Material aspects of triboelectric energy generation and sensors

The triboelectric nanogenerator (TENG) is a new type of energy generator first demonstrated in 2012. TENGs have shown potential as power sources for electronic devices and as sensors for detecting mechanical and chemical stimuli. To date, studies on TENGs have focused primarily on optimizing the systems and circuit designs or exploring possible applications. Even though triboelectricity is highly related to the material properties, studies on materials and material designs have been relatively less investigated. This review article introduces recent progress in TENGs, by focusing on materials and material designs to improve the electrical output and sensing performance. This article discusses the current technological issues and the future challenges in materials for TENG. The development of materials for a technology that uses the movement of the human body to provide power has been reviewed by scientists in South Korea. A triboelectric nanogenerator converts mechanical energy into electricity by harnessing the fact that two surfaces rubbing against one another can become electrically charged. This is known as the triboelectric effect. One exciting use for these nanogenerators is in wearable electronics, where the motion of the body provides the power. Unyong Jeong and colleagues from Pohang University of Science and Technology have reviewed recent progress in material advances in the four main elements of a triboelectric nanogenerator: the charge-generating layer, the charge-trapping layer, the charge-collecting layer, and the charge-storage layer. These improvements all aim to increase the electrical output of such devices. Over the last decade, triboelectric nanogenerator (TENG) has been verified to be an effective way of converting daily mechanical energy into electric power or detecting various stimuli in the external environment. To promote the material researches in TENG, we introduce recent progresses in materials and material designs to improve the power generation and sensing performance. Also, we discuss on the future challenges and suggest possible approaches to solve the challenges.


Introduction
During the last decade, triboelectric nanogenerators (TENGs) have been verified to effectively convert daily mechanical energy into useful electric power. Not only natural energy such as rain or wind but also everyday body motion such as hand touching or walking can serve as a mechanical energy source for triboelectrification [1][2][3] . In TENG devices, charges are separated on the contact surfaces, and an electrical potential is generated between the surfaces. The alternating potential resulting from the dynamic mechanical motions can be stored in a storage unit or can be used for powering electric devices 4 . In addition, the potential profiles or the current profiles of the TENG can also be used as sensing signals to monitor the mechanical or chemical stimuli exerted on TENGs 5 . To date, system design and circuit design have been extensively studied to improve the output performance of TENGs. Four different modes (contact-separation mode [6][7][8] , sliding mode 9,10 , single-electrode mode 11,12 , and freestanding mode 13,14 ) have been established, and a few device structures by using elastic spacers [15][16][17][18] or arcshape substrates 19 have been suggested. Ideal cyclic device operation and the corresponding circuit design have been proposed theoretically and tested experimentally 20,21 . Analog circuits have been developed to analyze the irregular TENG profiles and to minimize the power loss caused by the signal irregularity 22 . Relatively, material design to improve the TENG performance has not been investigated thoroughly. Microscopic understanding of the electrification mechanism is not well established, and the materials used for TENGs are limited to a few species [23][24][25] . Most studies on materials have focused on the morphological optimization of the contact surfaces 3,26,27 .
In this review article, we introduce recent progress in the material design for TENGs. The material aspects of enhancing the electric power output are discussed first, and the material designs for sensor applications are discussed next. This review ends with future challenges and suggestions for materials for TENG devices.

Material approaches to enhance the triboelectric power
In this section, we introduce the progress in materials for four representative component layers in TENGs: the charge-generating layer, the charge-trapping layer, the charge-collecting layer, and the charge-storage layer ( Fig. 1). The approaches for designing the materials used in those layers are discussed, and future directions are suggested.

Charge-generating layer
Because the performance of TENGs is proportional to the charge density of the contact surface 28 , increasing the charge generation has been the main strategy to improve the output electric power. The square of the charge density is the main parameter used to quantify a TENG's performance as a material figure of merit 29 . Several approaches from the materials perspective have been reported  . The approaches include increasing the contact area by fabricating microscale or nanoscale surface structures, surface functionalization to increase the surface charge density or to facilitate triboelectric charge transfer, modification of the surface property by electrical poling or doping, and creation of new materials by molecular synthesis or nanocomposite formation.
Increasing the contact surface area is a straightforward route to improve the total number of generated charges. When the two surfaces make contact by mechanical force, the actual contact area is much smaller than the surface areas of the two surfaces 37 . Fabricating microstructures on the surface has been widely investigated as a means to increase the contact area. Various methods to fabricate the microstructures have been reported, including forceassembled colloidal arrays 30 , soft lithography 31 , anodic aluminum oxide (AAO) 32 , block-copolymer assembly 33 , and surface nanomaterial fabrication 34,35 . The microstructure on the hydrophobic surface not only improves the output performance of a TENG but also improves the sustainability of the performance in humid environments. These approaches are reviewed in several reports 24,30 . To date, most studies on the microstructured surfaces have focused on the increase in surface area 24,[30][31][32][33][34][35][36] . Although triboelectrification is based on charging during contact separation or surface friction, adhesion between the contact materials has rarely been investigated. Recently, Lee et al. reported that increasing the adhesion energy can enhance the surface charge generation 37 (Fig. 2a). They formed poly(dimethylsiloxane) (PDMS) micropillars on a PDMS substrate and used a PDMS thin film to cover the top of the pillars. The large elastic deformation of the PDMS thin film during contact and separation stored the applied mechanical energy; therefore, the charge generation on the adhesive surfaces was more effective than that on nonadhesive surfaces. To clearly understand the enhancement of charge generation by microstructure formation, the relationship between the deformation dynamics of the microstructures and the charge distribution near the microstructures should also be investigated. These studies need to control the viscoelasticity and surface adhesion of the microstructures. Recently, liquid-solid contact electrification has been studied on microstructured surfaces 38,39 . Combining the liquid-solid contacts and the solid-solid contacts may provide a good strategy to separate the effects of the increase in the surface area from the effect of the microstructure deformation dynamics. In addition, liquid-liquid contact electrification between two pure liquids can generate triboelectricity 40 . The droplets falling through a freestanding liquid membrane can provide continuous power generation (peak power of 137.4 nW) and can be applied to many targets, including raindrops, microfluidics, and irrigation currents.
