Emerging Self‐Powered Autonomous Sensing Triboelectric Fibers toward Future Wearable Human‐Computer Interaction Devices

Wearable electronic technology is developing rapidly and has been widely used in human‐computer interaction, smart homes, telemedicine, rehabilitation training, sports monitoring, object tracking, etc. Fibers, as the basic elements of clothing, have become important carriers of wearable electronics. The fiber‐shaped triboelectric nanogenerator (F‐TENG) is typically a 1D structure that is highly flexible and can be woven from 1D to 2D or even 3D textiles. F‐TENG has both the structural characteristics of fibers and the function of energy conversion of the triboelectric nanogenerator (TENG). Therefore, it can be worn on the body both as an energy converter to convert the mechanical energy of human movement into electrical energy and as a self‐powered sensor to convert human movement information into electrical signals. Herein, this review comprehensively introduces the recent progress of F‐TENG, including the scale preparation method of fibers, the weaving method of fibers, triboelectric‐based multifunctional fiber, and various fibers for energy harvesting and self‐powered sensing. Finally, the challenges and opportunities in the field of F‐TENG are discussed.


Introduction
Fibers and textiles have been employed extensively by mankind for thousands of years. They play an important part in the development of human society and in everyday activities due to their unique properties. First, textiles can withstand a variety of complex mechanical deformations and show excellent resistance to fatigue during wear. Second, textiles are soft, comfortable and can be used on complex curved surfaces. Third, textiles with high porosity have excellent breathability and moisture permeability. Fourth, textiles have excellent compatibility, with a variety of color combinations and different pattern designs. With the development of wearable electronics and people's individual needs, textiles are rapidly developing toward functionality and intelligence. Textiles are receiving a variety of functions: such as heat management, [1][2][3][4][5] energy harvesting, [6][7][8] energy storage, [9][10][11] pressure sensing, [12,13] response driving, [14,15] and luminous color changing. [16][17][18] Triboelectric nanogenerators (TENGs) are a technology that converts mechanical energy into electrical energy based on the coupling effect of triboelectric and electrostatic. [19,20] TENGs, as a novel energy harvesting method, have the advantages of lightweight, low cost, flexible material selection, and simple structural design. And they can harvest the neglected energy of life, converting small irregular mechanical energy into electrical energy such as human activity, vibration, sliding, rolling, mechanical triggering, water drops falling, wind energy, water energy, etc. [21][22][23][24] The combination of TENG and textiles has developed a new type of intelligent textile with two main functions: mechanical energy harvesting and self-powered sensing. [25][26][27][28][29] The fiber-shaped triboelectric nanogenerators (F-TENGs) can effectively adapt to complex deformations such as bending, twisting, and stretching. [13,[30][31][32] The small size of the fiber gives the F-TENGs unique properties such as ultra-flexibility, tissue adaptability, and weaveability, allowing them to be used in a variety of scenarios, particularly in emerging areas related to wearable systems. [33][34][35][36] Due to its excellent mechanical properties and stable electrical output, [37][38][39] research on related F-TENGs is increasing and F-TENGs continue to progress in many aspects. [33,40] As shown in Figure 1, there have been significant advances in the fabrication, energy harvesting and self-powered sensing of F-TENG. In terms of fabrication, large-scale preparation is an important aspect of the fiber's journey toward practical applications. [41][42][43] Here, large-scale preparation methods are summarized. Moreover, the weaving method of F-TENG is described and the various methods are compared. In terms of energy harvesting, the enhancement of the power output is the main topic. Followed by solving the mismatch between the time and form of power output and consumption, including the use of fiber-shaped supercapacitors to store electrical energy and the development of direct current F-TENG (DC F-TENG). In terms of self-powered sensing, fibers are essential for the human body and F-TENG can be utilized to monitor human movement. Here, the latest innovative applications of F-TENG are summarized. In addition, fibers with multiple functions are introduced. Finally, the challenges that need to be solved are presented.

The Working Principle and Working Mode of TENG
The electricity generation process of TENG can be explained by the coupling triboelectric effect and electrostatic effect. During the contact-separation process, electrons are transferred to the material with the higher electron gaining capacity, thus creating an electrical potential difference between the two electrodes, which drive the flow of electrons between the two electrodes to form an alternating current. [44,45] As illustrated in the Figure 2a, the electron-cloud-potential-well model is utilized to explain the working mechanism of TENG from a microscopic perspective. [46] At first, the two atoms are kept separate from each other, with their respective electron clouds remaining separate. The potential trap binds the electrons tightly to the orbit and keeps them from escaping. When two atoms come close together, the electron clouds between the two atoms overlap and electrons can then be transferred from one atom to the other thus creating an electrical potential difference.
Depending on their working modes, TENG can be divided into vertical contact-separation mode, lateral sliding mode, singeelectrode mode, and freestanding triboelectric-layer mode, [47] as illustrated in Figure 2b. The vertical contact-separation mode is the most basic mode of the TENG. The device is composed of two electrodes, which are composed of two polymer film coated with metal electrodes on the back. The lateral sliding mode has the same structure as the vertical contact-separation mode, but the motion mode is different. The lateral sliding mode is more flexible than the vertical contact-separation mode and is suitable for collecting mechanical energy in the form of plane movements, turntable rotations, roller rolls, etc. It can also be used to monitor displacement, sliding angles, acceleration, etc. The singleelectrode mode consists of an electrode, and when a moving object comes into contact with the electrode, a potential difference a) The electron-cloud-potential-well model for electron transfer process. Reproduced with permission. [46] Copyright 2018, Wiley. b) Four working modes of TENG. Adapted with permission. [47] Copyright 2014, Royal Society of Chemistry.
is formed between the electrode and the ground (zero potential), and the electrons are driven to flow between the two to form an electric current. The single-electrode model is simpler in structure by eliminating one electrode. Although the electric quantity is reduced by nearly half, it is more flexible. Therefore, this mode effectively avoids the limitation of active space and broadens the application range of TENG. Based on the single-electrode mode, the freestanding triboelectric-layer mode is further developed. Compared with the single-electrode mode, it no longer needs to be grounded, so all electrodes can move freely. The various working modes have their own characteristics, which can adapt to different working environments and realize the collection of mechanical energy in numerous occasions and forms.

