Human Body Electrode Enabled Direct Current Triboelectric Nanogenerator for Self‐Powered Wireless Human Motion and Environment Monitoring

Triboelectric nanogenerator (TENG) is a promising technology, which can convert biokinetic energy into electricity and be utilized as self‐powered sensors and power sources for wearable electronics. The existing designs of conventional TENGs require complex fabrication processes and device structures, and they need to be attached on human body for wearable application, which is uncomfortable and may lead to malfunction under intense body moment. Here, a direct current TENG is proposed by utilizing natural human body, basketball, shoes, and ground floor. A unidirectional peak voltage and current output up to 700 V and 23 µA can be generated when a player plays a basketball, which can lighten up an array of 240 LEDs, and charge a 100 µF capacitor to 3.2 V in 1 min. The output of TENG is utilized to identify different movements of a basketball player using machine learning algorithm with an accuracy up to 96.7%. Moreover, the human body enabled direct current TENG (HBDC‐TENG) is used as a self‐powered sensor and an energy harvester for a wireless sensing system, which can collect human motion and environmental information, and transmit them wirelessly. The HBDC‐TENG has a great significance for self‐powered wearable electronics, providing a viable solution for human motion status and ambient environment monitoring.


Introduction
With the development of the Internet of Things, wireless wearable electronics are attracting more and more attention, which DOI: 10.1002/aelm.202300713plays a pivotal role in motion monitoring, rehabilitation, healthcare and medicine, etc. [1][2][3] At present, the power supply of wearable electronics is still mainly based on batteries, which has many disadvantages including environmental pollution, limited lifetime, high maintenance costs, etc. [4,5] Energy harvesting technology is one of the promising approaches to powering IoT owing to the advantages including low cost, environmentally friendly technique, etc.The ambient environmental energy can be harvested based on electromagnetic effects, [6] piezoelectric effect, [7,8] thermoelectric effect, [9,10] photovoltaic effect, [11] and triboelectric effect. [12,13]Among them, the triboelectric nanogenerator (TENG) is an effective energy harvester with many merits such as small size, high energy output, and long durability, and can be used for powering IoT and wearable electronics. [14,15]s shown in Figure 1a(i), TENGs have already been widely utilized as energy harvesters in wearable electronics.Conventional TENGs can be integrated into various places in humans for energy harvesting for wearable applications.For example, Lin et al. proposed a downy-structure-based TENG, which can be used to harvest arm swinging energy realizing self-powered heart-rate monitoring. [16]Lin et al. proposed a triboelectric TENG embedded in the insoles to effectively collect the energy from human walking and running, which is used for the sustainable operation of temperature sensors, heart rate monitoring devices, pedometers, etc. [17] Hao et al. have tried to integrate a natural biodegradable wood-based TENG into the floor to harvest walking energy for applications such as self-powered doorbell. [18]t is clear that all the existing designs of TENGs have limitations.[21] Second, complex device structures are needed for converting mechanical energy into electric energy.Third, they need to be attached on human body, which leads to uncomfortable under intense motion, and some of the designs even need to make modifications to existing surrounding objects such as floor, which is not cost-effective.Overall, the complexity of existing designs hinders the employment of TENGs in practical application.TENG also has been found promising for monitoring behaviors of human body with the advantage of a wide range choice of materials.By selecting suitable materials, stretchable TENG with coaxial structure can be designed for smart electronic skin from various biodegradable materials. [22,23]ompared with other sensors, it can not only monitor physical activities, but also collect the biomechanical energy generated by human movement in various deformation states and operate in a self-driven sensing mode.However, performance of most current TENG-based sensors is seriously affected by the choice of materials.On the other hand, there exist a lot of intrinsic triboelectrification effects on human body during movement.For example, triboelectrification occurs between ball-ground and shoeground when a person plays a basketball and it also occurs between body and cloths, between cloths themselves due to the body motion.These are natural tribo-materials and intrinsic triboelectrification, and could be the ideal tribo-material combinations for developing wearable TENG.
In this work, a human body enabled direct current TENG (HBDC-TENG) is proposed as shown in Figure 1a(ii), which only utilizes the components in a basketball player to construct the TENG.Human hand, basketball, and ground floor are employed as tribo-materials, while the human body is utilized as an electrode and conductive path for collecting and transmitting electric energy.While dribbling, a high direct current (DC) electrical output with a peak voltage and current up to ≈700 V and ≈23 μA can be generated, respectively, which can spontaneously lighten up an array of 240 light emit diodes (LEDs), and charge up a 100 μF capacitor to ≈3.2 V in 1 min.HBDC-TENG is exploited to power a wireless human motion and environment monitoring system and wirelessly transmitted to a mobile phone.Besides, a basketball player movement identification system is developed to distinguish the player behavior.This work provides an innovative way to remotely monitor environment and motion of the basketball player, recognizing athletic status of the basketball player, demonstrated good application prospect.

