Progress in triboelectric nanogenerators as self-powered smart … · REVIEW This section of Journal of Materials Research is reserved for papers that are reviews of literature in

REVIEW

This section of Journal of Materials Research is reserved for papers that are reviews of literature in a given area.

Progress in triboelectric nanogenerators as self-powered smartsensors

Nannan Zhang, Changyuan Tao, and Xing Fana)

College of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400044, People’s Republicof China

Jun Chenb)

Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, USA

(Received 20 November 2016; accepted 7 April 2017)

Personal, multifunctional, and smart electronic devices/systems are indispensable components of theinternet of things for modern information collection and exchange, which play a key role infacilitating the development of human civilization. Traditional technique for powering these sensornodes mainly relies on batteries, which may not be favorable owing to the limited battery lifetime,large sensor population, wide distribution, as well as the potential of environmental detriment.Extricated from external power sources, triboelectric nanogenerators (TENGs) based active sensorshave been extensively spread into a variety of fields for self-powered high-performance sensing,featured as being lightweight, extremely cost-effective, and environmentally friendly. In this article,current progress of TENGs as smart sensors for self-powered touch detection, vibration and acousticsensing, biomedical applications, as well as human-machine interfacing, has been comprehensivelyreviewed, from aspects of materials usage, device fabrication to practical applications. The latestrepresentative achievements regarding the TENG based self-powered sensing systems were alsosystematically presented. In the end, some perspectives and challenges for the TENG based self-powered smart sensors were also summarized.

I. INTRODUCTION

Smart sensors are effective means to collect usefulinformation for us from ambient environment, which isgetting more and more important especially with the fastdevelopment of internet of things (IoT). Smart sensorshave already penetrated into every corner of our dailylife, such as industrial production, marine exploration,environmental protection, resource investigation, medicaldiagnosis, biological engineering, even the protection ofcultural relics, and so on. It is no exaggeration to say thatvarious sensors are indispensable to the complicatedengineering systems for many modern projects, whichis critical to the human economic development and socialadvancement.1–9

Over the past decades, various kinds of sensors forhealth monitoring, environmental protection, infrastruc-ture detecting, and security have been developed.10–14 Itis said that the world will have trillions of sensor cellspositioned on the world by 2020.15 The recent develop-ment of IoT and sensor networks rapidly changes the

traditional understanding about power need. Poweringthese sensor nodes relies on rechargeable batteries, whichmay not be the favorable solution for IoT. Since theincreasing amount of batteries owing to the rapid explo-sion of mobile electronics may result in challenges forbatteries recycling and replacement as well as the concernsof potential environmental pollution.16–23 Mechanicalmotion ubiquitously exists in ambient environment andpeople’s daily life. In recent years, it becomes an attractivetarget for energy harvesting as a potentially alternativepower source to battery operated electronics.24–34

As one of the most ubiquitous natural phenomena,triboelectrification is often regarded as a negative effectthat in most cases was avoided by many technologies.Recently, Zhong Lin Wang’s group in Georgia Instituteof Technology has invented a fundamentally new energytechnology, triboelectric nanogenerator (TENG), relyingon the coupling of contact electrification and electrostaticinduction. As an inventive energy collector, TENGharvests energy through contact electrification of twoopposite tribo-polarity materials, which can convert themechanical motions of two triboelectric layers intoelectrical potential difference between electrodes, anddrive the electrons to flow back and forth in the externalcircuit.35–44 The materials used for the fabrication ofTENG are common and widely available, such as paper,

Contributing Editor: Paul MuraltAddress all correspondence to these authors.a)e-mail: [email protected])e-mail: [email protected], [email protected]: 10.1557/jmr.2017.162

J. Mater. Res., Vol. 32, No. 9, May 15, 2017 � Materials Research Society 20171628

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fabrics, polydimethylsiloxane (PDMS), polytetra-fluoroethylene (PTFE), aluminum (Al), polyvinylchlor-ide (PVC), and fluorinated ethylene propylene (FEP)45–52.And it was demonstrated to collect energy from a widerange of mechanical motions, for example, humanmotion,53 vibration,54 rotation,55 wind blowing,56

sound wave,57 water flow,58 rain drop,59 ocean wave,60

and so on.61–63 The working modes of TENG are ratherdiverse and could be applied to harvest most ofconventional mechanical/kinetic motions in our dailylife. Not only they can convert mechanical motions intoelectrical signals, TENGs can also be used as self-powered sensors to detect displacement, velocity,metallic ion, humidity, temperature, and ultraviolet(UV) intensity.64–66 In this paper, we presented a com-prehensive review of the self-powered smart sensorsbased on TENG with a start of introducing the TENGworking principles, then the device structural design,materials usage, and the smart sensors for practicalapplications, with an end of summarization of theperspectives and challenges for the TENG based self-powered smart sensors.

A. Working principles of the TENG

The working principle of TENGs relies on a couplingeffect of the contact electrification and the electrostaticinduction. As known, contact electrification is a widelyexisted phenomenon that one material becomescharged after it contacts another material with oppositeelectron affinity.67,68 Though it is one of the most

experienced effects in daily life, the mechanism behindtriboelectrification is still under exploration.69–71 It isgenerally believed that during the physical contact oftwo materials, some of the bonded atoms have a ten-dency to keep extra electrons and some a tendency togive them away, producing triboelectric charges at theinterface.72–74 The triboelectric charges on dielectricsurfaces can be a driving force to make electrons flowback and forth among electrodes.75–77 And four differ-ent working modes of TENGs have been establishedand presented,78 as illustrated in Fig. 1.

1. Vertical contact-separation mode

As illustrated in Fig. 1(a), two dissimilar dielectricfilms with back-coated electrodes are facing each other.79

The physical contact between a pair of dielectric filmswill generate opposite triboelectric charges on theirsurfaces. Once a physical gap induced between the twosurfaces by the external mechanical motion, a potentialdifference was created. The periodic contact and separa-tion between the two charged surfaces will inducea periodic electric potential difference between the pairedelectrodes, which will drive the electrons to flow backand forth in the external circuit.

2. Lateral sliding mode

The structure of this mode is the same as that of thevertical contact-separation mode. When two dielectric

FIG. 1. Four fundamental working modes of triboelectric nanogenerators: (a) vertical contact-separation mode; (b) lateral sliding mode; (c) single-electrode mode; and (d) freestanding triboelectric-layer mode. Reproduced with permission from the Royal Society of Chemistry.78

N. Zhang et al.: Progress in triboelectric nanogenerators as self-powered smart sensors

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films are brought into contact with each other, a relativein-plane sliding against each other also generates chargeson the contact surfaces [Fig. 1(b)]. Therefore, transversalpolarization occurs in the sliding direction, which drivesthe electrons on the back electrodes to flow to fullybalance the electrical potential difference created by thetriboelectric charges.80 A periodic sliding back and forthgenerates an alternating current output. This is the in-plane sliding mode TENG. The sliding motion can beextended to a cylindrical rotation or a disc rotation.81

3. Single-electrode mode

The previous two working modes need paired electro-des connected to external load, which will greatly confinethe movement of the triboelectric layers. In some cases,one of the triboelectric layers needs to be a mobile object,such as human walking on the floor.82 A single-electrodemode TENG was thus developed to solve this problemin which the only electrode in the TENG is grounded[Fig. 1(c)]. When the top object approaches or leaves thetriboelectric layer, it will also change the distribution ofthe local electric field, which would lead to a flow ofelectrons moving back and forth between the electrodesand ground. In this working mode, relative motion of thetriboelectric layer can also be classified as verticalcontact-separation and in-plane sliding motions.

