Textile or woven-structured triboelectric nanogenerators (TENGs) convert the mechanical energy generated from human motion into electrical energy and they can be used as efficient power sources for wearable and portable devices. However, the existing weaving-based woven-structured TENGs require multiple strands of fibers and have limited stretchability. In this paper, we propose highly stretchable and flexible single-strand fiber-based woven-structured TENGs (FW-TENGs). The proposed FW-TENGs can generate ∼34.4 µW/cm2 power from the continuous contact and separation from skin and demonstrate durability and potential for application in electronic devices. We successfully integrated the proposed FW-TENG into a shoe and harvested the mechanical energy generated from human motion. The FW-TENG is expected to find use in various applications such as e-textiles and smart clothing because it can be manufactured on a large scale.

The power source for typical electronic devices is electrical energy, which is stored in and used from batteries. Batteries are simple to handle and portable and can be used anywhere; however, they have a limitation because they require regular charging or replacement.1,2 With the recent developments in wearable low-power small-sized electronic devices, many studies have been conducted to overcome the limitations of battery size and lifetime. However, as the capacity of a battery is directly related to its size, the weights and sizes of the electronic devices using them will be limited.3,4 To address this problem, energy-harvesting technologies that convert the mechanical energy generated from human motion into electrical energy are attracting attention as new power sources.5–7 In order to effectively harvest the physical energy from the human body, the irregularity and discontinuity of human motion should be considered. Because the mechanical energy generated from human motion varies with the environment and passage of time, environment-adaptable energy-harvesting technologies are necessary.8 

Among the various related technologies, triboelectric energy harvesting is the most effective for converting the mechanical energy from living/working environments into electrical energy at low costs and with simple fabrication procedures.9–11 Compared with photovoltaic and thermal energy harvesting, it is less susceptible to weather and working environments and is more widely used to design self-powered, human-motion-based energy-harvesting systems than the piezoelectric energy harvesting systems, which require sophisticated control.12–14 

Conventional human-motion-based triboelectric nanogenerators (TENGs) have been fabricated to be integrated into a bag or attached to bands or joints of the human body to improve wearability and enable energy harvesting even in unconscious conditions.15 Yarn or fiber-based TENGs have the advantage that they can be fabricated in various two-dimensional (2D) structures and can be easily integrated with clothing; however, most fiber-based TENGs are less flexible and difficult to weave.16,17 In addition, in the study of woven-structured TENGs, multiple strands of fiber-based TENGs were required for weaving and an additional process was required to integrate the electrodes into one.18–20 

In this study, we propose a flexible single-strand fiber-based woven-structured TENG (FW-TENG) capable of generating power from free human motion. The proposed FW-TENG is fabricated by weaving a fiber-based TENG of silicone rubber as a negative triboelectric material and an Au-coated Cu thread as an electrode. The electrical power generation performance and outstanding durability are confirmed for FW-TENGs of various sizes, which generate power through contact with the skin, and their potential for generating electrical energy from human motion is demonstrated. Consequently, the application of FW-TENGs in ready-to-wear apparel products is expected to promote the use of self-powered electronic devices that are capable of being charged by continuous skin contact, without causing any inconvenience.

