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Textile-Based Triboelectric Nanogenerators for Self-Powered Wearable Electronics

Sung Soo Kwak, Hong-Joon Yoon, and Sang-Woo Kim*

Wearable smart electronic devices based on wireless systems use batteries as a power source. However, recent miniaturization and various functions have increased energy consumption, resulting in problems such as reduction of use time and frequent charging. These factors hinder the development of wearable electronic devices. In order to solve this energy problem, research studies on triboelectric nanogenerators (TENGs) are conducted based on the coupling of contact-electrification and electrostatic induction effects for harvesting the vast amounts of biomechanical energy generated from wearer movement. The development of TENGs that use a variety of structures and materials based on the textile platform is reviewed, including the basic com-ponents of fibers, yarns, and fabrics made using various weaving and knit-ting techniques. These textile-based TENGs are lightweight, flexible, highly stretchable, and wearable, so that they can effectively harvest biomechanical energy without interference with human motion, and can be used as activity sensors to monitor human motion. Also, the main application of wearable self-powered systems is demonstrated and the directions of future develop-ment of textile-based TENG for harvesting biomechanical energy presented.

DOI: 10.1002/adfm.201804533

a self-powered system.[1–8] However, in the case of energy harvesting technology that uses sunlight, it is difficult to con-tinuously and efficiently obtain power generation, because of temporal and spa-tial limitations, such as cloudy day, night time without sun, and room with weak light intensity. Therefore, in order to con-tinuously supply power to wearable smart devices, new energy harvesting technology is needed that can supplement or replace solar power generation technology. For this reason, a technique of harvesting the biomechanical energy generated by the movement of a person using the wearable devices and using the converted electric energy as the power of the devices has received much attention. Biomechan-ical energy occurs in various forms and sizes, such as walking, jumping, pulling, bending, etc., and occurs continuously while wearing a wearable device. There-fore, by converting such biomechanical

energy into electrical energy, it is possible to establish a self-powered system that is capable of continuous power supply to portable wearable devices.

1.2. Triboelectric Nanogenerators (TENGs) Based on Textile

There are energy harvesting techniques that convert mechani cal energy into electrical energy, but such techniques must meet several conditions, because they harvest the biomechanical energy generated in the human body. These conditions are lightweight, so that they can attach or wear the energy gener-ating element to the body in order to harvest the biomechanical energy, and have various structures or flexible characteristics, so as not to restrict the movement of the wearer or make them feel uncomfortable. Also, the energy generating element should be made of a material that is harmless to the human body, and is not toxic. However, conventional mechanical energy harvesting technologies do not meet these requirements, and have limita-tions. Electromagnetic induction generators (are made of heavy material, such as coils and magnetic bodies, and are limi ted in that they are not efficient in harvesting the mechani cal energy in the range of low frequencies that occur in human motion. Piezoelectric nanogenerators have a narrow range of materials in order to avoid harmful lead-containing materials or mate-rials that are easily broken, and because they are ceramic, have mechanical durability problems.[9]

Mechanical Energy Harvesting

Dr. S. S. Kwak, Dr. H.-J. Yoon, Prof. S.-W. KimSchool of Advanced Materials Science and EngineeringSungkyunkwan University (SKKU)Suwon 440-746, Republic of KoreaE-mail: [email protected]

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.201804533.

1. Introduction

1.1. The Need for Biomechanical Energy Harvesting Technologies

Recently, a variety of wearable smart devices have become more sophisticated and have a lot of functions, which means they consume large amounts of power, from several mill-watts to tens of watts. There is a difficulty in meeting the large amount of power consumed when operating a wearable device for a long time using only the existing battery. As a result, users have the inconvenience of frequently charging a small battery every few hours or using a large, heavy battery of several kilograms. To solve this problem, energy harvesting technology must be used as a power source to charge the battery in real time and to increase the use time, or to completely replace the battery when a sufficient level of power is secured, thereby enabling

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Recently, the TENG was invented. It is based on coupling between the triboelectric effect, in which charge transfer and electrification occur in contact or friction between two dis-similar materials based on the triboelectric series and elec-trostatic induction.[10,11] This new mechanical energy har-vesting technology has high output voltage value even at low frequency, which enables efficient energy conversion. The technology also has various device structures and working modes, allowing optimization of the form and size of the target mechanical energy.[12–15] In addition, it is easy to fab-ricate a large-scale device, because various materials, which are flexible, stretchable, biocompatible and cost-effective, can be applied, while the manufacturing process is cost-effective and simple.[16–19] Because of these advantages, this promising mechanical energy harvesting technology has attracted much attention, and has been reported in many papers that are applicable to various applications and enabling self-powered systems.[20–25]

Among these many TENG studies, several research groups have focused on the development of textile-based wearable TENGs that are particularly optimized for biomechanical energy harvesting by ensuring air permeability, and can be applied to real clothing. This paper reviews these various types of textile-based TENGs. In addition, some researchers have been conducted on textile-based hybrid generators that are complementary to each other through the combination of energy harvesting elements that convert other energy sources into electricity. Through inte-gration with the storage devices, researches on the development of a textile-based integrated energy system have also been carried out. These advances have provided a great opportunity to estab-lish self-powered wearable electronic systems.

1.3. Basic Working Mechanism and Four Working Modes of TENGs

The triboelectric effect, which has been known for thousands of years, is the process of electrification when two dissimilar materials come into contact and friction with each other. Tri-boelectrification is the most common phenomenon that can be easily seen in everyday life, and can be found in almost all mate-rials, such as metals, ceramics, and polymers. Nevertheless, a mechanism for the phenomenon of triboelectrification has not yet been clearly elucidated, and it is generally believed that a chemical bond is formed in part of the surface between friction materials, or charge exchange between materials occurs, due to difference of stability, work function, and electron affinity between two friction materials. There are hypotheses that these exchanged charges are molecules, ions, and electrons.[26–28] These exchanged charges are placed on each surface when two friction materials are dropped, and the charges on each surface have opposite directions. These charges on the surface act as electrostatic charges that induce electrical potentials on the elec-trodes located behind the friction material, and a potential dif-ference occurs between the two electrodes behind each friction material. In order to neutralize the potential difference, elec-trons flow through the external circuit connected between the electrodes. Then, when the two friction materials come close to contact again, the electrons flow back, and an alternative

current (AC) output signal is generated from these contacts and separations.[29–32] Based on this principle, four working modes of TENG have been invented, as explained in the following, and as shown in Figure 1.

1.3.1. Vertical Contact Mode

Two different layers of dielectric material are vertically laid up and down, with electrodes behind each layer of dielectric

Sung Soo Kwak received his Ph.D. degree from School of Advanced Materials Science and Engineering at Sungkyunkwan University (SKKU), Korea, in 2018. His research interests are fabrication and characteri-zation of piezoelectric and triboelectric nanogenerators for mechanical energy harvesting and their

multifunctional applications based on self-powered systems.

Hong-Joon Yoon received his Ph.D. degree under the supervision of Prof. Sang-Woo Kim at School of Advanced Materials Science and Engineering, Sungkyunkwan University (SKKU), Korea, in 2018. His research interests are fabrica-tion and characterization of piezoelectric and triboelectric nanogenerators for energy

harvesting and their application in self-powered devices.

Sang-Woo Kim is professor in the Department of Advanced Materials Science and Engineering at Sungkyunkwan University (SKKU). His recent research interest is focused on piezoelectric/triboelectric nanogenera-tors, photovoltaics, and 2D materials including graphene, MoS2, etc. Now he is a director of SAMSUNG-SKKU

Graphene/2D Research Center and is leading the National Research Laboratory for Next Generation Hybrid Energy Harvester.

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material. When external force is applied in the vertical direc-tion, the top layer and the bottom layer become close to each other, and then make contact with each other. Depending on the tribo-series, triboelectric charges in different directions are formed on the surface of each dielectric material. This poten-tial difference causes flow of electrons through external circuit, and when the two layers come close to contact, the flowing elec-trons flow back in the opposite direction, because the potential difference disappears.

1.3.2. Lateral Sliding Mode

The basic structure of the lateral sliding mode is similar to the vertical contact mode, consisting of two layers of two different dielectric materials followed by an electrode, but there is no gap in the vertical direction. Also, unlike the vertical contact mode where the surfaces of the two different dielectric materials make contact and separate at one time, when the external force is applied in the horizontal direction, the surfaces of the two dielectric materials in the lateral sliding mode make sequential contact. After the triboelectric charges are formed on both sur-faces of both dielectric materials by triboelectrification, in the process of successive contact and separation of the two dielec-tric layers, triboelectric charges on the surfaces, except for the contact area, form an electric field on the back electrode. Then, an electron flow occurs to neutralize the potential difference between the two electrodes. The sliding mode can be applied to planar motion, or rotation of disk or cylinder.

1.3.3. Single Electrode Mode

Unlike the working modes in which the two electrodes are connected to each other as described, this working mode is a

structure having a single electrode connected to the ground, and this electrode serves not only as an electrode, but also as a triboelectric material in contact with a dielectric material. Charge on the surface of the dielectric material is generated by the triboelectrification in the contact between the electrode and the dielectric material. The dielectric material with electro-static charges on its surface pulls or pushes electrons from the ground to the electrode, according to the change in potential generated in the process of making contact with and separating from the electrode. This working mode can be applied to both vertical and lateral movements, and the dielectric material layer can move freely, so it can be applied in various places.

1.3.4. Freestanding Mode

The freestanding mode is composed of moving object and a pair of patterned electrodes underneath a dielectric material, and the size of the electrode pattern is the same as the size of the moving object. After the triboelectification between dielec-tric layer and moving object, as the charged moving object approaches or move away, asymmetric charge distribution on each electrode is created, and this leads to electron flow through the external circuit connecting the two electrodes for neutralization.

2. Fiber-Based TENGs

2.1. Entwined Structure with Fibers

Since the first report of TENG by Wang’s group in 2012, TENG papers have proposed various materials and device structures. However, most of the papers were made of polymer and metal in film type, which causes lack of flexibility and stretchability,

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Figure 1. Four working modes of TENGs. Reproduced with permission.[30] Copyright 2015, Wiley-VCH.



low permeability to air and sweat, inconvenience to wearer, and adverse effect on maneuverability. Applying these film-based TENGs to real clothing was found to not be suitable for har-vesting biomechanical energy. Thus to address these problems, the development of a variety of textile-based TENGs that can be integrated into wearable smart apparel has been proposed.

Zhong et al. first introduced a cost-effective, metal-free, and fiber-based generator that converts biomechanical motion and vibrational energy into electricity by utilizing the tribo-electric effect and electrostatic induction effects, as shown in Figure 2.[33] The fiber-based TENG is a structure entwined with cotton threads having two different structures and character-istics. One is a cotton thread coated with carbon nanotubes (CNTs) by using homemade multiwalled CNT ink through a simple and cost-effective dipping and drying method, while the other is a cotton thread coated with polytetrafluoroethylene (PTFE) and a CNT.[34] The two threads were coated with home-made multiwalled CNT ink and a PTFE aqueous suspension through a simple and cost-effective dipping and drying method. The coated CNT thread (CCT), with diameter of ≈240 µm and density ≈0.207 mg cm−1, has good flexibility and conductivity with a resistance of ≈0.644 kΩ cm−1, and serves as electrodes (Figure 2b). In the PTFE and CNT-coated thread (PCCT) with diameter of ≈500 µm, PTFE has been used as a dielectric mate-rial and as a friction material, because it is known to be the most negatively charged material in the tribo-series, as well as being able to theoretically maintain static charge on the surface for decades (Figure 2c). The PTFE produced negative electro-static charges on the surface by the polarization through oxygen plasma treatment, so that it was possible to make surface potential of PTFE from (9 to 660) V by polarizing by plasma treatment for about 40 min. The increased surface potential is

reduced to about 470 V after 30 h, but can be maintained for longer than 20 days, making it applicable to long-term sustain-able devices.[35] CCT and PCCT were entangled in a helix struc-ture, with both ends fixed to a commodity cotton thread, and these twisted fiber-based TENGs were fabricated in a fabric for application to clothing using a weaving method (Figure 2d).