A large difference in surface potentials between two contact surfaces results in a large triboelectric output 41 ; thus, functionalization of the material surface is one of the key approaches to increasing the charge density of TENGs. Surface modification through ion doping, radical injection, and plasma treatment can have a significant impact on the charge generation [42][43][44][45] . When a fluorinated ethylene propylene (FEP) substrate was exposed to negatively ionized air, the electron transfer from an aluminum to the FEP surface was greatly increased 42 (Fig.  2b). After injecting oxygen radicals into the PDMS surface, sodium hydroxide treatment converted the nonpolar Si-CH 3 bonds to polar Si-O bonds, resulting in a 15-fold enhancement in power over the untreated PDMS 43 . Fluorinated PDMS and fluorinated poly(ethylene terephthalate) (PET) substrates prepared by tetrafluoromethane (CF 4 ) plasma treatment considerably improved the energy density 44,45 . Self-assembled monolayers with electron-donating and electron-withdrawing functional end groups effectively changed the surface dipoles of the contact layer 46,47 , hence significantly enhancing or reducing the triboelectric output.
Changing the intrinsic material properties (dielectric constant, polarity, work function, etc.) has been pursued through nanocomposite formation, electrical poling, Fig. 1 Representative components of a triboelectric nanogenerator (TENG) affecting triboelectric power generation. The chargegenerating layer (red), the charge-trapping layer (green), the chargecollecting layer (gray), and the charge-storage layer (white) can affect power generation in TENG. chemical doping, and material synthesis. Poly(vinylidene fluoride-co-trifluoroethylene) (PVDF-TrFE), which is a well-known ferroelectric polymer, can be polarized by electrical poling 48 . Lee et al. demonstrated that after electrical poling, PVDF-TrFE was ranked higher in the positive triboelectric series than human skin 49 . In an effort to improve charge generation, nanocomposite materials have been studied extensively. Electrical poling of a nanocomposite consisting of PVDF-TrFE and high-k barium titanate (BaTiO 3 ) nanoparticles resulted in an 18fold increase in charge generation compared with that of pure PVDF-TrFE 50 (Fig. 2c). Nanocomposites of poly (vinylidene fluoride) (PVDF) and zinc oxide (ZnO) nanorods 58 or zinc stannate (ZnSnO 3 ) nanocubes 59 exhibited 60 and 200% higher instantaneous output currents than the bare PVDF film-based TENGs, respectively. These significant output increases were attributed not only to the enhancement in β-phase content in a ferroelectric PVDF but also to the enhancement in surface roughness and hydrophobicity. When PVDF was doped with an electrolyte with an asymmetric ion pair, the triboelectrification property dramatically changed. Fig. 2 Utilization of the charge-generating layer for power enhancement of TENGs. a Increased triboelectrification by enhancing the conformal contact and adhesion energy with a film-covered pillar structure (modified from ref. 37 , copyright permission from Elsevier). b Maximizing surface charge density achieved by ionized-air injection (modified from ref. 42 , copyright permission from Wiley-VCH). c Boosting power generation via control of ferroelectric polarization and dielectric properties (modified from ref. 50 , copyright permission from Wiley-VCH). d Enhanced current density output with the synthesis of grafted copolymers for dielectric constant control (modified from ref. 53 , copyright permission from American Association for the Advancement of Science). e Triboelectricity of regenerative natural substances covered with lipid layers (modified from ref. 54 , copyright permission from Wiley-VCH). f Triboelectric series of 2D layered materials (modified from ref. 57 , copyright permission from Wiley-VCH).
Depending on the type of doping ions, PVDF varied from the most negative material to the most positive material 51 . Kim et al. reported that deposition of gold (Au) nanoparticles coated with positively charged 4-(dimethylamino)pyridine (DMAP) on the PDMS contact surface enhanced the charge density by a factor of 2 52 . This study revealed that the electron transfer from the Au nanoparticles to the dielectrics became easier because the effective work function of the Au nanoparticle was decreased by the permanent dipole induced at the DMAP-Au interface. A material with a high dielectric constant can be a good candidate for high-density charge generation. Grafting of poly(tert-butyl acrylate) (PtBA) in the PVDF polymer chain remarkably increased the charge density in proportion to the mole fraction of PtBA in the graft copolymer 53 (Fig. 2d).
Since Johan Carl Wilcke published the first triboelectric series in 1757, little change has been made in the material species. Recently, several new materials have been added to the triboelectric series. Kim et al. reported that lipid layers on the surface of regenerative natural substances (skin, hair, leaves, and cells) are responsible for the highly positive triboelectricity 54 (Fig. 2e). Similar to this study, other studies have utilized leaves and petals as the charge-generating layers of TENGs 55,56 . The charge density of a lipid-based TENG was four times higher than the charge density of a nylon 6-based TENG. These naturally regenerative materials open up the possibility of energy harvesting without damaging or changing the natural environment. Kim et al. also revealed that commercially available synthetic lipids have similar triboelectric characteristics regardless of the functional groups in the lipid molecules 54 . Unfortunately, the reason why the lipids have such a high positive charge density is not yet understood. In-depth studies on the charge distribution of a single molecule, the effect of the self-assembled structure of the lipids, and the effect of crystal deformation under mechanical contacts are needed. Two-dimensional (2D) materials (MoS 2 , MoSe 2 , WS 2 , WSe 2 , graphene, graphene oxide, etc.) are relatively new materials and have been tested for TENGs. Seol et al. investigated the triboelectric behavior of various 2D materials and placed their ranks in the triboelectric series 57 (Fig. 2f).

