A flexible silicone tube arrangement structure triboelectric nanogenerator for tennis training monitoring

Amid the rapid advancement of Internet of Things (IoTs) technology, there is a growing urgency in the market for self-powered flexible electronic devices. This is particularly true for devices that can address the short lifespan, frequent replacement, and potential explosion risks associated with traditional batteries.^1–3 The field of intelligent sport sensors is particularly eager for devices with long endurance and high efficiency.^4,5 For example, wearable sensors play a crucial role in the daily monitoring of tennis players, providing real-time multi-dimensional body and movement data, mainly including heart rate monitors,⁶ accelerometers,⁷ gyroscopes,⁸ and GPS devices.⁹ Through the comprehensive analysis of data from these sensors, coaches and athletes can develop personalized training plans to improve athletic performance and reduce the risk of injury. While wearable sensors are crucial for monitoring the daily activities of tennis players, they also come with certain drawbacks. First, the accuracy of data may be affected by factors such as vigorous exercise or improper device wear, which can affect the accuracy of heart rate monitors and accelerometers. Second, prolonged wearing may cause discomfort, and the size and weight of the device may limit the athlete’s movements. Battery life is also an issue as frequent charging may interrupt the continuity of data. In addition, the processing and privacy protection of large amounts of data require strong computing power and specialized software, while ensuring that personal information is not leaked or abused. Finally, high-quality sensors are expensive and require regular maintenance and calibration, which increases the economic burden.

As a cutting-edge technology, triboelectric nanogenerators (TENGs) can capture a wide range of irregular mechanical energy, especially human movements, thereby achieving self-sufficiency in flexible electronic devices.^10–20 The TENG devices can be used in intelligent sports to monitor the athlete status in real time and improve training effectiveness and competitive performance. Compared with other technologies, such as thermoelectric²¹ and piezoelectric,²² triboelectric technology has significant advantages in material selection, output performance, and working modes.^23,24 As a result, it has found extensive use in wearable energy and sensing systems, as well as in advanced sports equipment. Meaningfully, triboelectric sensors have significant advantages when applied to tennis sports. Compared to traditional sensors, TENGs have self-powered ability, reduce battery dependence, and solve the battery life problem. Meanwhile, their high sensitivity and accuracy can capture subtle motion details, providing more accurate data. The lightweight and flexible design makes integrating into athlete clothing or equipment easy without affecting comfort and freedom. In addition, triboelectric sensors can also incorporate multiple functions, such as pressure sensing,²⁵ vibration monitoring,²⁶ and position tracking,²⁷ providing comprehensive motion data analysis. Environmental protection and sustainability are other significant advantages; utilizing environmental energy for self-power generation reduces the generation of electronic waste. Through these advantages, TENG sensors have improved the accuracy and reliability of data collection, enhanced the convenience and sustainability of equipment, and contributed to the advancement of sports science and technology. However, current TENG devices with rigid structures struggle to adapt to the multi-directional, irregular movements of the human body. Their implementation in wearable applications is relatively complex, which hinders the development of wearable TENG systems.²⁸ To address these issues, researchers have developed various new TENG structural designs. From the future perspective, a folding structure TENG (including an X-shaped structure,²⁹ diamond shaped structure,³⁰ honeycomb shaped structure,³¹ etc.) can adapt to multi-directional mechanical deformation through folding and unfolding and provide a larger contact area in limited space, thereby improving energy harvesting efficiency. The flexibility and efficiency of the folding structure TENG give it broad application prospects in wearable devices. Moreover, flexible hydrogel TENGs are also an ingenious design that attracts much attention.³² The hydrogel’s high flexibility and excellent mechanical properties allow it to adapt to complex surfaces and significant deformation. Hydrogel materials’ high transparency and biocompatibility make this TENG design particularly suitable for close-fitting wear and skin surface applications, such as health monitoring³³ and motion tracking.³⁴ Moreover, a fibrous TENG is highly favored and considered ideal because of its excellent shape adaptability, breathability, and weaving capability. It can be woven into breathable electronic textiles or seamlessly integrated into various intelligent clothing systems.³⁵ To enhance practicality and efficiency, fiber TENGs must exhibit good ductility, endure mechanical deformation, and possess high triboelectric effects to deliver reliable and optimal output over long-term use. Accordingly, multiple optimization strategies have been proposed, including using deformable structural design,³⁶ stretchable electrodes,³⁷ and fiber triboelectric material³⁸ to manufacture this fiber/yarn. However, despite some progress, there are still some shortcomings in the fiber structure TENG. First, the manufacturing process of a fiber structure TENG is complex and costly, making large-scale production difficult. In addition, its output performance is easily affected by environmental factors such as humidity and temperature in practical applications, leading to the instability of triboelectric effects. Second, even with stretchable materials and structural designs, a fiber TENG may still experience material fatigue and damage under long-term mechanical deformation, affecting its performance. Besides, a fiber structure TENG also faces challenges in terms of integration and application. To address these issues, future research should further optimize material selection and structural design, improve the TENG’s durability and environmental adaptability, and develop more economical and efficient production processes to achieve large-scale applications.

