Innovative Technology for Self‐Powered Sensors: Triboelectric Nanogenerators

Internet of Things and wearable technology's quick development have opened up a vast market for sensor systems. However, typical sensors' external power supplies' short lifespan and expensive maintenance restrict them from being used more widely. Triboelectric nanogenerators (TENGs), a recently created mechanical energy harvesting and self‐powered sensing device, show enormous promise to get over these restrictions. TENG can be used not only to power sensors instead of conventional chemical batteries but also be utilized to actualize sensing by taking advantage of the unique characteristics of the friction layer itself. Triboelectric nanogenerators efficiently provide crucial infrastructure for a new generation of sensing devices that gather data using several self‐powered sensors in abundance. The recent progress in the development of TENGs applied in the sensor field is reviewed. First, the working mechanisms of solid‐solid TENG and solid–liquid TENG are introduced. Subsequently, the development of TENG‐based sensing systems and their application progress in self‐powered temperature sensors, self‐powered pressure sensors, self‐powered humidity sensors, self‐powered atmosphere sensors, self‐powered wireless sensors, interface wetting status monitoring, solution property monitoring, and friction condition monitoring are highlighted. Finally, current challenges and open opportunities are discussed.


Introduction
The gathering and analysis of data in the Internet of Things (IoT) field has experienced revolutionary changes and entered the digital era with the rapid development of big data and cloud computing in recent decades. [1][2][3][4][5] An essential component of IoT development is efficient data collecting and processing. Illustration to show the outline of this review paper, covering various TENG device models and applications in the sensor field. Wireless sensor: Reproduced with permission. [61] Copyright 2020, American Chemical Society. Pressure sensor: Reproduced with permission. [62] Copyright 2017, American Chemical Society. Humidity sensor: Reproduced with permission. [63] Copyright 2021, Elsevier. Gas sensor: Reproduced with permission. [64] Copyright 2021, Elsevier. Position sensor: Reproduced with permission. [65] Copyright 2020, American Chemical Society. Oscillation sensor: Reproduced with permission. [66] Copyright 2017, American Association for the Advancement of Science. Hall vehicle sensor: Reproduced with permission. [67] Copyright 2018, Wiley-VCH. Active motion sensor: Reproduced with permission. [68] Copyright 2017, Wiley-VCH.
technology will create a wide range of application possibilities in the IoT space, necessitating several uses for distributed monitoring technology. [31][32][33][34][35][36] The use of TENG in self-powered sensing applications appears to have a significant influence on the advancement of smart cities and IoT networks. [37][38][39][40][41][42][43][44] As a result of its capability to provide multifunctional power generation from a variety of sustainable energy sources, TENG energy has received a lot of attention. TENG's energy is derived from a variety of sources, including ocean waves, tides, wind energy, human motion, and more. [45][46][47][48][49] As a result, a number of TENG designs have been enhanced to be very effective, demonstrating excellent potential for harvesting multimodal energy from the environment. TENGs' adaptable and thorough qualities can also be a significant help for a variety of hybrid energy solutions, including wind wave, wind, solar, and solar water. [50][51][52][53][54] Therefore, TENGs will be at the vanguard of creating the next generation of sustainable energy by designing the capability to capture ambient energy using huge networks under relatively weak drives. [55][56][57][58][59][60] In this review, as shown in Figure 1, the basic mechanism of TENG is reviewed, including the generation mechanism of triboelectric charges at the solid-solid and liquid-solid interfaces. Secondly, the development of TENGs as self-powered sensing is reviewed, and the latest research progress in the application of TENG self-powered technology in various sensing fields is highlighted. Finally, the existing problems, most promising applications, and future development of TENG are discussed and summarized.

Development of Triboelectric Research
As shown in Figure 2, the understanding of triboelectric electrification (TE) shows a research and development trend from perceptual to rational, from macro to micro, and from basic to application. In the early experiments, because people knew little about the mechanism of contact electrification, there were many factors that affected the experimental results, such as easy contamination of samples, electrical leakage caused by atmospheric humidity, electric sparks caused by charge accumulation, and other discharges, which made some The loss of the original charge resulted in poor reproducibility of the experiment. [69][70][71][72] With the deepening of the understanding of the principle of contact electrification, such experiments began to operate in a vacuum environment, which overcomes the above difficulties to a certain extent. [73][74][75][76][77] However, TE is a very complex process, involving not only the fundamental processes that occur in tribology but also interfacial charge exchange and tunneling, especially before the advent of Kelvin probe force microscopy, the lack of nanoscale probes was a problem in the field. [78][79][80][81] The main limitation makes research quite difficult. Second, the TE effect occurs between the interfaces formed by all substances, and it is quite challenging to propose a unified physical model covering a wide range of materials and phases. [82][83][84][85] Triboelectric electrification at solid-liquid interface has become a key research topic in triboelectric electrification because it is closely related to the fields of electrochemistry, electrodynamics, catalysis, and surface interface science. [86][87][88][89][90] Many physical and biological phenomena are also related to charge transfer at the solid-liquid interface, such as electrowetting, colloidal suspensions, photovoltaic effect, photosynthesis, etc. [91][92][93][94] Therefore, a deep understanding of the charge transfer mechanism at the solid-liquid interface has universal value. [95][96][97][98][99] At present, scholars hold different views on the mechanism of solid-liquid triboelectric electrification. The fundamental reason is that the nature of charge carriers is not understood. Many scholars have put forward different views based on a large number of experimental data. [100][101][102][103][104] These disagreements arise akin to a blind man touching an elephant, with each person's conclusion depending on which part of the elephant they touched.