The power generation mechanism of the fiber-based TENG simplifies the complex helix structure, and is described in the equivalent circuit connected with the external load of R, as shown in Figure 2e. The surface of PTFE, the outer layer of PCCT, has negative triboelectric charges (Q), which are caused by contact and friction with the CNT of CCT. In the initial state, the triboelectric charges on the surface of PTFE act as an elec-trostatic charge, and generate a potential on the CNT inside the PCCT, thereby generating a potential difference between the CNT of the PCCT and the CNT of the CCT. In order to neu-tralize the potential difference, positive charges (Q1, Q2) are moved and redistributed in two CNT layers of PCCT and CCT, respectively. By the law of conservation of charge, the value of Q is the same as the sum of Q1 and Q2, where Q = – (Q1 + Q2). When the fiber-based TENG is stretched, the reduction of d, the gap distance between CCT and PCCT fibers, induces more positive charge on the CNTs of the CCT, resulting in imme-diate charge flow. Conversely, when the fiber-based TENG is released, the device recovers to its original shape, and the gap (d) increases, forming a reverse charge flow, unlike the stretching situation. This stretching–releasing process of the fiber-based TENG generates an AC-like output signal.

The output of fiber-based TENG with 9.0 cm and 8 twisted helix structures has been evaluated by varying stretching strain and frequency (the period of stretching) while the output cur-rent is measured at an external load of 80 MΩ.[33] Various

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Figure 2. a) Schematic diagram showing the fabrication process of fiber-based TENG. SEM images of b) a CNT-coated cotton thread and c) (PTFE) and CNT-coated cotton thread. d) Real image of fiber-based TENG. f) Output current–time curve of a fiber-based TENG with stimulation strains of 0%, 0.54%, 1.08%, 1.61%, and 2.15% at a constant frequency of 5 Hz. g) Output current–time curve of a fiber-based TENG with stimulation frequency of 1.3, 2, 3, 4, and 5 Hz at a constant strain of 2.15 Hz. h) Durability test of fiber-based TENG for 5 h (≈90 000 cycles). Reproduced with permission.[33] Copyright 2014, ACS Publications.



stretching strains (0%, 0.54%, 1.08%, 1.61%, and 2.15%) were applied to the fiber-based TENG at a frequency of 5 Hz. Output current will not be generated if there is no deforma-tion. When strains were increased from 0.54% to 2.15%, the output current was increased from 3.98 to 11.22 nA (Figure 2f). At different frequencies (1.3, 2, 3, 4, and 5 Hz) with a strain of 2.15%, peak value of output current increased with increasing frequency. However, the amount of transferred charge was the same (Figure 2g). In the mechanical deformation of contact and separation between two friction materials of TENG, as the frequency increases, the amount of charge transferred to the external circuit is constant, but the charge current increases and the output current value increases.[36] When two fiber-based TENGs are connected in parallel, each output can be linearly superimposed to increase the total output current. Very stable power generation was possible when 90 000 cycles of stimula-tion were applied at a frequency of 5 Hz and a stress of 2.15% (Figure 2h).

It has also been shown that a single element fixed to the finger can identify the motion with total amount of transferred charges and output current generated in the five different bending–releasing operations.[33] The resulting output was ≈0.91 µW, enough to power electronic devices such as liquid crystal displays. This demonstrates that fiber-based TENG can be used as a self-powered active sensor for monitoring human body motions during patient’s rehabilitation and sports training as well as for harvesting biomechanical energy. Furthermore, fiber-based TENG can be integrated into lab coat. A wireless homemade body temperature sensor system has been reported.

This is the first to demonstrate that self-powered smart apparel is possible by harvesting biomechanical energy with fiber-woven TENG to power a mobile medical system.

2.2. Coaxial Structured Fiber

Unlike the above-mentioned fiber-based TENGs that produce output current by contact and separation between two fibers of different materials, other fiber-based TENGs have a coaxial structure in which two different materials come into contact but separate from each other within a single fiber.[37] These coaxial structures have already been widely used in energy harvesting and storage devices in the form of other fibers such as solar cells, supercapacitors (SCs), and lithium ion batteries. They have higher stability under bending and stretching than two fiber-twisted structures. They are also sensitive to small move-ments. The first reported single-fiber-based TENG with a coaxial structure consists of Al wires with vertically aligned nanowires and polydimethylsiloxane (PDMS) tubes with nanotextured sur-faces as shown in Figure 3.[37] The Al wire is used as a core while the PDMS tube is used as a shell, a completely sealed device. Nanowires on the surface of Al wire were ZnO grown by hydrothermal method. Au thin film was then deposited thereon as an electrode of TENG. The interior of the PDMS tube was subjected to a reactive ion etching (RIE) process to make nano-wire arrays of 1–2 µm in length. The Al thin film was used as another electrode of TENG by enclosing the outer surface of the PDMS tube. By forming these nanowires on the inside of the

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Figure 3. a) Schematic diagrams showing the fabrication process for single fiber-based TENG with coaxial structure. Scale bars in SEM images of insets are 1 and 2 µm. b) Output voltage and current of single fiber-based TENG. c) Calculated electrostatic potential depending on radius of Al wire and PDMS tube with COMSOL software. Dotted lines of the output voltage are experimentally measured. d) Comparison of output voltage of unsealed and sealed single fiber-based TENG under relative humidity of 95% RH. Reproduced with permission.[37] Copyright 2015, ACS Publications.



shell and on the surface of the core, the contact area between two surfaces is increased, resulting in improved power output.

Output voltage and current of fiber-based TENG with a coaxial structure of about 5 cm in length have been measured under a compressive force of 50 N at a frequency of 10 Hz.[37] Results revealed an AC type output of about 40 V and 10 µA as shown in Figure 3b. Since the gap between the core and the shell affects the output of TENG, change of triboelectric poten-tial according to the radius of the PDMS tube and the Al wire is calculated through COMSOL simulation. The highest tribo-electric potential is obtained at radii of 6 and 2 mm, respec-tively. This result has been proved experimentally. Since the flexibility of fiber is an important factor for application to wear-able devices, PDMS tube with radius of 2.5 mm and Al wire of 0.1 mm have been used to fabricate the device (Figure 3c). In addition, both ends of the fiber are sealed to form a com-pletely sealed structure. This allows stable output even under harsh conditions with relative humidity over 95% as shown in Figure 3d.

Fiber-based TENGs have been fabricated to a fabric through weaving method with each fiber connected in parallel.[37] The output current increases as the number of devices increases, resulting in output voltage of 40 V and current of 210 µA in a 14 cm × 14 cm fabric. The instantaneous power peak value reached a maximum of 4 mW at the connection with an external resistance of 10 MΩ. Stretching test results confirmed that the fabric composed of fiber-based TENGs was restored to its original state without plastic deformation at stretching strains up to 25%.[38,39] As the fibers were stretched, the radius of the PDMS tubes decreased, resulting in a 47.6% reduction in output at 22% elongation. However, when the device is released, the reduced output power is restored and stable elec-tric output performance is shown. Applying this to real apparel results in an output difference of about elbow folding, resulting in an output current of 7 µA at a 130° bend angle.

2.3. Wrinkled Structured Fiber

Another fiber-based TENG with a coaxial structure has been newly reported by adding a wrinkle structure to increase the stretchability (Figure 4).[40] It can be more easily applied to the body with high strains such as fingers, elbows, and knee joints. To fabricate this new type of fiber-based TENG with high stretch-ability, a stretchable electrode was first designed in the form of polyurethane (PU) fibers wrapped in 6,6 nylon yarn coated with silver (Ag) at 30 µm in diameter. The stretchable electrode fiber had an average diameter of 440 µm, a resistance of 10.4 Ω in the initial state, and a length of 10 mm (Figure 4b).[41,42] When the fiber is pulled, the fiber stretches in the longitudinal direc-tion and contracts in the radial direction. However, electrical path of such silver-coated nylon yarn is constant. It has high electrical conductivity and electrical stability. Polyvinylidene fluoride-co-trifluoroethylene (PVDF-TrFE) with high negativity in tribo-series was selected as a negative triboelectric material. Electrospinning method was used to fabricate PVDF-TrFE mats composed of nanofibers with an average diameter of 750 nm. It was used to wrap around the Ag-coated nylon/PU fiber.[43,44] CNT sheets of multiwalled CNTs fabricated by a chemical vapor

deposition method that serve as another electrode of TENG are then used to wrap the stretched PVDF-TrFE/Ag-coated nylon/PU fiber by 180% stain. In this process, PVDF-TrFE with non-elastic property is capable of absorbing 180% of the large ten-sile strain by aligning randomly oriented nanofibers in the electrospun mats at the first fabrication stretching so that when the strain is released, PVDF-TrFE mats will have uniform and closely packed wrinkles. These wrinkled CNT/PVDF-TrFE shells consist of CNT sheets and PVDF-TrFE mats with thicknesses of 1 and 30 µm, respectively. Its resistance in the initial state was 5.03 kΩ, which was constant at 50% strain (Figure 4c). As a result, a stretchable fiber-based TENG with a microdiameter of about 490 µm in diameter was fabricated using two stretchable electrode materials: an Ag-coated nylon/PU fiber and a CNT/PVDF-TrFE shell.

In the initial state without strain, negative and positive tri-boelectric charges are formed on each surface according to the tribo-series at the contact of PVDF-TrFE and Ag-coated nylon in the fabrication process. When an external force is applied and the fiber is stretched, a gap is generated between the PVDF-TrFE mats and Ag-coated nylon due to difference in Poisson’s ratio between the CNT/PVDF-TrFE shell and the Ag-coated nylon/PU fiber. Due to negative triboelectric charges on the surface of PVDF-TrFE, a potential difference between the two electrodes of CNT and Ag is generated and current is gener-ated through the external circuit. When the external force is removed and the fiber contracts again, PVDF-TrFE and Ag are brought into contact again and the charges that have migrated between these two electrodes can flow reversely to generate an AC-like output (Figure 4d).

As stretching strain increases, the gap between the CNT/PVDF-TrFE shell and the Ag-coated nylon/PU fiber increases, thus increasing the distance between the electrode and the tri-boelectric material, leading to an increase in electrical output performance. Experimental evaluation of electrical output using linear motors to obtain accurate stretching strain has proven that the output voltage is increased from 13 mV at 10% strain to 24 mV at 50% strain as shown in Figure 4e. In addi-tion, since the frequency of human movement in everyday life is less than 10 Hz, the electrical output is evaluated at a fixed stretching strain of 50% within frequency ranging from 3 to 10 Hz. As a result, the output is increased from 9 to 24 mV as frequency increases (Figure 4f).