Working Mechanism of HBDC-TENG
The utilized triboelectric materials and components for forming the HBDC-TENG are the human body, a basketball, and the ground floor.The person wears a pair of shoes which isolates the body from the earth ground, otherwise the body is grounded.Basketballs are normally made of rubbers, and its electron affinity is between those of the human body and ground floor in the triboelectric series.Human body is normally on the positive side of the triboelectric series, while that of floor materials such as concrete and wooden are on the negative side of the triboelectric series relative to the human body.
The working principle of the HBDC-TENG under open circuit condition is schematically shown in Figure 1b.In the initial stage i, the ball is starting to move down.In stage ii, the ball fully contacts insulator (wooden floor) on the ground, and the basketball and wooden floor are positively and negatively charged, respectively.In stage iii, the positively charged basketball moves upwards and contacts the hand.As the electron affinity of the basketball is larger than that of body, the negative charges in the hand are transferred to the basketball to neutralize the positive charges in the basketball.The body is positively charged due to the loss of negative charges.In stage iv, the basketball contacts the wooden floor again, and the same number of negative charges are transferred from the basketball to the wood floor as shown in stage ii.In stage v, as in stage iii, the positive charges in the basketball are neutralized again due to the charge transfer between the basketball and the hand.The working principle of the HBDC-TENG under short circuit condition is detailed in Note S1 (Supporting Information).In summary, the charges are gradually accumulated both on the hand and insulator under open circuit condition, while the charge accumulation only occurs on the insulator under short circuit condition.To validate the proposed working mechanism of HBDC-TENG under short/open circuit conditions, the experimentally measured charge accumulation curves in the hand and wooden floor are shown in Figure 1c(i,ii), respectively.It is clear that charge accumulation only occurs in the wooden floor (insulator) under the short circuit condition (100 kΩ load between hand and earth ground), while the charges are accumulated both on the hand and insulator under the open circuit condition.To further verify the charge accumulating phenomenon, the basketball is dribbled multiple times without connecting to the circuit.During this experiment, charges are accumulated in the human body gradually.The potential of the body is then measured by using an oscilloscope with a 100 MΩ probe.The peak voltage output after 5-30 times dribbling is recorded as shown in Figure S2 (Supporting Information), which increases from 500 to 2300 V when dribbling time increases from 5 to 30 times.A 1000 V peak output voltage can be measured after dribbling ten times as shown in Movie S1 (Supporting Information), verifying the charge accumulating in the human body under the open circuit condition and complete isolation of body from the earth ground.It should be noted that although the output of HBDC-TENG can reach thousands of voltages, the peak current output is very limited at tens of μA maximum, which is far less than the lowest safe current for human body (10 mA, IEC 60479-1), thus no harmful impact will happen to human body.
Theoretically, a DC voltage output can be measured from HBDC-TENG as shown in Figure 1b.To verify that, the experimental and simulated voltage outputs under a 100 MΩ load are compared as illustrated in Figure 1d, showing a high similarity.It should be noted that the HBDC-TENG generates a DC voltage output, which results from a specific electron affinity combination of the human body, basketball, and floor type.A lower peak and a higher peak in Figure 1d are generated in stage (ii) and (iii), respectively, Movie S2 (Supporting Information) demonstrates a detailed relationship between the movement of HBDC-TENG and voltage output.The discrepancy between the voltage curves of the experimental and simulated results is attributed to two reasons: 1) Due to the finite input impedance of the electrometer and charge emission to ambient air, the accumulated charges attenuate gradually after each contact, therefore, the charge leakage is inevitable in the experiment as shown in Figure S3i (Supporting Information).However, the simulation ignores charge leakage as shown in Figure S3ii (Supporting Information).2) The difference in actual and simulated configured tribe-material properties can also lead to voltage output discrepancy.
Finite element model-based simulation is carried out using COMSOL Multiphysics to validate the proposed working principle.As shown in Figure S3ii (Supporting Information), for simulation, the accumulated charge in the hand and wood have nearly the same trends and values as the experimental results.The detailed simulation parameters are listed in Table S1 (Supporting Information) and the model settings are shown in Figure S4 (Supporting Information).As shown in Figure 1e, the electrical potential distributions are simulated corresponding to the four stages (Figure 1b-ii-v) under an open circuit condition.It should be emphasized that the human body must be completely isolated from the earth ground, otherwise the charges accumulated in the body will escape to the ground, and no or very small electrical potential could be built up in the body.
In summary, the HBDC-TENG must have three different tribomaterials (i.e., the human body, basketball, and floor material) with different electron affinities.To achieve a unidirectional output, the electron affinity of the basketball must between of the human body and floor material.Thus, the basketball is acting like an electron pump, which can pump electrons from the human body to the floor material, achieving a unidirectional transfer of electrons.Overall, the HBDC-TENG integrates TENG and the human body and has a unidirectional output during basketball dribbling, which does not need additional electronic and mechanic components, thus reducing power consumption compared to conventional TENG.