Working Mechanism of Triboelectric Nanogenerators
In 2012, the TENG based on the mechanism of contact electrification and electrostatic induction came out, which can convert the ubiquitous but wasted mechanical energy in the environment and life, such as human motion, walking, vibration, mechanical energy, etc. Triggering, wind, raindrops, and other energies are converted into electrical energy, and because of its low cost and simple preparation, it is possible for triboelectricity to be used by humans, which has rekindled people's research interest in the mechanism of triboelectricity. [105][106][107] The results show that the current area power density of TENGs can reach 500 W m −2 , and the conversion efficiency is >50%. Moreover, TENG can also be used as a self-powered sensor, exploiting the dependence of its electrical signal on external stimuli, with potential applications in robotics, flexible electronics, and artificial intelligence. [108][109][110][111][112]   Working mechanism of triboelectric nanogenerators. a) The structure of an integrated generator in bending and releasing process and related electrical measurement tests. b) Photographic images of a flexible TENG. c) Mechanical bending equipment. d) Proposed mechanism of a TEG (see text for details): charges are generated by frictioning two polymer films, which results in the creation of a triboelectric potential layer at the interfacial region (indicated by dashed lines). Reproduced with permission. [21] Copyright 2012, Elsevier. e) Structure of a thin-film triboelectric generator (TF-TEG). f) Electricity-generating process of the TF-TEG. Reproduced with permission. Copyright 2015, [113] American Chemical Society. In general, TENGs for self-powered sensors are mainly divided into two categories: solid-solid TENGs and solid-liquid TENGs. Figure 3 selects the two most representatives of these two modes to illustrate their respective structures and working mechanisms. The charge generated during the triboelectric process is often considered a negative effect in scientific research or technical applications and in many cases a waste of energy. [114][115][116][117] By switching ideas, Wang's team demonstrated a simple, low-cost, and efficient way to use the charging process in friction to convert mechanical energy into electrical energy that drives small electronics. As a result, the first solid-solid TENG came into being. [118][119][120][121] The TENG is constructed by superimposing two polymer sheets made of materials with significantly different triboelectric properties, with metal thin films deposited on top and bottom of the assembled structure. [122][123][124][125][126] The working mechanism of the solidsolid TENG is shown in Figure 3d. TENG's ability to generate triboelectric charge is often attributed to the coupling effect of contact charge and electrostatic induction. When the polyethylene terephthalate (PET) and Kapton were pressed together, the pressure bent the PET into full contact with the Kapton, resulting in positive charges on the PET surface and negative charges on the Kapton surface. When the external pressure was reduced, the PET partially disengaged from the Kapton, and the conductive layer generated opposing charges to the tribolayer due to the www.advancedsciencenews.com www.advsensorres.com electrostatic induction effect. Electrons flow from the Kapton/Au electrode to the PET/Cu electrode, with the PET/Au layer providing a negative charge and the Kapton/Au layer providing a positive charge. There is no current in the circuit until the PET and Kapton are completely separated and the charges have achieved equilibrium. Similarly, when pushed, a reverse current flow from the Kapton/Au electrode to the PET/Au electrode was recorded. When the TENG experiences contact separation owing to the coupling of contact-charged cations and electrostatic induction, current is produced alternately. The output voltage of this flexible polymer TENG is as high as 3.3 V and the power density is 10.4 mW cm −3 . [127][128][129][130][131] Different from solid-solid TENGs, the triboelectric electrodes of solid-liquid TENGs are usually composed of liquid and polymer tribolayers. As shown in Figure 3e,f, a flexible and area-scalable energy harvesting method to transform kinetic energy wave energy is described by X. Zhao et al. [113] Induced currents between electrode arrays are produced as a direct result of the direct interaction of dynamic waves and large-area nanostructured solid surfaces. The integrated technique makes sure that every pair of electrodes' induced currents can sum positively, which can greatly boost output power and allow electrode arrays to be integrated over a larger area. In addition to powering small electronic devices, the resultant current also permits efficient impressed current cathodic protection. If dynamic waves are accessible and a steady power source is present, this thinfilm-based device could be a workable solution for both on-and offshore locales.
The working mechanism of solid-liquid TENG is similar to that of solid-solid TENG. The interfacial "electric double layer model" (EDL), typically provides an explanation for the solidliquid contact electrification phenomenon. If the ions on the solid surface are positively charged (ionization, adsorption), some opposing ions (anions) will be adsorbed on the solid wall surface (inner Helmholtz layer and outer Helmholtz layer) as a result of chemical/physical interactions to form a tight layer (Stern layer), whose thickness is no greater than a few Angstroms (Å). Due to thermal motion and the influence of the electric field, the remaining anions and other electrolyte ions are Boltzmann distributed close to the electrode surface, forming a "Gouy-Chapmandiffuse layer" that lasts until the electrolyte solution. In diluted solutions, it can be up to several hundred angstroms (Å) thick, but in concentrated solutions, it can be disregarded. A negative triboelectric charge will be produced on the PTFE surface upon direct contact with water. The distribution of the triboelectric charge is thought to be uniform. The negative triboelectric charge is screened by positive ions in the water, causing temporary electron flow away from electrode A, which raises the water front and partially covers the area on top of electrode A. As a result, the electric potential of electrode A rises, and electrodes B and C display a positive net-induced charge. The distribution of the induced charge across the three electrodes will change as the water front continues to spread. On these electrodes, the induced charge density changes quantitatively. While the induced charge of electrode C has a reversed sign when the water front propagates halfway between electrodes A and B, electrodes A and B only have negative and positive induced charges, respectively. Due to the use of rectifying bridges, the induced current in the external circuit always flows from the anode to the cathode, resulting in a single current peak regardless of how the charge distribution changes. The hydrophobic solid surface will repel water as the water front recedes and reveal the negative triboelectric charge. Similar logic dictates that another current high will be created by the receding process.
Since the development of TENGs, it has been successfully verified that not only has the function of energy harvesting, but also the electrical output converted by itself varies with the degree of applied environmental factors. This indicates that it can function without an external power source and can be used to sense applied environmental conditions on its own. This characteristic is the essence of self-driven sensors. TENG can be used as an energy module to supply other sensors because its primary usage is as an energy harvester to power small electronic devices. Furthermore, the current and voltage signals of the TENG will be affected by the material choice, motion, environment, interface state, etc., therefore the change in electrical output can be employed as a sensor to reflect changes in material properties, motion, environment, interface state, etc. The next section will provide an overview of the design and development of TENG in the field of sensing from a variety of angles, including environmental considerations, different types of friction layers, the structure of those layers, changes in liquid composition, and variations in friction state.

Influence of Temperature
Both solid-solid or solid-liquid TENGs can be used as power modules for self-powered sensors. [132][133][134][135][136][137][138] In addition, depending on the special response of the tribolayer material itself to temperature, humidity, atmosphere, or pressure, the changes in the TENG signal can also be used for sensor fabrication. [139][140][141] As shown in Figure 4a-c, on the basis of fluorinated alumina covered in polycaprolactone (PCL), a novel temperature response liquid-solid TENG is reported by X. Li et al. [142] for programmable triboelectrification. The temperature controls the PCL conformation, giving the substrate a variable surface component and flexible control over the liquid-solid triboelectricity. The short-circuit current and open-circuit voltage of the PCL-based TENG are reduced by >40 times as the temperature increases from 20 to 40°C . The electrical output can rise to its initial level once more at a temperature of 20°C. Additionally, a month later, the electrical signal is still stable and reversible. In addition to water, organic liquids like ethylene glycol also respond favorably to temperature in terms of electrical production. The surface rebuilding phenomena of amphiphilic polymers in response to environmental stimuli-the PCL temperature response mechanism-is discussed. The wettability modifications and the triboelectrical performance are also found to be strongly correlated. Thus, a trustworthy approach of triboelectric monitoring of the surface reorganization and interfacial wettability changes of the amphiphilic polymer is proposed. Last but not least, a PCL-F-3D AAO liquid-solid TENG based on wearables is created to measure human body temperature and reflect health.