In the detection of human motion, it is important to acquire information about the direction as well as the size of motion in the monitoring system. Kinematic sensing textile that can obtain both size and direction components in human motion has been fabricated by plain weaving method using 11 fibers with each fiber coated with elastomeric styrene–butylene–sty-rene to insulate it from other fibers, as shown in Figure 4g.[40] After applying a stretching strain of 50% in x-, y-, and diagonal directions to the fabricated kinematic sensing textile, voltage responses of fibers in x- and y-axes have been measured (Figure 4h). When elongated in x- and y-directions, the voltage response in each direction can be detected from each fiber. In diagonal stretching, both x- and y-directions are detected. It has been confirmed that the magnitude of the voltage response in diagonal stretching is smaller than that in x- or y-unidirectional stretching. This stretchable fiber-based TENG can predict the

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direction of human motion by using two vertical perpendicular fibers as well as energy harvesting. It can be applied to accurate motion sensor (Figure 4e).

2.4. Elastomeric Fiber

Another novel structure of fiber-based TENG, similar to TENG previously reviewed, has been introduced to further enhance the

device’s stretchability as shown in Figure 5.[45] In the stretchable conductor, silicone rubber is one important part of this fiber-based TENG. It is a stretchable polymer used as the core of the fiber. CNT/polymer fiber as one stretchable electrode of TENG is then fabricated by coating with conductive CNT ink on sili-cone rubber fiber. Another silicone rubber thin film that acts as a friction material for TENG is also formed on the stretchable electrode. Finally, Cu microwire as another stretchable electrode for TENG is wound around the polymer/CNT/polymer fiber to

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Figure 4. a) Schematic diagram of wrinkled fiber-based TENG. SEM images of b) the silver-coated nylon yarn-covered PU fiber with scale bar of 200 µm and c) CNT sheet with scale bar of 2 µm. d) Power generation process of wrinkled fiber-based TENG. e) The measured output voltage of the wrinkled fiber-based TENG depending on stretching strains ranging from 10% to 50% with a frequency of 10 Hz. f) The measured output voltage depending on frequencies ranging from 3 to 10 Hz at a stretching strain of 50%. g) Optical image of wrinkled fiber-based TENGs woven into a glove (scale bar: 3 mm). h) Schematic diagram of experiments. i) Voltage response of x- and y-axes of the wrinkled fiber-based TENG when stretching strain was applied in x-, y-, and diagonal directions. Reproduced with permission.[40] Copyright 2016, Springer Nature.



form a coil (Figure 5a). The schematic structure of this fiber-like TENG is shown in Figure 5b. In scanning electron micro-scopy (SEM) images shown in insets of Figure 5b, thicknesses of the silicon thin film and the CNT layer are 200 and 80 µm, respectively. In stretching-releasing motion of this fiber-like TENG, contact and separation between Cu and silicone rubber thin film will occur as can be seen in Figure 5c,d.[46,47] In addi-tion, fiber-like TENG is not only stretchable, but also very flex-ible in bending and twisting motion (Figure 5e).

Electrical conductivity and stretchability of fiber-like TENG have been investigated in detail as shown in Figure 5f,g.[45] When stretching strain is increased, changes in electrical resistance and diameter of the inner fibers are confirmed. As strain increases from 0% to 100%, the resistance is almost unchanged from 0.47 to 2.01 kΩ cm−1. Thus, even fiber-like TENG can have stable electrical properties at high strain. SEM images of the morphology of inner fibers with and without 50% stretching strain are shown in Figure 5h,i, respectively. There is almost no phase separation. This demonstrates that CNTs with multiple electrical paths can have stable electrical conductivity at high strain rates. The distance between Cu and silicone rubber thin film affects the output of fiber-like TENG. The distance between these two materials is caused by change in diameter (Δd) of the inner fiber according to the stretching strain. Figure 5g shows that, as the stretching strain increases, Δd increases linearly. This not only leads to excellent elastic stability, but also generates higher output performance at higher stretching strains.[48] The working mechanism of fiber-like TENG is the same as that in vertical contact mode. Due to triboelectric charge formed on the sur-face of the silicone rubber, a potential difference between the

two electrodes occurs as the diameter of inner fiber changes. To achieve electrical balance, flow of electrons to the external circuit will occur (Figure 5k). Using COMSOL software, the potential distribution according to the distance between the cu coil and the silicone rubber in the stretching motion has been simulated.[45] Results confirmed that a Voc of ≈140 V was generated between the two electrodes in a 50% stretching strain as shown in Figure 5n. The flow of electrons through the external circuit by contact and separation in the short cir-cuit is in the opposite direction. This has been experimentally proven to be able to generate an AC-like output current in stretching-releasing.[45]

3. Fabric-Based TENGs

In single or multiple fiber-based TENG, the device is folded or stretched by the mechanical energy at the finger, elbow, or knee so that the contact and separation between two tribo-electric materials occurring in vertical direction are restricted to the vertical contact mode among working modes of TENG. However, there are various types of biomechanical energy that occur in everyday life, including movement of arms, walking, deformation of clothes, and so on, resulting in ver-tical and lateral movement and friction between different body parts or clothes or between skin and clothes. In these cases, various fabric-based TENGs have been developed to harvest these various biomechanical energies using various working modes of TENG, including lateral sliding mode, freestanding mode, single electrode mode, and vertical con-tact mode.[49–51,56,57]

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Figure 5. a) Fabrication process and b) schematic diagram of fiber-like TENG. Insets show SEM images of inner fiber (silicone rubber/CNT/silicone rubber fiber) with scale bar of 1 mm and 500 µm. Optical images of fiber-like TENG at c) released state and d) stretched state. e) Digital images of fiber-like TENG at stretching, bending, and twisting motions. f) Resistance and g) diameter of inner fiber depending on stretching strains ranging from 0% to 100%. SEM images of CNT layer in h) released state and i) stretched state. j) Photos of fiber-like TENG with 0% and 100% of stretching strain. k) Power generation mechanism of fiber-like TENG. l–n) Simulation results of potential distribution of the fiber-like TENG. o) The short circuit current with AC type output signal of the fiber-like TENG under stretching and releasing motion. Reproduced with permission.[45] Copyright 2016, Wiley-VCH.



3.1. Woven Structured Fabric

A woven structured TENG based on freestanding mode was first developed utilizing contact and friction between fabrics or fabric and other objects by human motion as shown in Figure 6.[50] This fabric-based woven structured TENG consists of homemade conductive Ag fiber fabric as electrode material with commodity nylon fabric and polyester fabric as positive and negative triboelectric materials, respectively. A smaller-cut Ag fiber fabric (5 mm in width and 27 cm in length) was attached to the center of the nylon fabric and the polyester fabric (7 mm × 29 cm) using double-sided tape while the lead wire was connected to the Ag fabric. Another nylon fabric and a polyester fabric were then attached to the opposite side

of the Ag fabric. Thus, two different fabric straps were fabri-cated in which the friction material completely surrounded the outside of the electrode material. Two types of straps were fabricated into a single fabric by plain weaving method (Figure 6a). To understand the driving mechanism of this fabric-based woven structured TENG, the structure was simpli-fied by a pair of nylon/Ag fiber fabric (5 cm × 5 cm) and poly-ester/Ag fiber fabric (5 cm × 5 cm). The working mechanism is divided into vertical contact mode using freestanding acryl plate (10 cm × 11 cm) and lateral sliding mode using polyester fabric (5 cm × 5 cm) as freestanding triboelectric layer. First, in the vertical contact mode, the acryl plate is brought into contact with the triboelectric materials by external force. The triboelec-tric effect forms positive charges on the surface of the nylon but

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Figure 6. a) Schematic diagram showing the fabrication process and structure of fabric-based TENG. b) Digital image of fabric-based TENG. c) Optical images of nylon fabric, polyester fabric, and silver fiber fabric. Each structure and working mechanism of the fabric-based TENG under d) vertical contact mode and e) freestanding mode are shown. Application of fabric-based TENG to harvest different mechanical energy from human motion and the generated short circuit current: f) footsteps, g) shaking of clothes, h) leg joints bend, and i) arm joint bend. Reproduced with permission.[50] Copyright 2014, ACS Publications.



negative charges on the surface of the polyester. On the surface of the portion of acryl that is in contact with each nylon and polyester, charges opposite to those will form on the surface of nylon and polyester. As the external force is removed and acryl plate is separated, a potential difference is generated between the electrode of the nylon and the electrode of the polyester. Electrons will flow through the external circuit connected to the two electrodes to electrically neutralize it. When the acryl plate begins to come close again, the former electrical equilibrium is broken and the potential difference is reversed so that elec-trons can flow back through the external circuit. Using a linear motor, the triboelectric output of the simplified fabric-based TENG has been evaluated at displacement of 40 mm, accelera-tion of 10 m s−2, and maximum velocity of 20 m s−1. An open-circuit voltage (Voc) of 90 V and a short-circuit current (Isc) of 1 µA are generated (Figure 6d).[50]

In the lateral sliding mode, the freestanding layer of poly-ester is negatively charged on the surface of polyester by tri-boelectrification in contact with nylon and positive charges are formed on the surface of nylon. An external force in the horizontal direction causes the freestanding polyester fabric to slide toward the polyester/Ag fiber fabric, causing a potential difference between the two electrodes and creating a flow of electrons. It is assumed that friction between the freestanding polyester fabric and the polyester/Ag fiber fabric does not cause charge transfer on the surface because they are the same mate-rial. When the freestanding polyester fabric completely overlaps with the top of the polyester electrode, the flow of electrons will stop. However, when the freestanding polyester fabric moves toward the nylon, electrons will flow in the opposite direction. It has been experimentally verified that triboelectric output with Voc of 45 V and Isc of 0.35 µA is generated at the speed of 30 m s−1 and the acceleration of 30 m s−2 using a linear motor (Figure 6e).

This fabric-based woven structured TENG is triggered by a freestanding triboelectric layer. There is no restriction on movement. Thus, it can be applied to a wearable device for har-vesting various energy. In addition to acrylic and polyester used, the freestanding triboelectric layer can also be paper, polymer, cotton, skin, and so on. To demonstrate that fabric-based woven structured TENG can harvest various kinds of mechanical energy, it has been integrated into actual shoes, coats, and pants to evaluate the output.[50] First, a TENG device was attached to the sole of the shoe to harvest mechanical energy generated at the foot step while an acryl plate serving as a freestanding tri-boelectric layer was placed on the bottom. An output current of about 0.3 µA is generated in one footstep and can be directly used as power source of nine LEDs. It can also be used as an active sensor to calculate the number of steps made by a person based on output signal in one footstep (Figure 6f). Second, a TENG device was applied inside a coat to harvest mechanical energy generated when clothes were shaken. The output cur-rent was about 0.75 µA (Figure 6g). To harvest the mechanical energy generated by movements of legs and arms, a TENG device has also been integrated into the knee part of pants and arm parts of a coat. It has been shown that alternative cur-rents of about 0.9 and 0.75 µA are produced as the contact area between TENG and pants or coat when human motion varies (Figure 6h,i).