Charge-trapping layer
The triboelectric surface potential at one surface induces the opposite electrostatic potential at the interface with the counter electrode. The surface charges can be drifted by the electric field and combined with the induced opposite charges. This charge combination interferes with surface charge accumulation and causes a sharp decrease in the triboelectric potential. Although it is important to block the charge combination, studies of the insulating interlayer and charge-trapping layer are relatively new. This section introduces the recent advances in materials for blocking the charge combination.
In polymer insulators, charge traps are formed by physical defects such as amorphous free volume, crosslinking points, and imperfections in the crystal lattice and chemical defects such as dangling bonds and functional groups in the polymer chains 60,61 . Previous studies have reported that polymers containing aromatic rings in their chains, such as in polystyrene (PS) and polyimide (PI), have many trapping sites due to the nonuniform energy levels along their main chains 62 . The addition of aromatic polymer films between the PVDF layer and the collector electrode trapped triboelectric charges and increased the triboelectric output by 7-9 times compared with the device without the interlayer 63,64 (Fig. 3a). PDMS has been found to have deeper charge traps and a larger trap density than the aromatic polymers ( Fig. 3b). Adding a PDMS thin layer below the charge-generating layer increased the power density of TENGs by 178 times. In addition, the high stretchability of the PDMS layer enabled the fabrication of stretchable TENGs 65 . Various metal oxide thin films have been reported to block the charge combination due to their high permittivity, a large number of oxygen vacancies, and the high interface density of states. Adding a titanium oxide (TiO X ) layer maintained the high surface charge density by suppressing the charge combination; thus, a significant enhancement in TENG performance was obtained 66 . A TENG with an indium zinc oxide (IZO) interlayer was also reported to provide a large interface density of states and to function as a charge reservoir 67 .
Nanomaterials have larger surface-to-volume ratios than thin-film interlayers and are likely to provide more charge trapping. It has been shown that monolayer MoS 2 acts as a triboelectric electron acceptor and restricts the charge combination so that polymer composites containing monolayer MoS 2 nanosheets improved the power density of TENGs 68 . In a similar mechanism, composites of reduced graphene oxide (rGO) nanosheets effectively trapped electrons and resulted in a large output enhancement 69 . Black phosphorous (BP), decorated with hydrophobic cellulose oleoyl ester nanoparticles, served as a synergistic electrontrapping additive. These printable and coatable composites introduce the possibility of manufacturing high-performance textile TENGs 70 (Fig. 3c). The large interface between the conductive fillers and the polymer matrix in nanocomposites can act as charge-trapping sites. In PVDF nanofibers, the addition of Ag nanowires (Ag NWs) promoted the formation of a β-crystalline phase in the PVDF chains and trapped the electrostatically induced charges at the metal-dielectric interfaces. The synergistic effect increased the output performance of TENGs 71 (Fig. 3d). Incorporating nanocrystals, such as metal organic frameworks (MOFs) 72 and titania monolayers (Ti 0.87 O 2 ) 73 , into the charge-generating layers enhanced the TENG output power due to the synergetic effects between efficient electron capture and the increase in the dielectric constant of the nanocrystals.

Charge-collecting layer
When two dielectric surfaces come into contact, the generated charges are electrostatically driven to flow from one electrode to another electrode 28 . When an electrode contacts a dielectric, the electrode acts as both a chargegenerating layer and a charge-collecting layer 42 . Solid and flexible electrodes, including metals 74,75 , graphene 18 , and indium tin oxide (ITO), are well established 17,76 . Relatively, the practical application of deformable electrodes is still very limited because deformable TENGs and TENG sensors require reliable deformability in electrodes. Some of the approaches to produce deformable electrodes and their uses for stretchable TENGs are introduced in this section.
There have been extensive studies on deformable electrodes. In-plane serpentine metal electrodes and out-ofplane buckled electrodes have been widely used in stretchable electronic devices [77][78][79][80][81] . Considering the large dimensions of TENGs and the high mechanical impacts/ frictions applied on the device, such wavy metal electrodes are not appropriate for general use in TENGs. Conductive elastic nanocomposites are alternative electrodes because low-cost printing over a large area is possible, and the composite is mechanically stable under repeated mechanical impacts and friction [82][83][84] . Elastomer composites made of metal flakes or metal nanosheets have often been used to fabricate deformable TENGs 85,86 . The Au nanosheet elastomer composite electrode has stable electrical properties under various mechanical environments 84 . The Au nanosheets were embedded in a PDMS matrix and used as a charge-collecting layer in TENGs 65,87 (Fig. 4a). During the 10,000 repetitive cycles of elongational strain (30%), the initial voltage output was held constant (Fig. 4b).
Liquid metals are another choice for highly stretchable electrodes [88][89][90] . Due to their fluidic nature at room temperature, liquid metals can be injected into a microchannel fabricated in an elastomer matrix. Figure 4c shows a single-electrode TENG 91 . When human skin separates from a rubber surface, charges are generated, and electrons flow from the liquid metal to the ground. When human skin approaches the rubber, the electrons flow in the reverse direction to the liquid metal. The single-electrode TENG is highly stretchable and deformable without causing rupture of the electrode 91 (Fig. 4d). Not only the liquid metal but also the liquid-state conducting polymer, a solution of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and physiological saline, was reported as a shape-adaptive and stretchable charge collector in TENGs 92,93 . Utilization of the charge-trapping layer for power enhancement. a Accumulation and decay processes of the trapped triboelectric charges with/without the PS dielectric interlayer (modified from ref. 63 , copyright permission from the American Chemical Society). b Chargetrapping mechanism with a PDMS deep-trap interlayer (modified from ref. 65 , copyright permission from Elsevier). c Black phosphorus layer coated with hydrophobic nanoparticles used for trapping the electrons (modified from ref. 70 , copyright permission from Nature Publishing Group). d Enhancement of charge trapping and charge induction by adding silver nanowires (Ag NWs) into the PVDF nanofiber matrix (modified from ref. 71 , copyright permission from Wiley-VCH).