Here, we designed a silicone tube-based triboelectric nanogenerator (ST-TENG) by simply arranging planes on a flexible substrate for low-frequency mechanical energy harvesting. Specifically, by using conductive copper paint on the silicone tube surface and inside of the silicone tube, the triboelectric pair is formed between the silicone tube layer and the copper layer. In the meantime, the conductive copper paint is used as a conductive electrode for the electrical output end of the ST-TENG. The structural surface of this silicone tube arrangement can effectively obtain the triboelectric contact area, and the simple integrated preparation scheme effectively avoids the preparation difficulties caused by the complex structure. At a working frequency of 6 Hz, the ST-TENG has an open-circuit voltage (V_oc) of 122.51 V, a short-circuit circuit (I_sc) of 15.05 µA, and a transfer charge (Q_sc) of 33.74 nC. Due to its unique tubular surface structure, the ST-TENG exhibits highly sensitive responses to both pressure and bending angles, demonstrating significant potential for applications in human motion monitoring. The ST-TENG device can provide factual and accurate monitoring data for tennis training by installing sensors on the waist, wrist, and feet of tennis players to monitor the intensity of various parts of their movements.

The silicone tube was sourced from Dongguan Ruihong Silicone Products Co., Ltd. Conductive copper paint was acquired from Huizhou Jincaihong Technology Co., Ltd. The flexible plastic substrate was purchased from Guangdong Honghua New Materials Co., Ltd., and the flexible thermoplastic polyurethane (TPU) adhesive was obtained from Dongguan Yantai Chemical Technology Co., Ltd. Additional materials, including wires, hot melt adhesives, resistors, and capacitors, were procured from local shopping malls.

The design of the S-TENG device mainly relies on the flexible silicone tube, and the silicone tube serves as both the supporting material and the triboelectric material, as described in Fig. 1(a1). To form a triboelectric pair, we applied conductive copper paint to the surface of the silicone tube and the inner side of the tube wall, as illustrated in Figs. 1(a2) and 1(a3). In detail, after the conductive copper paint is injected into the silicone tube, it is dried in a drying oven to evaporate the conductive copper paint solvent inside the silicone tube. Then the conductive copper paint will be evenly distributed on the inner wall of the silicone tube, forming a conductive electrode. In addition, conductive copper paint is applied to the surface of the silicone tube through a brush, and the conductive copper paint on the inner wall surface of the silicone tube is introduced through an injector. Figure 1(b1) presents the picture of the silicone tube without copper paint. Figure 1(b2) exhibits the photo of the outer surface of the silicone tube coated with conductive copper paint. Figure 1(b3) demonstrates the photography of injecting conductive copper paint into the interior of a silicone tube using a syringe. The preparation of triboelectric layers, conductive layers, and support structures has been achieved through a simple and fast preparation method. Furthermore, the flexible plastic sheet will be cut into 2.5 × 4 cm² pieces as the substrate for the ST-TENG device and used to carry triboelectric materials and conductive electrode materials, as illustrated in Fig. 1(c1). Then, TPU adhesive is applied on the flexible plastic substrate surface to fix the silicone tube, as presented in Fig. 1(c2). Finally, the two components carrying silicone tubes are assembled together to form the ST-TENG device, as illustrated in Figs. 1(d1)–1(d3).

Figures 1(e) and 1(f) showcase the two components that constitute the ST-TENG device. In addition, Fig. 2(a1) displays the scanning electron microscope (SEM) image of the cross section of the silicone tube coated with conductive copper paint. Figure 2(a2) reveals the microstructure of the silicone and copper layers, showing that the copper layer is tightly adhered to the silicone tube surface. Figure 2(a3) illustrates the surface microstructure of the conductive copper layer after drying, where rough surface textures are evident. For electrical output testing, the ST-TENG is mounted on a mechanical motor that provides continuous power. A Keithley 6514 electrometer is utilized to measure the V_oc, I_sc, and Q_sc of the ST-TENG device.