3.2. Lateral Arranged Fabric

Another fabric-based TENG with different structure based on a lateral sling mode utilizing friction in horizontal motion between fabrics as shown in Figure 7 has been reported.[51] This fabric-based TENG is composed of two layered structures with nylon cloth and Dacron cloth as triboelectric materials (Figure 7a). These cloths have microstructures that can affect the output increase because they are woven with numerous fibers (Figure 7b). In this fabric-based TENG, two cotton cloths are used as substrates while ten strap electrodes of the same size (2 cm × 28 cm × 0.05 mm) attached with adhesive are arranged in a grating structure with a gap of 1 mm between electrodes. The nylon cloth strap and the Dacron cloth strap of the same size (2.1 cm × 28 cm × 0.045 mm) are alternately put on 10 electrodes with double-sided tape (Figure 7c). The nylon-covered electrodes and the Dacron-covered electrodes are connected in parallel. The working mechanism of this fabric-based TENG is shown in Figure 7d. When an external force is applied to the device in a horizontal direction and the top and bottom layers are subjected to relative sliding, there is friction between the nylon cloth and the Dacron cloth, resulting in loss or gain of electrons on the surface of materials according to tribo-series. Positive charges are formed on the surface of the nylon cloth while the surface of the Dacron cloth is negatively charged. When the top layer is moved to the right, the Dacron cloth of A1 and A3 that are in contact with the nylon cloth of B1 and B3, respectively, will gradually go away in the lateral direc-tion. Triboelectric charges on the surface of the nylon cloth and the Dacron cloth will cause a potential difference in the rear electrodes, thus inducing the flow of electrons through the external circuit. When the relative displacement (S) is equal to the width d of the electrode and when nylon cloth and Dacron cloth are completely overlapped on the same material, charges induced in the electrodes are electrically screened to provide electrical equilibrium by fully screening the triboelectric charge on each surface of the nylon cloth and the Dacron cloth. Single alternate current peak occurs in one period of unidirectional motion in which the relative displacement (S) reaches 2d, cor-responding to the width of the two grating units. When a rela-tive displacement of 32 cm in one direction is applied under an average sliding speed of 1.7 m s−1 with an average pres-sure of 505 Pa, Voc and Isc of this fabric-based TENG can reach 2 kV and 0.2 mA, respectively. The triboelectric output in one motion has 16 peak signals and 43 Hz frequency. It can be easily adjusted by controlling the sliding speed and the width d of the grating unit (Figure 7e,f). Since the generated output has AC type power with high voltage and high frequency, it can be used as power source for electroluminescence (EL) lamp operating under these conditions. A tubular EL lamp with a length of 80 cm and a diameter of 2.5 mm directly connected to the device can emit bright green light as shown in the inset of Figure 7e. The AC-type output can be converted to a direct current type output simply by using a rectifying bride and the converted output can be stored in a capacitor. Using the output of such device in a single sliding motion in one direction, the charging rate is 69 µC s−1 and the charging rate per unit area of the device is 1.23 mC (m−2s−1) (Figure 7g). To assess the ability of this fabric-based TENG to harvest mechanical

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energy generated by human motion, two layers of the device have been attached to the waist and inner forearm of the coat so that the two layers are in good contact when arms swing naturally (Figure 7h). The output from the swing of the arm was rectified. It had a voltage of 0.7 V and a current of 50 µA (Figure 7i,j).

3.3. Fabric with Nanopatterned Surface

One of the biggest issues in the development of textile platform based TENG to power wearable devices is to increase the triboe-lectric output performance. As a method of controlling character-istics of triboelectric materials to increase the output of TENG, the surface of the triboelectric material has been modified to increase the friction or contact area. Surface modification methods used in many previous TENG studies include decorating nano- or

microsized particles or rods onto the surface of the triboelectric material or patterning the surface of triboelectric material into nano-/microstructures using RIE plasma treatment such as pat-tern mold. However, most previous TENG studies have used these surface treatment methods on plastic platforms, not tex-tile platforms for wearable devices.[52–55] In addition, mechanical durability due to very weak adhesion between textile and nano-structure has become a problem. Therefore, developing a textile platform-based triboelectric material with a mechanically robust nanopatterned surface through a simple and cost-effective pro-cess is very important for applying TENG to wearable devices. As shown in Figure 8, a fabric-based TENG with a nanopatterned surface that is completely flexible and foldable with excellent mechanical stability and high triboelectric output performance has been introduced for the first time.[56] The nanopatterned fabric-based TENG consists of two layers: (1) the top layer which is an Ag-coated fabric, and (2) the bottom layer which is an

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Figure 7. a) Schematic diagram showing fabric-based TENG with lateral sliding mode. b) SEM image of microstructured morphology of nylon cloth. c) Photograph of one layer of fabric-based TENG. d) Power generation mechanism of the fabric-based TENG with lateral sliding mode. e) Open-circuit voltage and f) short-circuit current of the fabric-based TENG at an average sliding speed of 1.7 m s−1. Inset is an EL lamp emitted by TENG. g) Charging curve of capacitor with 100 µF by rectified output of the fabric-based TENG. h) Photograph of a working fabric-based TENG integrated on the coat. i,j) Rectified output voltage and current of the TENG by swing arm. Reproduced with permission.[51] Copyright 2015, ACS Publications.



Ag-coated fabric of PDMS nanoparticles based on ZnO nanowire arrays. The top-layer Ag-coated fabric was used as a flexible and foldable electrode with positive triboelectric material. In the top layer, the Ag-coated fabric was used as a flexible and foldable electrode with positive triboelectric material. In the case of the bottom layer, ZnO nanowire array-templated PDMS nanopat-terns were used as negative friction materials. ZnO nanowires were grown on Ag-coated fabric by hydrothermal growth method while PDMS was coated process on the grown ZnO nanow-ires by dip coating process (Figure 8b). The nanopattern can increase the friction and contact area between the Ag and PDMS in a fabric of the same size, effectively creating many triboelec-tric charges on both surfaces. The exact structure and surface morpho logy of PDMS nanoparticles based on ZnO nanowire arrays on Ag-coated fabric have been analyzed using field-emission scanning electron microscopy (FE-SEM).[56] Results are shown in Figure 8c. The Ag-coated fabric was fabricated by plain weaving method while nanopatterns of PDMS were well formed using uniformly arranged ZnO NWs (average diameter

and length of 100 nm and 1 µm) as templates. As shown in Figure 8d, real image has confirmed high flexibility and fold-ability of nanopatterned fabric based TENG.

To compare the triboelectric output performance of this fabric-based TENG with and without nanopatterns, a fabric-based TENG of flat PDMS without nanopatterns was fabricated. The output voltage and output current were measured under the same condition with a vertical compressive force of 10 kgf using a pushing machine. Fabric-based TENG of nanopatterned PDMS produced higher output voltages and output currents of ≈120 V and 65 µA compared to ≈30 V output voltage and 20 µA output current for flat PDMS as shown in Figure 8e–h. Through this comparison, it was confirmed that increases in the output voltage and output current were due to increase in the friction and contact area. In addition, COMSOL simulation has been used to calculate charge distribution and potential dis-tribution generated by triboelectrification with Ag-coated fabric on the surfaces of flat PDMS and nanopatterned PDMS.[56] According to tribo-series, the triboelectric charge density on the

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Figure 8. a) Schematic diagram and b) fabrication process of nanopatterned fabric-based TENG. c) SEM image of the bottom fabric with nanopatterned PDMS. Inset shows high magnification SEM image that clearly shows ZnO NW-templated PDMS nanopatterns. d) Photograph of flexible and foldable nanopatterned fabric-based TENG. Comparison of output voltage and current of fabric-based TENGs with e) flat PDMS and f) nanopatterned PDMS. i) Schematic diagram showing multilayer-stacked fabric-based TENG. j) Output voltage and current measured as a function of the number of stacked layer. k) Self-powered jacket, including a commercial LCD, LEDs, and a remote control working by nanopatterned fabric-based TENG. Reproduced with permission.[56] Copyright 2015, ACS Publications.



surface of PDMS generated in the triboelectrification between PDMS and Ag is the same regardless whether the nanopattern is present. However, results of calculation from the simulation showed that nanopatterned PDMS had greater surface potential than flat PDMS because the amount of triboelectric charge was larger by increasing the surface area according to the nanopat-tern. These results of simulation showed a tendency to be con-sistent with results obtained from the experiment.[56]

To further increase the output of nanopatterned fabric-based TENG, stack structure with several layers has been designed (Figure 8i). In this stacked structure, ZnO NWs were grown on both sides of the Ag-coated fabric. PDMS nanopattern was formed by a dip coating process. Both sides of the Ag-coated fabric were used as a triboelectric layer. First, the nanopat-terned PDMS on both sides of the existing single-unit fabric-based TENG is placed and the fabric-based TENG of another single unit is turned over. Such two-sided nanopatterned PDMS is placed on the existing single-unit fabric-based TENG and another single-unit fabric-based TENG is overlaid on it. Thus, a multilayer stacked fabric-based TENG with a friction inter-face of four nanopatterned PDMS and Ag-coated fabric was made. The multilayer-stacked fabric-based TENG had four fric-tion interfaces between nanopatterned PDMS and Ag-coated fabric. The output voltage and output current of such multi-layer stacked fabric-based TENG were measured according to the number of stacked layers. As a result, both output voltage and output current were increased linearly when the number of stacked layers was increased. Such TENG with four stacked

layers achieved higher output voltage and output currents of 170 V and 120 µA than single unit device (Figure 8j). It also showed stable output without significant difference in mechan-ical durability evaluation over 12 000 cycles.[56]

It has been shown that nanopatterened fabric-based TENG with multilayer structure can be applied as a power source to operate wearable devices (Figure 8k).[56] The fabric-based TENG was integrated into a jacket containing LCD, LEDs, and remote control for keyless vehicle entry system. It has been shown that such TENG can turn on the LCD screen and driving all six LEDs at the same time by harvesting mechanical energy generated by tapping of hand. Finally, a commercial capacitor (1200 µF) was charged with only the output from fabric-based TENG. The charged power can be used as a power source for remote control for keyless vehicle entry systems, demonstrating its applicability for self-powered smart suites.