Electrolyte gels such as hydrogels and ion gels allow both high stretchability and high transparency. Hydrogels are composed of a hydrophilic polymer network swollen with water or ionic aqueous solution 94,95 . The weak adhesion between the conductive electrolyte and the electrification elastomer layer can be an issue in TENGs in terms of mechanical reliability. Liu et al. demonstrated a strong interface between the polyacrylamide−sodium alginate (PAAm−alginate) hydrogel and the silicone rubber 96 . The hydrogel was sandwiched between two elastomer films containing benzophenone. Benzophenone formed covalent bonds between the hydrogel and the silicone rubber (Ecoflex); hence, the layers maintained a strong interface under a large elongation (Fig. 4e). The output voltage of the single-electrode TENG showed no change during 5,000 cycles of repeated contact-separation motions. If the effect of humidity on electrolyte-based TENGs can be solved, the electrolyte will be a promising stretchable and transparent electrode for TENGs 97,98 .

Charge-storage layer
To use the generated triboelectricity as a power supply, the power must be stored in an energy-storage unit, and the AC power from TENGs must be converted to DC power. Typically, a circuit board containing a capacitor for power storage and a bridge rectifier for DC conversion has been used 99 . Since the absolute power of TENGs is small, application of these materials has been concentrated on wearable mobile sensors that do not require high electric power. Flexible or stretchable capacitors and rectifiers are suitable for mechanically compliant wearable devices. In addition, they should be integrated all-in-one on a flexible or stretchable substrate. This aspect of TENGs has rarely been investigated. This section introduces studies on the fabrication of flexible stretchable capacitors and rectifiers that require new types of materials.
A straightforward way to store triboelectric energy is to incorporate a rechargeable battery with a TENG. Nan et al. reported a high energy conversion efficiency from a TENG to a lithium-ion battery (LIB) 100 . The schematic in Fig. 5a represents a power-storage system consisting of a TENG and an LIB. Various cathode materials were reported, including Li 3 V 2 (PO 4 ) 3 100 , LiFePO 4 (LFP) 101,102 , LiMn 2 O 4 (LMO) 103 , and LiCoO 2 (LCO) 104 , to charge the triboelectric energy. They obtained a high conversion Fig. 4 Utilization of stretchable materials for charge collection. a Stretchable electrode made by embedding Au nanosheets in a PDMS matrix and b stable voltage output when cyclic elongational strain was applied (modified from ref. 87 , copyright permission from Elsevier). c Working mechanism of a TENG device by using a liquid metal electrode and d the high stretchability of the device (modified from ref. 91 , copyright permission from American Chemical Society). e Ionic conductor consisting of PAAm-alginate hydrogel and PDMS. The interface was modified by benzophenone for strong bonding between the layers (modified from ref. 96 , copyright permission from the American Chemical Society). efficiency (more than 80%). However, charging was performed with a rotary TENG at a high frequency (1300 Hz) of rotary sliding 105,106 . A conventional Li-ion battery requires a current at the mA level for charge/discharge 106 ; hence, direct charging in an LIB requires a coil transformer that increases the amplitude of the AC output current or a high-frequency stimulation (such as in highspeed rotating drum). Unfortunately, direct charging of a battery with a TENG is not practical due to the low current density that cannot overcome the charging energy barrier.
Thus far, the energy from TENGs has been stored in capacitors. A capacitor can be fabricated simply by inserting a dielectric layer between two electrodes. Since the dielectric capacitance is expressed by the equation C = kε o A/d, where k, ε o , A, and d are the dielectric constant, vacuum permittivity, electrode area, and dielectric thickness, respectively, the performance is easy to characterize by the dimensions and inherent permittivity. The fast charge/discharge performance of inorganic dielectric materials can be used for TENG capacitors to achieve a high-power density. Due to the poor flexibility of inorganic dielectric film capacitors, the growth of dielectric materials on flexible organic substrates and metal foils has been investigated 107 110 . As shown in Fig. 5b, the dielectric layer (green) between a TENG electrode (gray) and a capacitor electrode (light gray) was charged through the triboelectric layer (blue) and induction onto the TENG electrode. When the charged metal contacted the TENG electrode, the metal-to-metal contact released the stored charges and generated a large current (4.3 mA).
The electrolyte is an excellent capacitive material due to the formation of an electric double layer (EDL). Depending on the ion concentration, the capacitance can be controlled from nF to hundreds of μF 111 . The disadvantages of electrolyte capacitors are the frequency dependence and the temporal relaxation of the capacitance. Since the contact frequency of TENGs is typically below 100 Hz, the frequency dependence of the electrolyte (constant up to~100 Hz) is not a problem in TENGs 112 . The advantages of using an electrolyte capacitor are printability and patternability, as well as high flexibility and deformability. Combining TENGs and electrolyte capacitors can lead to new power sources, such as TENG-powered supercapacitors [113][114][115] . Wang et al. fabricated a flexible and wearable self-charging power system consisting of a fiber-based supercapacitor and a  100 , copyright permission from the American Chemical Society). b Dielectric capacitor with a P(VDF-TrFE) used for storing and releasing the electrostatic charges (modified from ref. 110 , copyright permission from Wiley-VCH). c Flexible and weavable self-charging power system by combining a fiber-based supercapacitor with TENGs (modified from ref. 115 , copyright permission from Wiley-VCH). d Triboelectric voltage modulation by printed ion gel patterns (modified from ref. 118 , copyright permission from Elsevier).