Large-Scale Preparation of F-TENGs
Exciting progress has been made in the research of F-TENG, which shows broad application prospects. It can be used for energy harvesting and self-powered sensing. However, some F-TENGs are limited to a small amount of fabrication in the laboratory and cannot be reached on a large scale. The lack of mature, efficient, and large-scale manufacturing technologies has greatly hindered the development of F-TENGs. Large-scale preparation is one of the key links from experiment to application, which is of great significance to both academia and industrial circles. In recent years, researchers have developed a set of methods for large-scale preparation of F-TENGs, including but not limited to, 3D printing method and spinning method.

3D Printings
3D printing has rapidly developed into an important manufacturing technology. It provides a powerful tool for the large-scale fabrication of complex structural devices. Among them, multimaterial 3D printing technology has been extensively utilized for the fabrication of wearable devices. For example, silicone-copper (Cu) TENG fibers were produced by micro-extrusion 3D printing processes (Figure 3a). [48] The fiber is a core-shell structure, the core layer is copper metal, and the shell is silicone rubber. Membranes and hollow structures can be printed by stacking the fibers, and their power generation performance is demonstrated by lighting LED and charging capacitors. The process enabled rapid prototyping of self-powered wearable triboelectric systems. In addition, Zhang et al. made e-textile by 3D printing ( Figure 3b). [49] The e-textile consists of core-sheath fibers, extruded by a coaxial spinneret and printed directly onto the textile using a 3D printer. Different core and sheath materials can be used to create versatile fibers. Here, the F-TENGs were fabricated using carbon nanotubes (CNTs) as the core and silk fibroin as the sheath layers. It has excellent properties for harvesting mechanical energy from human motion. Finally, Chen et al. developed . F-TENG made by 3D printing. a) Micro-extrusion 3D printing. Reproduced with permission. [48] Copyright 2020, Wiley. b) Printed directly onto the textile. Reprodiced with permission. [49] Copyright 2019, Elsevier. c) Direct-Ink-Writing 3D printing technology. Reproduced with permission. [50] Copyright 2021, Elsevier. a stretchable F-TENG with core-shell structure by Direct-Ink-Writing 3D printing technology (Figure 3c). [50] PDMS is used as a fiber material, and the core layer and shell layer are filled with graphene and PTFE particles, respectively. PTFE particles increase the electronegativity of PDMS, and graphene has conductivity as an electrode. They can also modulate the rheological properties of PDMS prepolymers to meet the rapid prototyping requirements of 3D printing technology. The fibers can realize the function of tactile sensing, and interlaced fibers make a sensing matrix that can respond to the contact position. In conclusion, with the further progress and maturity of 3D printing technology, it will greatly promote the large-scale application of F-TENG.

Spinning
Coating of conductive electrodes by various types of weaving machines is also a common method of preparing F-TENG on a large scale. As shown in Figure 4a, a negative Poisson's ratio yarn (NPRY) was made by ring spinning technology. [51] First, silver was plated on the surface of polyamide (PA) as a conducting electrode. Then silver-coated PA is wrapped on elastic yarn to make NPRY. Lastly, a yarn TENG (NPRY-TENG) was manufactured by inserting the NPRY into a silicone rubber tube. Through high-speed ring spinning technology, ≈2,000 m of NPRY can be prepared in just 1 h. In order to overcome the shortcomings of complex preparation process and the inability to continuously produce, Li et al. used a braiding machine to weave PVDF on the surface of silver-coated nylon fibers to make F-TENG with a core-shell structure (Figure 4b). [52] Yang et al. prepared F-TENG by combining core-spun spinning method and surface coating method, [53] as displayed in Figure 4c. First, a conductive corespun yarn is produced using a core-spinning method. The flexible conductive fiber is the core and the staple fiber is the shell. Next the triboelectric material is coated on the surface of the core spun yarn. Here, staple fibers can absorb the triboelectric material during the coating process, so the solution is highly retained in the core-spun yarn. This method is suitable for various conductive core wires (such as silver yarn, copper wire, stainless steel wire), short fibers (cotton, silk, polypropylene), and triboelectric materials such as PDMS, TPU, nylon, PVC, PVDF, etc.