4. Freestanding triboelectric-layer mode

The nonmobile triboelectric charges could sustaina long period of time on the contact surface.83 Whenthere is a symmetrical electrode under the dielectric layer,the objects close to and away from the electrodes willcause the electrons to flow between the two electrodes tobalance the local potential distribution [Fig. 1(d)]. Theconversion of mechanical motions into electricity doesnot need the direct physical contact between the twotriboelectric layers, which effectively prevents the mate-rials abrasion during TENG operation. This is a decentdesign to improve the durability of TENGs.

II. TENG AS A TACTILE SENSOR

The sensors for instantaneous detection of interfacialpressure are of critical importance to the applications oftouch screen, security monitoring, healthcare, and soon.84–86 Owing to the unique advantage of the single-electrode TENG that one of the triboelectric layer canmove freely, which makes it especially suitable for thedetection of touch from a foreign object.

Zhu et al.87 proposed a self-powered triboelectricsensor (TES) based on vertically laminated thin-filmmaterials [Fig. 2(a)]. To start with, the structure of TESis supported by a polyethylene terephthalate (PET) layer

consisting of two layers of indium tin oxide (ITO) on itsback and front sides. On the front side, FEP layer canproduce triboelectric charges through contact with otherobjects. The surface of FEP is modified by verticallyaligned polymer nanowires (PNWs). The average lengthof PNWs can reach 1.5 lm and a diameter of 150 nm.The PNWs layer is a significant part in achieving highsensitivity for low-pressure detection. A nylon film withexcellent mechanical and thermal performance plays asa protection layer on the back side.

Formed with general thin-film materials, the TES iscost-effective. With such structural design and fabricationprocess, the TES is also suitable for large-scale fabrica-tion. To evaluate the sensing performance of the device,a square-shaped TES in Fig. 2(c) was manufactured todiscover metal through cyclic contact and separation. Asshown in Fig. 2(d), the maximum output voltage of theTES can reach 35 V when the applied pressure is about0.03 kPa. With the increasing of contact pressure up to10 kPa, the output voltage reaches 50 V [Fig. 2(e)]. WithPNW modification, the TES obtains a high pressuresensitivity of 44 mV/Pa (0.09% Pa�1), and a maximumsensitivity of 1.1 V/Pa (2.3% Pa�1) in a low-pressureregion (,0.15 kPa). With flexible structure, the TES canbe fabricated with any size and shape, even onto curvedsurfaces. Additionally, a full wireless sensing system canalso be developed through integration of the TES witha signal processing circuit. The output voltage generatedby the TES to trigger an IC (integrated circuit) timerplays an important role in the whole system, whichcontrols a wireless transmitter. When a human handtouched the door handle, the remote system will imme-diately start to operate [Fig. 2(f)]. The system has beenutilized successfully in various applications, such ashuman-electronics interface, detection, remote operation,and security systems. In the process of replacing otherfunctional devices for the wireless transmitter in thecircuit, the sensing system has been successfully appliedin a wide range of applications, such as a touch-enabledswitch for a panel light shown in Fig. 2(g).

III. TENG AS A VIBRATION SENSOR

As one of the widely existed mechanical motions in ourdaily life, vibration holds a variety of forms and can befound from mechanical equipment, transportation, con-struction, and even ocean tide.88 Since the workingcondition of machineries and infrastructures can be eval-uated by their vibrations, numerous transducers have beendeveloped for monitoring and detecting ambient mechan-ical vibrations. Owing to the conversion from vibrationalmotions into electrical signals, TENG becomes an excel-lent self-powered sensor for ambient vibration detection.

The vibration frequency is usually easy to be quanti-fied by vibration sensors, but it is hard to measure the


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vibration amplitudes. Here, Wang et al. proposeda method to quantitatively measure the vibration ampli-tude by fabricating a contact-mode freestanding tribo-electric nanogenerator (CF-TENG).89 The structuraldesign of the device is schematically shown in Fig. 3(a).The acrylic was shaped by a laser cutter to form theframework of the TENG. Two front-to-front structuredAl-coated acrylic slices which served as electrodes of theCF-TENG are separated with a gap distance of 2 cm. Inbetween, another acrylic sheet served as the vibrationresonator connected by 4 pairs of springs at the four

corners to the top and bottom acrylic substrates. Toenhance the triboelectric charge density, the FEP surfaceis etched by the inductive coupling plasma (ICP) reactiveion etching, which produced nanowire structures [Fig. 3(b)]to increase the surface roughness and the effective surfacearea. The diameter of nanowires is about 100 nm, and thelength is about 1 lm [Fig. 3(c)]. As shown in Fig. 3(d),the linear motor followed sinusoidal motions. Because ofthe linearity for electricity generation, the profiles of Voc

are sinusoidal waves, but with a p/2 phase shift. Whenthe resonator vibration amplitudes were gradually

FIG. 2. A self-powered, flexible, and ultrasensitive tactile sensor: (a) schematic structure of the tactile sensor; (b) a scanning electron microscope(SEM) image of the polymer nanowires created on the surface of the FEP thin film; (c) photograph of the tactile sensor for sensing a foreign object;(d) measurement of open-circuit voltage with a cyclic contact force of 20 mN; (e) summarized results of the output voltage under different contactpressures. Inset: an enlarged view of the summarized results at low-pressure region; (f) a photograph showing that the TES was integrated witha signal processing circuit to turn on the siren alarm when a human hand touched the door handle; (g) a photograph showing that the TES wasintegrated with another signal processing circuit to turn on light when it was contacted by a human finger. Reproduced with permission from theAmerican Chemical Society.87



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increased up to 5 mm with a fixed vibration frequency of1 Hz, the increasing of Voc is propositional to thevibration amplitude, which can be expected from theirrelationships with the resonator moving distance. A goodlinearity in the electricity generation behavior makes theCF-TENG to quantitatively measure the vibrationalamplitude with utmost capability. To experimentallydemonstrate it, the electrical signals of the CF-TENGwere measured under a series of different vibrationamplitudes at constant vibration frequency. As shownin Fig. 3(e), with the expanding of vibrational amplitude,the peak-to-peak value of Voc increases directly, as wellas the amplitude of Isc when the shaker’s amplitude is

within the range below 1.5 mm. Figure 3(f) shows thatthe CF-TENG did well in monitoring the vibration froma wind blower’s operation. From the demonstration ofFig. 3(g), the generated electricity could power up to 60LEDs with a vibration amplitude of 1.5 cm and frequencyof 15 Hz.

