The triboelectric sensor based on PDMS/SGO for human running posture and physical fitness health monitoring

ABSTRACT Recently, research about the intelligent sports equipment based on triboelectric sensor for athlete posture monitoring has made progress. However, the exploration of new triboelectric materials remains an important challenge to be faced. Hence, we ingeniously design a polydimethylsiloxane (PDMS)/silanized graphite oxide (SGO)-based triboelectric nanogenerator (PS-TENG) for running posture and physical health monitoring. The introduction of SGO has improved the electron capture capability of PDMS, thereby achieving high output of PS-TENG. From the results, under an 60 MΩ resistance, the power density of PS-TENG reaches a peak of 1780 mW·m−2. Furthermore, athletes undergo various postures during the running process, so monitoring special postures is very meaningful, such as jogging, long jump, walking, etc. Especially, according to the self-charging mechanism, PS-TENG can indirectly evaluate physical energy analysis during running by storing electrical energy in capacitors. This triboelectric sensor can assist in posture monitoring and physical fitness monitoring during running.


Introduction
Recently, intelligent wearable electronic devices oriented towards miniaturisation, integration, and lightweight have shown a vigorous development trend [1][2][3].Meanwhile, due to high manufacturing and maintenance costs, traditional forms of solid-state chemical batteries are no longer suitable for supplying power to small wearable devices [4,5].Thus, it is crucial to develop environmentally friendly energy harvesting technologies with low preparation costs and convenient long-term maintenance.Currently, green power generation technologies, such as piezoelectric generator [6], thermoelectric generator [7], electromagnetic generator [8], and triboelectric nanogenerator (TENG) [9], have achieved research results.Among them, TENG power generation technology can convert low-frequency vibration energy [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25], which is superior to traditional power generation technology.Especially in terms of human motion energy collection, TENG devices can harvest and convert lowfrequency mechanical energy generated by daily human movements [26][27][28][29], including running, walking, jogging, etc. into electrical energy.Due to the lowcarbon and environmentally friendly nature of the energy generated by human movement, TENGs have potential application value in wearable electronic devices and sensing [30].TENG devices follow the working mechanism of the coupling of contact electrification and electrostatic induction and can efficiently convert biomechanical mechanical energy into electrical energy [31][32][33].By designing a reasonable supporting structure, two types of triboelectric materials can come into contact and separate from each other under external forces, thereby generating electrical energy output.At present, various textile-based TENG devices have been reported, demonstrating rich application prospects in human wearable electronics [34][35][36][37].These TENG devices can generate electrical energy during human movement and are used to drive low-power electronic devices such as light-emitting diode (LEDs), temperature and humidity sensors, and electronic watches [38].Through reasonable structural design, TENG devices can be integrated into shoes, clothing, and the ground.In addition to harvesting human motion energy, they can also monitor human motion posture.Furthermore, it is worth noting that the performance of triboelectric materials is a key factor affecting the TENG devices' performance, such as the development of high dielectric performance materials.
As is well known, sports are a collective term for various activities that can enhance physical fitness.But it can only achieve the goal of strengthening the body with moderate exercise.When the amount of exercise is insufficient, the exercise effect cannot be achieved.When the amount of exercise is too high, it can cause injury or even sudden death.Therefore, real-time monitoring of the movement process is necessary to avoid risks during the movement process.Real time monitoring of motion processes can be achieved through these new technologies [39].Among them, the Internet of Things (IoTs) technology, as a new type of information technology with comprehensive perception, reliable transmission, and intelligent processing characteristics, is widely used in various environmental monitoring, such as residential and parking lots.By applying IoTs technology to sports monitoring, various intelligent wearable sensor devices can be used to monitor physical movements or various indicators reflecting exercise intensity in real-time during the exercise process, thereby preventing the occurrence of various sports injuries in a more safe and effective environment.Thus, developing new triboelectric materials and applying TENG devices to sports monitoring is an important way to promote the application of TENG devices.
Herein, we ingeniously design a polydimethylsiloxane (PDMS)/silanized graphite oxide (SGO)-based triboelectric nanogenerator (PS-TENG) for running posture and physical health monitoring.Normally, PDMS is regarded as a triboelectric material for electron capture.However, due to the limited dielectric properties, material doping is required to further improve its performance.The introduction of SGO has improved the electron capture capability of PDMS, thereby achieving high output of PS-TENG.In detail, the PDMS/SGO film and Nylon film form the functional layers.Under an 60 MΩ resistance, the power density of PS-TENG reaches a peak of 1780 mW•m −2 .The V oc and I sc of PS-TENG can arrive at 1254 V and 43.8 μA, respectively.After 120,000 continuous testing, the PS-TENG can also still maintain stable output, indicating excellent reliability and stability.Furthermore, athletes undergo various postures during the running process, so monitoring special postures is very meaningful, such as jogging, long jump, walking, etc.Furthermore, according to the self-charging mechanism, PS-TENG can indirectly evaluate physical energy analysis during running by storing electrical energy in capacitors, for example, in the three states of fast running, slow running, and walking, the storage of electrical energy may vary.