As shown in Fig. 1(a), the FW-TENG consists of a single strand of a long fiber-based TENG. In the process of fabricating a fiber-based TENG, the liquid parts A and B of the silicone rubber are stirred in a 1:1 ratio to produce a silicone rubber fiber. The bubbles generated during stirring are removed from the vacuum state and the liquid of the silicone rubber is injected into a cylindrical mold having an inner diameter of 0.8 mm to be cured. The Au-coated Cu threads are used as electrodes to transfer the induced charges. The Au-coated Cu thread with a low resistance of 0.037 Ω/cm shows higher electrical conductivity than other metal-coated threads.21–24 It is also a metal-composite embroidery thread and is, thus, less affected by friction and corrosion. The Au-coated Cu threads are connected to the motor and wound onto the silicone rubber fiber at a constant speed and spacing. Finally, the surface is dip coated with silicone rubber and dried at room temperature for approximately 3 h. The silicone rubber is used as a negative triboelectric material and has superior chemical stability, heat resistance, and abrasion resistance when compared to the general organic rubber because of the inorganic properties of the main chain siloxane (Si—O) in the molecular structure. The silicone rubber is also used as a dielectric and encapsulating material for fiber-based TENG because of its excellent flexibility, stretchability, biocompatibility, mechanical properties, strong resistance to sweat and water, and high electron affinity. Prior to the weaving process, we compared the electrical output voltages according to the number of strands of the Au-coated Cu thread, as shown in Fig. 1(b), to design the optimal fiber-based TENG. Fiber-based TENGs consisting of one to six strands of the Au-coated Cu thread showed different output values for an external force of 0.5 N. The highest output value was obtained with three strands of Au-coated Cu thread. When more than four strands were used, the TENGs loosened under stretching conditions, resulting in damage to the coated silicone rubber and increasing the resistance. Therefore, we fabricated FW-TENGs with a 45 × 45 mm2 size, as shown in Fig. 1(c), based on the fiber-based TENG structure using three strands of Au-coated Cu thread through a weaving process using a weaving handloom. Figure 1(d) shows a scanning electron microscope (SEM) image of the side of an Au-coated Cu thread. The Au-coated Cu threads are composed of multiple twisted Au-coated Cu threads and polyester. Figure 1(e) shows the SEM image of a fiber-based TENG cross section fabricated with a radius of 0.6 mm. The silicone rubber fiber and silicone rubber coated on the surface are shown in the image. The thickness of the silicone rubber layer on the surface is 100 µm. As shown in Fig. 1(f), an Au-coated Cu thread was soldered on the breadboard to the anode and cathode of the light emitting diode (LED). Using a DC supply, a voltage of 2 V was applied to illuminate the LED, demonstrating the excellent electrical conductivity and soldering interconnection characteristics of the Au-coated Cu thread.

FIG. 1.

(a) Schematic illustration of the FW-TENG; the figure on the right is the fiber-based TENG. (b) Electrical output depending on the number of Au-coated Cu threads. (c) Photograph of the FW-TENG with a surface area of 45 × 45 mm2. (d) Top-view SEM image of the Au-coated Cu thread, scale bar: 300 µm. (e) Cross sectional view SEM image of fiber-based TENG, scale bar: 300 µm. (f) Photograph showing lighting of commercial LED connected to the Au-coated Cu thread.

FIG. 1.

(a) Schematic illustration of the FW-TENG; the figure on the right is the fiber-based TENG. (b) Electrical output depending on the number of Au-coated Cu threads. (c) Photograph of the FW-TENG with a surface area of 45 × 45 mm2. (d) Top-view SEM image of the Au-coated Cu thread, scale bar: 300 µm. (e) Cross sectional view SEM image of fiber-based TENG, scale bar: 300 µm. (f) Photograph showing lighting of commercial LED connected to the Au-coated Cu thread.

Close modal

As shown in the mechanism in Fig. 2, the FW-TENG generates electric current while balancing the charge by the movement of electrons from the electrode due to the friction between silicone rubber and the skin. According to the triboelectric series, silicone rubber has a negative characteristic with a high electron affinity, but the skin shows a positive characteristic with a low electron affinity. As shown in Fig. 2(a), when the skin and silicone rubber are in contact, the skin is positively charged and the silicone rubber is negatively charged. Thus, the silicone rubber generates a negative charge Q, and by induction of static electricity and preservation of charge, the Au-coated Cu thread of the FW-TENG electrode and the skin generate positive charges Q1 and Q2, respectively. Therefore, since Q = −(Q1 + Q2), the negative charge Q on the FW-TENG is considered to create an electric field. As shown in Fig. 2(b), when the two materials are separated, a potential difference occurs and electrons move from the electrode to balance the triboelectric potential. As shown in Fig. 2(c), when the two materials are maximally separated, they enter electrostatic equilibrium states in which electrons do not move. Finally, as shown in Fig. 2(d), when the two materials come into contact again, the electrostatic equilibrium collapses and the amount of charge moving from the electrode decreases. That is, Q1 increases and Q2 decreases, generating an electric current instantaneously. Hence, the FW-TENG is a single-electrode structure that converts motion into electric current, is independent of the kind of motion, and generates alternating current (AC) through a continuous contact–separation process.