Moreover, Hu et al.90 introduced a TENG based on thespiral structure (STENGs) as vibration and positioningsensor. As shown in Fig. 4(a), the STENG works ina vertical contact and separation mode. With light weight,proper strength, easy processing, acrylic is available forthe structural material. The two face-to-face round sliceswork to create electricity in Fig. 4(a) lined with red. One

FIG. 3. Contact-mode freestanding triboelectric nanogenerator: (a) schematic diagram showing the typical device structure of a CF-TENG;(b) enlarged view of the device structure showing the nanowire structure on the surface of the FEP films attached on the resonator plate; (c) SEMimage of the nanowire structures on the FEP films; (d) five groups of Voc profiles when the motor was set with sinusoidal motions at the samefrequency (1 Hz) but five different amplitudes; (e) simulated open-circuit voltages and short-circuit charge densities of the CF-TENG at different positionsof the resonator plate; (f) photograph showing the demonstration of using the CF-TENG to monitor the vibration of a wind blower during its operation;(g) 60 LEDs instantaneously driven by the CF-TENG. Reproduced with permission from the American Chemical Society.89



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FIG. 4. Triboelectric nanogenerator built on suspended 3D spiral structure as a vibration and positioning sensor: 3D TENG spiral structure (a) andschematic diagram of the device and its cross section view; SEM image of the surface morphology of (b) aluminum and (c) the Kapton film;(d) schematic diagram of the experimental setup shows three STENGs positioned at the three corners of a wooden board as a self-powered dynamicsensor; (e) schematic diagram of the experimental setup; (f) recorded voltage signals when the mass is released from L 5 5 cm at differentaccelerations; (g) measured voltage/current signals with different mass load momentum at L 5 5 cm; (h) comparison of measured voltage signalsunder different testing conditions. A linear relationship is revealed. Reproduced with permission from the American Chemical Society.90



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round plate connects the 3D spiral structure and the otherattached on the cube’s bottom. The diameter of nano-pores arranged on the aluminum surface in Fig. 4(b) isabout 30 nm for enhancing the effective contact area ofTENG. Figure 4(c) shows the nano-modified Kaptonsurface, which can enhance the output performance dueto the increasing of surface roughness and the effectivecontact area of the STENG. There are 3 STENGs fixed ona test board with a size of 1 � 24 � 48 in. for thevibration source position [Fig. 4(d)]. On the test board,nine positions were chosen to be monitored by 3STENGs, as the experimental setup illustrated in Fig. 4(e).The load weighed 100 g is used to hit the test board, withthe different height of L (L 5 1, 2, 3, 4, and 5 cm) andacceleration (a 5 1, 2, 3, 4, 5, 6, 8, and 9.8 m/s2). Asshown in Fig. 4(f), the dependence of the load fallingacceleration on the STENGs voltage output was studiedwith a height L of 5 cm. And the voltage output is anincreasing function of the acceleration, since the largerthe landing acceleration, the higher the momentum ontothe board. And the relevance of the monitored voltage/current signals and mass landing momentum was mea-sured and demonstrated in Fig. 4(g). Figure 4(h) pre-sented the measured results with different vertical fallingheights and accelerations, which all follow a linearrelationship between the output signal and the landingmass momentum.

IV. TENG AS AN ACOUSTIC SENSOR

Sound waves widely exist in the environment aroundus. And it has been lack of effective technology toconvert it into electricity since the sound wave itselfholds very low power density. Yang et al.91 developed anorganic thin-film based TENG to harvest environmentalacoustic wave as a sustainable power resource and self-powered acoustic sensor. As indicated in Fig. 5(a),relying on a Helmholtz cavity with a size-tunable narrowneck, the circular shape with a multilayer structure playsa critical role in acoustic energy harvesting. In the device,one electrode is the Al film with nanoporous surface andthe other is copper, which was back-coated on the PTFEfilm. As shown in Fig. 5(b), four TENGs as an array wereintegrated together to extend the whole device operationbandwidth from 10 to 1700 Hz, which made it possiblefor self-powered human voice recording. Figures 5(c1)and 5(c3) show time domain waveforms of the recordedsound by nanogenerator NG1 and NG4. As demonstratedin Figs. 5(c2) and 5(c4), the waveform of NG4 was roughsince its natural frequency is 1400 Hz, while NG1 onewas smooth owing to its natural frequency of 350 Hz,which is the dominant frequency response for the rangefrom 10 to 600 Hz. To reconstruct the original sound, theacquired acoustic signals of the array are weightedaccording to the relative amount of information obtained

from each source. The waveform of the rebuild signal andits corresponding short-time Fourier transform are shownin Figs. 5(d1) and 5(d2). Figure 5(e) indicates anas-manufactured TENG serving as a self-powered micro-phone to record sound.

Moreover, Fan et al.92 reported an ultrathin, rollable,paper-based TENG to harvest acoustic energy and recordsound in a self-powered manner. As illustrated in Fig. 6(a),the supporting substrate of TENG is a layer of multiholedpaper, which is coated with copper as electrode. FromFig. 6(b), we can know that the TENG is as thin as125 lm. With outstanding characteristics of thinness,flexibility, widen-bandwidth, and independence of reso-nator, the presented TENG is unique in acoustic energyharvesting. As shown in Fig. 6(c), the capacitor wascharged to 1.8 V just in 12 s by the power recycled fromthe sound of a cell phone. Figures 6(d) and 6(e),respectively, illustrate the principle of measuring thedirectional patterns of the flat and rolled TENG. Asdemonstrated in Fig. 6(f), the paper-thin TENG wascapable of recording sound and acting as a self-powered microphone. Different structures of the TENGcorresponded to different directional patterns [Fig. 6(g)].The flat one had a butterfly shaped directional patternwith mirror symmetry, while the rolled one has round-shape directional pattern, which is feasible for theatricstage live recording, military surveillance, and omnibear-ing acoustic energy harvesting.

V. TENG AS A BIOMEDICAL SENSOR

Smart sensors play an important role in our life bycatching characteristics of human physiology or behavior.Self-powered sensor based on TENG could also be usedfor biomedical monitoring purpose. Here, Bai et al.93

developed a membrane-based triboelectric sensor (M-TES)for air pressure detection and health monitoring. As shownin Fig. 7(a), M-TES is consisted of an acrylic sheet of1.6 mm as the substrate, a copper layer of 100 nm as theelectrode, FEP layer of 125 lm, and latex layer of 50 lm.The photograph in Fig. 7(b) indicates that the average sizeof nanorod on FEP layer is 130 nm of diameter and500 nm of length. The device is packaged with epoxy resinin the edges of layers. Figure 7(c) presents an image of theM-TES with an area of 3.2 cm2. As demonstrated in Fig. 7(d),the latex layer becomes expanded and then separatedaway from the FEP layer, when the inside pressure waschanged owing to the human motions via the air-conducting channel. With small size and lightweight,M-TES could be used everywhere for air pressuredetection. The sensor presents a resolution of 0.34 Pawith air pressure increasing and 0.16 Pa with air pressuredecreasing. With further miniaturization, the sensor willreach higher sensitivity, which makes it suitable for self-powered human biomedical monitoring.