fiber-based TENG (Fig. 5c) 115 . Carbon fibers covered with RuO 2 ·xH 2 O were assembled in a bundle, as shown in Fig.  5c. RuO 2 ·xH 2 O is a widely used material for electrochemical capacitors due to facile transport pathways for both protons and electrons, highly reversible redox reactions, wide electrochemical windows, and a long cycle life 116 . The specific capacitance of RuO 2 ·xH 2 O was much higher (1000 F/g) than the specific capacitance of bare ruthenium oxide (720 F/g) 117 . The fiber supercapacitor was charged by contact separation of the PDMS and PTFE layers. This fiber-type supercapacitor is a possible route to flexible self-charging power systems.
An additional advantage of using an electrolyte capacitor is the precise modulation of the output voltage by adjusting the capacitance of the electrolyte 118 . Lee et al. printed ion gel patterns on an electrode and used them as capacitors to modulate the maximum output voltage. An ion gel consisting of an ionic liquid (EMIM-TFSI) and a polymer (PVDF-HFP) was sandwiched between ITO electrodes that were connected to the TENG electrodes (Fig. 5d). During contact and separation in TENGs, the charges on the electrodes developed EDLs in the ion gel. The circuit diagram summarizes the parallel connection of the TENG capacitance (C TENG ), the ion gel capacitance (C gel ), and the oscilloscope resistance (R OSC ). The relaxation time (τ = RC) of the ion gel determined the measured voltage profile. Lee et al. changed the spike-like voltage peaks in TENGs to square voltage profiles 118 . The addition of a Ru(bpy) 3 (PF 6 ) 2 luminophore in the ion gel led to electrochemical luminescence. The voltage profile became more sluggish due to the increased capacitance by the redox reaction of Ru 2+ . Since the ion gel could be readily printed and patterned by UV, the capacitance of a single pattern was tunable according to the size of the pattern. In addition, the maximum voltage and the overall voltage profile were controlled separately through the serial connection and parallel connection of the ECL gel dot patterns.
Self-powered mobile healthcare sensors may be a major application of TENGs. These sensors require stretchable dielectric layers both for the TENG unit and for the capacitor. Kong et al. quantified the capacitance of commercially available elastomeric dielectrics such as PDMS, hydrogenated polystyrene-block-poly(ethylene butylene)block-polystyrene (H-SEBS), elastic PVDF-HFP, and polyurethane (PU) 119 . PDMS and H-SEBS exhibited dielectric behavior similar to that of conventional polymer dielectrics, whereas PVDF-HFP and PU showed high capacitances because of the mobile ions present in the elastomers. In addition, the capacitance was stable in a broad temperature range (20-160°C) 119 . These elastomeric dielectrics can be useful for application in stretchable TENGs and TENG sensors. The achievement of a high-k stretchable dielectric has rarely been studied.
Nanocomposites with high-k inorganic nanomaterials and stretchable electrolytes are two possible approaches. Both approaches need further studies for practical uses.

Material approaches for TENG sensors
The electrical signals obtained in TENGs can be used not only for electric power generation but also for sensing momentary stimuli. Research on the quantitative recognition of mechanical stimuli (touch, pressure, stretching, and torsion) has mainly been carried out with TENGs; in recent years, applications for chemical sensing have also been attempted. In this section, we describe the design of materials for TENG sensors and methods to deconvolute the signals in multimodal sensors.

Material design for mechanical stimuli sensors
Sensors that can detect mechanical stimuli are critical components in electronic skin, touch screen, healthcare devices, and habit-recognition security systems 120 . Mapping and imaging of stimuli are important for smart displays, flexible and wearable electronics, and human-robot interfaces 121 . Since the TENG sensors generate potential signals, sensing does not require external electric power, resembling the action potential generated in human mechanoreceptors. This potential generation can be effective in power savings when the entire sensing system needs a large number of sensors, as in the human tactile sensory system containing a tremendous number of mechanoreceptors 122 . The first material requirement for the sensor is to acquire a quantitative relationship between the sensing signals and the stimulation. The second material requirement is to obtain long-term reliability of the sensing signals.
Quantitative conversion of the potential signals into external tactile stimuli (pressure and contact) has been mostly sought by implementing the microstructures on the contact surfaces to obtain large output signals at low pressures 74,123-127 . Zhu et al. proposed a flexible sensitive tactile sensor that can detect a wide range of contact pressures from 0.03 to 10 kPa. They used vertically aligned FEP nanowires on contact surfaces 123 . This sensor was integrated with signal-processing circuits to detect the hand touch on the door handle and turn on a motionactivated lighting system. Differentiation between static pressures and dynamic pressures is important to recognize human body movements in daily activities. Lin et al. successfully recognized both static and dynamic pressures with a single TENG sensor 124 (Fig. 6a). Due to the microstructure on the PDMS contact surface, the sensor showed a low detection limit (2.1 Pa), high sensitivity (0.31 kPa −1 ), and fast response (<5 ms). The stable elastomer materials provided excellent mechanical stability (30,000 cycles). By integrating multiple sensor units on the contact surface, the local pressure distribution was mapped. Ren et al. proposed a TENG-based tactile sensor that can detect both normal and tangential forces 125 . The elastic tiny burr arrays on the contact interfaces not only can detect both normal and tangential forces but also can distinguish the direction of applied tangential forces with high sensitivity. Because the triboelectric phenomenon is the contact electrification, the TENG-based sensors are highly sensitive to low pressure and thus are suitable for detecting minute movements or biological pulses in the human body 74 nanowires, inspired by the structure of the human eardrums (Fig. 6b). They reported a bionic membrane sensor that not only monitored low-frequency cardiovascular arterial pulses but also acquired high-frequency throat sounds and vibrations 127 .