Additionally, electrospinning is used to create adjustable microarchitectures of a thermally induced shape memory polymer, such as microfiber mats, microspheres, and microspheresnanofibers (MSNFs), [143] as shown in Figure 4d-f. The tunable . Self-powered temperature sensor. a) The water on the surface of the PCL-F-3D AAO undergoes a reversible transition from non-adhesion to strong adhesion by changing the temperature, which is used to regulate liquid-solid triboelectricity. b) Water static CA of PCL-F-3D AAO at 20 and 40°C. c) Wearable liquid-solid TENG is used for detecting human body temperature. Reproduced with permission. [142] Copyright 2021, Wiley-VCH. d) Schematic illustration of the setup and working mechanism of a deformed mat-based water-TENG driven by hot water. e) The corresponding gradually recovered surface roughness and static contact angle of the mats under different recover ratios, indicating the dependence of healing process on the stimuli temperature of water. f) Output voltages of TENGs based on the mats with increased surface roughness at different impacting times (≈60 s) by hot water (≈45-95°C) with flowing rate of 5 mL s −1 . Reproduced with permission. [143] Copyright 2019, Elsevier. g) The electrical signal changes in situ as the temperature drops. h) Variation of current with temperature. i) The photographs showing the "unexcited" and "excited" state of the warning sensor under 33°C. Reproduced with permission. [144] Copyright 2022, Wiley-VCH. microarchitectured shape memory triboelectric nanogenerators (mSM-TENG) achieve enhanced and changeable triboelectric output (150-320 V, 2.5-4 A cm −2 ) due to increased frictional effects made possible by the high surface roughness. Due to its ability to take on a variety of temporary shapes when heated, the MFs mat is typically used as a skin-contact-driven shape memory TENG. Due to the self-restoring characteristics made possible by heat stimulation, the distorted mats can, at the micro level, revert to their original microstructures, extending the lifespan of durable TENGs. Waterproof mat-based TENGs with retentive rough surfaces are possible for energy harvesting from both cold and hot water with the help of a cellulose oleoyl ester. As a result, it is discovered that a distorted waterproof TENG can be recovered in shape under hot water. A water energy harvester with the ability to sense water temperature (25°C to 95°C) is made possible by the gradient surface roughness, which holds promise for self-powered waterproof wearable electronics and intelligent wastewater management systems. Besides the tem-perature response of the material itself, taking advantage of the energy harvesting feature of TENG, it can act as a power module to drive temperature-sensing devices. In addition, based on P(NIPAM-MMA) copolymer, a thermosensitive liquid-solid triboelectric nanogenerator (L-S TENG) is constructed for tunable triboelectrification, [144] as shown in Figure 4g-i. The interfacial wettability and triboelectricity of PNM modify the conformation between the acylamino and isopropyl groups through temperature modulation. The output of the PNM-based L-S TENG increases by a factor of 27 when the temperature increases from 20 to 60°C, while the contact angle of PNM increases from 22.49°t o 82.08°. For this PNM-based L-S TENG, other organic liquids, such as glycol, display a favorable response to temperature. It has been established that polymers such as polymethylmethacrylate, polytetrafluoroethylene, and polyimide do not possess such thermo-sensitive characteristics. A droplet-based wireless warning system powered by PNM is also created and activated for the purpose of monitoring a certain temperature. Machine knitting process and f) knitted pressure sensors. g) Smart glove for pressure sensing. h) Voltage signals recorded at different sensors. Reproduced with permission. [150] Copyright 2020, Elsevier. i) Schematic illustration of the SI-TENG with "chain-link" fence-shaped structure and rhombic unit design. j) Photograph of a glove with five pressure sensors implanted on its fingertips. k) The developed output interface for the personalized intelligent prosthesis. Reproduced with permission. [151] Copyright 2018, Wiley-VCH. l) Illustration of ultrathin H-TENGs and its stacked structure and packaged module. m) Real visual demonstration of self-powered pressure sensor arrays (6 × 3; 1 cm × 1 cm of dimension for each unit cell) for detecting a modeled foot pressure (inset: visually illustrated peeled-off device). n) Open-circuit voltage (V oc ) of the H-TENG (3 mm) depending on different compressive forces. Reproduced with permission. [152] Copyright 2016, Wiley-VCH. o) The basic structure and brief operation process of U-shaped TENG. p) The open-circuit voltage versus frequency with different vibration amplitudes (3, 4, 5, and 6 mm). q) The relationship between the V OC and the torsion angle of the silicone tube. Reproduced with permission. [153] Copyright 2017, Elsevier.

Influence of Pressure
For self-powered wearable electronics that smoothly connect with smart fabrics, the use of the triboelectric effect of tribolayer to actualize the experience of pressure is of significant interest. [145][146][147][148][149] As shown in Figure 5a-h, machines are used to interlace strong Cu-coated polyacrylonitrile (Cu-PAN) yarns and parylene-coated Cu-PAN (parylene-Cu-PAN) yarns via multiple textile industry compatible technologies, producing devices with stitched, woven, and knitted structures simultaneously. [150] These textile pressure sensors based on triboelectric nanogenerators (TENGs) have machine washability and excellent breathability. Different textile structures used in TENG-based pressure sensors are examined both in their as-fabricated state and after machine washing. A correlation between the textile structure and the essential properties of the sensors is then provided. The sensitivity, linearity, saturation trend, and washability of the obtained sensors will be impacted by the material/device designs, mechanical conditions, manufacturing techniques, etc. The creation of a smart textile glove featuring embroidered pressure sensors allows for the demonstration of grip posture detection under varied conditions. In addition, a stretchable and washable skin-inspired triboelectric nanogenerator (SI-TENG) is developed by Z. Zhao et al. for both biomechanical energy harvesting and versatile pressure sensing, [151] as shown in Figure 5i-k. The SI-TENG is capable of driving small electronic devices and lighting up 170 light-emitting diodes with a maximum average power density of 230 mW m −2 . The SI-TENG is used as a self-powered multipurpose sensor to track human physiological signals like speech vibrations and arterial pulse.