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Furthermore, Yang et al.94 reported a self-poweredbionic membrane sensor (BMS), which is flexible,lightweight, low-cost, simply fabricated, and capable of

operating in a multimodal way for biomedical monitoringand biometric authentication. As illustrated in Fig. 7(e),the BMS holds a multilayered structure with PET layer at

FIG. 5. Triboelectric nanogenerator for self-powered active acoustic sensing: (a) sketch and structural design of the triboelectric nanogenerators;(b and c) demonstration of the TENG acting as a self-powered microphone; frequency responses from the nanogenerator array, which consists offour NGs with various designed natural frequencies, aimed to enhance the overall working bandwidth; (c1 and c2) sound waveforms of the signalsacquired by NG1 and NG2, respectively; (c3, c4) short-time Fourier transforms of the acquired signals by NG1 and NG2, respectively; (d) soundwaveform and corresponding short-time Fourier transform of the signals acquired by the array of NGs; (e) photograph that shows an NG working asa self-powered microphone for sound recording. Reproduced with permission from the American Chemical Society.91



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the bottom as substrate and the ITO layer as electrode.Then, a layer of nylon was laminated on the ITO,followed by a tented PTFE layer on the top to forma conical cavity. A pair of holes with diameters of

0.5 mm was punched through the BMS bottom layers,serving as channels to communicate with the environ-mental air [Fig. 7(f)]. To promote the triboelectrification,nanowires were arranged on the surface of the PTFE

FIG. 6. Paper-based TENG for acoustic energy harvesting and self-powered sound recording: (a) schematic illustrations of the paper-based TENG;(b) photograph of the multihole paper electrode; (c) recycling the acoustic energy from the cell phone via charging a 2 lF capacitor; schematicillustrations to show the measurement of the directional patterns of the (d) flat and (e) rolled paper-thin triboelectric nanogenerator and right sidesare the photographs of the as-fabricated device; (f) photograph that shows a paper-thin triboelectric nanogenerator as a self-powered microphone forsound recording. Inset is the acquired electrical signals; (g) shape dependent directional patterns of the paper-based TENG with flat and rolledstructure, respectively. Reproduced with permission from the American Chemical Society.92



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layer [Fig. 7(g)]. To operate, the vibration mode of thePTFE layer in the BMS changed in diverse frequencies.Multiple BMSs were utilized to detect human pulsein different artery positions, which can capture moredata for human cardiovascular risk estimation. Figures 7(h)and 7(i) demonstrated the capability of BMS,respectively, for self-powered noninvasive arterial pulsemeasurement and as a superior self-powered throatmicrophone.

VI. TENG FOR HUMAN-MACHINE INTERFACING(HMI)

HMI is an effective way to convert human activitiesinto electric output to command robot to detect thedesired state by means of a biopotential system. Withthe contact electrification between human skin andPDMS, the output voltage and current density of theTENG device can, respectively, reach to 1000 V and8 mA/m2, which indicated that it can also be applied asactive sensors.95 Furthermore, Chen et al. developed anintelligent keyboard (IKB), which can be used to guar-antee the computers against unauthorized accessing.96

With a typical single-electrode TENG as the corecomponent, IKB can transfer the type motions intoelectrical signals relying on the contact electrificationbetween human finger and keys, which can identifypersonal typing signals that could be identified aspersonalized physiological data. As shown in Fig. 8(a),

the key functional element of IKB is consisted of PETlayer, ITO electrodes, and FEP layer. To enhance theeffective contact area, nanowires array was purposelycreated on the FEP surface, as shown in Fig. 8(b).Furthermore, the FEP surface was intrinsically hydro-phobic and with the increased surface roughness, the FEPsurface becomes superhydrophobic and self-cleaning.Figure 8(c) demonstrated the photograph of an as-fabricated IKB equipped on a commercial keyboard.The IKB system can successfully convert the humantyping motions into electrical signals. Each key on theIKB system was individually addressable by a multichan-nel measurement system. Figure 8(d) showed the outputvoltage signals from different channels. And the outputvoltage of the channel from each key striking can delivera peak-to-peak value of 10 V. By signal processing viathe Pauta criterion method and visualized by the Lab-View programming, the stroked key could display on thescreen simultaneously without observable delay [Fig. 8(e)]. In addition, by using the collected personal typingpatterns, two error rates, false rejection rate (FRR) andfalse acceptance rate (FAR), were used to evaluate theproperties of the biometrics authentication system. FRRis the percentage of a genuine user rejected by the systemincorrectly, while FAR is the probability of impostorinputs accepted incorrectly. Figure 8(f) chooses Pearsoncorrelation coefficient as the classification threshold toevaluate the behavioral biometric authentication system.With the increase of threshold, FRR was elevating while

FIG. 7. Self-powered biomedical sensor based on a TENG: (a) illustration of the membrane-based self-powered sensors for pressure changedetection; (b) SEM image of the polymer nanowires on the FEP surface; (c and d) photograph of the as-fabricated real device. Reproduced withpermission from Wiley.93 (e) Schematic illustration of the self-powered bionic membrane sensor; (f) photograph of the as-fabricated bionicmembrane sensor; (g) SEM image of the PTFE nanowires; (h) photograph showing that the as-fabricated bionic membrane sensors were directlyattached to simultaneously monitor the pulse waves of the participant from his carotid artery, chest, and wrist; (i) photograph showing that thebionic membrane sensor was worn against the participant’s neck acting as a self-powered throat microphone. Reproduced with permission fromWiley.94



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FAR becomes decreasing, and their intersection repre-sented the equal error rate (EER) point. In contrast withthe traditional technology, the biometric authenticationsystem obtained an obviously lower EER value of 1.34%at the threshold of 0.37. In the meanwhile, Fig. 8(g)indicated the receiver operating characteristic (ROC)curve of the biometric authentication system.

Regarding TENG based motion sensor for HMI, Yanget al.97 presented a motion sensor fabricated with flexible,

recyclable, skin-friendly materials to track human musclemotion. As illustrated in Fig. 9(a), five separated flexiblebiopotential electrodes were made up to form the self-powered motion sensor. And the PDMS film, as thefabricating material, was patterned with micropyramidswith an edge length of 4 lm, as shown in Fig. 9(d).Figures 9(b) and 9(c), respectively, demonstrated an as-fabricated motion sensor array and the one fixed on thesurface of a shoulder joint. Owing to its independence,

FIG. 8. Personalized keystroke dynamics for self-powered human-machine interfacing: (a) schematic illustrations of the key functional elementand inset shows an enlarged schematic of FEP nanowires on the top surface; it should be noted that these drawings are not to scale; (b) SEM imageof FEP nanowires and inset shows contour of the resting droplet for surface static contact angle measurement (the scale bar is 500 nm);(c) photograph of a flexible and transparent key functional element (the scale bar is 3 cm); (d) the system acquired output voltage signals when thekey “T” was stroked; (e) a photograph demonstrating the IKB for real-time keystroke tracing and recording (the scale bar is 5 cm) and inset showsan enlarged view of the key “G” being stroked (the scale bar is 2 cm); (f) evaluation of the performance of the biometric authentication system usingtriboelectrification enabled keystroke dynamics; the variation of FAR and FRR is related to the threshold. Inset: an enlarged view of the EER point;(g) receiver operating characteristic (ROC) curve of the biometric authentication system using triboelectrification enabled keystroke dynamics.Reproduced with permission from the American Chemical Society.96



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addressability, and self-powering, the motion sensor hasused to record electric output signals, which can preciselyquantify the freedom degrees, positions, and capacities ofmuscle motions. With an excellent signal-to-noise ratio of1000, the output current density and voltage generated bymotion sensor arrays can reach up to 10.71 mA/m2 and42.6 V. Therefore, the device can serve as a motionsensor to precisely record and convert the motions ofhuman joints, such as elbow, knee, heel, and even fingers,and thus distinguish it as a superior and unique inventionin the field of HMI.

VII. TENG AS A CHEMICAL SENSOR

The material surface of the triboelectric layers of theTENG is highly related to the device electrical signaloutput. While it is effective to modify the triboelectricsurface with some chemical species or alternatingchemical elements, and thus it could be developed intoself-powered active chemical sensors for such as ionconcentration and UV illumination detection.