Melt Spinning
F-TENG is usually a core-shell structure, the core layer is usually a conductive electrode, and the shell layer is a triboelectric material. Melt-spinning is a mature spinning technology. The existing melt spinning technologies for making F-TENG can be divided into three categories: spraying triboelectric materials on metal wires, adding conductive electrodes after spinning a tubular triboelectric layer, and coaxial spinning. As illustrated in Figure 5a, the core-shell triboelectric yarns are made by blowing silicone rubber on the surface of stainless steel wire. [54] Silicone rubber tubing is intrinsically elastic and the stainless steel wire is spirally structured, so the fiber is superbly stretchable and flexible. This spinning method is continuous and scalable, and silicone rubber and stainless steel wire are commercially available low-cost materials. Ye et al. used electrostatic spinning to add triboelectric layers to the surface of stainless steel yarns. [55] As shown in Figure 5b, during spinning, stainless steel fibers pass through a metal funnel, and two symmetrical electrospinning  [51] Copyright 2021, Royal Society of Chemistry. b) High-speed braiding method. Reproduced with permission. [52] Copyright 2022, Wiley. c) Core-spun spinning method and a surface coating approach. Reproduced with permission. [53] Copyright 2021, American Chemical Society.
units begin to deposit nanofibers on the rotating funnel, forming a nanofiber web that completely covers the core fibers. In addition, researchers have utilized the fibers to achieve real-time recognition of material types. And the fibers can be used in intelligent sensing systems to control various electrical systems.
Thermal stretching involves softening and stretching a thermoplastic material by heating it at a high temperature. This method has the advantages of simplicity, flexibility, and scalable preparation. In the past 2 years, some researchers have prepared F-TENGs on a large scale by thermal stretching methods. As exhibited in Figure 5c, researchers fabricate F-TENG by hot stretching. [43] The production process includes 3 steps: the styrene ethylene butylene styrene (SEBS) pellets are hot-pressed into a film, the PVA pellets are molded into a rod shape by an injection molding machine, and the SEBS film tightly wraps the PVA core into a preform; the preform is thermally stretched into fibers, and then soaked in 80°C hot water to fully dissolve the inner core to form a hollow tube; the liquid metal GaIn alloy is injected into the hollow channel and connected at both ends of the fiber with copper wires to form an ultra-flexible, superstretched F-TENG. This technology is not only suitable for doping liquid metal into fibers, but can also be extended to other electronic devices that can be developed in a similar way. In addition, wet spinning can be utilized to prepare coaxially structured F-TENG on a large scale. [56] Figure 5d shows a schematic diagram of the coaxial wet spinning process. The researchers used triaxial wet spinning needles with inner, middle, and outer diameters of 400, 1,950, and 3,000 m, respectively. Correspondingly, three spinning solutions are PVDF-HFP-TFE [poly(vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene)] /EGaIn particle/MEK in the inner channel, PVDF-HFP-TFE/PEGDA [poly (ethylene glycol) diacrylate]/Irgacure184/MEK in the middle channel, and DI water in the outer channel, respectively.

Weaving Method of F-TENG
In daily life, fibers often need to be woven into textiles for use. In the weaving process of F-TENG, different weaving methods such as plain weave, [7,[57][58][59] knitting, [60,61] and 3D weaving [62][63][64][65][66] can be adopted depending on the need. Plain weave fabrics are lighter, more breathable, and less prone to deformation. The knitted fabric is soft and has good wrinkle resistance and breathability, as well as greater extensibility and elasticity. The 3D woven fabric has a larger space for movement and more sufficient contactseparation between fibers, so there is a larger electrical output.

2D Weaving
Plain weave technology is simple and easy to manufacture, and it is a commonly used weaving method in industrial production. At the same time, F-TENG is also often used to weave textiles in this way. As shown in Figure 6a, Zhao et al. wove coppercoated polyethylene terephthalate (Cu-PET) warp and polyimide (PI)-coated copper-PET (PI-Cu-PET) weft into textile triboelectric nanogenerators (T-TENGs). [67] The warp and weft yarns in a plain weave are interwoven with the other one. When the human Figure 5. F-TENG is made by melt-spinning. a) Blow-molding. Reproduced with permission. [54] Copyright 2019, Springer Nature. b) Electrostatic spinning. Reproduced with permission. [55] Copyright 2022, American Chemical Society. c) Thermal drawing. Reproduced with permission. [43] Copyright 2021, Springer Nature. d) Coaxial wet-spinning. Reproduced under terms of the CC-BY-NC license. [56] Copyright 2021, The Authors, published by American Association for the Advancement of Science. body moves, the warp and weft yarns of the fabric collect electrical energy by contact separation. Knitting needle is the process of forming a coil of yarn and connecting it into knitted fabric. Knitted fabrics have greater extensibility and elasticity. Contact and separation between coils during stretch-release can generate electrical signals. Therefore, the knitted fabric can effectively detect the stress and strain. As displayed in Figure 6b, conductive yarn and commercial nylon yarn are woven into the fabric by knitting. [68] Attaching the knitted fabric to the corresponding position: neck, wrist, and fingertips can monitor the pulse signals of separate parts of the human body. In order to compare the properties of fabrics made by diverse weaving methods, Zhao et al. fabricated t-TENGs with different textile structures by sewing, weaving, and knitting, and tested various properties by controlling variables (Figure 6c). [69] Sewn, woven, and knitted structures are achieved by machine interweaving copper covered with polyacrylonitrile (Cu-PAN) yarns and Cu-PAN covered with polypropylene (polypropylene-Cu-PAN) yarns. Studies on textiles with different structures have shown that the contact between yarn, the initial mechanical state and the overall structural stability of different structured textiles are different. The sensitivity, working range, and washability of the F-TENG depend to a great extent on the textile structure.