3.4. Fabric with Nanostructured Surface

In addition to the previous method, another fabric-based TENG with increased triboelectric output by forming nanostructure on the surface of textile platform has been developed (Figure 9).[57] This device uses Al nanoparticles and nanostructured PDMS as triboelectric materials with super flexible Au-coated fabrics. Al and PDMS are selected because it is possible to transfer a large amount of charge when making contact between two materials which can make a large difference in polarity. Au-coated fibers

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Figure 9. a) Schematic diagram showing the fabrication process and structure of fabric-based TENG with nanostructured surface geometry. SEM images of b) the top plate with Al NPs on the surface of fabric and c) the bottom plate with nanostructured PDMS on the surface of fabric. d) Working principle of the fabric-based TENG. e) Comparison of electrical potential distributions of the fabric-based TENG according to nanosurface configura-tions. Reproduced with permission.[57] Copyright 2015, Elsevier.



are woven by weaving method to have microscale roughness. They are used as substrate for textile platform with 7 cm × 7 cm size in top and bottom plates. On the surface of the Au-coated fabric of the top plate, Al NPs was deposited at a deposition rate of 0.2 Å s−1 and a deposition thickness of 4 nm using thermal evaporation at room temperature and pressure of 10−7 Torr.[58] SEM images showed that the size of the deposited Al NPs was ≈10 nm and the surface of the micro fiber woven fabric was covered with Al NPs (Figure 9b).[57]

In the case of the bottom plate, PDMS solution was coated by spin-coating at 200 rpm for 60 s and cured in a 60 °C oven for 4 h. Through an RIE plasma process in which a radio wave frequency of 100 W was applied for 20 min in a chamber filled with a mixed gas of 13 sccm of O2 and 37 sccm of CF4, nano-structured configurations with a diameter of about 150 nm and a length of 2.5 µm were uniformly distributed onto the surface of the PDMS (Figure 9c).[57]

The prepared top and bottom plates were assembled using a spacer between the two plates to face each other. In bending motion, the top and bottom plates of such fabric-based TENG have relative bending deformations, resulting in contact and friction between the two plates because the working mode is based on a single electrode mode. When a compressed force is applied, Al NPs and PDMS surfaces are partially contacted, resulting in positive and negative triboelectric charges on the surface of Al NPs and PDMS, respectively, according to tribo-series. The triboelectric charges formed on the surface of the PDMS can retain for a long time due to insulating properties of the PDMS. As the two plates are released, a decrease in posi-tive triboelectric charge of the Al NPs surface will occur and electrons will flow from the ground to Al NPs. The surface of Al NPs will become electrically neutral when it is returned to the initial state. When the two plates approach each other due to external force, positive charges will be induced in Al NPs by negative triboelectric charges on the surface of PDMS and electrons will flow from Al NPs to ground (Figure 9d). To confirm the effect of triboelectric output performance with or without nanostructured surface configurations of the top plate, COMSOL simulation has been used to numerically calculate the difference in surface potential between the flat and the nanostructured Al surface of top electrode.[57] PDMS with the same surface was used for the bottom plate and surface tribo-electric charge density of 60 µC m−2 was assumed. These fibers were hemispherical (the morphology of fibers in the Ag-coated fabric) with a radius of 20 µm while Al NPs with a radius of 200 nm were stuck to the fiber. The electrical surface potential of the nanostructured surface was about 1.8 times higher than that of the flat surface. Results showed that the nanostructured surface had an electrical surface potential of 383 V, which was about 1.8 times higher than that of the flat surface. Theoretical calculation confirmed that increasing the roughness of the sur-face could increase the electrical potential (Figure 9e).[57]

To experimentally demonstrate this theoretical result, the output voltage and output current of the top plate with three different surface morphologies were measured. Three types of top plates were prepared with an Al thin film having a flat surface, an Al-coated woven fabric with microscale surface roughness, and a nanostructure surface with Al NPs attached to the fabric. These three types of devices were repetitively

compressed and released at bending length of 3 cm and speed of 20 mm s−1 while Voc and Isc of these devices were measured at a resistance of 105 Ω. Al thin film with flat surface gener-ated the lowest output voltage and output current of 64 V and 17 µA, respectively. The microstructure of Al-coated woven fabric generated an output voltage of 200 V and an output cur-rent of 73 µA. The fabric with nanostructured Al NPs generated the highest output voltage of 259 V and output current of 78 µA, which were 4 and 4.6 times higher, respectively, than those of a flat Al thin film. The improvement in triboelectric output per-formance has been proven to be influenced by differences in morphology, consistent with theoretical calculation results.[57]

To verify the energy harvesting ability of such nanostructured fabric-based TENG in human activity as a wearable device, a nanostructure fabric-based TENG has been integrated into the elbow of stretchable arm sleeve and the electrical response is then measured.[57] The external stress caused by bending and releasing the arms generated an output voltage of 139 V and an output cur-rent of 39 µA. Such output can be used to drive LED bulbs. These results demonstrate that self-powered wearable electronic devices can be developed using fabric-based TENG as a power source.

4. Knitted Fabric-Based TENGs

A single yarn is formed by twisting several fibers and a fabric can be made using several yarns. Weaving, knitting, and non-woven methods have been widely used to make fabric. In the weaving method, yarns arranged in the vertical and hori-zontal directions are interlaced so that it is difficult to have a high stretchable characteristic or various deformations. Thus, there is a limit to apply to wearable devices with extreme harsh working environments such as stretching, bending, and twisting. To have a high stretchable property in the weaving method, fiber or yarn itself with high stretchable property can be used. However, these materials are not used in the textile industry. Thus, they are not applicable to real clothing. Non-woven fabric can be made by a process in which parallel or randomly arranged yarns are bonded with an adhesive and subjected to a drying heat treatment without a woven process. This nonwoven method is also difficult to have high stretchable properties. It has limitations to harvest mechanical energy gen-erated by human motion with various complex deformations applied to actual clothing. In addition, the weaving method and nonwoven method have a problem in that complicated wiring occurs because each aligned yarn or fiber serving as electrode must be electrically connected. However, when knitting fabric is used, it is composed of one yarn without requiring com-plicated wiring. It has the potential to be easily deformed by a structure in which interconnected loops of yarns are con-tinuously arranged. The knitting structure can have excellent stretchability due to many interdependent loops. In addition, spaces in each loop can be stretched in many directions.[59,60]

4.1. Basic Knitting-Structured Fabric

A facile and scalable knitted fabric-based TENG capable of harvesting energy from human motion to power wearable

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electronics as shown in Figure 10 was first reported.[61] This stretchable and shape-adaptive fabric is knitted from a single energy harvesting yarn. This energy harvesting yarn has twisted three commercial conductive stainless steel/polyester fibers serving as electrodes while silicon rubber used as triboelectric material is uniformly coated on the surface of the conducting yarn (Figure 10a). The stainless steel/polyester yarn is soft with excellent conductivity. It is suitable for use as electrode mate-rial of wearable device. Silicon rubber is super soft, biocompat-ible, flexible, highly stretchable, and mechanically stable. It is one of the easiest materials to obtain electrons in tribo-series. It is suitable for use as dielectric material of wearable TENG. In cross-section SEM image of the silicone rubber-coated con-ductive yarn as shown in Figure 10c, the three-ply twisted stainless steel/polyester fiber was located almost at the center of the silicone rubber while the silicone rubber was uniformly coated. The prepared energy harvesting yarn was knitted into a TENG fabric using weft-knitting technology. As shown in Figure 10b, the basic loop structure of fabricated knitted fabric consists of three parts: needle loop, leg, and sinker loop. The stitch is divided into a knit stitch and a purl stitch. The knit stitch is formed from the back to the front through the loop made before while the purl stitch is the loop from the front to the back. Because these meandering and suspended loops can easily extend in many directions, knitted fabrics can have higher stretchability than woven or nonwoven fabrics. In addi-tion, the space of each loop makes it easy for knitted fabrics to be elongated, widened, or distorted by external or internal force as shown in Figure 10f. Figure 10d shows the shape of a single loop of a knitted fabric in these deformations.

The working mechanism of knitted fabric-based TENG is illustrated in Figure 10g. It is simplified to a single energy harvesting yarn, the basic unit of a knitted fabric. The internal

conductive yarn is connected to the ground by a metal wire. Its working mechanism is based on a single electrode mode. When a moving active object such as a hand, foot, or glove touches a silicone rubber, a triboelectrification occurs at the interface between the two materials and an equal amount of triboelec-tric charges with opposite polarities is formed on each surface. In contacted state, two opposite charges are screened at the interface so that no electric potential difference occurs. When the two surfaces begin to separate, they will create a potential difference between the inner electrode and ground due to elec-trostatic induction effect by the negative triboelectric charge on the surface of the silicone rubber. This potential difference will cause electrons to flow from the inner electrode to the ground to induce positive charges to the inner electrode. When the active object is far away, a new electrical equilibrium is reached and the electron flow will stop. When the active object begins to approach the silicone rubber again, the electrical balance is broken and electrons will flow from the ground to the inner electrode. When the active object and the silicone rubber come into complete contact, charge neutralization is performed again as in the initial state. AC type triboelectric output occurs in the contact and separation motion between the active object and the yarn coated with silicone rubber. To quantitatively understand the driving mechanism of such device described above, the potential distribution in contact and separation has been simu-lated using COMSOL multiphysics software (Figure 10h).[61]

To optimize triboelectric output performance, the energy-harvesting yarn is controlled by characteristics of two structural elements: characteristic of the silicone rubber and the diameter of the yarn. Four different silicone rubbers, Ecoflex 0010, 0020, 0030, and 0050, were used. Each of them had different hard-ness and viscosity. Ecoflex 0010 has the highest viscosity but the lowest hardness. On the contrary, Ecoflex 0050 has the lowest

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Figure 10. a) Schematic diagram showing the fabrication process of knitted fabric-based TENG. b) Loop structure and type of knitted fabric. c) SEM images of three-ply twisted stainless steel/polyester fiber blended yarn (upper), the cross-section of this blended yarn, and the cross-section of silicone rubber-coated yarn (lower) (scale bar: 1 mm, 500 µm, and 1 mm, respectively). d) Schematic diagram showing various deformations of a single knitting loop. e) Top view of knitted fabric. f) Photographs of the knitted fabric-based TENG under various tensile deformations, such as initial (0), elongated (1), widened (2), and distorted (3). g) Simplified working mechanism of a single yarn of knitted fabric-based TENG. h) Simulation results of potential distribution of single energy harvesting yarn using COMSOL software. i) Open-circuit voltage and short-circuit current of the knitted fabric-based TENG at frequency ranging from 1 to 5 Hz. Reproduced with permission.[61] Copyright 2017, ACS Publications.



viscosity but the highest hardness. The coating diameter was fixed at 6 mm and the tapping frequency was fixed at 2 Hz. The triboelectric output of energy harvesting yarns coated with four kinds of silicone rubber was then measured according to com-pressive force. As a result, the energy harvesting yarn coated with Ecoflex 0200 showed the highest triboelectric output per-formance compared to others. This means that harder sili-cone rubbers are less effective in contacting with counterpart materials with less contact area while higher viscosity silicone rubbers have lower separation distance in the contact and sep-aration process, thus affecting power reduction. Next, the tri-boelectric output performance was measured according to the coating thickness of the silicone rubber using Ecoflex 0020. As the diameter of the silicone rubber-coated yarn increased, the contact area increased, resulting in improved triboelectric output performance.[61] However, knitted fabric TENGs are fab-ricated with energy harvesting yarns coated with Ecoflex 0020 with an appropriate diameter of 3 mm because it is difficult to apply thick yarns to real clothing.

To evaluate the triboelectric output performance of fabricated knitted fabric TENG, a linear motor has been used to periodi-cally contact and separate with the acrylic plate.[61] The active contact area is 40 × 40 mm2. The tapping force is 11 N and the maximum moving distance is 40 mm. Under these fixed measurement conditions, Voc and Isc of the knitted fabric-based TENG were measured at various frequencies ranging from 1 to 5 Hz. Despite changes in frequency, Voc was observed to be ≈150 V without showing significant changes. On the other hand, peak value of Isc increased with increasing frequency and the output generated was increased from about 0.55 µA at 1 Hz

to about 2.9 µA at 5 Hz (Figure 10i).[61] With increasing external load resistance, the maximum voltage output is increased but the maximum current output is decreased, resulting in a peak power density of 85 mW m−2 at 100 MΩ.[61]

4.2. Various Knitting-Structured Fabrics

The knitting technique has various knitting patterns compared to weaving or nonwoven method. It can control the geometrical configuration and basic loop of the yarn according to knitting fabric made with any knitting pattern using any yarn. Thus, it is possible to change characteristics of fabric such as surface characteristics and stretchability of knitted fabric.[62–65] Using the knitting technique, a fully stretchable knitted fabric-based TENG with high stretchability has been developed, although the material of yarn does not have intrinsic stretchable properties. The characteristics of fabric and TENG have been evaluated according to various knitting patterns as shown in Figure 11.[66] For the choice of materials, the material should be available in the textile industry. It also should have positive or nega-tive properties in the tribo-series. Ag with high conductivity was selected as a positive triboelectric material and electrode material. PTFE with the most negative triboelectric proper-ties is widely used in the textile industry. It was chosen as the dielectric material of TENG. Fabrics were knitted with 12-gauge (needle per inch) using commercial PTFE (198 denier, Swicofil Co.) and Ag (280 denier, X-Silver Co.) yarns using an industrial knitting machine. Knitted fabric-based TENG with a double arc shape and a size of 10 × 10 cm2 were fabricated by using knitted

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Figure 11. a) Schematic diagram showing rib-knitted fabric based TENG. b) Nomenclature of the knitted fabric. c) SEM image of the plain-knitted fabric. Photograph of d) plain-knitted fabric, e) double-knitted fabric, and f) rib-knitted fabric with high magnification shown in insets. g) Schematic dia-gram showing stretched structure of plain-, double-, and rib-knitted fabrics. h) Contact surfaces of plain-, double-, and rib-knitted fabric depending on the applied transverse strain. i) Measured output voltage of three knitted fabric-based TENGs depending on the applied transverse strain. Reproduced with permission.[66] Copyright 2017, ACS Publications.



