale blue energy, and direct high‐voltage power sources. A roadmap is proposed for the research and commercialization of TENG in the next 10 years.

Harvesting energy from the environment is crucial for the independent, wireless, and sustainable operation of nanodevices. This is a key requirement for building self-powered nanosystems. The living environment of nanodevices is diverse ranging from natural to in vitro and in vivo. Mechanical energy is one of the most abundant and popular energies in the environment, which can range from wind energy to mechanical vibration, sonic/ ultrasonic waves, noise, fluidics, biomotions, muscle stretching, and more.Harvesting energy using piezoelectric materials has been demonstrated some time ago, [1][2][3] but these structures are rather large and a tiny physical motion, such as the contraction of a blood vessel is not strong enough to drive the generator. More importantly, the demonstrated cantilever-based microelectromechanical systems (MEMS) work only under a specific driving resonance frequency that is determined by the cantilever. In a real biological system, the mechanical disturbance has a large frequency range and the mechanical vibration is time dependent.We have demonstrated a few approaches for harvesting mechanical energy from several different sources, [4][5][6] one of which was based on an alternating-current (AC) nanogenerator using a two-ends-bonded piezoelectric nanowire (NW).[7] The NW is laterally bonded on a flexible substrate, and the physical deformation of the NW is directly driven by the shape change of the substrate that is induced by external dynamic mechanical sources. When a ZnO NW is subject to a periodic mechanical stretching and releasing, the mechanical-electric coupling effect of the NW, combined with the gate effect of the Schottky contact at the interface, results in a alternating flow of the charge in the external circuit. The single-wire generator (SWG) acts as a ''charging pump'' that drives the electron motion in accordance to the mechanical deformation of the NW.Recently, we have applied the AC generator to harvest mechanical energy from body movement under in vitro conditions.[8] However, the applications of the nanogenerators under in vivo and in vitro environments are distinct. Some crucial problems need to be addressed before using these devices in the human body, such as biocompatibility and toxicity. To directly interface nanowires with cells, our studies have indicated that ZnO nanowires can be safely used for in vivo applications and they are biodegradable. [9] In this Communication, in vivo biomechanical-energy harvesting using an AC nanogenerator has been achieved for the first time. We demonstrate the implanting of the nanogenerator in a live rat to harvest energy generated by its breath and heartbeat. This study shows the potential of applying nanogenerators for the scavenging of low-frequency dynamic muscle energy created by very small-scale physical motion for the possible driving of in vivo nanodevices.The fabrication process of a SWG was presented in detail in our previous publication.[7] The piezoelectric ZnO NW was grown using a physical-vapor deposition process an...

We demonstrate a new type of integrated nanogenerator based on arrays of vertically aligned piezoelectric ZnO nanowires. The peak open-circuit voltage and short-circuit current reach a record high level of 58 V and 134 μA, respectively, with a maximum power density of 0.78 W/cm 3 . The electric output was directly applied to a sciatic nerve of a frog, inducing innervation of the nerve. Vibrant contraction of the frog's gastrocnemius muscle is observed as a result of the instantaneous electric input from the nanogenerator.KEYWORDS: Nanogenerator, ZnO, energy harvesting, functional electrical stimulation T he modern life is inexorably dependent on emerging technologies in stand-alone portable systems designed to provide complete and personal solutions.1 Integration of microto-nanosized sensors, actuators/transducers, and medical implants leads to ultraminiaturized and multifunctional smart systems that are expected to provide unprecedented life quality for human kinds.2−4 For such a system that consumes much less power than do their bulky counterparts, it is not only significant but also very feasible to harvest ambient energy to build self-powered systems that can operate independently and sustainably. Here, we achieved real-time functional electrical stimulation (FES) of a sciatic nerve of a frog by a new type of ZnO-nanowire (NW)-based nanogenerator (NG) that produced electricity from biomechanical energy. The electric output from the NG reached a record high of 58 V and 134 μA, with a maximum power density of 0.78 W/cm 3 . It was sufficient to directly and instantaneously induce innervating of the motor never and hence contraction of the frog's gastrocnemius muscle. Our demonstration suggests potential applications of the nanogenerator in biomedical and neurological fields, such as the power source for neuroprosthetic devices.Since 2005, we have been developing "self-powered nanotechnology" by ZnO-nanowire-based NGs. 5,6 By virtue of the piezoelectric effect of ZnO NWs, the NGs target ambient mechanical energy, transforming it into electrical energy. As a result of worldwide efforts, such a concept is being developed into a practical technology with a variety of demonstrated applications.7−13 However, a major limitation was that the output power of a NG was still not sufficiently high enough for real-time operation of conventional electronics.The previously designed NGs utilized the Schottky barrier between metal−semiconductor contacts, which was required for charge accumulation. 5,14−16 In this work we developed a novel design and process flow for fabricating an integrated NG based on position-controlled vertical ZnO NWs. The Schottky contact was replaced by a thin insulating layer that prevents the current leakage through the internal structure. The process flow for fabrication is shown in Figure 1. A precleaned silicon substrate was consecutively deposited with an ITO layer and a ZnO seed layer by RF sputtering (Figure 1a−c). Not only does the ITO layer play a role as a conductive electrode, but ...