3D Weaving
Textile-TENG (T-TENG) will play a key role in the field of energy and self-powered sensing. However, low electrical output power has largely limited the development of T-TENG. The 3D woven structure is an effective way to increase the electrical output. [62] It can effectively increase the electrical output of T-TENG by enlarging the movement space of triboelectric layer. As illustrated in Figure 7a, the 3D T-TENG consists of two knit-wear layers that alternate parallel to each other and cross at the joints. [70] Conductive silver (Ag) yarn is used as the conductive material, while nylon 66 (PA66) is positive triboelectric material and polytetrafluoroethylene (PTFE) is the negative triboelectric material. The 3D T-TENG is fabricated by a computerized flat knitting machine. In the initial state, the two layers of the 3D T-TENG are separated due to the elasticity of the material. When it is stretched, the distance between the upper and lower layers is lowered to zero and the surfaces touch. When the external force is released, the two layers separate and the fabric returns to its original state. The T-TENG can be seamlessly integrated into knitwear to convert the mechanical energy generated by human movement into electrical energy. Figure 7b shows the 3D woven stretchable hierarchical interlocked fancy-yarn (HIFY-TENG). [71] 3D HIFY-TENG Figure 6. 2D weaving of F-TENG. a) Plain weave. Reproduced with permission. [67] Copyright 2016, Wiley. b) Knitting. Reproduced under terms of the CC-BY-NC license. [68] Copyright 2020, The Authors, published by American Association for the Advancement of Science. c) A comparison of stitching, weaving, and knitting. Reproduced with permission. [69] Copyright 2020, Elsevier.
uses industrially scalable processing, fast and cost-efficient weaving methods. Polyurethane (PU) yarn is used as torso yarn, which has good and tensile properties. Wing yarn is a core-shell construction yarn, consisting of a conductive core yarn and an insulating core yarn, which are made by mass-produced twisting methods. Conductive core yarn is silver-plated polyamide (PA) yarn and shell yarn are polyester (PET), polyimide (PI), and PA yarn. And shell yarn wrapped around the surface of the core yarn. When the 3D HIFY-TENG is stretched, the wings are straightened, which makes the wing yarn contact with the torso yarn. When the tension is released, owing to its double helix structure, the 3D HIFY-TENG returns to its original state, allowing the wing yarn to separate from the torso yarn. An intelligent fitness system was designed by integrating 3D HIFY-TENG into the smart yoga belt. Implemented several functions such as statistical analysis of exercise frequency, real-time exercise detection, and self-powered posture correction alarm.
As displayed in Figure 7c, a high-power-output 3D orthogonal woven TENG (3DOW-TENG) was designed. [72] 3DOW-TENG consists of three different yarns: a twisted stainless steel/polyester warp yarn forming the middle conductive layer, PDMS coated energy harvesting weft yarn forming the top and bottom dielectric layers, and a non-conductive z-yarn placed in the direction of thickness to form the adhesive layer. In 3D orthogonal woven fabrics, the warp and weft yarns are side by side to each other and are not interlaced. Z-yarns unite the warp and weft layers by interlacing them at the top and bottom. Thanks to the 3D structure design, the maximum peak power density of 3DOW-TENG can reach 263.36 mW m −2 at a contact frequency of 3 Hz, which is several times higher than that of conventional 2D textile TENGs. The 3DOW-TENG can be utilized as a self-powered sensor to monitor the body's movements. Here, a smart dance blanket has been designed that simultaneously converts biomechanical energy and senses body movements. As . 3D weaving for F-TENG. a) Dual electrode energy textile with a double layer structure. Reproduced with permission. [70] Copyright 2020, Elsevier. b) 3D-braided stretchable hierarchical interlocked fancy-yarn. Reproduced with permission. [71] Copyright 2022, Wiley. c) 3D orthogonal wove TENG. Reproduced with permission. [72] Copyright 2017, Wiley. d) 3D five-directional braiding structure. Reproduced with permission. [73] Copyright 2020, Springer Nature.  [74] Copyright 2021, American Chemical Society. b) Water wave energy. Reproduced under terms of the CC-BY license. [75] Copyright 2022, The Authors, published by American Association for the Advancement of Science. c) Mechanical energy for human movement and solar energy. Reproduced with permission. [76] Copyright 2016, Springer Nature. d) Mechanical energy for human movement and electromagnetic energy. Reproduced with permission. [77] Copyright 2021, Wiley. illustrated in Figure 7d, based on a 3D five-way weave structure, Dong et al. have developed a new type of e-textile with a 3D structure. [73] In this study, commercial silver-plated nylon yarn is used as the electrode and PDMS elastomer is used as the dielectric material. PDMS-coated energy yarn as braided yarn and eight-axis winding yarn as axial yarn, and the four-step rectangular braided process was utilized to make the 3D-braided TENG (3DB-TENG). The braided yarn passes through the section and moves forward in the axial direction, interweaving with the axial yarn through position transitions. Moreover, the orientation of braided yarn is not disordered, but follows four basic directions, forming a multitude of spatial diamond-shaped support frames. The innermost core axial yarn is located in the center of the support frame. And a frame-column structure is created between the braided support frame and the innermost core column to provide sufficient space for them to contact and separate. The 3DB-TENG has good permeability, significant compression resistance, and the 3D structure enhances its output power and improves pressure sensitivity. 3DB-TENG can achieve an open circuit voltage of 90 V and a peak power density of 26 W m −3 at a loading frequency of 3 Hz and an applied force of 20 N.

Harvesting of Various Energy
F-TENG can effectively convert mechanical energy into electrical energy, but the electrical output of F-TENG is limited. In order to enhance the electrical output, the researchers combined the F-TENG with other devices to develop fiber-shaped devices that can harvest multiple forms of energy. In addition to collecting mechanical energy, it can also collect wind energy, raindrop energy, water wave energy, solar energy, and electromagnetic energy. As displayed in Figure 8a, T-TENGs have been created that can harvest manifold forms of mechanical energy, including wind energy, raindrop energy, and mechanical energy from human movement. [74] This water-repellent, dual-mode textile is an association of a vertical contact-separation mode TENG and a freestanding triboelectric-layer mode TENG. The vertical contact-separation mode TENG is utilized to harvest the mechanical energy generated by raindrops, wind and human movement, and the freestanding triboelectric-layer mode TENG is used to harvest the raindrop energy. Two TENGs are sewn together, with the freestanding triboelectric-layer mode TENG on the top layer and the vertical contact-separation mode TENG being a two-layer structure on the lower layer. Raindrops can generate a voltage of ≈4.3 V and a current of ≈6 A. Human motion can generate an open circuit voltage of over 120 V and a peak power density of ≈500 mW m −2 . The F-TENG gives the sensor the flexibility and structural versatility to be employed in more complex scenarios or under special operating conditions. As shown in Figure 8b, the underwater F-TENG is illustrated and demonstrates collecting water wave energy and sensing underwater. [75] The fiber has two electrodes distributed in the inner layer of the fiber. The outermost layer is a waterproof layer, can protect from the influence of water. The F-TENG can respond to different waveforms, including square, tidal, and sine waves. And its voltage signal has the exact same waveform as the water waveform, indicating that the F-TENG can convert water wave mechanical energy into electrical energy in response to water waves. A peak power density of ≈95.5 W m −1 is achieved for fibers with a length of 5 cm by the coupling of surface polarization and ferroelectric polarization.
In addition to harvesting all types of mechanical energy, some fibers can also simultaneously harvest additional forms of energy such as solar and electromagnetic energy. Solar cells are an effective way to capture solar energy. By combining F-TENG with fiber-shaped solar cell, researchers can collect both mechanical energy and solar energy to greatly increase electrical output. As illustrated in Figure 8c, Chen et al. produced an all-solid, foldable, and sustainable hybrid energy harvesting textile. [76] Hybrid energy textiles are breathable and stable and can draw energy from both ambient sunlight and the body's biomechanical movements. A 4 cm × 5 cm hybrid energy textile can deliver a steady 0.5 mW when a human body is walking in sunlight with an intensity of 80 mW cm −2 . What is more, the power textile can be used for large-area applications such as curtains and tents. As shown in Figure 8d, Lai et al. reported a stretchable liquid metal fiber that can harvest mechanical energy (energy from human movement) and electromagnetic energy (energy radiated by electronic devices). The fiber is set by a melt extrusion method in which SEBS hollow fibers are prepared and liquid gallium-indium alloy is injected into the hollow fibers as a conductor. The mechanical energy collected has an output of 160 V m −1 and 360 W m −1 , respectively, while the electromagnetic energy collected from the laptop has an output of 8 V m −1 and 8 W m −1 . The energy collected in both modes can power wearable electronics.