The first work of TENG based chemical sensing wasreported by Lin et al.98 And it showed the feasibility ofambient Hg21 ions detection. By using a TENG ina vertical contact-separation mode, Au nanoparticles(NPs) were fixed on the metal slice to increase theeffective contact area. Furthermore, 3-mercaptopropionicacid (3-MPa) molecules were used to modify the NPs to

strengthen the Au–S interactions. Then, the gap betweenthe triboelectric polarity of AuNPs and Hg21 ions makesfeasible the Hg21 ions detection. Based on the rationaldesign, the electrical output of TENG achieved up to105 V in voltage and 63 lA in current with a devicedimension of 1 � 1 cm. With a detection limitation of30 nM, the reported nanosensor holds a linear range from100 nM to 5 lM for ambient Hg21 ions detection.

Moreover, Li et al.99 brought the conception to anapplication for self-powered phenol detection andelectrochemical degradation. In this work, b-cyclodextrin(b-CD) was selected both as a surface modificationchemical to enhance the TENG electrical output and alsoas the recognition element for phenol molecules de-tection. As shown in Fig. 10(a), acrylic was used as thestructural supporting backbone of the device, whichhas a multilayered structure. PTFE thin film was usedas the triboelectric layer with back-coated copper as theelectrode. b-CD was packaged onto the TiO2 nano-wires as the surface modified component. Figure 10(b)demonstrated the charge transfer between b-CD andTiO2. Figure 10(c) shows a TENG with b-CD surfacemodification with a dimension of 4 � 4 cm. Theperformance of the TENG based phenol sensor wasmeasured via dropping different concentrations ofphenol solutions onto the device. Figures 10(d) and10(e), respectively, showed the dependence of short-circuit current density Jsc and open circuit voltage Voc

FIG. 9. Triboelectrification based motion sensor for human-machine interfacing: a sketch (a) and photographs (b and c) of motion sensor array;(d) SEM image of the patterned PDMS film with pyramids features. Reproduced with permission from the American Chemical Society.97



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on different phenol concentrations, which both de-clined with the increasing of the phenol concentrationwithin a range from 10 lM to 100 lM. The decay ofthe electric output was largely because of a modifiedsurface triboelectric behavior, and phenol would re-place the position of TiO2 to contact with PTFE. Asshown in Fig. 10(f), the relative variation of outputvoltage and current with phenol concentrationsindicated that a sensitivity of 0.02 lM�1 was obtained.Furthermore, control experiments were carried out toevaluate the selectivity of phenol sensors as comparedwith other organic species, as shown in Fig. 10(g).With a constant concentration of 50 lM for all the

organic species, the output performance of phenol waslarger than all the other organic species, which in-dicated an outstanding selectivity of the sensor forphenol detection. In addition, a further step was takento electrochemically degrade the phenol in a self-powered manner in a large container, owing to thehigh electrical output performance of the TENG. Thiswork paved a new and efficient pathway for thedetection and degradation of the environmental chem-ical waste, which represented a solid advancement inthe fields of wastewater treatment, ecological sanita-tion, environmental assessment, monitoring, andsustainability.

FIG. 10. b-Cyclodextrin (b-CD) improved TENG for self-powered phenol detection and electrochemical degradation: (a) a sketch of the TENGwith b-CD surface modification for phenol detection; (b) schematic diagram for illustrating the charge transfer from the hydroxyl groups of b-CD toTiO2 nanowires; (c) a photograph of the as-fabricated device for phenol detection (the scale bar is 2 cm); (d and e) under a fixed 80 lM b-CDsurface modification, dependence of the current (d) and voltage (e) output on the phenol concentrations; (f) the sensitivity and detection range of theas-developed b-CD enhanced TENG for phenol detection in terms of both current and voltage output; (g) the selectivity of the as-developed b-CDenhanced TENG for phenol detection; the inset shows an illustration of the reaction mechanism between b-CD and different types of organicspecies. Reproduced with permission from the Royal Society of Chemistry.99



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FIG. 11. A self-powered electrochromic (EC) device driven by a TENG: (a) schematic structure of the WO3-based EC device; (b) SEM image of the WO3

film; (c) photographs of the EC device; (d) optical image of the colored EC; (e) the optical image of the bleached EC; (f) It and Vt of coloring (left pair) andbleaching processes (right pair); (g) the transmittance spectra of the EC in as-fabricated, colored, and bleached states. Reproduced with permission from theRoyal Society of Chemistry.100



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VIII. TENG AS POWER SOURCE FORSELF-POWERED SENSING SYSTEM

A rapid development of personal electronics and sensornetworks was to some extent hindered by the need ofportable and sustainable power sources, which are nowmostly relying on batteries. For batteries, the limitedlifetime and potential environmental detriment makes themunfavorable for sensor network with large node populationand wide-range distribution. Integrating miniaturizedenergy-harvesting device into the sensor system to realizethe self-powered operation would be a superior solution.

Electrochromic (EC) devices are able to change theoptic performance reversibly through electrons injectionand extraction powered by the applied potential in theexternal circuit. Yang et al.100 demonstrated a self-powered electrochromic device powered by a TENG.

As shown in Fig. 11(a), an EC cell consists of multiplelayers. The outer ones served as electrodes are glasscoated with FTO, and between the two electrodes is a ofWO3 layer with a thickness of about 250 nm [Fig. 11(b)] andpolyelectrolyte. From the photograph of Fig. 11(c), wecan see that the completely encapsulated EC devices havea transmittance even over 70%. In the discharging pro-cess, the voltage across the EC device and the currentflowing through it were simultaneously monitored, withthe results presented in Fig. 11(f) (left section). Thecurrent climbs to the highest point and then drops sharplywith the capacitor continuous discharging, at the sametime the device becomes dark in Fig. 11(d), which meansthe EC unit starts the process of coloring. Oppositely,voltage and current of the device [right section of Fig. 11(f)]are reversible and the EC unit gains the transmittanceagain [Fig. 11(e)], when the EC unit was reversely

FIG. 12. Self-charging system for sustainable operation of mobile electronics: (a) structure of the designed multilayer TENG; (b) photograph of anas-fabricated TENG; (c) short-circuit current output and (d) open-circuit voltage output of the as-fabricated TENG; (e) demonstration of a self-powered temperature sensor; (f) demonstration of a self-powered scientific calculator. Reproduced with permission from the Nature PublishingGroup.101



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connected. Due to the increasing resistances of chargetransfer from Li1 ions flowing into the WO3 film, the ECcell’s voltage also increases, which was continuouslydriven by a TENG. As shown in Fig. 11(g), the ECdevice finally switches from coloring to bleachingconstantly with a clearly observable difference in DTvis.The reported self-powered EC system can be applied inmonochrome displays or electronic billboards.