The preparation process of SGO
The preparation process of silane graphite oxide (SGO) includes three steps.First step, place a three necked flask containing 0.5 g of natural flake graphite, 2.0 g of phosphorus pentoxide, and 120 mL of concentrated sulphuric acid in an ice water bath, and continuously stir to add 15.0 g of potassium permanganate in 15 batches, with an interval of 30 min between each addition.Then, control the reaction temperature to react at 0°C for 24 h.After the low-temperature reaction is completed, transfer the three necked bottle to a 35°C water bath for 2 hours, and then add 230 mL of deionised water dropwise.After the intermediate temperature reaction, heat the water bath to 98°C and react for 15 min.After the high-temperature reaction is completed, slowly add hydrogen peroxide (30%) until the colour of the mixture changes from brown to bright yellow and there are no more bubbles generated.Finally, the reaction mixture is subjected to acid pickling, centrifugation, dialysis, ultrasonic dispersion and freeze drying to obtain black graphite oxide (GO) powder for standby.The second step is to weigh 0.5 g of graphite oxide in 30 mL of acetone and disperse it with an ultrasonic cell grinder.After 5 min, add 1.0 g of IPDI and 0.2 mL catalyst DBTDL, stir evenly with glass rod, pour into a 100 mL round bottom flask, and stir continuously for 10 h.Then, dried in a vacuum drying oven for 24 h.The black product obtained was recorded as the intermediate graphite oxide (MGO) for standby.The third step is to take 0.5 g of MGO and disperse it uniformly in 30 mL of acetone by ultrasound.Add 2.0 g of HO-PDMS and 0.2 mL DBTDL, and reflux at 60°C.After the reaction, remove the unreacted HO-PDMS with acetone centrifugal cleaning, and then dry it in a vacuum drying oven for 48 h to obtain greyish black silane graphite oxide (SGO) powder for standby.Fig. S1 in Supporting Materials shows the FTIR spectra of GO and SGO.According to results, the characteristic band of carboxyl groups in GO appeared at 1705 cm −1 .After interacting with the silane coupling agent, double peaks corresponding to symmetric and asymmetric vibrations of the -CH 2 -group appeared at 2928 cm −1 and 2847 cm −1 .In addition, it is expected that there will be -CH 2 -vibrations in the amino group at around 3305 cm −1 .Thus, GO successfully reacted with silane coupling agent.

The preparation process of PDMS/SGO composite film
As shown in Figure 1(a1-a3), PDMS/SGO composite film is cast by casting.Take a certain amount of SGO powder and place it in 15 g of n-heptane (300 W ultrasound for 10 min) to evenly disperse the SGO powder in n-heptane.To reduce the modulus of PDMS elastomer, and weigh the polymer and curing agent in a mass ratio of 14.87:1.Take 9.37 g of Sylgard 186 PDMS into a beaker, add SGO dispersion and 0.63 g of curing agent, stir evenly, vacuum defoaming, and then pour into a mould, as shown in Figure 1(a1,  a2).Cure at 60°C for 6 h to obtain PDMS/SGO composite film (thickness: 0.5 mm) with 15% SGO content, as shown in Figure 1(a3).

The preparation process of PS-TENG device
As illustrated in Figure 1(a4), a piece of conductive copper foil with wires is laid on the PDMS/SGO surface.Then, pour a layer of PDMS onto the surface of the copper foil as the packaging layer, as shown in Figure 1(a5, a6).Finally, assemble PDMS/ SGO@Conductive copper and Nylon/Conductive copper into PS-TENG devices, as illustrated in Figure 1(b).In this design, the PDMS/SGO film and Nylon film make the triboelectric pairs, as illustrated in Figure 1(c).The picture of PDMS/SGO film is demonstrated in Figure 1(d).Furthermore, the surface texture features of PDMS/SGO film and Nylon film can be present in Figure 1(e, f), where the rough surface is conducive to the storage of frictional charges.

Characterization and measurements
The PS-TENG electrical output were evaluated by mechanical motor with different forces and frequencies.The WTS electronic universal testing machine was used to measure the force signal.An electrometer (Keithley 6514) was used to measure the short-circuit current of PS-TENG and the voltage on the capacitor.The oscilloscope (SDS1202X-C) is used to test the open-circuit voltage of PS-TENG.

