A flexible triboelectric sensor based on P(VDF-co-HFP)/MXene for breath and posture monitoring in basketball motion

ABSTRACT

Recently, sports monitoring sensors based on flexible wearable technology have attracted much attention. Here, we reported a P(VDF-co-HFP)/MXene-based triboelectric nanogenerator (PM-TENG) to harvest bio-mechanical energy. The introduction of MXene can significantly improve the dielectric constant of P(VDF-co-HFP), thereby achieving higher electron harvesting ability. The PM-TENG can obtain the maximum instantaneous power of 1.68 mW contacted with a resistance of 4 MΩ. Furthermore, the PM-TENG can be also integrated inside the mask to monitor changes in respiratory status before and after basketball exercise. Meanwhile, the PM-TENG installed inside the shoes can be used to distinguish different gaits in basketball, which will be used to assist in basketball training. This flexible sport sensor demonstrates potential application value in basketball training assistance.

KEYWORDS:

• Triboelectric nanogenerators (TENGs)
• self-powered sensor
• intelligent sport
• basketball training
• human posture

Introduction

It is worth noting that intelligent wearable devices based on flexible electronic technology play an important role in fields such as human posture monitoring, biomedicine, human-machine interaction, and distributed sensor networks [1–3]. However, the development of flexible wearable electronic devices is under pressure from power supply [4]. In other words, with the diversification of wearable electronic devices’ functions, the demand for electrical energy is increasing, and the limited working hours seriously affect the use of portable devices. Usually, small intelligent devices rely on traditional chemical batteries for power supply, but chemical batteries have a limited lifespan and can generate additional environmental pollution [5–8]. Nevertheless, the chemical battery is still the central power supply device of small intelligent electronic devices, which will restrict the development of small electronic equipment [9,10]. In addition, smart wearable intelligent electronic devices have potential application value in sports monitoring, so it is significantly meaningful to develop flexible electronic devices. To better promote the application of wearable electronic monitoring devices in the sports field, it is crucial to improve flexible power sources with high power generation efficiency [11,12]. It is worth noting that human movement generates usable mechanical energy. If advanced energy harvesting technology is used to convert this mechanical energy into electrical energy, it will promote the development of wearable electronic devices [13]. So far, various power generation effects have been applied in the field of power generation technology, such as electromagnetic effects, photoelectric effects, temperature difference effects, piezoelectric effects, and triboelectric effects [14]. Among them, triboelectric nanogenerator (TENG) based on contact electrification and electromagnetic induction principle is considered as a advanced mechanical energy harvesting methods [5,15–24].

The flexibility, simple preparation, high stability, and low cost of the TENG device endows TENG device with rich types of working structure. Many optimization methods can enhance the output performance of TENG devices, including dielectric regulation, nanocomposite materials, research and development of new triboelectric materials, and diverse structural designs [25,26]. Among them, developing frictional materials with high dielectric constants is a crucial way to improve the output performance of TENG devices. By optimizing the dielectric constant of materials, the frictional charge on the surface of frictional materials can be increased [27–29]. According to previous work [30], the triboelectric material dielectric constant is a crucial factor influencing the TENG electrical output. Recently, triboelectric materials based on two-dimensional materials such as MXene and graphene oxide have been reported, and research results indicate that MXene is a potential triboelectric material for preparing TENG devices [31,32]. MXene has excellent mechanical and electrical properties, which endows TENG devices with excellent durability. Besides, P(VDF-co-HFP) thin films with high dielectric properties are considered fine dielectric materials and have advantages in biocompatibility and mechanical reliability [33]. Recently, MXene has been introduced as a doping material into triboelectric materials, including polyvinylidene fluoride (PVDF), polydimethylsiloxane (PDMS), polyvinyl alcohol (PVA), and other materials to enhance their triboelectric properties [34–36]. One potential method is to introduce MXene into P(VDF-co-HFP) to achieve the preparation of high-performance triboelectric materials.