Optimization of HBDC-TENG
When dribbling a polyurethane (PU) basketball wearing a pair of shoes on a wooden floor with 40%RH (relative humidity), unidirectional voltage and current output up to 700 V and 23 μA can be generated under a 100 MΩ load as shown in Figure 2a,b, respectively.The schematic and equivalent circuit for measuring the voltage signal using oscilloscope are shown in Figure S5b,c (Supporting Information), respectively.The resistances between different positions of the human body and ground floor R body are measured as shown in Figure S6 (Supporting Information), which is ≈0.45-1.45MΩ and can be considered conductive as the impedance of HBDC-TENG is much larger than this.Therefore, the conductive adhesive pad can be placed anywhere on the surface of the human body with identical voltage outputs as shown in Figure S5d (Supporting Information).The transferred charge on the human body can reach ≈200 nC per dribbling as presented in Figure 2c.As shown in Movie S3 (Supporting Information), a large voltage output can be recorded when dribbling a basketball on a wooden floor.Figure 2d illustrates the peak current I peak and instantaneous power P (P = I peak 2 R) of HBDC-TENG under different load resistance R, showing a maximum instantaneous power of ≈5 mW at ≈30 MΩ load resistance.
For optimizing the output performance of HBDC-TENG, basketballs, and floors made from different materials are investishown in Figure 2e, the voltage outputs of HBDC-TENG with different basketball materials are compared on a wooden floor wearing a pair of basketball shoes with a 75 cm maximum dribbling height at 50% RH.The peak is ≈200, ≈150, and ≈80 V for a basketball made from PU, ZK microfiber (ZK), and polyvinyl chloride (PVC), respectively, and the optimal basketball material for high TENG output is PU.
For HBDC-TENG performance evaluation using PU basketball on different kinds of floors, wooden, plastic, and concrete floors are chosen, and the voltage outputs are measured and compared as shown in Figure 2f with the same experimental condition used in Figure 2e.The absolute peak voltage is decreased from ≈180 to ≈60 V when dribbling on a wooden and a plastic floor respectively.More importantly, the polarity of voltage output is reversed, which is attributed to the opposite electron affinity of the plastic and wood grounds relative to that of the human body as shown in Figure S7 (Supporting Information).For dribbling on a concrete ground, the voltage output dramatically decreases to ≈10 V.For various types of floors, the conductivity of the insulator between the basketball and the actual floor as shown in Figure 2f plays an important role in voltage output.The vanished voltage output results from the high conductivity of the concrete ground compared with the plastic and wooden floors, which is verified using simulation results as shown in Figure S8 (Supporting Information).Therefore, the optimal choice is the wooden ground, which is beneficial as most basketball matches are taken place on wooden grounds inside a hall.
Considering the shoe type influence on HBDC-TENG performance as shown in Figure 2g, the peak voltage is ≈180, ≈120, and ≈100 V for basketball shoes, running shoes, and leather shoes, respectively.However, the output voltage of HBDC-TENG is only ≈ 2 V when the player is on a wooden floor with bare feet.The voltage variation is mainly affected by the insulation property of the shoe soles, which has a much better insulation property than bare feet due to the high conductivity of the human body.The insulation properties (i.e., thickness and the dielectric constant of the sole material) of sole materials will also affect the voltage output of HBDC-TENG.The voltage output is systematically investigated using different shoes on the wooden or plastic ground as shown in Figure S9 (Supporting Information), verifying the unidirectional output performance of HBDC-TENG.
Dribbling heights can have a significant impact on the output of HBDC-TENG as shown in Figure 2h.When the dribbling height increases from 35 to 95 cm, the peak voltage output gradually increases from −90 to −390 V.The performance improvement can be attributed to the larger contact area under a higher dribbling height, leading to more transferred charges.Humidity is another important factor that remarkably affects performance of the HBDC-TENG as shown in Figure 2i.The peak voltage output decreases from ≈700 to ≈70 V with the humidity increasing from 40 to 60%RH.This experiment suggests that at high RH ambient condition, the moisture adsorbed on the surfaces of the components leads to leakage of the generated charge and is not in favor of TENG. [24]mpared to a conventional TENG, the HBDC-TENG can produce larger voltage output and instantaneous peak power, especially when playing basketball, and is suitable for high-voltage applications.Figure 3a illustrates the cyclic test results of HBDC-TENG under ≈50% humidity, showing a good stability.Figure 3b shows a photograph of a 240 LED array that are lighten up when dribbling on the wooden ground in the laboratory (Movie S4, Supporting Information).To assess the charging behavior of HBDC-TENG, the charge curve of HBDC-TENG on a 100 μF capacitor directly using a power management unit (PMU, Figure 3c) is illustrated in Figure 3d.It should be emphasized that rectifier is not used owing to the unidirectional property of the outputs of HBDC-TENG, and less energy will be lost in the PMU. [25]Without PMU, the 100 μF capacitor can only be charged to 0.2 V in 1 min.However, the 100 μF capacitor can be charged at 3.2 V in 1 min with the PMU. Figure 3e shows charging curves of different capacitors with capacitance varying from 100 to 330 μF using the PMU circuit.As the capacitance value increases, the charging speed decreases.For a 330 μF capacitor, it can only be charged to 1.7 V in 1 min.The above results show that the energy harvested by HBDC-TENG can be efficiently stored for subsequent applications, such as the wearable electronic devices as shown later.