As known, the characteristics of human biomechanicalmotions are fluctuant in amplitudes and variable in lowfrequencies. Recently, Niu et al.101 reported an effectiveself-powered system by harvesting human biomechanicalenergy as active sensors. Basically, the system consists ofa high-output TENG, a manipulation of the circuit totransfer the chaotic biomechanical energy into power atan efficiency of 60%. For the energy-harvesting part,Kapton was structured into a zigzag shape by 10–15layers. In every layer, the thin Al foil was acting as oneelectrode and the copper back-coated onto FEP was theother electrode [Fig. 12(a)]. And the as-fabricated TENGhas small volume and was lightweight [Fig. 12(b)].Figures 12(c) and 12(d), respectively, show that thevoltage and the charge output can reach up to 700 Vand 2.2 lC when the TENG was packaged into shoes toharvest energy from human walking. A systematicallystructural optimization could assure that all individualparts can operate collaboratively. Additionally, the totalenergy efficiency can reach up to 60% after solving thepuzzle of impedance mismatch by using a designedpower management circuit, which greatly enhanced theefficiency approximately two orders compared with thatof the direct charging. By capturing power fromhuman motions, the self-powered sensing system canbe applied in many areas. As demonstrated in [Figs. 12(e)and 12(f)], respectively, a commercial thermometer anda scientific calculator were sustainably driven by thepower supplying system.

IX. CONCLUSION AND FUTURE WORK

As the rapid advancement of modern technologies,mobile and portable sensor systems with applications incommunication, personal healthcare, and environmentalmonitoring contributed largely to construct a world ofbetter life. Power supplying to the sensor nodes in thenetwork with large population and wide-range distribu-tion could be a challenge. Self-powered smart sensorsbased on TENGs without external power sources couldbe of wide-range interest in many fields. On one hand, theenvironmental energies harvested by the TENG couldprovide sustainable electrical power to drive the mobileelectronics and sensor networks. On the other hand, theelectrical current and voltage signals generated by theTENG represent the dynamic and static variation corre-sponding to the change of mechanical motions or other

environmental factors. It provides a superior solution forconstructing active sensors without the need of externalpower sources. Various applications have been success-fully demonstrated in the areas of micro-electromechanicalsystems, HMI, touching technology, security systems, andhuman-motion sensing.

With the introduction of new materials and surfaceengineering technologies, both the efficiency of TENGas a power source and sensitivity of TENG as activesensors are expected to be significantly improved. Asmoving toward the future applications of TENGs,several main issues and problems still need to be welladdressed.102,103 First, since the TENG output perfor-mance is highly subjected to environmental factors,such as temperature, humidity, pressure, particle con-taminations and so on. Effective packaging technologyof the TENG unit is vitally important to make com-mercialized products, especially for application ina variety of hash environments. Effects should bedevoted to take care of possible air gaps due to themotion part without affecting its sensitivity. Second, thedurability and output stability of the device can beimproved from both materials usage and the devicedesign. Third, based on the surface charging effect, theTENG based sensing application is also challengedwhen the device dimension is miniaturized, whichgreatly hinders its further development into the micro-electromechanical systems. More advanced device de-sign and fabrication techniques are highly desired toaccomplish a higher spatial resolution of the TENGbased active sensors.

In summary, this article gives a comprehensive reviewof the recent progress for TENG based self-poweredactive sensing. The tremendous developments in this fieldcould lead a revolution to our way of life, although thismay be a long-term goal.

ACKNOWLEDGMENTS

This work was sponsored by the Program for NewCentury Excellent Talents in University of China (NCET-13-0631) and the Fundamental Research Funds for theCentral Universities (106112016CDJZR225514). Theauthors also thank Professor Zhong Lin Wang and hisgroup at Georgia Institute of Technology for the pioneer-ing work on triboelectric nanogenerators and self-poweredsensing systems.

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34. W. Yang, J. Chen, Q. Jing, J. Yang, X. Wen, Y. Su, G. Zhu,P. Bai, and Z.L. Wang: 3D stack integrated triboelectric nano-generator for harvesting vibration energy. Adv. Funct. Mater.24(26), 4090 (2014).

35. S. Park, H. Kim, M. Vosgueritchian, S. Cheon, H. Kim, J. Koo,T. Kim, S. Lee, G. Schwartz, H. Chang, and Z. Bao: Stretchableenergy-harvesting tactile electronic skin capable of differentiatingmultiple mechanical stimuli modes. Adv. Mater. 26(43), 7324(2014).

36. J. Zhong, H. Zhu, Q. Zhong, J. Dai, W. Li, S. Jang, Y. Yao,D. Henderson, Q. Hu, L. Hu, and J. Zhou: Self-powered human-interactive transparent nanopaper systems. ACS Nano 9(7), 7399(2015).

37. G. Zhu, J. Chen, T. Zhang, Q. Jing, and Z.L. Wang: Radial-arrayed rotary electrification for high performance triboelectricgenerator. Nat. Commun. 5(3), 3426 (2014).

38. P. Bai, G. Zhu, Q. Jing, Y. Wu, J. Yang, J. Chen, J. Ma,G. Zhang, and Z.L. Wang: Transparent and flexible barcodebased on sliding electrification for self-powered identificationsystems. Nano Energy 12, 278 (2015).

39. J. Chen, J. Yang, H. Guo, Z. Li, L. Zheng, Y. Su, Z. Wen, X. Fan,and Z.L. Wang: Automatic mode transition enabled robusttriboelectric nanogenerators. ACS Nano 9(12), 12334 (2015).

40. J. Bae, J. Lee, S. Kim, J. Ha, B.S. Lee, Y. Park, C. Choong,J.B. Kim, Z.L. Wang, H.Y. Kim, J.J. Park, and U.I. Chung:Flutter-driven triboelectrification for harvesting wind energy.Nat. Commun. 5, 4929 (2014).



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42. C.K. Jeong, K.M. Baek, S. Niu, T.W. Nam, Y.H. Hur,D.Y. Park, G.T. Hwang, M. Byun, Z.L. Wang, Y.S. Jung, andK.J. Lee: Topographically-designed triboelectric nanogeneratorvia block copolymer self-assembly. Nano Lett. 14(12), 7031(2014).

43. Y. Yang, H. Zhang, S. Lee, D. Kim, W. Hwang, and Z.L. Wang:Hybrid energy cell for degradation of methyl orange by self-powered electrocatalytic oxidation. Nano Lett. 13(2), 803 (2013).

44. Y. Yang, H. Zhang, Y. Liu, Z. Lin, S. Lee, Z. Lin, C.P. Wong,and Z.L. Wang: Silicon-based hybrid energy cell for self-powered electrodegradation and personal electronics. ACS Nano7(3), 2808 (2013).

45. G. Zhu, P. Bai, J. Chen, and Z.L. Wang: Power-generating shoeinsole based on triboelectric nanogenerators for self-poweredconsumer electronics. Nano Energy 2(5), 688 (2013).

46. Q. Yang, Y. Liu, C. Pan, J. Chen, X. Wen, and Z.L. Wang:Largely enhanced efficiency in ZnO nanowire/p-polymer hybrid-ized inorganic/organic ultraviolet light-emitting diode by piezo-phototronic effect. Nano Lett. 13(2), 607 (2013).

47. W. Tang, T. Jiang, F.R. Fan, A.F. Yu, C. Zhang, X. Cao, andZ.L. Wang: Liquid-metal electrode for high-performance tri-boelectric nanogenerator at an instantaneous energy conversionefficiency of 70.6%. Adv. Funct. Mater. 25(24), 3718 (2015).

48. B. Meng, X. Cheng, X. Zhang, and H. Zhang: Single-friction-surface triboelectric generator with human body conduit. Appl.Phys. Lett. 104(10), 103904 (2014).