As shown in Figure 5l-n, K. Lee et al. demonstrated that H-TENGs can operate under challenging environmental conditions, such as underwater, while being extremely thin and entirely packaged. [152] The fabrication of the hemispheres-arraystructured TENGs, which are primarily hemispheres-arraystructured films of micro/nanoscale thickness, included making devices with different hemisphere diameters. Because the hemispheres-array-structure functions as a spring to hold the top and bottom materials apart, no-spacer TENGs are effectively designed. Both practical and theoretical research was done to examine the effects of different external compressive forces and hemisphere diameter relationships on output performance. Due to the fact that the diameter is equal to the separation distance between the tribo-layers, both researches found the same conclusion: the electrical output performance of the H-TENGs increases with an increase in hemisphere diameter. They also successfully achieved a self-powered pressure mapping sensor based on H-TENGs. All types of planar and vertical forces resulting from human motions, such as standing, walking, running, and sliding, can be detected at minimal cost with a straightforward fabrication procedure because of this system's ability to gather pressures very accurately. In wearable electronics, for instance, H-TENGs may work well and steadily as a power supply and sensors independent of the environment, including dust and even perspiration. Additionally, based on solid-liquid contact triboelectrification and Pascal's law, a novel smart U-shaped triboelectric nanogenerator (TENG) has been reported, in which complex mechanical motions can be converted into liquid pressure and electrical signal, [153] as shown in Figure 5o-q. The Ushaped TENG is a mechanical energy harvesting device with a constant peak output voltage and current of around 20 V and 400 nA, respectively. It is driven by an inertial force and airflow. The relationship between the output performance and the water sliding conditions is carefully explored with a stable peak output performance at the point of resonance. The TENG with a U-shape can also operate as a smart multipurpose sensor to measure displacement, pressure, torsion, and other variables. The U-shaped TENG has exceptional sensitivity as a self-powered displacement sensor, with values of 0.91 V·mm −1 and 8.50 nA V·mm −1 . It also has a high sensitivity for a pressure sensor, with values of 4.41 V·kPa −1 and 72.94 nA·kPa −1 , respectively.

Influence of Humidity
Due to the migration of water molecules, the triboelectric charges on the polymer surface follow the accelerated dissipation of water molecules into the environment, which leads to a decrease in the electrical output of the TENG under high humidity. [156][157][158][159][160][161][162] The humidity sensor designed according to this principle has the characteristics of high sensitivity. As shown in Figure 6a-c, with a capacitive humidity sensor, a self-powered wireless and chipless sensor system based on TENG has been suggested and tested. [63] The mathematical model and comparable circuit have been developed for the sensor system with significant applications. The voltage output received by the receiver terminal has analytical solutions. Using PA-66 and FEP as the positive and negative triboelectric materials, respectively, a high-performance TENG integrated with a synchronization microswitch for impedance match with the resonant circuit was constructed and evaluated. This TENG with the microswitch has a maximum output peak voltage and peak power density of up to 1400 V and 16 000 W cm −2 , respectively. The self-powered wireless humidity sensor system based on TENG was created and described. The sensor system has a strong linearity for the measured humidity level, with a sensitivity of 1.26 kHz/%RH, and a quick response time with consistent and reproducible responses to humidity fluctuations. Furthermore, the sensor system's transmission distance can be increased to 50 cm by employing magnetic-core coils with a 1 cm diameter. The usage of magnetic core coils considerably reduces the sensor system's size and extends the wireless sensing range, which is advantageous for the broad use of these kinds of sensor systems.
Water molecules in the environment are a disadvantage for traditional polymers because the evaporation of water molecules increases the dissipation of triboelectric charges. However, for polyhydroxylated polymers, the formation of hydrogen bonds can convert free water in the environment to bound water, allowing water molecules to participate in triboelectric charging, and turning a disadvantageous factor into a favorable one. In addition, some water-absorbing inorganic compounds can also serve as carriers for hydrogen bond formation, immobilizing water molecules to participate in triboelectric electrification, thereby increasing the electrical output of TENG. This self-powered humidity sensor has a stable signal, large electrical output, and high sensitivity. As shown in Figure 6d-f, N. Wang et al. created a brand-new class of biofilm materials based on TENG that can function effectively and steadily in high-humidity environments by creating hydrogen bonds with water molecules. [154] Water molecules can participate in triboelectric charging as a more electropositive material to achieve a higher output of TENG in high-humidity environments by forming hydrogen bonds with hydroxyl-rich biomaterials like starch molecules. This fixation of water molecules on the surface results from the spontaneous formation of hydrogen bonds between these two molecules. The production of the starch film-based TENG rises with the level of environmental humidity, unlike the conventional polymer materials-based TENGs. With ambient humidity rising from 15% to 95%, the output current and voltage may grow from 6.2 A to 110 V to 16.6 A and 330 V, respectively. This is approximately 12 times greater than that of the conventional nylon-11-based TENG in a humidity of 95%. It is a crucial Figure 6. Self-powered humidity sensor. a) Schematic diagram of the TENG-based self-powered wireless sensor system. b) The capacitance value of the humidity sensor and resonant frequency of the sensor system as a function of humidity. c) A photo image of the humidity sensor fixed in the hermetic box for humidity sensing. Reproduced with permission. [63] Copyright 2021, Elsevier. d) Schematic diagram of the structure of starch-based TENG. e) The mechanism of moisture resistance of TENG. f) Photograph of humidity warning of starch-based TENG and the process of humidity warning. Reproduced with permission. [154] Copyright 2020, Elsevier. g) Schematic of the adsorption phenomenon of water molecules on the surface of the SnO 2 humidity sensing layer and schematic illustration of the chemisorption of the first H 2 O layer and the physisorption of the subsequent H 2 O layer on the surface of the SnO 2 humidity sensing layer. h) 1 × 3 CNES matrix working as the self-powered switch for controlling the operation of the electrical devices in the intelligent greenhouse IoT system, including light, ventilation fan, and humidifier. Reproduced with permission. [155] Copyright 2022, Elsevier.
addition to the TENG family and greatly broadens the application possibilities for energy harvesting and self-powered sensors in high-humidity environments, particularly on gloomy, foggy days or when they are submerged in water or perspiration.
In addition. it is reported by F. Yin et al. that a hybrid e-skin (CNES) that enables both touch and non-contact sensing comprises a flexible substrate layer shared by a triboelectric nanogenerator (TENG) and a humidity sensor, [155] as shown in Figure 6gf. The current response characteristics of the CNES are mainly explained by the adsorption phenomenon of water molecules on the surface of the SnO 2 humidity sensing layer. The hydrothermal process used to create the shared flexible substrate results in a porous surface with a nanoflake-like shape. CNES displays benefits in touch sensing and electrical output performance by adding an Ag nanowires (NWs) electrification layer to the top surface of the shared substrate. As a result, we are able to monitor for falling behaviors using a CNES that doubles as a touch sensor and manage humidity levels using CNES matrices that mimic self-powered switches. Additionally, the SnO 2 humidity sensing layer that is added to the lower surface of the shared substrate makes it possible for the CNES to detect human respiration signals without making touch, providing a trustworthy method for creating electronic skin for both contact and non-contact sensing. It is worth noting that for solid TENG, temperature and humidity affect the electrical output of TENG simultaneously. Since high temperature increases the charge dissipation rate of the tribolayer, this leads to a rapid decrease in the electrical output of the TENG. In addition, since the evaporation of water molecules will take away the surface charge of the tribological layer, humidity will also increase the charge dissipation rate of the tribological layer, resulting in a severe decrease in the electrical output of TENG. However, for solid-liquid TENG, since the tribolayers of solid-liquid TENG are composed of liquid (mainly water) and solid, humidity has little effect on the electrical output of TENG. Furthermore, temperature can affect the wettability of the friction layer of TENG, resulting in a change in the contact angle of the friction layer, which in turn affects the electrical output of TENG.