HBDC-TENG Based Basketball Player Movement Identification
Owing to the high performance of HBDC-TENG, it can be applied to monitor the movements of basketball players during match or exercise, that could assist the training and performance enhancement for the players.As shown in Figure 4 for demonstration, the voltage output of HBDC-TENG under five movements including dribble, cross-leg dribble, walk, dribble breakthrough, and step back are measured.Figure 4a shows that the peak output voltage of the HBDC-TENG is 180 V when the player is dribbling and standing still, and the output waveform is the same as mentioned above.As shown in Figure 4b, the peak output voltage of the HBDC-TENG is decreased to 80 V while crossleg dribbling and standing still, which is attributed to the less contact force between the basketball and ground/human hand as the lower body center of gravity during cross-leg dribbling.Besides, the first output peak voltage is higher than the second output peak voltage, which is another main difference caused by cross-leg dribbling.Figure 4c illustrates the output voltage when the player is walking without the ball.The single-electrode TENG, in which the triboelectric materials are the shoe soles and the ground (the human body acts as the electrode), generating an AC signal on the surface of the body during shoe-ground friction.The dribble breakthrough movement is the combination of dribbling and walking as shown in Figure 4d, and the output signal contains an AC component due to the walking movement compared to the signal from the dribbling in Figure 4a. Figure 4e shows that the peak output voltage of the HBDC-TENG is 120 V when the human performs a step back movement.With a greater force applied to the ground during the step back, the contact separation between the shoes and the ground generates greater friction at the shoe-ground interface, resulting in a higher AC signal.The different output voltage curves for the different movements are summarized in Figure 4f for a clearer demonstration.Using a machine learning algorithm, the movements of the basketball player can be accurately identified with a well-trained classifier.Long short-term memory (LSTM) is employed in the training, testing, and validating of the HBDC-TENG output voltage data.756, 209, and 214 random samples are used for the training, validation, and test sets, respectively.The model has good generalization ability and no overfitting or underfitting occurs.As shown in Figure 4g, with 100 epochs, the average prediction accuracy of the LSTM reaches 96.7% and 97.8% on the training and validating sets, respectively.The confusion matrix is shown in Figure 4h, demonstrating an excellent identification between different movements.To further illustrate the identification of different movements, the data are downscaled by principal-component analysis (PCA) and plotted as a 3D spatial scatter distribution, surrounding by 95% confidence intervals, as shown in Figure 4i.It can be found that by projecting the data into 3D space, the data of the same category naturally cluster into a pile, which also further verifies that the data obtained from HBDC-TENG is valid and distinguishable.