Fibers Used for Energy Harvesting and Storage
The output of the TENG is AC pulse current, and the output electric energy is bounded, which cannot directly drive some electronic equipment. In addition, in many cases, energy collection and energy consumption are not synchronized, which requires effective storage and management of the energy obtained. Therefore, in recent years, researchers have developed some selfcharging yarn integrating energy harvesting and storage, which can convert the mechanical energy of human motion into electrical energy and store it for use in electronic devices. [58,[78][79][80][81] The solution of combining wearable F-TENG with fiber-shaped capacitors has been validated. As displayed in Figure 9a, F-TENG, fiber-shaped solar cells and fiber-shaped supercapacitors are integrated together to increase power generation and store energy. [82] Not only it can harvest solar energy from ambient light and mechanical energy from human movement, it can also store the energy gained in a supercapacitor. A single fiber-shaped solar cell with an open circuit voltage of 0.74 V and a current density of 11.92 mA cm −2 corresponds to a conversion efficiency of 5.64%. The F-TENG can provide an output current of 0.91 mA through human movement. The fiber-shaped supercapacitor unit has a good specific capacitance (1.9 mF cm −1 ). This hybrid selfcharging textile not only offers energy conversion and storage capabilities, but also inexpensive and easy to manufacture. Furthermore, fiber-shaped hybrid self-charging devices can be easily woven into e-textiles. As exhibited in Figure 9b, researchers designed an asymmetrical coaxial structure of the fiber, consisting of a supercapacitor in the core layer and a TENG in the shell layer. [83] It not only collects mechanical energy from its surroundings, but also automatically stores the collected energy via supercapacitor fibers. Under the action of mechanical vibration or human motion, the triboelectric charge on the surface of the TENG creates a potential difference in the asymmetric electrolyte of the fibrous supercapacitor, allowing the charging process to take place spontaneously inside the fibers without any additional circuit connections. The fibers are scalable and the hybrid smart fiber can be scaled up to 50 cm while maintaining its self-charging capability. And self-charging hybrid smart fiber has excellent mechanical and chemical stability and can withstand standard washing processes. In addition, the TENG, supercapacitor (SC), and pressure sensor can be integrated into a coaxially structured fiber, as shown in Figure 9c. The core layer of the fiber is a supercapacitor. The outer layer is a single electrode TENG for energy harvesting, and the outer triboelectric layer and inner layer form a self-powered pressure sensor. [84] The TENG is composed of a composite of silver-plated nylon and PDMS. The supercapacitor is made of carbon fiber and poly(vinyl alcohol)/sulfuric acid (PVA/H 2 SO 4 ) electrolyte that acts as a core layer and is covered with a PDMS protective layer. The pressure sensor is developed on the basis TENG and allows self-powered sensing via a contact-separation mode. SC has a capacitance density of 13.42 mF cm −1 and excellent cycling stability (≈96.6% retention after 10 000 cycles). The TENG has a maximum power of 2.5 W and can power temperature sensors.
In addition to integrating the supercapacitor and the TENG device into the same fiber, it is also possible to weave the supercapacitor and the TENG into the same fabric by means of a weave. As illustrated in Figure 9d, the authors propose a novel selfcharging power textile (SCPT) consisting of a fabric TENG and a woven supercapacitor (W-SC) to simultaneously collect and store energy from human movement. [85] This one-piece self-charging power textile can be obtained by using a traditional weaving process with a plain weave. For the energy generating components, contact-separation mode, freestanding mode TENG was successfully established on the textile by weaving cotton. For the energy storage components, fibrous W-SC was prepared utilizing RuO 2 -coated carbon fibers and cotton threads. The design of the  [82] Copyright 2016, The Authors, published by American Association for the Advancement of Science. b) Asymmetric structure. Reproduced with permission. [83] Copyright 2020, Wiley. c) Coaxial structure. Reproduced with permission. [84] Copyright 2021, American Chemical Society. d) Plain weave. Reproduced with permission. [85] Copyright 2018, Elsevier. e) Knitting. Reproduced with permission. [86] Copyright 2017, American Chemical Society.
series/parallel W-SC can be easily adapted to different application scenarios. Furthermore, two components integrated in one SCPT by the same weaving technology are easily manufactured on a large industrial scale. This research provides an easy and efficient method for mass production of SCPTs, which makes a significant contribution to the further development of wearable electronics. As shown in Figure 9e, Dong et al. developed a highly stretchable and washable all-yarn self-charging textile that enables biomechanical energy harvesting and energy storage by knitting a F-TENG and a fiber-shaped SC into the same fabric. [86] Figure 10. Direct current fabric TENG a) fiber-based air breakdown. Reproduced with permission. [87] Copyright 2020, American Chemical Society. b) Based on the tribovoltaic effect. Reproduced with permission. [88] Copyright 2021, American Chemical Society. c) Fabric-based air breakdown. Reproduced with permission. [89] Copyright 2021, Royal Society of Chemistry. d) Integration of multiple breakdown units. Reproduced with permission. [90] Copyright 2022, Wiley.
The F-TENG is made by coating a stainless steel/polyester fiber blend yarn with silicone rubber. TENG fabric has a maximum instantaneous peak power density of 85 mW m −2 and can light up at least 124 light emitting diodes. The supercapacitor is fabricated using a dip-coating process in which two active materials, carbon nanofibers and poly (3,4-ethylenedioxythiophene)-poly (styrene sulfonic acid) (PEDOT: PSS), are successively deposited on the surface of carbon fibers, and an all-solid-state supercapacitor is prepared using H 3 PO 4 /poly(vinyl alcohol) (PVA) solid gel as the electrolyte. The prepared fiber-shaped capacitor and F-TENG are woven together to form a wearable self-charging energy textile. It is capable of capturing energy from human movement to continuously drive a thermometer or calculator.