49. J. Zhong, Q. Zhong, Q. Hu, N. Wu, W. Li, B. Wang, B. Hu, andJ. Zhou: Stretchable self-powered fiber-based strain sensor. Adv.Funct. Mater. 25(12), 1798 (2013).

50. Y. Su, G. Zhu, W. Yang, J. Yang, J. Chen, Q. Jing, Z. Wu,Y. Jiang, and Z.L. Wang: Triboelectric sensor for self-poweredtracking of object motion inside tubing. ACS Nano 8(6), 3843(2014).

51. Z. Wen, J. Chen, M.H. Yeh, H. Guo, Z. Li, X. Fan, T. Zhang,L. Zhu, and Z.L. Wang: Blow-driven triboelectric nanogeneratoras an active alcohol breath analyzer. Nano Energy 16, 38 (2015).

52. Z. Li, J. Chen, J. Zhou, L. Zheng, X. Fan, Z. Wen, C. Yu, andZ.L. Wang: High-efficiency ramie fiber degumming and self-powered degumming wastewater treatment using triboelectricnanogenerator. Nano Energy 22, 548 (2016).

53. F. Yi, L. Lin, S. Niu, P.K. Yang, Z. Wang, J. Chen, Y. Zhou,Y. Zi, J. Wang, Q. Liao, Y. Zhang, and Z.L. Wang: Stretchable-rubber-based triboelectric nanogenerator and its application asself-powered body motion sensors. Adv. Funct. Mater. 25(24),3688 (2015).

54. H. Zhang, Y. Yang, Y. Su, J. Chen, K. Adams, S. Lee, C. Hu,and Z.L. Wang: Triboelectric nanogenerator for harvestingvibration energy in full space and as self-powered accelerationsensor. Adv. Funct. Mater. 24(10), 1401 (2014).

55. Q. Jing, G. Zhu, P. Bai, Y. Xie, J. Chen, R.P.S. Han, andZ.L. Wang: Case-encapsulated triboelectric nanogenerator forharvesting energy from reciprocating sliding motion. ACS Nano8(4), 3836 (2014).

56. Z. Quan, C. Han, T. Jiang, and Z.L. Wang: Robust thin films-based triboelectric nanogenerator arrays for harvesting bidirec-tional wind energy. Adv. Energy Mater. 6, 1501799 (2015).

57. R. Yu, C. Pan, J. Chen, G. Zhu, and Z.L. Wang: Enhancedperformance of a ZnO nanowire-based self-powered glucose sensorby piezotronic effect. Adv. Funct. Mater. 23(47), 5868 (2013).

58. Z. Lin, G. Cheng, W. Wu, K.C. Pradel, and Z.L. Wang: Dual-mode triboelectric nanogenerator for harvesting water energy and

as a self-powered ethanol nanosensor. ACS Nano 8(6), 6440(2014).

59. H. Zhang, Y. Yang, Y. Su, J. Chen, C. Hu, Z. Wu, Y. Liu,C.P. Wong, Y. Bando, and Z.L. Wang: Triboelectric nano-generator as self-powered active sensors for detecting liquid/gaseous water/ethanol. Nano Energy 2(5), 693 (2013).

60. T. Jiang, L. Zhang, X. Chen, C. Han, W. Tang, C. Zhang, L. Xu,and Z.L. Wang: Structural optimization of triboelectric nano-generator for harvesting water wave energy. ACS Nano 9(11),12562 (2015).

61. Y. Fang, J. Tong, Q. Zhong, Q. Chen, J. Chou, Q. Luo, Y. Zhou,Z.L. Wang, and B. Hu: Solution processed flexible hybrid cellfor concurrently scavenging solar and mechanical energies. NanoEnergy 16, 301 (2015).

62. J. Chen, J. Yang, Z. Li, X. Fan, Y. Zi, Q. Jing, H. Guo, Z. Wen,K.C. Pradel, S. Niu, and Z.L. Wang: Networks of triboelectricnanogenerators for harvesting water wave energy: A potentialapproach toward blue energy. ACS Nano 9(3), 3324 (2015).

63. L. Zhang, B. Zhang, J. Chen, L. Jin, W. Deng, J. Tang,H. Zhang, H. Pan, M. Zhu, W. Yang, and Z.L. Wang: Lawnstructured triboelectric nanogenerators for scavenging sweepingwind energy on rooftop. Adv. Mater. 28(8), 1650 (2016).

64. S. Niu, S. Wang, L. Lin, Y. Liu, Y. Zhou, Y. Hu, and Z.L. Wang:Theoretical study of contact-mode triboelectric nanogenerators as aneffective power source. Energy Environ. Sci. 6(12), 3576 (2013).

65. X. Wang, L. Dong, H. Zhang, R. Yu, C. Pan, and Z.L. Wang:Recent progress in electronic skin. Adv. Sci. 2, 1500129 (2015).

66. S. Niu, Y. Liu, S. Wang, L. Lin, Y. Hu, and Z.L. Wang: Theoryof sliding-mode triboelectric nanogenerators. Adv. Mater. 25(43),6184 (2013).

67. Y. Zi, J. Wang, S. Wang, S. Li, Z. Wen, H. Guo, and Z.L. Wang:Effective energy storage from a triboelectric nanogenerator. Nat.Commun. 7, 10987 (2016).

68. C. Zhang, Z. Zhang, X. Yang, T. Zhou, C. Han, and Z.L. Wang:Tribotronic phototransistor for enhanced photodetection andhybrid energy harvesting. Adv. Funct. Mater. 26(15), 2554(2016).

69. C. Wu, X. Wang, L. Lin, H. Guo, and Z.L. Wang: Paper-basedtriboelectric nanogenerators made of stretchable interlockingkirigami patterns. ACS Nano 10(4), 4652 (2016).

70. J. Chun, J.W. Kim, W. Jung, C. Kang, S. Kim, Z.L. Wang, andJ.M. Baik: Mesoporous pores impregnated with Au nanoparticlesas effective dielectrics for enhancing triboelectric nanogeneratorperformance in harsh environments. Energy Environ. Sci. 8(10),3006 (2015).

71. S. Niu, Y. Liu, X. Chen, S. Wang, L. Lin, Y. Hu, and Z.L. Wang:Theory of freestanding triboelectric-layer-based nanogenerators.Nano Energy 12, 760 (2015).

72. J. Zhong, Y. Zhang, Q. Zhong, Q. Hu, B. Hu, Z.L. Wang, andJ. Zhou: Fiber-based generator for wearable electronics andmobile medication. ACS Nano 8(6), 6273 (2014).

73. M. Yeh, H. Guo, L. Lin, Z. Wen, Z. Li, C. Hu, and Z.L. Wang:Rolling friction enhanced free-standing triboelectric nanogener-ators and their applications in self-powered electrochemicalrecovery systems. Adv. Funct. Mater. 26(7), 1054 (2016).

74. L. Su, Z. Zhao, H. Li, J. Yuan, Z.L. Wang, G. Cao, and G. Zhu:High-performance organolead halide perovskite-based self-poweredtriboelectric photodetector. ACS Nano 9(11), 11310 (2015).

75. Z. Li, J. Chen, H. Guo, X. Fan, Z. Wen, M.H. Yeh, C. Yu,X. Cao, and Z.L. Wang: Triboelectrification enabled self-powered detection and removal of heavy metal ions in waste-water. Adv. Mater. 28(15), 2983 (2016).