Flexible and stretchable physical sensors capable of both energy harvesting and self-powered sensing are vital to the rapid advancements in wearable electronics. Even so, there exist few studies that can integrate energy harvesting and self-powered sensing into a single electronic skin. Here, a stretchable and washable skin-inspired triboelectric nanogenerator (SI-TENG) is developed for both biomechanical energy harvesting and versatile pressure sensing. A planar and designable conductive yarn network constructed from a three-ply-twisted silver-coated nylon yarn is embedded into flexible elastomer, endowing the SI-TENG with desired stretchability, good sensitivity, high detection precision, fast responsivity, and excellent mechanical stability. With a maximum average power density of 230 mW m , the SI-TENG is able to light up 170 light-emitting diodes, charge various capacitors, and drive miniature electronic products. As a self-powered multifunctional sensor, the SI-TENG is adopted to monitor human physiological signals, such as arterial pulse and voice vibrations. Furthermore, an intelligent prosthetic hand, a self-powered pedometer/speedometer, a flexible digital keyboard, and a proof-of-concept pressure-sensor array with 8 × 8 sensing pixels are successively demonstrated to further confirm its versatile application prospects. Based on these merits, the developed SI-TENG has promising applications in wearable powering technology, physiological monitoring, intelligent prostheses, and human-machine interfaces.

Functional polymers possess outstanding uniqueness in fabricating intelligent devices such as sensors and actuators, but they are rarely used for converting mechanical energy into electric power. Here, a vitrimer based triboelectric nanogenerator (VTENG) is developed by embedding a layer of silver nanowire percolation network in a dynamic disulfide bond-based vitrimer elastomer. In virtue of covalent dynamic disulfide bonds in the elastomer matrix, a thermal stimulus enables in situ healing if broken, on demand reconfiguration of shape, and assembly of more sophisticated structures of VTENG devices. On rupture or external damage, the structural integrity and conductivity of VTENG are restored under rapid thermal stimulus. The flexible and stretchable VTENG can be scaled up akin to jigsaw puzzles and transformed from 2D to 3D structures. It is demonstrated that this self-healable and shape-adaptive VTENG can be utilized for mechanical energy harvesters and self-powered tactile/pressure sensors with extended lifetime and excellent design flexibility. These results show that the incorporation of organic materials into electronic devices can not only bestow functional properties but also provide new routes for flexible device fabrication.

It has been demonstrated that substantial electric power can be produced by a liquid-based triboelectric nanogenerator (TENG). However, the mechanisms regarding the electrification between a liquid and a solid surface remain to be extensively investigated. Here, the working mechanism of a droplet-TENG was proposed based on the study of its dynamic saturation process. Moreover, the charge-transfer mechanism at the liquid−solid interface was verified as the hybrid effects of electron transfer and ion adsorption by a simple but valid method. Thus, we proposed a model for the charge distribution at the liquid−solid interface, named Wang's hybrid layer, which involves the electron transfer, the ionization reaction, and the van der Waals force. Our work not only proves that TENG is a probe for investigating charge transfer at interface of all phases, such as solid−solid and liquid−solid, but also may have great significance to water energy harvesting and may revolutionize the traditional understanding of the liquid−solid interface used in many fields such as electrochemistry, catalysis, colloidal science, and even cell biology.

Growing demand in portable electronics raises a requirement to electronic devices being stretchable, deformable, and durable, for which functional polymers are ideal choices of materials. Here, the first transformable smart energy harvester and self-powered mechanosensation sensor using shape memory polymers is demonstrated. The device is based on the mechanism of a flexible triboelectric nanogenerator using the thermally triggered shape transformation of organic materials for effectively harvesting mechanical energy. This work paves a new direction for functional polymers, especially in the field of mechanosensation for potential applications in areas such as soft robotics, biomedical devices, and wearable electronics.

The phenomenon of contact electrification (CE) has been known for thousands of years, but the nature of the charge carriers and their transfer mechanisms are still under debate. Here, the CE and triboelectric charging process are studied for a metal–dielectric case at different thermal conditions by using atomic force microscopy and Kelvin probe force microscopy. The charge transfer process at the nanoscale is found to follow the modified thermionic‐emission model. In particular, the focus here is on the effect of a temperature difference between two contacting materials on the CE. It is revealed that hotter solids tend to receive positive triboelectric charges, while cooler solids tend to be negatively charged, which suggests that the temperature‐difference‐induced charge transfer can be attributed to the thermionic‐emission effect, in which the electrons are thermally excited and transfer from a hotter surface to a cooler one. Further, a thermionic‐emission band‐structure model is proposed to describe the electron transfer between two solids at different temperatures. The findings also suggest that CE can occur between two identical materials owing to the existence of a local temperature difference arising from the nanoscale rubbing of surfaces with different curvatures/roughness.