In addition to simple pressure stimulation, TENG sensors can detect other dynamic mechanical stimuli, such as shear, stretch, bend, and twist. These sensors are designed to be compliant with the deformation of an object and not to interfere with the stimulation. The main use of these stretchable TENG-based sensors is monitoring human body motions in joints, limbs, and muscles [128][129][130] . Jin et al. designed a finger-motion sensor to detect bending and unbending of the finger joints in real time 128 . The sensor was composed of FEP and PET layers. Similarly, Lu et al.  124 , copyright permission from the American Chemical Society). b Eardrum-inspired triboelectric sensor detecting both cardiovascular arterial pulse and throat sound vibrations (modified from ref. 127 , copyright permission from Wiley-VCH). c Hydrogel-based triboelectric sensors for the detection of bending, stretching, and twisting motions (modified from ref. 133 , copyright permission from Wiley-VCH). designed a stretchable strain sensor with a ladder structure of the PTFE film and a stretchable aluminum electrode fabricated on an elastic rubber substrate 129 , and Chen et al. proposed an on-skin stretchable TENG sensor consisting of an elastic dielectric film and a carbon grease electrode that can perform tactile sensing at high strain without being disturbed by human movements 130 . These sensors were attached to the human skin and used to monitor the movement of joints and muscles. To attain high conductivity and prevent a decrease in the conductivity of the sensor electrode in the stretched state, ion gels have been proposed as an electrode for the TENGbased sensors [131][132][133] . Pu et al. used a polyacrylamide (PAAm-LiCl) hydrogel containing lithium chloride as an electrode and demonstrated a skin-like tactile sensor 131 . This soft sensor was transparent and highly stretchable (stretchable up to 1160%) and could sense pressures as low as 1.3 kPa. Hydrophobic ion gels have also been utilized in TENG sensors. Zhao et al. used the poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPS)-based ion gel as both the electrode and triboelectric contact layer to fabricate a transparent stretchable strain sensor 132 . They inserted a micropatterned PDMS layer between the PAMPS ion gel electrode and the contact layer. The contact area between the PDMS micropattern and the ion gel layer changed according to the applied strain; thus, the magnitude of the electrical signals had a quantitative correlation with strain. Xu et al. fabricated a cylindrical stretchable Al electrode containing polyvinyl alcohol (PVA) hydrogel balls encapsulated by PDMS 133 (Fig. 6c). A Ni fabric electrode was placed inside the hydrogel ball and wired to the Al electrode. The changes in the contact area between the PDMS and the Al electrode resulted in corresponding changes in the magnitude of the potential; thus, the sensor could provide quantitative information on body motions (bending, twisting, and stretching).

Material design for signal selectivity
Mechanical stimulation often simultaneously includes several force components, such as normal force, pure shear, and torque. The sensors for electronic skin and haptic devices typically hold information about pressure and strain [134][135][136][137] . For practical use of the sensor, the sensor should selectively respond to a specific force component or deconvolute specific signals from a bundle of mixed signals. For instance, the finger-motion strain sensor must not respond to touch; otherwise, the sensor will give incorrect operation signals. Signal selectivity is crucial in biological sensors for healthcare monitoring. The issue in the haptic device is the real-time recognition of the motion dynamics. The sensor should transfer the speed information of a mechanical stimulation for motion control of an object. However, in some other cases, the sensor should be selectively dependent on the total strain or total pressure regardless of the motion dynamics. Therefore, the dynamic information and the strain/pressure information should be obtained separately.
Although the signal selectivity is critical for practical uses, it has rarely been investigated in TENG-based sensors. One way to obtain multiple information selectively is by combining different sensors. For example, a combination of a TENG sensor and an EMG (electromagnetic generator) sensor enabled detection of the magnitude and the velocity of a body motion 138 . The peak voltage value in the TENG sensor provided information on the maximum force, while the peak current in the EMG sensor provided the maximum velocity. To make the sensor system simple, material design is necessary for proper signal deconvolution from a multimodal sensor. Li et al. demonstrated a dual-mode sensor that can separate touch from pressure and provide hardness information 139 (Fig.  7a). The sensor could determine the hardness of a contact material by analyzing the shape of the current profile obtained at the moment of contact. Hard materials created sharp rectangular current peaks, whereas soft materials generated broad current peaks. Since the open-circuit voltage changed linearly with the external force (Fig. 7b), simultaneous detection of pressure and hardness was realized when a foreign object was pressed on a TENG sensor. Very recently, Choi et al. reported a stretchable TENG sensor that could extract information about pressure, strain, moving distance, and moving velocity of the sensor 140 . They fabricated a multilayer stretchable sensor that could separate signals from the sliding mode and the contact mode (Fig.  7c). The key to the success was blocking the signal interference when stretching and pressing were exerted simultaneously. The electrode for the sliding mode was placed in the inner part, and the electrode of the contact mode was placed at the upper part. For the sliding mode, a PTFE thin film was used due to its low friction coefficient. To block the signal interference, they placed a stretchable electrode beneath the top PDMS surface. The stretchable electrode was made of a PDMS/Au nanosheet composite film. Figure  7d shows the potential peaks measured from the sliding mode (measurement 1) and from the contact mode (measurement 2). The sliding mode responded only to stretching without any signal change by pressing events, whereas the contact mode showed large peaks by pressing but negligible peaks by stretching.

Material design for chemical sensors
Contact electrification can change when chemicals are adsorbed on the contact surface. The electrical outputs should be dependent on the amount and the chemical species of the adsorbent. In situ composition changes through doping may be another possible way to cause changes in the triboelectric outputs. These characteristics introduce the possibility of fabricating TENG-based chemical sensors, either in a liquid phase or in a gas phase.