Here, we propose a P(VDF-co-HFP)/MXene-based triboelectric nanogenerator (PM-TENG) to harvest bio-mechanical energy. The P(VDF-co-HFP)/MXene film and PET film form the triboelectric pairs. Additionally, the PM-TENG can be integrated into masks and shoes for monitoring respiratory status and movement posture during basketball training. The introduction of MXene can significantly improve the dielectric constant of P(VDF-co-HFP), thereby achieving higher electron harvesting ability. The PM-TENG can obtain the maximum instantaneous power of 1.68 mW contacted with a resistance of 4 MΩ. From the results, the PM-TENG can generate high performance, including open-circuit voltage (V_oc, 225 V), current density (J_sc, 110 mA/m²), and transfer charge (Q_sc, 180 nC). Moreover, the PM-TENG can be also integrated inside the mask to monitor changes in respiratory status before and after basketball exercise. Meanwhile, the PM-TENG installed inside the shoes can be used to distinguish different gaits in basketball, which will be used to assist in basketball training.

Experimental section

Fabrication process of MXene powder

Firstly, to prepare the etching solution, 20 mL hydrochloric (9 mol) acid and 1 g of lithium fluoride were mixed, and then, stir them at 300 rpm for 10 min. Subsequently, 1 g of Ti₃AlC₂ powder was added to the etchant solution, stirring was carried out through magnetic stirring for one day (Temperature: 35°C). Afterwards, use centrifugation technology and deionized water to wash the acidic solution until the pH reaches greater than 6. Then, the obtained precipitate was placed in ionic water and the mixture was subjected to ultrasonic treatment, ultimately obtaining a multi-layer Ti₃AlC₂ sample. Finally, centrifuge the mixed solution in a nitrogen environment to obtain a clear upper layer solution. Place the sample in an oven for drying treatment (Temperature: 75°C, time: 25 h) to obtain MXene powder.

Preparation of P(VDF-co-HFP)/MXene film

Firstly, dry MXene powder was subjected to ultrasonic treatment at 88% amplitude in N-dimethylformamide (DMF) for 75 min to obtain a uniformly suspension. Next, centrifuge the solution (rate: 9000 rpm, time: 15 min) to obtain MXene precipitate. Mix the harvested MXene sediment with 16% (w/v) P(VDF-co-HFP) in ACT/DMF (2:3) solution to form a P(VDF-co-HFP)/MXene solution with a concentration of up to 20% (w/v), and stir evenly for 2 h (Figure 1(a)). Subsequently, spin coat the solution on the surface of copper foil to prepare P(VDF-co-HFP)/MXene film.

Figure 1. (a) the molecular structure schematic diagram of P(VDF-co-HFP)/MXene’s synthetic composition. (b) schematic diagram of PM-TENG’s structure. (c) the SEM image of P(VDF-co-HFP)/MXene film surface. (d) the physical picture of PM-TENG device. (e) XRD analysis of P(VDF-co-HFP) under different MXene contents. (f) XPS spectrum of P(VDF-co-HFP) composite material with 10% MXene.

Preparation of PM-TENG device

We use two plastic substrates and flexible plastic film as the PM-TENG support structure. On the top substrate, stick a layer of conductive copper foil electrode as the induction electrode for PM-TENG device. Next, a piece of PET is pasted on the electrode surface as a positive triboelectric material. Paste a layer of conductive copper foil electrode on the bottom substrate as the induction electrode for PM-TENG devices. Next, the prepared P(VDF-co-HFP)/MXene film is pasted on the surface of the induction electrode as a negative triboelectric material. It is remarkable that before pasting the frictional layer, the wire is connected to two electrodes to achieve electrical output. The structure configuration diagram of PM-TENG device can be shown in detail in Figure 1(b).

Electrical measurement and characterization of the PM-TENG

Figure 1(c) shows the scanning electron microscopy (SEM) image of P(VDF-co-HFP)/MXene film surface. Besides, the physical image photo of PM-TENG is present in Figure 1(d). An oscilloscope equipped with a high impedance probe is used to test the V_oc of PM-TENG devices. The I_sc of PM-TENG is measured by using SR570 current preamplifier, and transferred charge of PM-TENG is measured by using Keithley 6514 electrometer. A mechanical vibrator used to provide quantitative motion for PM-TENG device.