Conversion of Direct Current Mode
TENGs have shown promising potential for biomotion energy harvesting. Combining TENGs with textiles is an effective way to achieve smart fabrics. However, most conventional alternating current (AC) fabric TENGs must use stiff and unfriendly rectifier bridge to obtain direct current (DC) to store and power electronic devices. For this reason, researchers designed a DC F-TENG with the flattest structure (Figure 10a), which cleverly exploits the harmful electrostatic breakdown phenomenon on clothing to capture biomotion energy. [87] The textile-based TENG uses air breakdown phenomenon to collect DC energy and apply it to the power supply. This DC F-TENG consists of normal PA yarn as warp and weft yarn and PA conductive yarn as weft yarn. A small DC F-TENG (1.5 cm × 3.5 cm) can easily light up 416 serially connected LED. A larger DC F-TENG (6.8 cm × 7 cm) has a V OC , I SC , and Q SC of 4,500 V, 40 A, and 4.47 C, respectively. Furthermore, thanks to a special operating mechanism, the DC F-TENG shows efficient energy conversion by harvesting the energy of human movement and storing it directly in the fabric SC without the need for any rectifier bridge. The use of mechanical energy to generate direct current (DC) is key to the realization of self-powered wearable electronic devices. As showed in Figure 10b, a flexible textile-based DC-TENG based on the tribovoltaic effect is reported. The dynamic Schottky interface is capable of absorbing dissipated frictional energy, generating unbalanced electron-hole pairs, and outputting a voltage to an external circuit, thus enabling charge transfer at the metal-semiconductor polymer interface on textile substrates. [88] This textile DC-TENG is based on a dynamic Schottky junction between an aluminium block and a poly (3,4-ethylenedioxythiophene) coated textile and can output 0.45-0.70 V. The voltage and current can be enhanced by simply connecting several DC-TENG in series or in parallel. Seven DC-TENG connected in series can continuously power an electronic meter without any regulation circuitry.
The air breakdown-based DC-TENG not only requires a rectifier bridge, but also increases the electrical output. As illustrated in Figure 10c, a lightweight, highly flexible, and wearable high-power output fabric direct current TENG (FDC-TENG) has been developed by simply coating two electrodes on the top (breakdown electrode) and bottom (triboelectric electrode) of a polyester-cotton fabric. [89] The friction electrode is used to generate triboelectric charge, which are transferred by the breakdown electrodes breaking through the air. The finger-sized FDC-TENG is capable of lighting up 99 bulbs and 1053 LEDs and can easily drive watches and calculators directly without rectification or capacitor charging. In addition, the current output can be increased exponentially by increasing the amount of the breakdown electrodes. To further increase the electrical output, Cheng et al. increased the number of units and designed a power management module to match the DC output (Figure 10d). [90] The alternating current and high impedance of frictional electric nanogenerators greatly limit their practical application. In this paper, an energy textile consists of a high output DC triboelectric power textile (DC-TPT) and a miniaturized energy management module (EMM). This multi-array DC-TPT has 9 repetitive cells and obtains a transferred charge of 5.5 C per cycle. With an energy conversion efficiency of 82.6%, the EMM reduces the impedance of the DC-TPT from 200 to 1.6 MΩ, which improves the charging performance of energy storage devices by a factor of 2117. With its superior electrical output and efficient energy management strategy, after sliding the DC-TPT over the arm for just 1.6 s, the watch can operate continuously for 172 s and can transmit wireless signals up to 281 m away by sliding it for 2 min. In addition, four 30 W lamps can be lit when the EMM is in pulse output mode.