To endow the chemical selectivity of the sensor, the contact surface should be modified to capture a target chemical selectively. Lin et al. demonstrated a TENGbased Hg 2+ ion sensor 141 . They coated Au nanoparticles on the contact surface. 3-Mercaptopropionic acid (3-MPA), which is known to selectively bind to Hg 2+ ions, was self-assembled on the Au nanoparticles. They found that the sensor responded selectively to the concentration of Hg 2+ ions when it was dipped in a solution containing various metal ions. When the triboelectric output was measured after drying the sensor in air, the triboelectric output from the sensors decreased as the concentration of Hg 2+ increased. A similar approach was used to detect organic molecules. Titanium dioxide (TiO 2 ) nanowires were coated on a Ti electrode, and their surfaces were modified with β-cyclodextrin (β-CD), which has a specific binding to phenol 142 (Fig. 8a). The output signal decreased with increasing concentrations of phenol, with a sensitivity of 0.01 μM −1 and a sensing range of 10-100 μM. By placing the sensor in phenol-containing wastewater, the sensor not only detected the phenol concentration but also electrochemically purified the wastewater into clean water. Liquid-solid contact electrification, instead of conventional solid-solid electrification, opens a new possibility in chemical sensing. Choi et al. proposed a concept of a smart pipette tip to measure the electrolyte concentration of the dispensed solution.
They utilized the spontaneously generated electric signals during the pipetting process 143 . The smart pipette tip was used for in situ prediction of both the concentration and morphology of palladium (Pd) nanoparticles in the electrolyte. Likewise, a microfluidic ion concentration sensor was demonstrated by Jeon et al. 144 . The electrical signals generated through the contact between a polymeric channel surface and a moving fluid were used to detect the ion concentration in the fluid (Fig. 8b). The output signal decreased quantitatively with increasing concentration of sodium chloride (NaCl) in water with a good concentration resolution and reliability in the range from 5 × 10 −3 to 0.1 M.
TENG-based gas sensors use the changes in output signal when chemicals are adsorbed on the contact surface. Cui et al. reported an ammonia (NH 3 ) gas sensor by using the absorption and desorption in polyaniline (PANI) nanofibers 145 (Fig. 8c). PANI has been known as a highly sensitive NH 3 gas-sensing material 146 . When PANI is exposed to NH 3 gas, it is deprotonated, and its conductivity decreases. In the TENG sensor, the PANI nanofibers worked both as NH 3 gas-sensing materials and as triboelectric electrodes. The sensor had a good resolution with a detection limit of 500 ppm and was insensitive to other volatile gases. Similarly, a hydrogen (H 2 ) gas sensor was fabricated 147 . Zinc oxide nanorods decorated with Pd nanoparticles were used as both the H 2 sensing material and the contact material in the sensor. The chemisorbed oxygen ions (O 2-) formed by the reaction Fig. 7 Multimodal TENG sensors. a Dual-mode TENG sensor detecting touch and hardness of the material by analyzing the shape of the current profile and b the linear relationship between the sensor output voltage and the external pressure (modified from ref. 139 , copyright permission from the American Chemical Society). c Stretchable TENG sensor discriminating pressing and elongation in one system and d its voltage output responses to pressure (Measurement 1) and strain (Measurement 2) (modified from ref. 140 , copyright permission from Elsevier).
between the Pd/ZnO electrode and the oxygen molecules in the air reacted with H 2 molecules and formed water (H 2 O) molecules. The higher H 2 concentration in the air resulted in more H 2 O molecules formed on the contact surfaces, causing a decrease in the electrical output of the sensor. In this mechanism, the sensor showed a minimum detecting concentration of 10 ppm and a short response time of 100 s.
Gas sensors with a high sensitivity and selectivity similar to those of the human nose, called "electronic noses", have been studied and are expected to provide sustainable healthcare services. Based on TENGs, electronic noses that are capable of detecting several gases without the need for external power have been reported. Kim et al. reported an electronic nose with a highly selective gas detection capability 148 (Fig. 8d). They fabricated arrays of ZnO nanowires that were coated with nickel oxide (NiO) nanoparticles and used them as the contact materials. The catalytic oxidation between NiO nanoparticles and gas molecules generated electrons. Electron transfer to the semiconducting ZnO nanowires increased the triboelectric charge density at the contact surface, thereby increasing the output signal with increasing gas concentration. Since the catalytic reactions occurring on the ZnO nanowires and NiO nanoparticles differ depending on the type of gas molecules, these electronic noses distinguished between four volatile organic compound (VOC) gases with a detection limit of 0.1%. A flexible electronic nose that can detect ethanol in the air was realized from the PANI/PTFE/PANI sandwich nanostructure 149 . When the PANI was exposed to ethanol, the interaction between nitrogen atoms of the PANI and the hydroxyl (-OH) groups of ethanol altered the conformation of PANI chains and affected charge transportation through the chain. The degree of rearrangement in the PANI chain depended on the concentration of ethanol, thus changing the triboelectric output. Due to its high sensitivity to ethanol, this electronic nose could be used as a mobile alcohol sensor without requesting external electrical power.