Figure 2(b) illustrates the schematic diagram of the connection between the ST-TENG device and the external load, where each conductive electrode of silicone is interconnected with each other. The working mechanism of the ST-TENG can be simplified as a contact electrification model between the silicone tube layer and the conductive copper layer [Fig. 2(c)]. In the initial state [Fig. 2(c1)], when the silicone tube surface comes into contact with the copper surface under external force, the electrons will transfer from the copper layer on the bottom silicone tube surface to the top silicone tube surface. When the silicone tube surface separates from the copper surface under external force, electrons located in the copper layer on the inner wall of the top silicone tube will transfer to the copper layer on the surface of the bottom silicone tube, forming an induced current, as illustrated in Fig. 2(c2). When all positive charges in the copper layer on the surface of the bottom silicone tube are transferred, the induced current in the ST-TENG becomes zero, and the separation distance between the copper layer on the surface of the top and bottom silicone tubes reaches its maximum, as presented in Fig. 2(c3). When external force is applied again to the ST-TENG, electrons will transfer from the copper layer on the surface of the bottom silicone tube to the conductive copper layer on the inner wall of the top silicone tube, forming a reverse current, as shown in Fig. 2(c4). To evaluate the frictional electric pair combination advantage of silicone tube layer@copper, we measured the electrical output of the silicone tube layer and different triboelectric materials under the same mechanical conditions, as shown in Fig. 2(d). The V_oc of TENGs with different triboelectric pairs of silicon tube@copper, silicon tube@paper, silicon tube@PET, silicon tube@Kapton, and silicon tube@PTFE can reach 113.31, 99.99, 76.54, 62.95, and 72.23 V, respectively, as illustrated in Fig. 2(e). Hence, the triboelectric pair of silicone tube@copper exhibits excellent matching effect. The output performance of the TENG is determined by the triboelectric properties of the material, contact strength, and the material itself. Due to the differences in material strength and surface texture between the silicone tube we are using and the previous silicone film, the difference in the triboelectric sequence compared to before is caused by the different materials used, resulting in differences in output.^39,40

When silicone tubes are subjected to pressure, they deform, increasing the triboelectric contact area. Therefore, developing the output of the ST-TENG under different pressures is necessary. As described in Fig. 3(a), the V_oc of the ST-TENG can be 85.83, 96.79, 105.47, and 117.25 V when the external force is 2, 4, 6, and 8 N, respectively. Then, the I_sc of the ST-TENG can be 3.51, 4.55, 5.86, and 7.25 µA when the external force is 2, 4, 6, and 8 N, respectively, as presented in Fig. 3(b). Under the same changing conditions shown in Fig. 3(c), the Q_sc of the ST-TENG can reach 14.7, 21.06, 27.09, and 30.54 nC, respectively. When the ST-TENG is subjected to pressure, the silicone tube is flattened, resulting in an increase in contact efficiency and an increase in the ST-TENG output. In addition, ST-TENG sensors will face various environmental factors during use, among which the most important is the relative humidity of the environment. Relative humidity can affect the surface charge density of the ST-TENG and interfere with the performance of ST-TENG sensors. Hence, it is necessary to test the output of the ST-TENG in different humidity environments. Figure 3(d) illustrates that the V_oc of the ST-TENG can be 85.83, 110.1, 122.85, and 134.67 V when the relative humidity is 60%, 50%, 40%, and 30%, respectively. The I_sc of the ST-TENG can reach 2.42, 4.03, 4.98, and 6.25 µA when the environmental relative humidity is 60%, 50%, 40%, and 30%, respectively, as presented in Fig. 3(e). Furthermore, under the same relative humidity variation, the Q_sc of the ST-TENG can be 19.27, 25.93, 29.95, and 32.29 nC, respectively, as illustrated in Fig. 3(f). The high humidity environment causes a loss of triboelectric charges, which in turn significantly diminishes the output performance of the ST-TENG.

The rate at which the triboelectric layers make contact and subsequently separate can significantly affect the flow of electrons within external circuits. This dynamic interaction governs the efficiency of charge transfer between the layers, thereby influencing the overall performance and output of the triboelectric generator. The speed and frequency of these contact-separation events play a crucial role in determining the magnitude and stability of the generated electrical current. Hence, it is necessary to investigate the impact of different frequencies on ST-TENG output. When the frequencies provided by the mechanical system are 2, 3, 4, 5, and 6 Hz, the V_oc of the ST-TENG is maintained at around 122.51 V, according to results shown in Fig. 4(a). As presented in Fig. 4(b), the I_sc of the ST-TENG can reach 6.89, 9.77, 11.57, 12.87, and 15.05 µA when the contact and separation frequencies between the silicone tube layer and the copper layer are 2, 3, 4, 5, and 6 Hz, respectively. Moreover, the Q_sc of the ST-TENG can remain at ∼33.74 nC, as shown in the results in Fig. 4(c), when the motion frequency varies between 2 and 6 Hz. Moreover, the expansion of the separation gap between silicone layer and copper layer will also give the ST-TENG a higher output voltage, as shown in the results in Fig. 4(d). The 40 000 consecutive reliability tests have shown that the ST-TENG has excellent stability and demonstrates the potential for long-term operation [Fig. 4(e)]. In addition, after 20 days of placement, the conductive electrode still has good performance, which can be observed through output performance of the ST-TENG. The energy harvested by the ST-TENG can be stored in a small commercial capacitor through energy management methods [Fig. 4(g)]. According to charging curves shown in Fig. 4(h), small capacitors (1, 2, and 4.7 µF) with different capacitance values reflect different effects on ST-TENG energy harvesting; the smaller the capacitance, the faster the charging speed. Besides, the increase in working frequency from 3 to 5 Hz will bring faster energy output to the ST-TENG, as shown in the results in Fig. 4(i).