PTFE fabric and knitted Ag fabric. The knitted PTFE fabric con-sists of the top and bottom triboelectric layer while the knitted Ag fabric is composed of the electrode at the middle layer and the back electrode of the PTFE fabrics at the top and bottom layers (Figure 11a). The knitted PTFE fabrics and the knitted Ag fabrics of the top and bottom were stitched at regular inter-vals in the direction perpendicular to the stretching direction to avoid interfering with the stretching deformation. The knitted Ag fabric in the middle layer was made to be slightly shorter than top and bottom layers while the top and bottom layers were slightly bent to have a double arc shape by sewing on both ends of three layers. Characteristics of such knitted fabric-based TENG have been described using nomenclature in the knit-ting technique as shown in Figure 11b.[66] FE-SEM measure-ments were performed to determine the exact loop dimension of the knitted fabric. The loop of plain-knitted PTFE fabric in unstretched state had a wale width of about 1520 µm, a course height of about 2660 µm, and a diameter of 300 µm for the PTFE yarn (Figure 10c).[66]

To study the mechanical behavior of knitted fabric according to the knitting pattern and find the optimized knitting structure to improve the stretchability and surface contact area, knitted fabrics were prepared using three different knitting patterns: plain knit, double knit, and rib knit.[66] These three types of knit-ting patterns are widely used in the commercial textile industry. In tilted and top-view optical images of plain, double, and rib knits, these three kinds of knitted fabrics showed different loop connections and different knitting structures, resulting in different surface properties such as morphology, density, and roughness (Figure 11d–f).

In addition, knitted fabrics can have excellent stretchability due to interdependencies between loops. The degree of stretch-ability varies depending on the knitting structure. To evaluate the effect of knitting patterns on the stretching and recovery properties of fabric, three types of knitted fabrics were analyzed to determine the extent to which they could be recovered from a given stretching strain. Rib knitted fabric was able to stretch up to 30% stretching strain without plastic deformation while plain and double knitted fabrics were able to stretch up to 10% and 20% of stretching strain, respectively.[67] A simple geomet-rical model can be used to illustrate why three knitted struc-tures have different stretching properties (Figure 11g). Unlike the plain knitting structure, the double knitting structure con-sists of a purl side layer and a knit side layer with a middle region connecting the two layers. When the knitted fabric is stretched, the middle region expands laterally. Thus, the double knitting structure has higher stretchability than the plain knit-ting structure. Unlike a double knitting structure, the rib knit-ting structure consists of two loops on the knit side and two on the purl side, creating a folded or overlapped part in the middle region. These hidden parts are laterally unfolded and exposed during stretching. They help the rib knitting structure to have a higher stretchability than the double knitting structure.

The difference in stretchability due to knitting structure also affects the contact area as shown in Figure 11h. As rib and double knitting structures are stretched, the hidden parts of the middle region are exposed. When stretching strain increases, the exposed area is additionally generated and can increase the contact area. In order to obtain correct value of the increased

contact area, the contact and noncontact parts are separated and the ratio of the enlarged area is calculated considering the shape of the stretched knitting structure. By multiplying the ratio of the enlarged area and the surface density of PTFE as the fabric is stretched, the exact contact area of each knit-ting structure with increasing stretching strain has been cal-culated.[66] Contact areas of plain, double, and rib knitted fab-rics reached 108, 163, and 180 cm2, respectively, at their max-imum stretching state. Increased contact area will increase the amount of triboelectric charges on the surface of knitted fabric. Thus, rib knitted fabric has higher surface potential. Results of output voltage measurement according to increase of stretching strain confirmed that the device using rib knitted fabric had the highest output voltage.[66] This result was consistent with the increasing tendency of contact (Figure 11i).

5. Textile-Based Integrated Energy Devices

Many fiber or fabric-based TENGs have been developed to harvest biomechanical energy from human motion to power wearable electronics. However, since most human movements have a low frequency of less than 10 Hz, the amount of total generated energy may be insufficient to meet the continuously consuming power of wearable electronics, although instan-taneous power output can be high. Therefore, it is necessary to study hybrid energy device by combining different types of energy harvesting device based on textile to complement TENG for harvesting mechanical energy.[68–72] These hybrid energy devices can simultaneously harvest multiple energies from the environment, enabling energy resources to be used effectively and complementarily, thereby increasing energy conversion efficiency and total amount of energy output. The energy con-version efficiency of TENG is defined as the ratio between the generated electrical output energy and the input kinetic energy applied to TENG. The power output from the TENG is typically in the form of short duration pulses. Consequently, the root-mean-square (RMS) value (as the effective value) of the output voltage and current is to be much lower than the peak value.[73] Further TENG generates low current output but high voltage output, which is the opposite for solar cells. Therefore, sys-tematic device design for fabricating hybrid energy devices is greatly demanded.

5.1. Integration of TENG and Solar cell

A fabric-based all-solid hybrid energy device capable of simul-taneously harvesting both energy from solar irradiance and mechanical motion has been developed with economically fea-sible materials and scalable manufacturing technology as shown in Figure 12.[74] Polymer fiber-based solar cells are used as com-ponents of TENG so that light and mechanical energy can be harvested at the same time. A fabric-based hybrid energy device with a single fabric structure was fabricated by weaving two polymer wire-based energy harvesting devices, including both fabric-based TENG and solar cells. Polymer fibers have excel-lent mechanical strength and flexibility. Thus, they are used as basic components of a fabric-based hybrid energy device.

Adv. Funct. Mater. 2019, 29, 1804533



Fabric-based TENG and solar cells have enlarged structures as shown in Figure 12a,b, respectively. SEM image of the photo-anode part of the fabric-based solar cell is shown in Figure 12c. The counter electrode is made by a low-temperature wet pro-cess while the polymer fiber is coated with Cu. Wire-type photo-anodes, Cu-coated PTFE stripes, and Cu-coated polymer wires serving as electrodes for both TENG and solar cells were woven using an industrial weaving machine with a shuttle-flying pro-cess. Figure 12d shows a real image of a woven fabric. A hybrid energy device that can be mixed with commercial wool fibers at a thickness of about 0.32 mm is flexible, colorful, and wear-able. This device is made to have breathability and durability by using fibers of different colors with arbitrary size and various weaving patterns. It has been demonstrated that this hybrid energy device can simultaneously harvest energy of sunlight and mechanical energy of human motion when worn regard-less whether the devices is encapsulated or not.[74] In addition, each output performance of solar cell and TENG has been com-pared according to three types of weaving pattern (plain, twill, and satin), the intensity of light applied, contact method, and the number of strings of TENG and solar cell.[74] After evalu-ating individual characteristics of each type of TENG and solar cells, electrical connections have been optimized to maximize energy efficiency of hybrid energy device. TENG generates low current output but high voltage output due to its high internal impedance. This is the opposite for solar cells with low internal impedance. This impedance mismatch increases leakage cur-rent, resulting in significant decrease in the final output power of the hybrid energy device. As shown in Figure 12e, electrical interactions between TENG and solar cells can be optimized by introducing three electrical connection methods: series connec-tion, parallel connection, and rectifying diode connection. Due to high internal impedance of series connection, the output of fabric-based hybrid energy device is dominated by the voltage

and current output of TENG, resulting in low electrical output performance. In the case of parallel connection, TENG is shorted due to low internal impedance of the solar cell which is inefficient for energy harvesting through hybridization. If a rec-tifier diode is used, it is possible to combine the output power of the TENG and the solar cell effectively without reducing the output of the solar cell by preventing short circuits in the con-nection of components such as a large resistor.

A fabric-based hybrid energy device with a total of 4 cm × 5 cm mixed with wool fibers containing TENG (4 cm × 4 cm) and solar cell (4 cm × 1 cm) components has been fabricated through optimized electrical connections and structures.[74] The solar cell fabric is made up of 15 wire-type solar cells with a photoanode length of 3 cm connected in series while the plain weaving pattern is electrically connected in the regu-lated unit. For quantitative characterization, a linear motor was used to create a mechanical motion. Figure 12f shows electrical output of a fabric-based hybrid energy device in a different environment. When a person wearing a fabric-based hybrid energy device rests in the sun without mechanical movement, the solar cell component can absorb sunlight and convert it to electrical energy. The TENG component will also work when there is movement of the person without sunlight. For example, when a person wears a fabric-based hybrid energy device and walking under the sun, the person can get electrical energy from sunlight and biomechanical energy from walking at the same time.

The output power of fabric-based hybrid energy device has been further investigated by external resistance as shown in Figure 12g. The current decreases when load resistance increases. However, the voltage increases when the resistance increases. As a result, TENG, solar cell, and hybrid energy device have the maximum power at certain value of load resist-ance. A fabric-based hybrid energy device at 4 cm × 5 cm in

Adv. Funct. Mater. 2019, 29, 1804533

Figure 12. Schematic diagram showing fabric-based hybrid energy device. a) Fabric-based TENG and b) photovoltaic fabric. c) SEM image of the photoanode of the fiber-based solar cell. Inset shows ZnO nanowire arrays grown on Mn-plated polymer wire. e) Illustration of three electrical con-nection methods (in series, in parallel, and regulated by rectifying diodes). f) Electrical output performance of fabric-based hybrid energy device under various conditions. g) Comparison of output power of hybrid energy device and individual components according to load resistance. h) Charging curve of commercial capacitor with 2 mF by the fabric-based hybrid energy device under natural daylight with mechanical energy. i) Directly charging a cell phone and j) continuously powering an electronic watch by using fabric-based hybrid energy device. Reproduced with permission.[74] Copyright 2016, Springer Nature.


1804533 (19 of 26) © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimAdv. Funct. Mater. 2019, 29, 1804533

size can harvest both photovoltaic energy from the sunlight with intensity of 80 mW cm−2 and the mechanical energy from walking that generates an average output power of 0.5 mW over a wider range of load resistances from 10 kΩ to 10 MΩ.[74] As a result, hybridization of TENG and solar cells enable fabric-based hybrid energy device to produce higher power than indi-vidual components, thus efficiently harvesting energy over a wider range of load resistances.