Results and discussion
The PS-TENG device (size: 2 cm × 3 cm) can be installed inside the shoe to obtain human motion energy, thus demonstrating the working mechanism of PS-TENG from the perspective of kinematics.The PS-TENG device installed inside the shoe can effectively achieve contact separation working mode.Figure 2 shows the detailed power generation mechanism of PS-TENG.More specifically, when the sole of the foot touches the ground, PS-TENG is in a compressed state, and the surface of PDMS/SGO film and Nylon film is in contact, with equal amounts of heterogeneous charges evenly distributed on the surfaces of the two layers of film, as shown in Figure 2(a).When athletes lift their legs, the PS-TENG device is in a relaxed state, PDMS/SGO film and Nylon film are separated, and the potential difference will drive current in the circuit, as shown in Figure 2(b).When the step is at its maximum state, the PS-TENG is in its maximum relaxation state, and there will be no current, as illustrated in Figure 2(c).When the sole of the foot touches the ground again, PS-TENG will be compressed again, causing the surface of PDMS/SGO film and Nylon film to come into contact with each other, generating reverse current in the circuit until it returns to its normal unstretched state (shown in Figure 2(d)).When the human body is running, the PS-TENG installed inside the shoes will generate continuous alternating current.
To systematically study the ability of PS-TENG devices to collect mechanical energy, we tested the output performance of PS-TENG under different pressure effects and mechanical frequencies.In detail, with a constant mechanical frequency of 3 Hz, when the force improves from 1 to 20 N, the open-circuit voltage of PS-TENG gradually increases from 626 V to 1036 V.And the short-circuit current of PS-TENG increases from 17.76 μA rises to 34.32 μA, as shown in Figure 3(a, b).The reason for this result is that higher pressure can bring more triboelectric charges, resulting in higher electrical output.Also, when the force is set to 20 N, considering the the low-frequency capture advantage of PS-TENG, as the mechanical frequency grows from 1 Hz to 4 Hz, the open-circuit voltage and short-circuit current of PS-TENG will increase from 284.4 V to 15.96 μA rises to 1254 V and 43.8 μA, as illustrated in Figure 3(c, d).Higher mechanical frequencies can transfer more charges in a shorter time, resulting in higher output voltage and current.
Furthermore, we measure the output of PS-TENG under different loads by connecting different resistors.Set the external pressure to 20 N and the mechanical frequency to 4 Hz.Connect loads with different resistance values to the circuit to test the output voltage of PS-TENG under different loads and calculate the output current.The experimental results show that the larger the external load resistance, the higher the output voltage of PS-TENG, while the output current shows the opposite trend, as shown in Figure 4(a).Moreover, the output power of PS-TENG under different resistance loads was calculated using P=U*I, as illustrated in Figure 4(b).The U represents the output voltage of PS-TENG under different loads, and I represents the output current of PS-TENG under different loads.From the results, with an external 60 MΩ resistance, the power density of PS-TENG reaches a peak of 1780 mW•m −2 , which is superior to previous research work [40][41][42].Next, to study the energy storage capability of PS-TENG, we use a rectifier circuit to store the electrical energy generated by PS-TENG in different capacitors, as present in Figure 4(c).From the charging curve results in Figure 4(d), the PS-   TENG exhibits good charging ability.This indicates that PS-TENG has great potential in supplying power to various small electronic products.
As is well known, sports are a collective term for various activities that can enhance physical fitness.But it can only achieve the goal of strengthening the body with moderate exercise.When the amount of exercise is insufficient, the exercise effect cannot be achieved.When the amount of exercise is too high, it can cause injury or even sudden death.The physical health status of runners is related to their gait, and monitoring their gait during running can indirectly reflect their physical health status.Hence, scientific exercise is necessary.In addition to issuing personalised exercise prescriptions to guide exercise, real-time monitoring of the exercise process is also necessary to avoid risks during the exercise process.Thus, the monitoring sensors for human running posture are particularly important.In this work, the PS-TENG can used to monitor various postures of the human body during running.In detail, the PS-TENG device can be installed at the shoe bottom.Figure 5(a) illustrates the voltage output signal (maximum peak value: 540 V) of PS-TENG when the athlete is in a jogging state.During jogging, the lifting and lowering of the feet are slightly slow, resulting in a symmetrical output signal.Figure 5(b) shows the output signal (maximum peak value: 398 V) of PS-TENG when the athlete stomps their feet.Furthermore, during the jumping movement, the output signal (maximum peak value: 360 V) of PS-TENG shows a different peak, indicating that the amplitude of each high jump is different for athletes, as shown in Figure 5(c).When an athlete steps in place, PS-TENG generates a clear impact signal (maximum peak value: 550 V), as the athlete lifts their feet faster, as shown in Figure 5(d).Figure 5(e) shows the output signal (maximum peak value: 510 V) of PS-TENG when the athlete is in a running state.Due to the vibration of the shoes themselves during running, there is a lot of noise in the output signal of PS-TENG.When the athlete is in a walking state, the output signal (maximum peak value: 380 V) of PS-TENG exhibits symmetrical characteristics and the signal noise is low, which is caused by the lower vibration of the shoes when the athlete is walking, as shown in Figure 5(f).We installed PS-TENG inside the shoes and evaluate the reliability of PS-TENG by testing the TENG output performance of athletes after different running hours, calculating the output voltage after different working cycles.Furthermore, PS-TENG exhibits excellent stability during prolonged working hours, as illustrated in Figure 5(g).
In addition, the electrical energy generated by PS-TENG device can be stored in capacitor (20 µF), which in turn supply power to low-power electronic device, for example, electronic watch, as present in Figure 6(a).From the charging/discharging curve in Figure 6(b), when the voltage of the capacitor reaches 2.6 V, close the switch and the electronic watch begins to operate.Furthermore, when the switch is turned on, the capacitor is charged until the voltage reaches 2.6 V again.When the switch is closed, the electronic watch starts working again.This charging feature of PS-TENG can be used for physical fitness monitoring during running.When athletes are running, jogging, and walking, there will be differences in the energy stored by capacitor (2 µF) owning to the different power generation effects of PS-TENG under different motion states, as shown in Figure 6(c).Further calculations can obtain the energy stored by the capacitor within the same time period.From the results in Figure 6(d), through calculating W=CU 2 /2, the capacitor can store 42.25 µJ, 79.57µJ, and 195.44 µJ, when athlete is run fast, jogging, and walking.