FIG. 2.

[(a)–(d)] Schematic diagrams illustrating the working principles of the FW-TENG under contact–separation motion.

FIG. 2.

[(a)–(d)] Schematic diagrams illustrating the working principles of the FW-TENG under contact–separation motion.

Close modal

The proposed FW-TENG can be weaved in various sizes and shapes by using a weaving handloom. The FW-TENG was fabricated in various sizes and experiments were conducted to analyze the electrical output characteristics on applying a repetitive external force. The output voltage was measured by connecting an electrode to the oscilloscope (MSO9104A), and the output current was measured using a precision source/measurement device (B2911A). Figure 3 shows the output voltage and current signals generated from the FW-TENG by skin friction for an applied force of 1 kgf. As shown in Fig. 3(a), an output voltage of 42 V and a current of 5 µA were generated by a 45 × 45 mm2 FW-TENG. In the same experimental environment, as shown in Fig. 3(b), an FW-TENG with 70 × 35 mm2 size produced the maximum electrical output of 53 V and 15 µA; an output of 72 V and 18 µA was measured from the FW-TENG of size 75 × 75 mm2 [Fig. 3(c)]. From these results, it can be inferred that the efficiency of electric power generation is improved as the size of the FW-TENG increases; however, this improvement is nonlinear. This may be because of a characteristic of triboelectricity, which induces saturation above a certain level of force. Therefore, it is important to select an optimal TENG size that offers efficient electrical output when a constant force is applied. In addition, to investigate the effective electric power, FW-TENG was connected to electrical resistors and the electrical output was measured. As the resistance increased, the output voltage increased and was saturated at the open-circuit voltage when the resistance was infinitely large. Conversely, the output current decreased as the resistance increased [Fig. 3(d)]. As a result, the output power showed a maximum value (about 34.4 µW/cm2), calculated from the equation (W = V2peak/R), at an external resistance of 1 MΩ [Fig. 3(e)].

FIG. 3.

Comparison of the electrical outputs according to the surface area of the FW-TENG: (a) 45 × 45 mm2, (b) 70 × 35 mm2, and (c) 75 × 75 mm2. (d) Output voltage and current with different resistors as external loads. (e) Dependence of the output power on external load resistances.

FIG. 3.

Comparison of the electrical outputs according to the surface area of the FW-TENG: (a) 45 × 45 mm2, (b) 70 × 35 mm2, and (c) 75 × 75 mm2. (d) Output voltage and current with different resistors as external loads. (e) Dependence of the output power on external load resistances.

Close modal

Durability and stability are important parameters for the TENG as well as electrical energy generation. When a repetitive external force was applied using a pushing tester (JIPT-100), the FW-TENG exhibited a stable output performance. The contact area of the pushing tester applying the external force to the FW-TENG was 36π mm2. As shown in Fig. 4(a), an output voltage was generated during 5000 pushing cycles with a force of approximately 0.6 kgf. When comparing the output voltages of the initial state and the state after 5000 pushing cycles, a constant value was confirmed, without distortion. Likewise, the current signal generated during 5000 pushing cycles in the same experimental environment is shown in Fig. 4(b). Hence, the FW-TENG is of practical value as a reliable energy harvester since there is no reduction in the electrical output after an external force is applied. In addition, to verify the flexibility of the FW-TENG, a triboelectric output test was conducted before and after specific stretch-release cycles by using a tensile tester [Fig. 4(c)]. Figures 4(d) and 4(e) show the comparisons of the voltages and currents of the FW-TENG before and after the stretching tests wherein it was stretched by 66.6% through 5000 cycles. The results show that a stable triboelectric performance was obtained. Thus, it can be concluded that the FW-TENG shows outstanding flexibility without degrading the electrical output.