To demonstrate the potential of using fabric-based hybrid energy device as a sustainable power source for practical application, lightweight and thin fabrics with various pat-terns and colors are fabricated into cloths. As shown in Figure 12h–j, a capacitor with 2 mF is charged to a max-imum of 2 V in a minute by harvesting energies generated by shaking hand under natural light, allowing the mobile phone to be charged directly and the electronic watch to be continu-ously driven.[74]

Several methods have been studied to increase the practical applicability of textile-based mechanical energy harvesting devices as energy source for wearable electronics. Among them, there is a method of harvesting energy separately or simultaneously by combining an energy harvesting device that can harvest other energy sources such as light energy gener-ated in the vicinity in addition to mechanical energy, thereby complementing outputs and increasing total energy genera-tion amount. Another method is to store electrical energy with nonuniform peak shape generated in textile-based TENG by various biomechanical energies, and to supply uniform power to wearable electronics by storing the wearable system itself. As energy storage in wearable systems, safe and environmentally friendly SCs with high power density, fast charge/discharge rates, and long cycle life are best suited. In addition, SCs have been greatly improved in recent years to levels comparable to lead-acid batteries.[75–80]

5.2. Integration of TENG and Fabric-Based SC

A fabric-based wearable integrated energy device consisting of TENGs and SCs as shown in Figure 13 has been developed.[81] The TENG can harvest both vertical and horizontal mechanical energy generated between arms and the torso by arranging them in carbon fabrics of top and bottom plates using four complementary materials: PU and polyimide (PI) with rough-ness quadratic mean (Rq) of 158 and 23.5 nm are arranged on the top plate attached to the inside of the arm while PDMS (Rq = 49.4 nm) and Al (Rq = 200 nm) are arranged at the same interval as the top plate on the bottom plate attached to the torso (Figure 13c,d). Each material has a relative characteristic of tri-boelectrification. Al is used as a conductive material to transfer electrons to the polymer surface. Electrical energy generated by TENG is stored in a fabric-based SC with a symmetric structure of polyvinyl alcohol (PVA)/H3PO4 gel electrolyte sandwiched between two carbon fabric/CNT/RuO2 electrodes (Figure 13e). Fabric-based TENG and fabric-based SC were first character-ized individually. Fabric-based TENG could generate output voltages and output currents up to 6 V and 55 nA, respectively, from rubbing as horizontal mechanical energy, and about 15 V and 130 nA, respectively, from contact/release friction as ver-

tical mechanical energy. It has been confirmed that fabric-based SC can be stably and uniformly stored even under various mechanical deformation.[81]

Fabric-based integrated energy devices including TENG and SC have been stitched together with conductive carbon threads on commercial cloths such as knit shirts (Figure 13f).[81] The output performance of TENG in human motion that occurs in everyday life such as walking and running has been measured. An output voltage of 33 V and an output current of 0.25 µA were generated at an average speed of 1.5 Hz (Figure 13g).[81] The generated electrical energy was stored in the SC (Figure 13h). It has been shown that the slope of charge accumulation varies linearly with increasing frequency of friction from 0.67 to 4 Hz. It can be used to monitor wearer’s human activity with a fabric-based integrated energy device. Figure 13i shows rec-tified output current and charge accumulation resulting from stretching, walking, running, sprinting, and cool-down walking in a person wearing this integrated energy device. Slopes of charge accumulation in each of these human activities were 0.48, 8.4, 2.2, 5.3, and 9.6 nC s−1, respectively.[81] The charging behavior for three types of capacitors (1, 10, and 100 nF) was compared. It was found that fast charge and discharge capac-itor (1 nF) was more sensitive to monitor human activity while large capacitor (100 nF) was more suitable for long-term moni-toring (Figure 13j).

5.3. Integration of TENG and Fiber-Based SC

Another type of integrated energy device that incorporates energy generation and storage by fabric-based TENG and all-solid-state yarn-based SC as shown in Figure 14 has been reported.[82] The Ni layer was coated directly on the surface of a common polyester yarn using electroless plating to create a highly conductive 1D-type fabric. It was used as an electrode and current collector in a yarn-based all-solid-state SC. As an active material, a reduced graphene oxide (rGO) film was coated on the electrode surface which was Ni/polyester yarn. Of these two rGO/Ni/polyester yarns thus prepared, one yarn was first coated with a PVA/H3PO4 gel-type electrolyte while the other one was parallel assembled and coated with the same electrolyte. PVA/H3PO4 acted as a solid electrolyte and separator. The fabricated SC has a symmetrical structure (Figure 14a). A polyester yarn with a diameter of 500 µm coated with a Ni thin layer had conductivity with a resistance per length of about 1.48 Ω cm−1, which was much smaller than a conductive yarn of a carbonaceous material made by conventional dip-coating or dry-spinning (20 Ω cm−1). In addition, weight increase due to Ni coating was about 60% of the pristine polyester yarn, which was very low compared to metal wire. Thus, after the Ni coating, the flexibility and lightness of the pristine polyester yarn were maintained. Figure 14b shows a photograph of a 1 m long polyester yarn coated with a Ni layer on a cylinder. Figure 14c is an optical image showing pristine polyester yarn, Ni-coated yarn, and rGO-coated Ni/polyester yarn arranged from above to com-pare three yarns. The white polyester yarn was changed to silver color after the coating of Ni. It turned black after rGO coating. rGO film was formed by hydrothermal reaction



directly from the surface of the Ni-coated yarn in GO aqueous solution.[83,84] To increase the conductivity of the coated rGO film, it was subjected to an additional reduction process by ascorbic acid. Raman spectroscopy and XRD analysis of these prepared final rGO films confirmed the reduction of GO and formation of rGO.[82] In SEM image of the pristine polyester yarn shown in Figure 14d, two polyester fibers with a diam-eter of 20 µm are twisted to form a yarn. However, Ni-coated yarn almost had no change in diameter (Figure 14e). The rGO-coated yarn had a thick layer of rGO film that the initial shape of the polyester fiber was invisible (Figure 14f). High

magnification SEM image of rGO-coated yarn confirmed that the rGO film was wavy on the Ni-coated polyester fiber (Figure 14g).[82]

Fabric-based TENG that can harvest biomechanical energy from human motion to charge yarn-based SC has been fab-ricated by weaving the Ni-coated polyester straps and par-ylene/Ni-coated polyester straps arranged vertically and hori-zontally, respectively.[82] Parylene/Ni-coated polyester straps were connected with electrode A while Ni-coated polyester straps were connected with electrode B. Figure 14h shows that the fabric-based TENG is driven in a contact–separation

Figure 13. a) Schematic diagram showing arm swings with TENG and SC equipped. b) Circuit diagram of the integrated energy device including fabric-based TENG and all-solid-state yarn-based SC. Schematic diagrams and photographs of individual components: c) top plate of TENG, d) bottom plate of TENG, and e) SC. Insets show AFM images. f) Photograph of the integrated energy device applied to a knit shirt and connected by conduc-tive thread. g) Open-circuit voltage and rectified current generated by fabric-based TENG from arm swings. h) Charge accumulation of capacitors by the fabric-based TENG with different frequencies. i) Demonstration of human activity monitoring system. The generated output signal was recorded during jogging. j) Charging behavior of capacitors with various capacitances by the fabric-based TENG. Reproduced with permission.[81] Copyright 2014, Wiley-VCH.



mode with a common cotton cloth. It shows a real image of a fabric-based TENG with a size of 10 cm × 10 cm. Yarn-based SCs have stable energy storage characteristics even in mechanical deformation. Output voltage and output current of fabric-based TENG have been measured when external mechanical energy is applied.[82] Yarn-based SCs and fabric-based TENGs were individually analyzed and integrated into a single fabric to demonstrate the potential of self-charging power fabric. Three SCs connected in series were woven on the side of the fabric-based TENG (Figure 14i). Because this integrated energy device consists of yarn as the build block that makes up the fabric, it is possible to fabricate more com-plex structures. Equivalent circuit of this self-charging power fabric is shown in Figure 14j. Bridge diode is used to rectify the enhanced AC-type output of the TENG and charge it to the SC. It is not very flexible. However, it has a very small size of about 0.5 × 0.5 cm2. When the contact–separation motion was applied to the fabric-based integrated energy device using a common cloth at a frequency of 5 Hz, the SC was charged to 2.1 V in 2009 s due to output generated by TENG. The charged SC could be discharged at 1 µA for 811 s. Applying mechanical energy with a frequency increased to 10 Hz, the charge time could be reduced to 913 s and

the charged energy could be maintained for 808 s at 1 µA (Figure 14k).[82]

5.4. Integration of TENG, Solar Cell, and SC

Two methods for creating an integrated energy device have been proposed. The first was to harvest two energy sources (light energy and mechanical energy) through hybridization of solar cell and TENG while the second was to store elec-trical energy generated in an integrated SC. A fabric-based integrated energy device that combines these two methods has been developed (Figure 15).[85] This fabric-hybridized self-charging power system could easily convert solar and mechanical energy into electrical energy using fiber-shaped dye-sensitized solar cells (DSSCs) and fiber-shaped TENGs. The generated electrical energy can then be stored as chem-ical energy in fiber-shaped SCs (Figure 15a). This hybridized self-charging fabric not only has reasonable energy conver-sion efficiency and storage capacity, but also has advantages of low cost and easy fabrication process. The hybridized self-charging fabric has a two-layer structure with three dif-ferent types of functional devices, including fiber shaped

Figure 14. a) Schematic diagram showing all-solid-state yarn-based SC composed of two rGO/Ni/polyester yarns in parallel with symmetric structure. b) Photograph of Ni-coated polyester yarn with length of 1 m. c) Photographs of pristine polyester yarn (white), Ni-coated polyester yarn (silver), and rGO/Ni/polyester yarn (black). SEM images of d) a pristine polyester yarn, e) a Ni-coated polyester yarn, and f,g) an rGO/Ni/polyester yarn. The scale bar is 3 cm for image (b), 500 µm for image (d–f), and 50 µm for image (g). h) Schematic diagram and photograph of a fabric-based TENG moving in vertical contact mode with a common cotton cloth. i) Photograph of a self-charging power textile woven with a fabric-based TENG and yarn-based SCs. Enlarged view of the fabric woven using two black yarn-based SCs. j) Equivalent circuit of the self-charging power fabric for wearable electronics. k) Charging and discharging curves of three yarn-based SCs in series by the fabric-based TENG at motion frequencies of 5 and 10 Hz with discharge at 1 µA. Reproduced with permission.[82] Copyright 2016, Wiley-VCH.



DSSC and fiber-shaped TENG integrated into conventional fabric. The top layer of the hybridized self-charging fabric is woven with multiple fiber-shaped DSSCs to harvest solar energy. Materials and dyes of DSSC can be controlled to reach optimum results under a variety of lighting conditions, making them suitable for indoor and outdoor applications. Thus, they have been used in a variety of photovoltaic cells. In addition, DSSCs can be applied to a variety of substrates.