Results and discussion

Figure 1(e) demonstrates several peaks inside the P(VDF-co-HFP)/MXene polymer when MXene is added to P(VDF-co-HFP). Based on the specific characteristics of the peaks, MXene will reduce the crystallinity of P(VDF-co-HFP). According to the XPS spectrum of the C1s region in Figure 1(f), the five peaks correspond to chemical bonds such as C-Ti, C-C, C-O, C-F, and C-F₂, respectively. PM-TENG device is composed of two different materials, cathode and anode, which show different electron affinity during the contact process. Under the triboelectric process, the negative triboelectric material attracts electrons, while the positive triboelectric material loses electrons, as illustrated in Figure 2. The working mechanism of PM-TENG is as follows: firstly, the negative triboelectric material and the positive triboelectric material are separated and maintained in a neutral state, as shown in Figure 2(a1). When the external force is applied, P(VDF-co-HFP)/MXene film comes into contact with PET film, as shown in Figure 2(a2). Due to the triboelectric effect, charge transfer occurs in the contact area between P(VDF-co-HFP)/MXene film and PET film, resulting in the formation of PET film with positive surface charge and P(VDF-co-HFP)/MXene film with negative surface charge. After removing the external force driving the PM-TENG device, due to the separation distance between PET film and P (VDF-co-HFP)/MXene film, a potential difference is generated, resulting in the generation of current in the circuit, as present in Figure 2(a3). When the separation distance between PET film and P(VDF-co-HFP)/MXene film is large enough, no current will be generated in the circuit, as shown in Figure 2(a4). When PET film and P(VDF-co-HFP)/MXene film are driven close to each other again, a reverse current will be generated in the circuit, as shown in Figure 2(a5).

Figure 2. (a) structural diagram of PM-TENG working principle. (b, c) the relationship between output voltage/transfer charge of PM-TENG and separation distance simulated by using COMSOL software. (d) the simulated voltage of PM-TENG under different thickness triboelectric layer. (e, f) the P(VDF-co-HFP)/MXene polymer dielectric constant. (g) the formation of microcapacitors and conducting channels.

Furthermore, we used COMSOL finite element simulation software to study the influence of material size on TENG device performances. According to the simulation results (the inset of Figure 2(b)), the potential distribution of PM-TENG can be seen, which shows the potential difference between PET film and P(VDF-co-HFP)/MXene film in the PM-TENG device. When two triboelectric materials come into contact with each other, the electric potential approaches zero. However, as the separation distance increases (0.1 mm ~5 mm), the potential difference of PM-TENG also increases, as shown in Figure 2(b). In addition, when the gap between PET film and P(VDF-co-HFP)/MXene film increases, the total transfer charge on the electrode will also increase, as illustrated in Figure 2(c). When the distance between two dielectric thin films increases from 0 mm to 1.5 mm, the transfer charge maintains an increasing trend. When the distance between two dielectric films is greater than 2 mm, the simulation results show that the transfer charge will reach saturation. Thus, when the separation distance between the two dielectric thin films is 2 mm, the charge transfer rate of the system will reach its maximum value. Therefore, in the following simulation, a maximum separation distance of 2 mm is selected as the condition to avoid the impact of separation distance on the output performance of TENG devices. Obviously, the thin dielectric film will produce a stronger electrostatic induction effect, leading to improved output performance [37]. Hence, we studied the influence of dielectric layer thickness on output voltage through simulation methods. According to the results (Figure 2(d)), when the dielectric film thickness is 55 μm, the TENG can achieve optimal performance. One of the key characteristics of PM-TENG output performance is the dielectric constant, so increasing the dielectric constant of PM-TENG triboelectric materials can improve the output performance. Figure 2(e,f) shows the P(VDF-co-HFP)/MXene dielectric constants felt at different filler concentrations and the variation of dielectric loss with frequency. As the concentration of MXene from 0 to 20% in P(VDF-co-HFP)/MXene increases, the P(VDF-co-HFP)/MXene dielectric constant can gradually increase. The dielectric loss can also increase. The P(VDF-co-HFP)/MXene dielectric constant is about four times that of pure P(VDF-co-HFP), owning to the micro-capacitance effect. When MXene reaches the threshold fraction, a conductive network is designed within the polymer matrix. At this point, the polymer pad will change its phase with the conductor. When an electric field is applied, charges accumulate on the surface between MXene and P(VDF-co-HFP) polymer matrix, generating microscopic dipoles (Figure 2(g)). As the P(VDF-co-HFP) content increases, the interlayer spacing of MXene expands, and polymer chains are inserted between MXene nanosheets.