Conclusion
In summary, we ingeniously design a polydimethylsiloxane (PDMS)/silanized graphite oxide (SGO)-based triboelectric nanogenerator (PS-TENG) for running posture and physical health monitoring.By introducing SGO, the triboelectric electrical performance of PDMS is improved, thereby enhancing the output capability of TENG device.In detail, the PDMS/SGO film and Nylon film form the functional layers.Under an 60 MΩ resistance, the power density of PS-TENG reaches a peak of 1780 mW•m −2 .The V oc and I sc of PS-TENG can arrive at 1254 V and 43.8 μA, respectively.Also, the PS-TENG can used to monitor various postures of the human body during running, such as, jogging, long jump, walking, etc.Furthermore, according to the self-charging mechanism, PS-TENG can indirectly evaluate physical energy analysis during running by storing electrical energy in capacitors, for example, in the three states of fast running, slow running, and walking, the storage of electrical energy may vary.This study aims to investigate the real-time monitoring of sports information that will be used for runners in the future, which will help prevent accidents such as sudden death caused by physical health status.At the same time, it can achieve the evaluation of athletes' physical fitness and reasonable arrangement of exercise volume.

Figure 1 .
Figure 1.(a1-a6) the preparation process of PDMS/SGO@Conductive copper film.(b) the diagrammatic sketch of PS-TENG device.(c) the structural configuration of triboelectric pairs.(d) the picture of PDMS/SGO film.The SEM image of (e) PDMS/SGO film surface and (f) Nylon film surface.

Figure 2 .
Figure 2. (A-d) the working principle of PS-TENG device.

Figure 4 .
Figure 4. (a) the output voltage and current of PS-TENG with various resistances.(b) the output power of PS-TENG under different resistances.(c) the equivalent-circuit model of PS-TENG charging for capacitor.(d) the charging curves of PS-TENG charging for different capacitors.

Figure 3 .
Figure 3. (a) the open-circuit voltage and (b) short-circuit current of PS-TENG under different force.(c) the open-circuit voltage and (d) short-circuit current of PS-TENG under mechanical frequency.

Figure 5 .
Figure 5. (a) the output voltage signal of PS-TENG when the athlete is in various motion postures, such as (a) jogging, (b) stomp, (c) jumping, (d) marking time, (e) running, and (f) walking.(g) reliability study of PS-TENG under prolonged continuous working conditions.

Figure 6 .
Figure 6.(a) the equivalent-circuit model of PS-TENG charging for electronic.(b) the charging/discharging curve of 20 µF capacitor for powering the electronic watch.(c) the charging curves of 2 µF capacitor under run fast, jogging, and walking.(d) Electric energy stored in 2 µF capacitor under different sports states of athletes, such as run fast, jogging, and walking.