F-TENG for Self-Powered Sensing
F-TENG can not only convert all kinds of mechanical energy into electric energy for energy harvesting, but also can be used as self-powered pressure sensors to monitor mechanical activities and physiological information about the human body. [91][92][93][94][95][96][97] Fibers and fabrics are essential to the human body. F-TENG combining fibers with TENG can be utilized to monitor human motion without affecting normal human activity. As displayed in Figure 11a, a pressure sensing array based on F-TENG is fabricated that can be utilized to monitor human movement and real-time tactile sensing. [98] The F-TENG is manufactured by depositing conductive and dielectric materials layer by layer on a stretchable fiber substrate, so it is stretchable and diameter controlled. The fibers have good mechanical and electrical output properties, and can be woven with common fibers to form sensing arrays, which can be used for tactile sensing and has potential applications in fields such as human-computer interaction and robotics. F-TENG can also be utilized to gesture recognition by attaching them to fingers, which generate electrical signals when fingers move. Figure 11b shows a wearable sign language speech translation system, which translates sign language into audio speech in real time. [99] Wearable sign-to-speech conversion system consists of a yarn-based stretchable sensor array (YSSA) and a wireless printed circuit board. The YSSAs are employed for electrical signal acquisition, and the circuit board is used for signal processing and transmission. The wearable sign-to-speech conversion system has the advantages of high sensitivity, low weight, high stretchability, and low cost. With machine learning, the system achieves a recognition accuracy of 98.63%. In the same way, F-TENG can be used for gait recognition. As illustrated in Figure 11c, 3D fabrics with excellent compression properties are integrated into commercial carpets to make identity recognition carpets. [73] The smart carpet is a sensing array that records the trajectory of the human body. When a person steps on the carpet, his walking path will be recorded. There will be two states (correct password, incorrect password) corresponding to this. When an incorrect password is entered more than 3 times, a warning will sound. Self-powered identification carpet system is a simple, efficient, and self-powered visitor identification system that is an essential safeguard for maintaining internal security and preventing external intrusion.
F-TENG can be utilized not only for human-computer interaction and human motion monitoring, but also for monitoring some physiological information about the human body, such as respiration and pulse. Figure 11d illustrates a respiratory monitoring system based on F-TENG. [100] A helical fiber strain sensor (HFSS) is integrated on the body's chest cavity, and the expansion and contraction of the chest cavity caused by human breathing drive the HFSS to output electrical signals. The PTFE winding Ag-coated nylon fibers (PTFE/Ag-braided fibers) and the nylon winding Ag-coated nylon fibers (nylon/Ag-braided fibers) are alternately wound around a stretchable substrate fiber to form the HFSS. PTFE/Ag-braided fibers and nylon/Ag-braided fibers realize contact separation in the stretch-to-release process of HFSS. The self-powered intelligent alarm module can distinguish different breathing modes, and when the patient stops breathing for more than 6 s, the preset mobile phone can be called for help. As shown in Figure 11e, triboelectric all-textile sensor array (TATSA) made by knitting with conductive yarn and commercial nylon yarn. The TATSA has high pressure sensitivity (7.84 mV Pa −1 ), fast response time (20 ms), stability (> 100 000 cycles), and machine washability (> 40 washes). [68] When the TATSA are sewn onto different parts of the garment, it can simultaneously monitor arterial pulse waves and respiratory signals. Based on TATSA, the authors have developed a personalized intelligent health monitoring system to continuously acquire and preserve physiological signals for cardiovascular disease (CAD) analysis and sleep apnea syndrome (SAS) assessment.

Multifunctional Coupling of Fibers
Fibers and fabrics are developing in the direction of functionality and intelligence. The fabrics of the future comply with the Figure 11. F-TENG for self-powered sensing. a) Human-machine interaction. Reproduced with permission. [98] Copyright 2020, Wiley. b) Gesture recognition. Reproduced with permission. [99] Copyright 2020, Springer Nature. c) Gait recognition. Reproduced with permission. [73] Copyright 2020, Springer Nature. d) Respiratory monitoring. Reproduced with permission. [100] Copyright 2022, American Chemical Society. e) Pulse monitoring. Reproduced under terms of the CC-BY-NC license. [68] Copyright 2020, The Authors, published by American Association for the Advancement of Science.
requirements of wind protection, warmth, and aesthetics while at the same time being multifunctional. In recent years, researchers have developed a range of functionalized fibers and fabrics based on TENG, [101][102][103][104] which collect mechanical energy while also being luminous, hydrophobic, flame retardant, radiation resistant, and corrosion resistant. As displayed in Figure 12a, inspired by Meissner chlorophos, this study proposes a multifunctional F-TENG with self-emission and energy harvesting. This fiber (SLEH-TF) can spontaneously emit visible light in the dark while harvesting biomechanical energy to power the device. SLEH-TF is a wearable energy provider, self-powered sensor, and humanmachine device interface. [105] It can convert biomechanical energy into usable electrical energy. A 5 cm SLEH-TF can output V OC = 15 V, I SC = 500 nA; the electrical output of a palm-sized SLEH-TF textile is V OC = 250 V and I SC = 80 A. Persistent vis-ible light (50 mcd m −2 , 120 min) can be emitted after 5 min of exposure to daylight. In addition, the SLEH-TF can be used to detect pressure (with a sensitivity of 11.48 V N −1 and 119.54 nA N −1 (<1 N)). As exhibited in Figure 12b, researchers prepare F-TENG that can harvest water droplet energy. [106] A conductive stretching fiber was prepared using the wet-spinning technique. The conductivity of the fiber is maintained even in the stretching mode. Utilizing the AuCNS@MCP fiber as a sensing unit, ultrasensitive pressure sensors with high sensitivity (0.94 kPa −1 ), fast response time (39 ms), and excellent durability can be prepared. By coating the fiber surface with PDMS, TENG with a highly elastic hydrophobic fiber structure can be constructed, which can capture energy from water droplets. When water is continuously dripped onto the fibers, a continuous output voltage can be generated. By controlling the water flow rate, we can observe that  [105] Copyright 2022, Elsevier. b) Hydrophobic. Reproduced with permission. [106] Copyright 2021, Elsevier. c) Corrosion resistant. Reproduced with permission. [107] Copyright 2021, Wiley. d) Flame-retardant. Reproduced with permission. [108] Copyright 2020, Wiley. e) Electromagnetic shielding. Reproduced with permission. [109] Copyright 2021, Wiley.
the output voltage and current increase as the water flow rate increases, demonstrating the potential application of the TENG as a self-powered sensor for measuring water flow. In addition, the TENG has good durability under uninterrupted water flow conditions and the output voltage of the TENG does not drop significantly under 4 h of continuous water flow. As illustrated in Figure 12c, a smart chemical protective suit with good breathability and chemical resistance is designed to protect the wearer from chemical spill incidents in laboratories, chemical plant, and special workshops. [107] PTFE has excellent properties such as good acid and alkali resistance, oxidation resistance and non-toxicity. The F-TENG is a composite yarn with PTFE filament as the shell yarn and conductive polyamide yarn as the core yarn, which turns it into corrosion resistant. A smart protective suit is manufactured based on the fiber. This smart suit is equipped with human movement energy harvesting and self-powered safety monitoring systems with functions such as acid and alkali resistance, self-powered chemical leak detection, operator vital signs monitoring.
Fire is one of the most common hazards threatening public safety and improving rescue capabilities remains a huge challenge. As shown in Figure 12d, a 3D honeycomb structured woven fabric TENG (3D fabric TENG) is developed using a flame retardant wrapped yarn as the raw material. [108] First, a very durable and sustainable flame retardant single-electrode triboelectric yarn (FRTY) has been spun using a stretchable hollow spinning fancy twist technique. FRTY core sheath yarn is made by wrapping a 32s fineness polyimide yarn around the surface of a conductive core yarn. Next a 3D fabric TENG with a honeycomb structure was woven based on FRTY. The resulting 3D fabric TENG can be utilized in smart carpets as a self-powered escape and rescue system, pinpointing the location of survivors and indicating escape routes, and assisting in the timely search and rescue of victims. In addition to its flame-retardant properties, the 3D honeycomb structure TENG has potential applications in carpet noise reduction due to its unique porous structure. Radiation is pervasive in our lives and harmful to the human body, and the integration of radiation-protective materials into fibers gives them a radiation-protective function. As shown in Figure 12e, Shen et al. designed a coaxially structured F-TENG with electromagnetic shielding by polymerizing pyrrole and silicone rubber encapsulation material on the surface of a polyamide yarn. [109] The fibers not only convert human motion energy into electrical energy, but also act as an electromagnetic shield. The electromagnetic shielding fabric made by knitting F-TENG has an electromagnetic shielding effectiveness of 32.49 dB at 8.2-12.5 GHz and a maximum instantaneous peak power density of 142.27 W m −1 . In addition, a real-time human-computer interaction system has been developed based on this fiber: an intelligent calculator with multiplication and division functions, which convert electronic signals into digital signals and perform calculations. In order to compare the different studies, we have summarized the working modes, material selection, fabrication methods, peak power density, and pressure sensitivity of the 1D, 2D, and 3D TENGs. As shown in Table 1, we are not able to find obvious patterns, because they are different in material selection, fabrication methods, and testing conditions (e.g., diameter and length of fibers, contact force, and contact frequency), and it is impossible to know how a particular factor affects the performance of the device. However, we can draw on their structural settings, material selection, etc. It can be seen from Table 2 that the pressure sensitivity of 3D textile-TENGs is higher than that of 2D textile-TENGs, because the triboelectric layers of 3D textile-TENGs have a wider space for movement, which makes the contact-separation between the triboelectric layers more adequate.