Compared to the triboelectric material of PVDF, the P(VDF-co-HFP) and MXene can enhance PM-TENG’s output performance. According to results (Figure 3(a-c)), the output of TENG (PVDF@PET) can obtain the lowest outcome, and the electrical outcome of TENG (P(VDF-co-HFP(20%))/MXene@PET) can reach the highest performance. Thus, the P(VDF-co-HFP)/MXene enhances the TENG performance by improving the triboelectric performance. The adjustable mechanical vibrator (frequency: 6 Hz, force: 7 N) is used to drive PM-TENG to generate electricity, and the working mode of PM-TENG devices is based on the contact separation model. In the design of PM-TENG devices, P(VDF-co-HFP)/MXene film acts as the negative triboelectric part, and PET film acts as the positive triboelectric part. Figure 3(c-e) shows the open-circuit voltage, circuit density, and transfer charge of the PM-TENG device. From the results, the PM-TENG can get the Voc is 225 V, the Jsc is 110 mA/m2, and the Qsc is 180 NC. Figure 3(f,g) illustrates the specific Voc and Isc signals, respectively. Figure 3(h) shows the influence of MXene with different concentrations on electrical output performance. After adding MXene to P(VDF-co-HFP), the output voltage of PM-TENG increased by more than four times. However, with the increase of MXene content, the triboelectric performance of P(VDF-co-HFP)/MXene decreases, leading to a decrease in output performance. Thus, the optimal content of MXene in P(VDF-co-HFP)/MXene is set at 10%.

Figure 3. (a–c) the electrical output of four TENGs based on PVDF@PET, PVDF/MXene@PET, P(VDF-co-HFP(10%))/MXene@PET, and P(VDF-co-HFP(10%))/MXene@PET. The (d) V_oc, (e) J_sc, and (f) Q_sc of PM-TENG. The output signal of (f) V_oc and (g) J_sc of PM-TENG. (h) the influence of MXene content on the output performance of PM-TENG device.

Figure 4(a) illustrates the influence of the content of MXene on the charge density of PM-TENG. The experimental results show that when the content of MXene is 10%, PM-TENG can achieve the best output performance. To analyse the effect of P(VDF-co-HFP)/MXene film thickness on the PM-TENG output performance, different P(VDF-co-HFP)/MXene film thicknesses from 0.01 mm to 0.1 mm were tested and the PM-TENG output voltage was checked. The P(VDF-co-HFP)/MXene film with different thicknesses can influence the output voltage (Figure 4(b)). The output voltage of PM-TENG gradually increases from 118 V to 324 V and reaches its maximum value at 0.05 mm P(VDF-co-HFP)/MXene film thickness. Also, when the thickness of P(VDF-co-HFP)/MXene film grows from 0.06 mm to 0.1 mm, the open-circuit voltage of PM-TENG decreases. This result confirms that PM-TENG with a P(VDF-co-HFP)/MXene film thickness of 0.05 mm exhibits the best output performance. Figure 4(c,d) shows the V_oc and J_sc response as a PM-TENG operating frequency from 1 Hz to 9 Hz under the condition of applying a 7 N force. The experimental results indicate that under low frequencies from 1 Hz to 6 Hz, the output voltage of PM-TENG is proportional to the operating frequency. When increasing the impact frequency by using an electric vibrator, the Voc increases from approximately 28.8 to 324 V due to the maximum surface charge accumulated on P(VDF-co-HFP)/MXene film surface, and the Jsc increases from approximately 8.4 mA/m2 to 168 mA/m2. It is worth noting that at higher mechanical frequencies, the triboelectric charges cannot be quickly dissipated, resulting in a decrease in the transfer charge in the electrode, thereby reducing the output performance of PM-TENG. External forces can also affect the PM-TENG device’s performance, as present in Figure 4(e). However, when the applied external force is greater than 9N, the electrical output of the PM-TENG device will remain stable.

Figure 4. (a) the influence of MXene content on the Q_sc of PM-TENG. (b) the influence of P(VDF-co-HFP)/MXene film thickness on V_oc of PM-TENG. (c, d) the influence of working frequency on V_oc and J_sc of PM-TENG device. (e) the influence of external force on V_oc of PM-TENG device.