Challenges in materials of TENGs
Although the triboelectric effect is one of the most frequently experienced in everyday life, research on energy harvesting and sensor applications has been conducted in recent years since the first report of TENGs in 2012. The performance of TENG devices and TENG sensors has made tremendous advances during the short Fig. 8 TENG-based chemical sensors. a Phenol detection in wastewater by using triboelectrification with a β-cyclodextrin surface (modified from ref. 142 , copyright permission from the Royal Society of Chemistry). b Ion concentration sensor by using the liquid-solid contact electrification in a microfluidic system (modified from ref. 144 , copyright permission from Wiley-VCH). c Mechanism of sensing NH 3 gas in a PANI-based TENG gas sensor (modified from ref. 145 , copyright permission from Elsevier). d Self-powered electronic nose based on the triboelectrification and the heterogeneous catalytic reaction (modified from ref. 148 , copyright permission from Wiley-VCH).
period of years since their first report 150 . TENG devices are expected to be a promising energy harvester for small devices, with efforts devoted to finding new mechanical power sources, combining with other types of devices, and exploring new applications. Advances in device performance are possible with a clear understanding of the charge generation mechanism; however, the mechanism of triboelectrification is still under debate 151 . It is generally believed that after two different materials are in contact, a chemical interaction is formed between the molecules on the surfaces, and the charges are transferred from one side to the other side to equalize the electrochemical potentials 152 . However, contact between two identical surfaces has also been reported to generate triboelectricity 153 . A possible explanation based on the thermionic-emission model has recently been reported, suggesting that the hotter material is positively charged and the cooler material is negatively charged through an electron transfer during contact electrification 154 . The question of "what are the transferred charges?" remains controversial for electrons 155 and ions/molecules 156 .
Synthesis and discovery of new triboelectric materials 53,54,57 are necessary. In particular, positive triboelectric materials lack choices compared with the negative triboelectric materials 51,157 . Various lipids have been reported to be ranked as the highest positive materials in the triboelectric series, regardless of the type and position of the functional groups in molecules 54 . Wax molecules consisting of aliphatic carbons exhibit similar positive electrification. Considering that polyethylene, which has the same chemical bond structure, does not have strong positive surface charges, the surface electrification cannot be understood from the molecular structure, and the molecular orientation and crystal structure must be considered together. In addition to new material development, precise surface characterization of the existing triboelectric materials is also required in future studies. Although there have been many studies using PDMS, PVDF, PI, PET, and nylon as the contact surfaces, detailed information on the surface of the materials is not provided in articles. PDMS is a representative material used for negative charge generation; however, the surface molecules differ depending on the preparation process. It is well known that a thin liquid layer of unreacted small molecules is formed on the PDMS surface when cured at 80°C in the air. The unreacted chemicals disappear when cured at a high temperature (180°C) in vacuum, and the modulus increases ten times 158 . The differences in the surface materials may cause a significant difference in the triboelectric property. The surfaces of the commercially available PI and PET films are often treated with other materials for easy film manufacturing or to prevent electrostatic charges. The lack of detailed information on the surface treatment results in the problem of general reproducibility in other groups even though the structure of the TENG device is simple.
Charge-trapping layers have not been studied in detail compared with charge-generating layers. A material that is superior in terms of both charge generation and charge trapping (such as PI 64 or PDMS 65 ) is more suitable for manufacturing a low-cost, high-performance TENG. More studies are needed on the fundamental principles of the charge trap mechanism in polymer films. Mechanically robust and stretchable electrodes can replace conventional brittle electrodes in TENGs. To overcome the humidity problems of the hydrogel electrodes, hydrophobic ion gels can be alternative stretchable transparent electrodes for TENG devices. In addition, the dynamics of the ionic molecules in the bulk and the electrode interface should be understood, which is in the beginning stage in theoretical studies. It is also important to improve the electrical stability of the wiring interface between the TENG electrodes and the other components (capacitors, rectifiers, and microprocessing units). Since metal wiring is not usable in connection with stretchable electrodes, stretchable interfacing conductive materials should be developed to guarantee high deformability of TENG devices without electrical disconnection under large strains 135 .
TENG sensors are typically used as dynamic pressure sensors with spike-like signals 19,159 . Some studies demonstrated the possibility of static pressure sensors with a square-shaped constant voltage signal 123,124 . However, the square voltage profiles were obtained by using a high-impedance electrometer (>1 GΩ) 17,160 , which is not realistic in typical electronic devices because the resistance of conventional devices is in the range of kΩ to 10 MΩ. Currently, without the aid of a highimpedance electrometer, TENG sensors cannot be used as static pressure sensors 161 . To detect both dynamic and static stimuli together, as in human sensory mechanoreceptors, combining a TENG sensor with a capacitive or resistive system is necessary 162,163 . However, TENGbased self-powered sensing can be useful in the human body, where dynamic movements and stimuli mainly occur. Implantable TENGs have been utilized to generate electricity from the biomechanical motions in the body 15,164 , or to acquire physiological information by obtaining electrical signals from the heart rate 165 , pulse 166 , respiratory rate 167 , and bladder swelling 168 . As implantable TENGs and sensors are relatively new to wearable TENGs, biocompatible and desirable materials for implantable TENG devices must be developed. The hygroscopic environment, long-term stability, and stretchability of the device are additional material properties of implantable devices; thus, unconventional material studies are necessary to fabricate practical implantable TENG devices. In addition, in the case of tactile sensing, there is relatively little research on the use of the triboelectric phenomenon of the material itself 169 . Electrolytes can be used as a deformable capacitor that can finely tune the output voltage of TENGs. The spike-like AC high voltages in TENGs are not useful in electronics, and the voltage should be converted to a constant DC voltage. To tune the voltage and effectively harvest the charges in the electrolyte capacitor, an electrolyte material with a large charge relaxation time and charge resistance is needed 94,97 . In TENG-based chemical sensors, sensing materials or systems that are not affected by environmental changes (temperature, humidity, ambient vibrations, etc.) are desirable. TENG-based gas sensors have utilized signal changes by incorporating gas molecules into triboelectric surfaces. The selective adsorption of gas molecules in polymers and metal oxides has been extensively studied 170,171 . Adopting such materials as the substrate in the TENG-based sensors can improve the chemical selectivity and increase the chemical species to be detected.