Conclusion and Perspectives
Since its invention in 2012, the TENG has gained rapid development due to its unique advantages. Among them, F-TENG is developed by combining traditional textile technology with TENG technology. F-TENG combines the flexibility and practicality of fibers with the energy conversion function of TENG, and has also gained significant progress in both wearable energy harvesting and self-powered sensing. F-TENG as a self-powered sensor, can convert mechanical motion into electrical signals, the electrical output can fully meet the sensing needs. F-TENG as a power supply, AC output needs to be converted into DC through the rectifier bridge. The electrical energy can directly drive some small electrical appliances such as: electronic digital watches. But now it cannot power some high-power electronic devices such as mobile phones, which have to be combined with power storage devices in the future.
This paper summarized recent developments in F-TENG in the direction of large-scale preparation, weaving mode, latest applications in the field of self-powered sensing, electrical output performance enhancement and multi-functionalization. In addition, we have summarized several challenges and opportunities that F-TENG faces in practical applications. As displayed in Figure 13, the following five areas can be used as references for research, and more work should be focused on a key issue.

I. Scaled preparation
Scale-up preparation is an important part of the process of F-TENG from the laboratory to commercialization, and there has been researching on the large-scale preparation of F-TENG and satisfactory progress has been made, which has been summarized above. However, they are usually high costs, complex processing technology, low efficiency and unstable. Finding an industrially compatible, low production cost, green and sustainable preparation method is the direction that needs to be considered.   Figure 13. Schematic illustration of the developing directions of F-TENGs.
Adv. Sensor Res. 2023, 2, 2200044 www.advancedsciencenews.com www.advsensorres.com flexible circuit management system, and designs an energy storage unit that matches the output of F-TENG.

I. Sustainability
Green, environmentally friendly and sustainable, is a constant topic. This should be taken into consideration in the manufacture, use, and recycling of F-TENG. To develop environmentally friendly, recyclable F-TENG has far-reaching implications for carbon neutrality.

I. Enhancement of comfortability
Another obstacle to the commercialization of F-TENG is the absence of comfort. Owing to the special application areas of F-TENG, which has high requirements for comfort, and there is still a certain gap between the F-TENG made now and the fiber used in our daily life. This is mainly due to the immaturity of the fabrication process, such as the introduction of rigid electrodes in the fabrication process, resulting in poor comfort and practicality of the fibers prepared. Wearability and functionality are often mutually exclusive and it is difficult to achieve synergy between the two. While taking into consideration functionality, we should strive to produce fibers with excellent mechanical properties, comfortability and practicality, from the perspectives of material selection and manufacturing methods.

I. Explore emerging applications
Fibers and fabrics are a necessity for people and its deep integration with people's life. Smart clothing will have a broad application prospect. For example, health monitoring, motion capture, identity recognition, and smart home, will make a significant contribution to the development of wearable electronics.