Figure 5(a) illustrates the electrical output (output voltage and current) of PM-TENG under various loads, and the experimental results indicate that the PM-TENG electrical output is based on the composite Ohm’s law. The comparison of PM-TENG peak power and power density for different externally connected loads is expressed in Figure 5(b). When a resistor with a resistance value of 4 MΩ is connected to a PM-TENG device, the instantaneous maximum output power of PM-TENG will reach 1.68 mW, and its rectified voltage will be 82 V (Figure 5(c)). The output performance of PM-TENG, including Voc and Isc can maintain stable output values over 20,000 cycles at a frequency of 6 Hz (Figure 5(d,e)). The results indicate that TENG’s output performance is stable, that is, there is no measurable degradation during long-term operation, and it indicates that PM-TENG has excellent durability. Moreover, the PM-TENG can power the hygrometer by using a commercial capacitor (24 μF), as shown in Figure 5(f). Furthermore, the PM-TENG can be also integrated inside the mask to monitor changes in respiratory status before and after basketball exercise, as shown in Figure 5(g). Figure 5(h) shows the working principle for respiratory monitoring of PM-TENG. In detail, when a person breathes, it drives the collision and friction between the P(VDF-co-HFP)/MXene film and the mask cloth, leading to a voltage signal that reflects the breathing situation. Figure 5(i) shows the breathing situation of basketball players before and after exercise, which can clearly distinguish the changes in breathing, which will help analyse the physical fitness changes of athletes during basketball training.

Figure 5. (a, b) the dependence of output (voltage, current, power) of PM-TENG on different resistances. (c) the output voltage signal of PM-TENG with a match load of 4 MΩ. (d, e) stability testing for long-term work of PM-TENG. (f) the charging/discharging curves of a 24 μF capacitor for powering the humidity/temperature sensor driven by PM-TENG device.

Basketball, as a skill-oriented adversarial sports event, requires a high level of skill from basketball players. Among them, basketball gait analysis can be a crucial indicator for improving the posture of basketball players. Hence, it is necessary to develop wearable sports sensors suitable for monitoring the gait of basketball players. In this design, we used a PM-TENG device to serve as the motion sensor for basketball posture monitoring, as present in Figure 6 (a1-a3). Specifically, the PM-TENG device is integrated into the shoes. And when basketball players exercise, it generates corresponding voltage signals to provide feedback on their posture, as illustrated in Figure 6(b). The various athlete’s postures (including walking, running, and jumping) can be determined by PM-TENG’s output signals, such as the peak and frequency characteristics of the output signals, as shown in Figure 6(c–e). Based on this sensing information, the training effect can be summarized and analysed after training to achieve efficient basketball training. Figure 5(f) presents the posture information under continuous motion, which can clearly distinguish the human motion posture based on the sensing signal. Thus, the PM-TENG can play the role of basketball posture monitoring.

Figure 6. (a1-a3) Photos of athletes doing shooting sports. (b) The output signal of PM-TENG installed inside the shoe. (c-e) The PM-TENG voltage output signal under various motion posture. (f) Sensing signals of PM-TENG under basketball players in continuous motion

Conclusion

In summary, we reported a PM-TENG based on P(VDF-co-HFP)/MXene. Compared to the PVDF, the P(VDF-co-HFP)/MXene has high dielectric properties, which bring TENG device higher output performance. The P(VDF-co-HFP)/MXene film and PET film form the triboelectric pairs. The introduction of MXene can significantly improve the dielectric constant of P(VDF-co-HFP), thereby achieving higher electron harvesting ability. According to results, the PM-TENG can obtain the maximum instantaneous power of 1.68 mW contacted with a resistance of 4 MΩ. The experimental results indicate that PM-TENG can provide high output performance, including open-circuit voltage (225 V), current density (110 mA/m²), and transfer charge (180 nC). Furthermore, the PM-TENG can be also integrated inside the mask to monitor changes in respiratory status before and after basketball exercise. Meanwhile, the PM-TENG installed inside the shoes can be used to distinguish different gaits in basketball, which will be used to assist in basketball training.

Disclosure statement

No potential conflict of interest was reported by the authors.