Thus, they have significant advantage for use as a component of TENG. The bottom layer of the hybridized self-charging fabric consists of fiber-shaped SCs that can store the gener-ated electrical energy. One unit of fiber-shaped DSSC in the top layer and one unit of fiber-shaped SC in the bottom layer are connected to form a unit of fiber-shaped TENG to harvest mechanical energy generated from human motion. Since eth-ylene vinyl acetate (EVA) tubing is transparent and flexible,

Figure 15. a) Schematic diagram showing a fabric-hybridized self-charging power system. b) Schematic diagram and c) photograph of a fiber-shaped DSSC. The scale bar is 1 cm. d,e) SEM images of TiO2 nanotube arrays on the Ti wire. Scale bars are 100 mm and 100 nm. f) Schematic diagram and g) photograph of a fiber-shaped SC. The scale bar is 1 cm. h,i) SEM images of RuO2·xH2O-coated carbon fiber electrode. Scale bars are 100 and 5 µm. Demonstration of the fabric-hybridized self-charging power system and its operation under j) outdoor and k) indoor, and l) movement conditions. m) Circuit diagram of the fabric-hybridized self-charging power system for wearable electronics. n) Charging curve of SCs. Blue area corresponds to the charging curve by DSSCs while the red area corresponds to the charging curve by the hybridization of DSSC and TENG. The top inset shows an enlarged curve during charging period by only the DSSC while the bottom inset shows rectified ISC of TENGs. o) Normalized QSC values of TENGs, ISC values of DSSCs, and capacitances of SCs bent between 0° and 180° during 1000 cycles. Inset shows photographs of two final bending statuses (both scale bars: 1 cm). Reproduced with permission.[85] Copyright 2016, AAAS.



it has been used as the basic unit of a hybridized self-charging fabric.

One unit of fiber-shaped DSSC with a length of 10 cm con-sists of a working electrode in which N719 dye-sensitized TiO2 nanotube arrays are formed on a Ti wire and a carbon fiber coated with Pt as a counter electrode. These two electrodes are then sealed into a Cu-coated EVA tubing containing the electro-lyte based on I−/I3

−. Cu-coated EVA tubing serves as a holder for the fabrication of fiber-shaped DSSCs and as an electrode for fiber-shaped TENG. The structure of fiber-shaped DSSC is schematically shown in Figure 15b. A real image of a single-unit fiber-shaped DSSC is shown in Figure 15c. Figure 15d shows an SEM image of a Ti wire with a diameter of about 200 nm after anodic oxidation. TiO2 nanotube arrays with an average diameter of about 50 nm that are grown vertically on the surface of the Ti wire are shown in a high magnification SEM image of Figure 15e.

Fiber-shaped SC is an energy storage device which has a symmetrical structure in which two carbon fibers coated with RuO2·xH2O are assembled in parallel in an EVA tubing filled with an electrolyte of H3PO4/PVA. In addition, PDMS is used as a dielectric material of TENG after being Cu coated as another electrode of TENG on the EVA tubing as a holder (Figure 15f). Using a cellulose-based paper membrane, these two bundles of carbon fibers are separated. In the fabrication of fiber-shaped SC without any binder, long-ordered carbon fibers with chemically stable and high conductivity are used as electrodes and substrates. RuO2·xH2O was synthesized on the surface of the carbon fiber bundle using the vapor-phase hydrothermal method. Figure 15h shows a low-magnification SEM image of carbon fibers coated with RuO2·xH2O. SEM image of high magnification confirmed that RuO2·xH2O had a surface morphology similar to that of cracked mud on a single carbon fiber with an average diameter of 10 µm (Figure 15i).[85]

The fiber-shaped TENG consists of a pair of Cu-coated EVA tubing of DSSC and PDMS/Cu-coated EVA tubing of SC. Struc-ture optimization of TENG fabric has been carried out through evaluation of the output of TENG with three kinds of weaving pattern: 1 × 1, 3 × 3, and 5 × 5 nets. Individual analysis and evaluation proceeded before these three fibrous devices were operating simultaneously. Several units of fiber-shaped DSSCs and fiber-shaped SCs were woven and fabricated into a single hybridized self-charging power fabric, each unit connected in series and in parallel. Fiber-shaped TENG was made after both fibers were connected. When this hybridized self-charging power fabric was applied to T-shirts, it was found that light energy and kinetic energy from wearer’s outdoor activities and indoor activities could be harvested and stored (Figure 15j–l).[85] It is also important that the DSSC is efficient in harvesting light of weak intensity. Equivalent circuit of a hybridized self-charging power fabric is shown in Figure 15m. The bridge rec-tifier can convert AC type output of TENG and help save it in the SC. The diode can block the output of TENG from passing through the DSSC. Three switches are used to control the cir-cuit. Rectifiers, diodes, and switches used in circuits are not flexible. However, they are small enough to be used as buttons or logos for clothes. To verify the performance of a hybridized self-charging power fabric, each fabric with a 3 × 3 network

structure was woven with three fiber-shaped DSSCs or six fiber-shaped SCs connected in series. When switch S2 was off and switches S1 and S0 were on, the voltage of SCs was lin-early increased from 0 to 1.8 V for about 69 s by stable output of the DSSCs. However, due to low output voltage of DSSC, the SC remained at 1.8 V which limited the practicality of the device.[85] To solve this problem, the high voltage output of TENG complements the disadvantage of DSSC and allows the SC to be charged to have high voltage. When switch S2 was turned on, the voltage of SC was constantly charged and increased by output of TENG (Figure 15n). Finally, the dura-bility of the hybridized self-charging power fabric was evalu-ated during 1000 cycles of bending motion using a linear motor (Figure 15o). Even after 1000 cycles, the Qsc value of TENG, the Isc value of DSSC, and the capacitance of SC showed excellent stability without any noticeable decreases. Research on these energy harvesters and their integration sys-tems based on various textile platforms is one of the major challenges in the development of self-powered wearable elec-tronics that require lightweight, flexible, stretchable, washable characteristics.

6. Summary and Conclusions

Recently, with development of IoT and autonomous smart sys-tems, electronic devices have become smaller and more com-pact and portable with complex and diverse functions. They are also becoming independent due to wireless systems. These developments have been applied to various aspects of our daily life. They play a major role in raising the quality of life. How-ever, since improved smart electronic devices consume a large amount of electric energy, the battery used as the conventional power source can cause inconvenience such as limitation of use time and frequent charging. These inconveniences can act as a constraint to further development of smart electronic devices.

To solve this problem, researches on energy harvesting technologies have received much attention. It is essential that wearable electronic devices based on wireless systems can use energy harvesting devices to convert a large amount of various biomechanical energies from human motion into electrical energy as a power source.

In wearable electronics, TENGs are most suitable among mechanical energy harvesters because they have many advan-tages. They can be applied to a wide variety of materials, making it possible to select materials that are lightweight, envi-ronmentally friendly, and mechanically stable with high flex-ibility and stretchability. They also have simple structure, low manufacturing cost, and excellent energy conversion efficiency. By four modes of operation (vertical contact mode, single elec-trode mode, lateral sliding mode, and freestanding mode), it is possible to design an optimum TENG device to harvest various types of mechanical energy. Furthermore, friction between two fabrics or friction between fabric and skin/external objects fre-quently occurs in clothing. A textile-based TENG that can take advantage of the triboelectric effect caused by such frictions will be a perfect candidate as a power source for wearable devices.

Many textile-based TENGs have been studied through the development of various types and device structures, ranging



from fibers as basic parts of clothes to fabrics woven with fibers and yarns. By improving the flexibility and stretchability of the device, various biomechanical energies from human motion such as bending and pulling can be harvested more efficiently with increased applicability to real clothing. In addition, tri-boelectric output performance has been improved by nano-/micropatterning of surface morphology of friction material based on textile platform through various methods. Weaving and knitting techniques widely used in the textile industry have been used to control properties of woven fabrics according to various weaving and knitting patterns, thereby increasing the output of fabric-based TENG. Finally, hybrid energy device developed through a combination of TENG and solar cell based on textile platform can harvest two energy sources at the same time, enabling complementary power generation. In addition, a textile platform-based SC has been fabricated and integrated into a single fabric so that the generated electrical energy could be stored on its own, thus enhancing applica-bility as a power source for various wearable electronics. Textile platform-based TENGs have also demonstrated their potential as energy harvesting device as well as a self-powered sensor without requiring an external power supply to monitor human body movements. The output performance, size and stretching

capabilities of these various textile-based TENGs are summa-rized in Table 1.

Demand for flexible, stretchable, and wearable energy device similar to real clothing and fully integrated in real clothing without a sense of heterogeneity has led to increased interest in textile platform-based TENGs as power source for self-pow-ered wearable electronics. In response to this surging interest, many researches on TENGs and energy devices based on tex-tile platform have been carried out. However, in order to be used as a power source in a wide variety of high-performance applications, textile-based TENGs developed and reported so far still need to take a step further by increasing the gener-ated output power. Therefore, it is necessary to develop and improve the material which is high in surface charge density in triboelectrification, flexible and stretchable, which is friendly to the human body, and the structure of the device optimized for human motion. In addition, a wearable energy device should be developed as real clothing that can generate power from the entire garment and can be washed, not at the level where textile-based TENG is integrated into a part of existing clothes. To further expand and revitalize this interest, efforts are needed to continuously improve their performance to increase applicability.

Table 1. A summary of output performance, size, and stretchability of various textile-based TENGs.

Ref. Output performance of TENG Size Stretchability max.

Voltage Current Transferred charges Power

[33] – At 80 MΩ11.22 nA

0.16 nC At 80 MΩ0.91 µW

Length 9.0 cm 2.15%

[37] At 10

MΩ 40 VAt 80 MΩ

210 µA

– At 80 MΩ4 mW

196 cm2 25%

[40] 24 mV 8 nA – – Diameter/l440 µm/1 cm 50%

[45] At Voc

140 V

At Isc

75 nA cm−1

At Qsc

6.1 nC cm−1

At 320 MΩ5.5 µW

Length 10 cm 70%

[50] At Voc

95 V

At Isc

2.5 µA

At Qsc

9.6 µC m−2

– 25 cm2 –

[51] At Voc

2 kV

At Isc

200 µA

– – 588 cm2 –

[56] 170 V 120 µA – At 1 MΩ1.1 mW

– –

[57] 368 V 78 µA – At 20 Ω33.6 mW cm−2

49 cm2 –

[61] At Voc

150 V

At Isc

2.9 µA

at Qsc

52 nCAt 100 MΩ85 mW m−2

16 cm2 60%

[66] At 40 MΩ23.5 V

At 100 Ω1.05 µA

– At 5 MΩPRMS

a) 60 µW

100 cm2 30%

[74] At Voc

15 V

At Isc

0.9 µA

– At 10 MΩ0.75 mW

16 cm2 –

[81] At Voc

15 V

At Isc

130 nA

– 0.18 µW cm−2 45 cm2 –

[82] 40 V 17 µA – – 100 cm2 –

[85] At Voc

12.6 V

At Isc

0.91 µA

At Qsc

20.8 nC

– 100 cm2 –

a)RMS: root mean square.



AcknowledgementsThe authors acknowledge financial support from the Center for Advanced Soft Electronics (CASE) under the Global Frontier Research Program (2013M3A6A5073177) through the National Research Foundation of Korea Grant funded by the Ministry of Science and ICT, and the Industrial Strategic Technology Development Program (10052668, Development of wearable self-powered energy source and low-power wireless communication system for a pacemaker) and the Technology Innovation Program (10065730, Flexible power module and system development for wearable devices) funded by the Ministry of Trade, Industry and Energy (MOTIE, Korea).

Conflict of InterestThe authors declare no conflict of interest.

Keywordsbiomechanical energies, energy harvesting, textiles, triboelectric effect, wearable

Received: July 2, 2018Revised: September 18, 2018

Published online: November 23, 2018

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