Influence of Atmosphere
As of today, conducting polymers like polypyrrole and polyaniline (PANI) as well as semiconductors (WO 3 , ZnO, In 2 O 3 , SnO 2 , etc.) have been the most widely employed NH 3 sensing materials.
Additionally, measuring the electrical variation of the NH 3 sensing materials is the traditional way of gas detection. [165][166][167][168] Even though numerous studies have been done to improve the NH 3 sensor's sensing performance, some pressing issues, like fabrication complexity and excessive energy consumption, remain unresolved. Additionally, the drawback of commercial NH 3 sensors not having a continuous working function must be solved. A clever self-powered NH 3 sensor that is portable, easy to set up, and capable of continuous operation is therefore greatly desired. TENG has been widely used as self-powered sensors for efficiently detecting external changes using the current and voltage output signals, respectively, with applications such as pressure sensors, motion detection, vibration monitoring, chemical and environmental detection, and gas sensors. [169][170][171] This is due to the advantages of low cost, simple fabrication, sustainability, and high efficiency. In contrast to conventional sensors, such a sensor can function without the need for external power.
Although it has many advantages in the sensor field, TENG as the power supply and gas sensor measurement are separated for the majority of the gas sensors based on TENGs, nevertheless. The lack of integration between TENGs and gas sensors makes the device less portable and more difficult to utilize in everyday situations. [172][173][174] Therefore, it will be more practical to use the TENG's tribolayer material as the ammonia gas reaction site, and to measure the ammonia gas concentration by observing how the triboelectric signal changes. As shown in Figure 7a-e, a new triboelectric nanogenerator made of conducting polyaniline nanofibers (PANI NFs) has been used to create a flexible, portable, sensitive, and self-powered ammonia (NH 3 ) nanosensor (TENG). [163] The integration of the gas sensor and power supply into one unit has been accomplished. The PANI NFs with NH 3 sensing capabilities function in the TENG as both an electrode and a friction layer. The NH 3 sensor's design principle-a change in the electroconductivity of PANI-causes the output voltage of the TENG, which exhibits high output performance with a maximum short current circuit of 45.70 A and output voltage of 1186 V in air, to be significantly reduced after exposure to NH 3 at various concentrations. The limit of detection for this NH 3 nanosensor at room temperature is 500 ppm, and it also demonstrates good selectivity and sensitivity.
Additionally, for the purpose of harvesting random mechanical energy, a new conductive and elastic sponge-based triboelectric nanogenerator (ES-TENG) has been created by Y. Liu et al, [64] as shown in Figure 7f-o. This device integrates elastic and conductive materials on a flexible sponge to achieve the collection of mechanical energies, particularly for erratic and random motions. The conductive polyaniline nanowires (PANI NWs) are grown on the surface of the elastic sponge using a straightforward, diluted chemical polymerization of aniline. Sponge can capture the kinetic energy of disorganized motion with various amplitudes and from various directions thanks to its flexible deformation. The polyaniline nanowires on its surface and the porous sponge employed as the ES-triboelectric TENG's layer can provide a significant contact area and boost triboelectric efficiency. In addition, the conductive polyaniline coating on the surface of sponge can be used as the electrode of an ES-TENG to conduct electrons and produce outputs of 540 V and 6 A, respectively. These outputs can be used on the surfaces of various flexible objects to collect irregular and random mechanical energy that is present in ev-eryday life. Additionally, the ES-TENG can make it operate as a self-powered sensor for detecting poisonous NH 3 with a detection limit of up to 1 ppm and a response time of <3 s based on the NH 3 -sensing performance and the 3D reticular structure of the polyaniline nanowires on the elastic sponge. The ES-TENG has prospective applications in a variety of irregular and random mechanical energy harvesting and self-powered NH 3 sensors due to its microporous and nanowire architectures, elasticity, conductivity, and ease of manufacture.

Influence of the Tribolayer Structure
The condition monitoring of numerous systems, such as industrial automation, building structures, implantable sensors, human healthcare, and the environment, is greatly aided by wireless sensor networks (WSN). Powering such widely dispersed electronic devices has long been a key challenge because of the huge coverage area of WSN. It is difficult to use standard batteries for WSN because they have a number of flaws, including a short lifespan, difficult and expensive replacement, bulky size, and environmental risks. [175][176][177][178] In order to provide the necessary power for WSN, energy harvesting from ambient energies is a feasible solution. Kinetic energy is the most varied and pervasive type of ambient energy that exists in the world today. Compared with other energy conversion methods, electricity is produced by TENGs using contact electrification and electrostatic induction combination. It has been demonstrated that TENGs work extremely well in terms of energy conversion even in challenging conditions. Furthermore, TENG can achieve high power densities for a range of macro-and microscale applications with ease using lightweight, inexpensive materials.