FIG. 4.

Mechanical durability test for the FW-TENG under 5000 cycles with the first and last 0.5 s waveforms enlarged. (a) Open-circuit voltage and (b) short-circuit current. (c) Photograph of the experimental setup for stretch-release cycles. (d) Output voltage and (e) current before and after stretching by 66.6% through 5000 cycles.

FIG. 4.

Mechanical durability test for the FW-TENG under 5000 cycles with the first and last 0.5 s waveforms enlarged. (a) Open-circuit voltage and (b) short-circuit current. (c) Photograph of the experimental setup for stretch-release cycles. (d) Output voltage and (e) current before and after stretching by 66.6% through 5000 cycles.

Close modal

We now demonstrate the potential applications for harvesting and substantially utilizing the electrical energy generated from FW-TENGs. As shown in Fig. 5(a), a capacitor was connected to the rectifier circuit and the charge level was measured as the voltage. The capacitor accumulated the charge until the positive and negative charges were equal to the voltages supplied externally. Since the FW-TENG generates AC, a bridge rectifier circuit was used. An external force was applied to an FW-TENG with size 70 × 35 mm2, which produced the highest power per unit area, and a 1 µF capacitor was charged to 1.2 V for 25 s. According to the formula Q = CV, the amount of charge stored in the 1 µF capacitor over 25 s is approximately 1.2 µC. As shown in Fig. 5(b), using the accumulated charge, a commercial LED that requires an operating voltage of 1.8 V was lit up. In addition, the possibility of driving electronic devices was confirmed by connecting an electrical watch to an FW-TENG of size 70 × 75 mm2 and a rectifier circuit. In order to drive the electrical watch for a long time, a 1 µF capacitor was connected to the circuit to increase charge capacity. As shown in Fig. 5(c), the electrical watch was driven continuously through contact with the skin, under a pushing motion. In another application, the FW-TENG was attached to the heel of a shoe in order to convert the mechanical energy generated from the movement of the human body into electric energy. As shown in Fig. 5(d), a signal of instantaneous power was generated by the contact of the heel and the FW-TENG during a step. The FW-TENG can be applied to active sensors that calculate the number of steps, gait correction shoes, portable power sources, etc., and can be further extended to other applications. It has been proven that the FW-TENG successfully outputs electrical energy as a self-powered device and that it can be used as a promising power source for wearable devices.

FIG. 5.

(a) Circuit diagram of a full-wave bridge rectifier and charging curve for the capacitor. (b) The LED can be directly lit and is visible in bright environments. (c) The electrical watch was powered by the FW-TENG under the pushing mode. (d) FW-TENG integrated in a shoe, harvesting energy from steps.

FIG. 5.

(a) Circuit diagram of a full-wave bridge rectifier and charging curve for the capacitor. (b) The LED can be directly lit and is visible in bright environments. (c) The electrical watch was powered by the FW-TENG under the pushing mode. (d) FW-TENG integrated in a shoe, harvesting energy from steps.

Close modal

In summary, we proposed a flexible FW-TENG composed of silicon rubber and Au-coated Cu threads. As a single-electrode structure harvesting the mechanical energy generated from free motions, the proposed FW-TENG generated ∼34.4 µW/cm2 power from continuous contact with the skin. The FW-TENG demonstrated outstanding durability without degrading the electrical output during the application of repetitive external forces at 5000 pushing cycles. It was demonstrated to power a commercial LED and electrical watch with the output energy, as examples for practical applications. In addition, its potential for converting the mechanical energy generated from human motion into electric energy was illustrated. The theoretical and experimental results establish that the FW-TENG can be utilized as a self-powered system and can offer a new solution to power wearable or portable devices.

This research was supported by the Mid-career Researcher Program (No. 2016R1A2B3009423) through NRF grant funded by the MSIT (Ministry of Science and ICT) and the ITRC support program (No. IITP-2015-0-00390) supervised by the IITP, the grant from the MSIT, Korea.

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