76. W. Peng, R. Yu, Y. He, and Z.L. Wang: Theoretical study oftriboelectric-potential gated/driven metal-oxide-semiconductorfield-effect transistor. ACS Nano 10(4), 4395 (2016).



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77. J. Li, C. Zhang, L. Duan, L. Zhang, L. Wang, G. Dong, andZ.L. Wang: Flexible organic tribotronic transistor memory fora visible and wearable touch monitoring system. Adv. Mater.28(1), 106 (2016).

78. Z.L. Wang: Triboelectric nanogenerators as new energy technol-ogy and self-powered sensors-principles, problems and perspec-tives. Faraday Discuss. 176(11), 447 (2014).

79. W. Yang, J. Chen, G. Zhu, J. Yang, P. Bai, Y. Su, Q. Jing,X. Cao, and Z.L. Wang: Harvesting energy from the naturalvibration of human walking. ACS Nano 7(11), 11317 (2013).

80. L. Zheng, G. Cheng, J. Chen, L. Lin, J. Wang, Y. Liu, andZ.L. Wang: A hybridized power panel to simultaneously gener-ate electricity from sunlight, raindrops and wind around theclock. Adv. Energy Mater. 5(12), 1501152 (2015).

81. L. Lin, Y. Xie, S. Wang, W. Wu, S. Niu, X. Wen, andZ.L. Wang: Triboelectric active sensor array for self-poweredstatic and dynamic pressure detection and tactile imaging. ACSNano 7(9), 8266 (2013).

82. Y. Yang, H. Zhang, Z. Lin, Y. Liu, J. Chen, Z. Lin, Y. Zhou,C.P. Wong, and Z.L. Wang: A hybrid energy cell for self-powered water splitting. Energy Environ. Sci. 6(11), 2429(2013).

83. G. Zhu, J. Chen, Y. Liu, P. Bai, Y. Zhou, Q. Jing, C. Pan, andZ.L. Wang: Linear-grating triboelectric generator based onsliding electrification. Nano Lett.13(3), 2282 (2013).

84. G. Zhu, Z. Lin, Q. Jing, P. Bai, C. Pan, Y. Yang, Y. Zhou,and Z.L. Wang: Toward large-scale energy harvesting bya nanoparticle-enhanced triboelectric nanogenerator. Nano Lett.13(3), 847 (2013).

85. X. Zhang, M. Han, R. Wang, F. Zhu, Z. Li, W. Wang, andH. Zhang: Frequency-multiplication high-output triboelectricnanogenerator for sustainably powering biomedical microsys-tems. Nano Lett. 13(3), 1168 (2013).

86. G. Zhu, Y.S. Zhou, P. Bai, X. Meng, Q. Jing, J. Chen, andZ.L. Wang: A shape-adaptive thin-film-based approach for 50%high-efficiency energy generation through micro-grating slidingelectrification. Adv. Mater. 26(23), 3788 (2014).

87. G. Zhu, W. Yang, T. Zhang, Q. Jing, J. Chen, Y.S. Zhou, P. Bai,and Z.L. Wang: Self-powered, ultrasensitive, flexible tactilesensors based on contact electrification. Nano Lett. 14(6), 3208(2014).

88. J. Chen, G. Zhu, W. Yang, Q. Jing, P. Bai, Y. Yang, T.C. Hou,and Z.L. Wang: Harmonic-resonator-based triboelectric nano-generator as a sustainable power source and a self-poweredactive vibration sensor. Adv. Mater. 25(42), 6094 (2013).

89. S. Wang, S. Niu, J. Yang, L. Lin, and Z.L. Wang: Quantitativemeasurements of vibration amplitude using a contact-modefreestanding triboelectric nanogenerator. ACS Nano 8(12),12004 (2014).

90. Y. Hu, J. Yang, Q. Jing, S. Niu, W. Wu, and Z.L. Wang:Triboelectric nanogenerator built on suspended 3D spiral

structure as vibration and positioning sensor and wave energyharvester. ACS Nano 7(11), 10424 (2013).

91. J. Yang, J. Chen, Y. Liu, W. Yang, Y. Su, and Z.L. Wang:Triboelectrification-based organic film nanogenerator for acous-tic energy harvesting and self-powered active acoustic sensing.ACS Nano 8(3), 2649 (2014).

92. X. Fan, J. Chen, J. Yang, P. Bai, Z. Li, and Z.L. Wang: Ultrathin,rollable, paper-based triboelectric nanogenerator for acousticenergy harvesting and self-powered sound recording. ACS Nano9(4), 4236 (2015).

93. P. Bai, G. Zhu, Q. Jing, J. Yang, J. Chen, Y. Su, J. Ma,G. Zhang, and Z.L. Wang: Membrane-based self-powered tri-boelectric sensors for pressure change detection and its uses insecurity surveillance and healthcare monitoring. Adv. Funct.Mater. 24(37), 5807 (2014).

94. J. Yang, J. Chen, Y. Su, Q. Jing, Z. Li, F. Yi, X. Wen, Z. Wang,and Z.L. Wang: Eardrum-inspired active sensors for self-powered cardiovascular system characterization and throat at-tached anti-interference voice recognition. Adv. Mater. 27(8),1316 (2015).

95. Y. Yang, H. Zhang, Z. Lin, Y.S. Zhou, Q. Jing, Y. Su, J. Yang,J. Chen, C. Hu, and Z.L. Wang: Human skin based triboelectricnanogenerators for harvesting biomechanical energy and as self-powered active tactile sensor system. ACS Nano 7(10), 9213(2013).

96. J. Chen, G. Zhu, J. Yang, Q. Jing, P. Bai, W. Yang, X. Qi, Y. Su,and Z.L. Wang: Personalized keystroke dynamics for self-powered human-machine interfacing. ACS Nano 9(1), 105 (2015).

97. W. Yang, J. Chen, X. Wen, Q. Jing, J. Yang, Y. Su, G. Zhu,W. Wu, and Z.L. Wang: Triboelectrification based motion sensorfor human-machine interfacing. ACS Appl. Mater. Interfaces6(10), 7479 (2014).

98. Z. Lin, G. Zhu, Y. Zhou, Y. Yang, P. Bai, J. Chen, andZ.L. Wang: A self-powered triboelectric nanosensor for mercuryion detection. Angew. Chem., Int. Ed. 52(19), 5065 (2013).

99. Z. Li, J. Chen, J. Yang, Y. Su, X. Fan, Y. Wu, C. Yu, andZ.L. Wang: b-Cyclodextrin enhanced triboelectrification for self-powered phenol detection and electrochemical degradation.Energy Environ. Sci. 8(3), 887 (2015).

100. X. Yang, G. Zhu, S. Wang, R. Zhang, L. Lin, W. Wu, andZ.L. Wang: A self-powered electrochromic device driven bya nanogenerator. Energy Environ. Sci. 5(11), 9462 (2012).

101. S. Niu, X. Wang, F. Yi, Y. Zhou, and Z.L. Wang: A universalself-charging system driven by random biomechanical energy forsustainable operation of mobile electronics. Nat. Commun. 16(1),37 (2015).

102. J. Chen: Triboelectric Nanogenerators (Georgia Institute ofTechnology, Atlanta, 2016).

103. J. Chen and Z.L. Wang: Triboelectric nanogenerators as aninnovation on vibration energy harvesting and self-poweredvibrational sensing. Joule (2017), in press.



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