N. Wang et al. [26] report a modified cellulose material that achieves enhanced electrical output property at high humidity by acquiring universal dual-electric polarity augmented property. This material shows enhanced contact electrification performance when contacted with different electrode materials. Results show that the electrical output of the TENG composed of cyanoethyl cellulose (CE-cellulose) with polytetrafluoroethylene (PTFE) and nylon 11 increased by 4 and 8 times. Moreover, the triboelectric charge density of CE-cellulose-PTFE-based TENG at 95%RH is 533 C m −2 , which is a new record for the electrical output of TENG in a high-humidity environment. Based on this, a sensor system employing CE-cellulose-based TENG as a "switch" to control the charge-discharge of the capacitor in the system to drive the alarm device was designed, as shown in Figure 8a-d. In addition, in order to create extremely wearable and machine-washable F-TENGs, washable fabrics were created using straightforward liquid-phase fluorination utilizing homemade urethane perfluorooctyl silane (NHCOO-PFOTS), [179] as shown in Figure 8e,f. For extended periods of time, fluorinated silk can remain hydrophobic (contact angle >140°). A wearable TENG was created using fluorinated fabric for the purpose of harnessing human energy. After 45 000 cycles of contact-separation motions and 70 h of washing, a simple but reliable F-TENG made of fluorinated silk and nylon materials produced a maximum output power of 2.08 W m −2 at 10 M with barely any decay. Additionally, the F-TENG created here has exceptional anti-wear and selfhealing capabilities. F-TENGs sewn onto garments can use the electricity produced by arm swinging to power a digital watch.  [163] Copyright 2018, Elsevier. f) Preparation process of the fluorine-modified acrylate resin and the organic coating TENG. g) Influence of rotating speeds on the output current during the same spin time. (h-m) SEM images of PANI nanowires modified conductive elastic sponge, elasticity of conductive elastic sponge, and optical photographs of ES-TENG with irregular shape of the triboelectric layers. n) Influence of changes in the ratio of primer and curing agent on I sc at 3000 rpm. o) Effect of spraying process times on the output performance. Reproduced with permission. [64] Copyright 2020, Elsevier. p) Structure diagram of the self-powered CO 2 sensor. q-s) The output current curves of the TENG-GD under negative gas discharge in CO 2 for various d, where the load is 10 MΩ. Reproduced with permission. [164] Copyright 2018, Elsevier.
In addition, to generate power in the winter, a brand-new kind of ice-based triboelectric nanogenerator (ICE-TENG) has been created, as shown in Figure 8g-i. Because of its cleanliness, environmental protection, abundance of reserves, minimal friction, and self-healing capabilities, ice is a popular material for designing TENGs in cold winter, alpine, or snowy mountain environments. With a 4 mm thick ice layer, 30 N loadings, and a 5 Hz contact frequency, the short-circuit current, and voltage may be achieved, which can be helpful in some real-world applications like illuminating LEDs and charging capacitors. The ICE-TENG has exceptional stability and can produce 35 W of output power with a 20 M loading resistance. The coefficient of friction between ice and other friction pairs is minuscule, and when it falls below 0.08, it can even become extremely slippery. Therefore, the friction pairs experience low wear, which may be a factor in TENG's lengthy operational life. Additionally, the ICE-TENG has an excellent capacity for self-healing and keeps up its original output performance even after numerous damage and repair procedures thanks to the quick phase shift of ice. A single electrode ICE-TENG controlled by walking has been developed to harvest energy from the ice surface in order to simulate practical applications. The ICE-TENGs are made to Figure 8. Self-powered wireless sensor. a) Photo of TENG with nylon socks as tribolayer to power the whole system, two CE-cellulose-based singleelectrode TENGs attached to the insole. b) The photo of charging the capacitor while walking. c) While running, capacitor discharging, and signal receiver alarming. d) Enlarged view of signal receiver and cell phone. Reproduced with permission. [26] Copyright 2022, Elsevier. e) Preparation process of urethane perfluorooctyl silane (NHCOO-PFOTS) and fluorinated fabrics using the dip-coating method. f) Application of F-TENGs for wireless sensors for drowning warning. Reproduced with permission. [179] Copyright 2022, Wiley-VCH. g) Schematic illustration of the ICE-TENGs-based monitoring sensor. h) The short-circuit current of the ICE-TENGs with unbroken and broken ice layers. i) Photograph of the ICE-TENGs-based wireless warning system on the broken ice layer. Reproduced with permission. [180] Copyright 2022, Elsevier. build a warning system to notify people of the danger when the ice surface abruptly shatters due to the difference in electric output before and after the ice fragmentation. Such ICE-TENGs can also charge an electronic watch with footfall while lighting "ICE" LEDs. This demonstrates the possibility for self-powered systems to warn of danger, run out of charge, and gather energy.
One of the crucial properties of the material surface is wettability, which enjoys a high reputation in the fields of aerospace, oil-water separation, self-cleaning, and lubrication. As a result, it's important to keep an eye on how the interface wetness condition changes. [181][182][183] At present, there are many characterization methods of interface wettability, such as static contact angle, advancing angle, receding angle, rolling angle, surface adhesion, and other tests. However, these traditional wettability monitoring methods are relatively static, and the actual application process of materials is relatively complex, so the traditional methods cannot objectively evaluate the change of interface wettability during operation. Compared with the static transient characterization method, it is more valuable to know how the interface wettability of the material changes dynamically during the actual complex operation, and it is more valuable to further optimize the design and practical application of the material, for example, when the material is continuously working for a long time. If the change of the wetting state of the material surface can be predicted and the failure or deterioration of the superhydrophobic effect can be known in advance, the loss caused by the superhydrophobicity failure of the material can be avoided. However, there is currently no suitable means in the field to predict the dynamic change of the surface wetting state during the material operation. [184][185][186] For solid-liquid triboelectricity, the surface structure influences the wettability of the interface, the solid-liquid contact area, and the surface that separates and accumulates triboelectric charge. As a result, frictional charges are produced in proportion to the size of the solid-liquid contact area and the difference in solid-liquid polarity, whilst the interfacial wettability controls the complete separation and accumulation of charge. Therefore, by controlling the shape and content of the friction layer, solid-liquid frictional electricity can be controlled. Conversely, by analyzing the electrical signal of solid-liquid triboelectricity, we can monitor the wetting state of the tribolayer. As shown in Figure 9a-d, systematically investigated is the effect of surface structure, composition, and interfacial wettability on the solidliquid triboelectrification based on the friction of polypropylene (PP) sheets and deionized water. [187] By using a surface selfassembly technique, the PP films are surface functionalized Figure 9. Interface wetting status monitoring. a) Schematic diagram of PP NWs preparation process through a hot pressing method with AAO as template and surface chemical modification of PP film with APTES, OTS, and PFTS as modifiers. b) The water static contact angle of bare PP and polydopamine-modified PP. c) I sc of polydopamine-PP-based TENG. d) comparison of I sc between the bare PP and polydopamine-PP-based TENG. Reproduced with permission. [187] Copyright 2019, Wiley-VCH. e) The different surface roughness obtained on PP substrate after the hot molding process. f) Tribocurrent as a function of time. g) Typical TENG sloshing pressure. (h) Tribocurrent as a function of time. i) Steady-state tribocurrents. j) Drag-out dewetting map. Reproduced with permission. [188] Copyright 2022, Wiley-VCH.
with three distinct functional groups. The findings demonstrate that solid-liquid triboelectrification is more susceptible to surface components than surface structure. While from a macroscopic perspective, the interfacial wettability dominates in its electrical output, the triboelectrical output continues to decrease with the failure of the hydrophobic surface of the PP film. The wettability change of the solid-liquid interface is tested using the interface wetting state monitor that is described in this study, and the results are accurate and trustworthy.