On the basis of capacitor energy storage circuits, we have designed a circuit system to power low-power electronic devices, such as temperature/humidity sensors [Fig. 5(a)]. The specific working method is to first close switch K₁ in the circuit and disconnect switch K₂, through which the ST-TENG charges capacitors. After the electric energy in the capacitor is sufficient, close switch K₂, and the temperature/humidity sensor will start working. Figure 5(b) illustrates the charging and discharging curves of the capacitor during the non-working and working processes of the temperature and humidity sensor. Due to its flexible characteristics, the ST-TENG can match human movements. Accordingly, we have established a tennis training system based on the ST-TENG to monitor the daily tennis training of athletes and utilize the sensing function of the ST-TENG, as illustrated in Fig. 5(c1). Specifically, we will install the ST-TENG on the wrist and inside the waist belt to obtain corresponding motion information, as presented in Figs. 5(c2) and 5(c3). Due to its unique tube structure, the ST-TENG generates relative friction between the silicone tube and the copper layer surface when subjected to external bending, resulting in sensing signals for different bending angles from 30° to 90°. Based on the results shown in Fig. 5(d), the V_oc of the ST-TENG can reach 68.08, 83.58, 102.77, 120.01, and 140.39 V for bending angles of 30°, 45°, 60°, 75°, and 90°, respectively. Similarly, under the same conditions, the I_sc of the ST-TENG can achieve 3.42, 4.89, 5.54, 6.42, and 7.59 µA, as shown in Fig. 5(e). Thus, the ST-TENG exhibits exceptional sensing abilities for detecting bending angles, making it ideal for monitoring human joint movements, especially in tennis players. The joint movements in tennis players are often critical to the accuracy of their technical skills. Furthermore, the ST-TENG installed at the wrist provides very clear feedback on the characteristics of tennis player’s wrist movement [Fig. 5(f)]. When the wrist shake amplitude is relatively large, the output performance of the ST-TENG will be significantly enhanced. Similarly, the ST-TENG installed on the inside of the belt can sense the range of waist movement during tennis player training. The magnitude of the movement amplitude can be determined based on the peak signal, and the frequency of waist movement can be monitored based on the signal frequency [Fig. 5(g)]. The ST-TENG installed inside the shoes can monitor the gait of tennis players and reflect the step information during tennis training [Fig. 5(h)]. This study verifies the efficient power generation performance of the ST-TENG based power generation devices under different bending angles and human motion states, and it demonstrates their practical application in tennis sports. Moreover, this sports application provides important reference for further application of the ST-TENG in wearable devices and health monitoring.

In summary, we developed an ST-TENG using a silicone tube aimed at harvesting mechanical energy and monitoring tennis training. The ST-TENG is constructed using a flexible silicone tube coated with conductive copper paint, forming the triboelectric pair that captures low-frequency mechanical movements and converts them into sustainable electrical energy. At a working frequency of 6 Hz, the ST-TENG demonstrated impressive performance, with a V_oc of 122.51 V, I_sc of 15.05 µA, and Q_sc of 33.74 nC. The innovative tubular structure allows for high sensitivity and accuracy in detecting pressure and bending, making it suitable for real-time motion monitoring. The ST-TENG can be integrated into athletes’ clothing and equipment, such as wristbands, belts, and shoes, to monitor movements in various body parts. This integration provides detailed motion data, which is crucial for developing personalized training plans to enhance athletic performance and reduce injury risks. The study highlights the potential of ST-TENGs to improve the accuracy and sustainability of sports monitoring devices, marking a significant advancement in the field of intelligent sports technology.

The authors have no conflicts to disclose.

Xu Deng: Conceptualization (lead); Data curation (lead); Formal analysis (lead); Funding acquisition (lead); Investigation (lead); Methodology (lead); Project administration (lead); Resources (lead); Software (lead); Supervision (lead); Validation (lead); Visualization (lead); Writing – original draft (lead); Writing – review & editing (lead).

The data that support the findings of this study are available from the corresponding author upon reasonable request.