Wetting is frequently thought of as an inherent surface feature of materials, but understanding its genesis is challenging due to its intricate relationship with scale-dependent roughness. Only two of the many wetting behaviors are the Wenzel (W) state, where liquids come into close contact with the rough surfaces, and the Cassie-Baxter (CB) state, where liquids rest on air pockets created between asperities. The anti-contamination, anti-icing, drag reduction, etc. surface performance that is dictated by transitions from the CB to the Wenzel state is also entirely distinct; yet, nothing is understood about how transitions happen over time between the various wetting modes. To clarify the mechanisms relating triboelectric production to wetting dynamics and wetting-induced fluid penetration, a multiscale model was created by X. Zhang et al, [188] as shown in Figure 9e-j. Triboelectric decay was connected with the wetting/dewetting transition from Cassie-Baxter to Wenzel mode, which was mechanically caused by dynamic water pressure or by a retarded water drag-out process. This enables us to quantify a wide range of wetting dynamics processes for the first time (and related wetting areas). In contrast to untextured and nanostructured surfaces, the hierarchical PP surface with structured sidewalls offers a stable and Figure 10. Solution property monitor. a) Schematic illustration of the self-powered TENG. b) Schematic diagram of the deposition of Pdop film on PTFE and wettability of PTFE before and after modifying with dopamine. c) Short-circuit current as a function of time. Reproduced with permission. [189] Copyright 2016, Elsevier. d) Schematic diagram of the structure of L-TENG. e) The schematic image of the solid-liquid interface. f) Induction of the influence of several factors on the CE of the solid-liquid interface. Reproduced with permission. [190] Copyright 2020, Elsevier. g) Schematic of hydrophobic organic coating based TENG for efficiency. h) Output current of the device versus different liquid components. Reproduced with permission. [191] Copyright 2020, American Chemical Society. substantial tribo-nanocurrent as a result of either a quicker dragout dewetting or a larger reversible genuine wet contact area. On the other hand, the microtextured surface displays incremental infiltration dynamics in real-time, as revealed by the triboelectric signal as well. The fidelity of triboelectricity to examine the dynamic wetting qualities of surfaces is the study's most significant finding.

Influence of the Tribolayer Composition
Currently, the majority of TENG devices rely on solid-solid contact electrification, which frequently has problems with output stability and longevity due to significant abrasion of the micro/nanostructured electrodes. On the other hand, it has recently been shown that contact electrification between liquid and insulator polymer films can gather ambient mechanical energy, acting as a self-powered sensor. As shown in Figure 10a-c, for the purpose of detecting organics in water, a fully packaged water-solid triboelectric nanosensor based on the liquid-solid friction between a specific type of commercial PTFE membrane filter and water was created. [189] Due to its nano-fiber structure, great electro negativity, and outstanding hydrophobicity for rubbing with water for high output performance, the commercial PTFE filtration membrane was chosen as the tribo-layer. Without any modifications, the output voltage and current of this PTFE filtration membrane-based TENG may reach 12 V and 1.5 A, respectively. Fluid mechanics analysis is used to show the dependence of the output performance of water-based TENS on the water-to-cylinder volume ratios, vibration frequency, and amplitude. This TENS can function as a dopamine concentration sensor by doping a PTFE filter membrane. This innovative gadget is more sensitive and tolerable than the previously described solidsolid friction-based TENG. This TENG can also be used as a sensor to measure the concentration of ethanol by combining it with ethanol to lessen the water polarity and lower the triboelectric charges.
In addition, recent studies have shown that electron transfer and ion transfer have a dual effect on the contact electrification of the solid-liquid interface, whereas the interfered contribution of various factors has received little attention. This is based on the principle of electric double layer (EDL) theory and the twostep model of electron or ion transfer. In view of this, L. Zhang et al. talked about how surface elements affect the solid-liquid interface's triboelectric activity, [190] as shown in Figure 10d-f. In addition to altering the quantity of charge at the solid-liquid interface, changes in the composition of the solid surface can also alter the polarity of the triboelectricity. The surface material has a significant impact on the polarity and size of the triboelectricity. This knowledge of the mechanism and influencing elements of CE at the solid-liquid interface will be enhanced by the current work. Additionally, the tribolayer of a new kind of organic coating triboelectric nanogenerator is made out of acrylate resin. Fluorine-containing components were added to the acrylic resin to enhance the solid-liquid triboelectrification performance and the hydrophobicity of the coating. As a non-microstructuredependent film, its fabrication is straightforward, and a sizable area can be prepared by changing several anticorrosion and antifouling coatings that are frequently used in engineering. This compact organic-coated triboelectric nanogenerator has good stability and high output performance, and by capturing wave energy while at sea, it can easily light numerous commercial lightemitting diodes (LEDs) on a model ship. In addition, the research paper analyzes the influence of impurities in water on triboelectricity. Most liquids that can be directly collected and used in the environment, such as ocean, rainwater, industrial wastewater, river water with silt, etc., are not pure water. In this investigation, five different liquids-deionized (DI) water, NaCl solution, HCl solution, NaOH solution, and a combination with 5% sandwas chosen to investigate their effects on the OC-TENGs, [191] as shown in Figure 10g-h. The findings demonstrate that all TENGs based on the aforementioned five liquids have obvious output current and voltage, which may be efficiently employed to capture the friction energy between solids and liquids. The TENGs based on clean water had the highest yield, followed in decreasing order by those based on sediment-filled water, acidic water, salt-filled water, and alkaline water. The outcomes show that the electrolytes in water have an impact on the output of the OC-TENG. This is due to the fact that even when the coating is cut off from the water, water droplets can still adhere to it. More positive charges, such as dissolved ions, will remain on the films once there are electrolytes in the water, which leads to partial screening of the triboelectric charges on the coating. As a result, less electrical output will be produced than with deionized water. This study provides a theoretical basis for the study of liquid impurity sensors.
Generally speaking, friction processes will result in triboelectrification. Multiple applications require triboelectricity control to function. Triboelectric charge accumulation can produce extraordinarily high voltages, which can cause electronic component failure and explosions in flammable settings. [194][195][196][197][198][199] Additionally, the buildup of triboelectric charges on the friction surface results in a rise in energy usage. Therefore, it is crucial to reduce static charge creation and accumulation in industrial activities. In addition, the monitoring of friction coefficient and wear state during friction process is of great significance for industrial production. In order to realize the monitoring of friction state, X. Liu et al. used a novel system to investigate the influence of liquid lubrications on triboelectrification, [192] as shown in Figure 11ad. By boosting the triboelectric signal generated during the friction process on the friction testing apparatus, the friction coefficient was determined. By comparing the ground current and friction coefficient recorded throughout the experiment, the effect of the liquid lubricant on triboelectrification and tribological behavior was examined. To investigate the impact of various pressures and reciprocating frequencies on tribocharges, the load, and frequency of the friction testing machine were adjusted. To investigate how liquid lubricants affect the triboelectric characteristics of materials with different polarities, a steel ball was partnered with polyvinylidene fluoride, polyoxymethylene, and polytetrafluoroethylene separately to generate friction pairs. Liquid lubrication improved the stability and robustness of the material's triboelectric output as a result. This study looked into how lubrication and TE are closely related, with the goal of enhancing triboelectric signals while also lowering wear. It was crucial to keep track of the machine's lubricant levels to ensure its longevity. The lubrication of the friction pair in the mechanical system can be monitored online to increase machine life and safety through the alteration of the triboelectric signal, which reflects the status of the lubricant in the friction pair in real-time.
The friction force curve frequently contains too little information about the friction process to accurately depict the friction state of the friction pair. As a result, real-time sensor of the friction state is crucial in tribology since it enables us to comprehend the status of the friction pair and guarantee that the friction is operating normally. As a result, in order to characterize the friction interface state, additional test methodologies must be combined. From a tribological standpoint, L. Zhang et al. developed a new macroscale superlubric triboelectric nanogenerator (SL-TENG) based on a hydrogenated diamond-like carbon (DLC) sheet that can endure high-contact stress with an ultralow COF (0.01) and ultralow wear rate, [193] as shown in Figure 11e-g. Due to the tribovoltaic effect, the SL-TENG can produce direct current (DC) output and light up a number of light-emitting diodes (LEDs) without rectification. Since the strong relationship between tribology and triboelectricity, online superlubric friction monitoring and early warning can be achieved.

Conclusion and Future Perspective
This review focuses on the working mechanism of solid-solid and solid-liquid TENGs, and the latest research progress in the field of sensors and monitors. We first comprehensively introduce the models and working mechanisms of classic solid-solid and solid-liquid TENGs. Then, we describe the recent research progress of TENGs in the fields of temperature sensors, humidity sensors, pressure sensors, gas sensors, and wireless sensors. In addition, related studies on the monitoring of the friction state of solid-solid TENG and the monitoring of the wetting state of solid-liquid TENG at the interface are also mentioned. Finally, we compare the types and electrical outputs of these TENGs used in the sensor field, as shown in Table 1. The results show that the voltage range of the TENG used for the sensor is very wide (0.15-1186 V), which indicates that the electrical output of the TENG Figure 11. Friction condition monitoring. a) Mechanism of action of the friction pair. b,c) Current and friction coefficient of the steel ball and PVDF friction pair under dry friction and with 0.5 L PAO 4 lubrication. d) Repaired current and friction coefficient during 0.5 L PAO 4 lubrication after re-adding three times. Reproduced with permission. [192] Copyright 2022, Elsevier. e) Schematic of the fabrication of the SL-TENG based on a ball-on-disc friction mode with DLC film and steel ball as the friction pair. f) I sc curve collected with a current amplifier and its corresponding COF curve under different atmospheric changes. g) A circuit for sensing the failure of superlubric friction by using the unipolarity of the current and the corresponding optical photo of the LED lamp when nitrogen and oxygen are injected. Reproduced with permission. [193] Copyright 2022, Cell Press.  [ 200] Temperature sensor S-S 320 2019 J. Xiong et al. [ 143] Temperature sensor S-L 6 2021 X. Li et al. [ 142] Pressure sensor S-S 30 2016 K. Lee et al. [ 152] Pressure sensor S-S 160 2018 K. Dong et al. [ 151] Pressure sensor S-S 20 2019 Z. Zhao et al. [ 150] Humidity sensor S-S 330 2020 N. Wang et al. [ 154] Humidity sensor S-S 910 2021 L. Xu et al. [ 201] Humidity sensor S-S 100 2022 F. Yin et al. [ 155] Gas sensor S-S 1186 2018 S. Cui et al. [ 163] Gas sensor S-S 540 2021 Y. Liu et al. [64] Wireless sensor S-L 200 2016 A. Ahmed et al. [ 202] Wireless sensor S-S 15 2021 Y. Feng et al. [ 203] Wireless sensor S-S 400 2022 M. Feng et al. [ 179] Friction condition monitoring S-S 0.5 2022 L. Zhang et al. [ 204] Friction condition monitoring S-S 0.15 2022 X. Liu et al. [ 192] Interface wetting status monitoring S-L 6 2019 X. Li et al. [ 187] Interface wetting status monitoring S-L 0.2 2020 L. Zhang et al. [ 190] Interface wetting status monitoring S-L 2.5 2022 X. Zhang et al. [ 205] Adv. Sensor Res. 2023, 2, 2200058 www.advancedsciencenews.com www.advsensorres.com depends on the voltage value required by the driven sensor, not that the larger the electrical output, the better. TENG is a cutting-edge energy technology with several distinct advantages. In order to increase the efficiency of micromechanical energy harvesting, TENGs can be activated by minute external mechanical stimuli. This creates the ideal environment for selfpowered sensors to gain higher sensitivity. Second, mechanical energy harvesting and self-powered sensing are made possible by the ability of TENG to be affixed to curving and moving surfaces. Last but not least, TENGs are architecturally adaptable and can be employed as wearable and implanted electronics to actualize human motion detection and implantable medical monitoring or to harvest biomechanical energy to supply sustainable energy for portable electronic devices.
In future, the following research aspects are desired to be focused on. First of all, the research on DC triboelectric nanogenerators (DC-TENG) is imperative. It is well known that direct current (DC) outputs are more suitable for driving sensors and energy storage than alternating current outputs. Therefore, DC-TENG will be one of the focuses of future research. In the second place, to practically apply TENGs to the field of smart sensing, the miniaturization and modularization of TENGs-based sensing systems are two key challenges. To achieve practical and comfortable use, it is particularly necessary to make the TENG system smaller, lighter, and more efficient in the field of motion. The flexibility, stretchability, biocompatibility, and breathability of TENG devices should also be considered, especially for smart wearable devices. Finally, in the current development stage of TENGs-based intelligent sensing systems, the sensing accuracy of TENGs has been neglected since most researchers are committed to achieving sensing ability rather than improving it. With the rapid development of the smart sensing field, sensing accuracy will become increasingly important in its commercialization. The sensing accuracy of TENG is closely related to its electrical performance, and its electrical performance can be improved by means of material improvement, structural design, and power management. Given the great potential of TENGs in smart sensing systems, further advancements and improvements are likely in the near future. It is clear that by vigorously improving these systems, various multidisciplinary research areas, including environmental monitoring, food safety, smart home, and even defense systems, can be greatly innovated.