Wireless Human Motion and Environment Monitoring System
Human motion and environment monitoring would be one of the main application scenarios for wearable devices on basketball players as shown in Figure 5a.The HBDC-TENG can be used for a self-powered sensor and an energy harvester for a wireless sensing module simultaneously.A smartwatch is designed and fabricated for self-powered sensing applications as shown in Figure 5b, and the structural block diagram is shown in Figure S10 (Supporting Information).The output of HBDC-TENG can be recorded and analyzed in the smartwatch for dribbling count.Movie S5 (Supporting Information) demonstrates the self-powered dribbling counting in practice.It should be noted that in this self-powered sensing application, although the sensing part requires no external power, the data analyzing and visualization parts still need a battery, thus the system is not fully self-powered.Therefore, a fully self-powered wireless monitoring system is proposed, which can collect human motion and environment information and wirelessly transmit without extra batteries for powering.The energy is harvested using HBDC-TENG when the basketball player is playing at the match.The self-powered wireless monitoring system is fabricated on a flexible PCB adhered to the lower leg of the player as shown in Figure 5a.The two input terminals of the system are connected to the human body and shoe sole for grounding, respectively as indicated in Figure 5c.The circuit detail of the sensor chip is illustrated in Figure S11 (Supporting Information).It consists of a temperature/humidity sensor and an acceleration sensor that are used for environment and human motion monitoring, respectively.A wireless Bluetooth communication module is used for sensing information transmission.The HBDC-TENG can efficiently collect energy and power the wireless sensing system to transmit a wireless signal every 8 min.After that, the sensing signals including temperature/humidity, tri-axial acceleration, and the angle between the lower leg and ground are wirelessly transmitted to a mobile phone for data analysis and display as shown in Figure 5d, respectively.The historical sensed temperature/humidity, leg angle data is shown in Figure 5e,f, respectively.The human motion and environment monitoring information can be stably sensed and wirelessly transmitted to mobile phone based on HBDC-TENG, which can also be verified through system demonstration in practice as shown in Movie S6 (Supporting Information).

Conclusion
In summary, the HBDC-TENG generates DC voltage output during dribbling of basketball player, which can be attributed to the specific electron affinity combination of the human body, basketball, and ground type.Extensive experiments have been conducted to optimize the output of HBDC-TENG, and the optimal basketball material, ground type, shoe type, height between shoe sole and ground, humidity is PU, wooden ground, basketball shoes, 95 cm and ≈40% humidity, respectively.A unidirectional peak voltage and current output up to 700 V and 23 μA can be generated, respectively, which can spontaneously lighten up 240 LEDs, and charge a 100 μF capacitor to 3.2 V in 1 min.A basketball player movement identification system has been developed.A machine learning algorithm using LSTM is used and an accuracy of 96.7% is achieved for the identification of five different movements.Besides, A smartwatch is designed and fabricated for self-powered sensing applications.The output of HBDC-TENG can be recorded and analyzed in a smartwatch for dribbling count.In addition, a fully self-powered wireless sensing system is developed, which can monitor temperature, humidity, acceleration of lower leg, and angle between lower leg and ground.The HBDC-TENG can efficiently collect energy and transmit a wireless sensing signal every 8 min.All the results demonstrate that the HBDC-TENG has a wide range of potential applications in wearable electronics, including training posture calibration, fall detection, motion status recognition, etc.

Experimental Section
The Ag/AgCl wet electrode (Junchen, X-1) was adhered to the human body for electrical signal retrieving.An oscilloscope (Tektronix, TBS 1202B) was used to measure the voltage of HBDC-TENG with a 100 MΩ probe.A photograph of the wearable physiological motion sensor connected to the arm to measure the alternating current through the body and display motion information on the screen.c) A photograph of the wearable wireless sensing system attached to the lower leg, which collects energy via the HBDC-TENG and transmits the sensing information to a mobile phone.One terminal of the wireless sensing system is connected to the skin using electrodes, while the other terminal is grounded using a conductive strip adhered to the shoe sole.d) A photograph of the information transmitted wirelessly from the wearable wireless sensing system to the mobile phone.Historical e) temperature and humidity and f) angle between lower leg and ground.
An electrometer (KEITHLEY, 6514) was used to measure the current and transferred charge of HBDC-TENG.In wireless monitoring applications, humidity/temperature sensor (SENSIRION, SHT20) and acceleration sensor (ADI, ADXL335) were used for human motion and environment sensing.The wireless sensing system was fabricated on a flexible polyimide film with a thickness of 60 μm.

Figure 1 .
Figure 1.Working mechanism of the HBDC-TENG.a) Conventional TENG designs for wearable electronics.b) Schematic diagram of the working principle of the HBDC-TENG under open circuit condition.c) Experimentally measured charge accumulation in the i) hand and ii) wooden floor with different circuit conditions.d) The voltage output of the HBDC-TENG in experiment and simulation.e) Potential distribution of the HBDC-TENG obtained using COMSOL-based simulation for four different states under an open circuit condition in one motion cycle.

Figure 2 .
Figure 2. Output performance of HBDC-TENG and the affecting factors.a) voltage output and b) current output of HBDC-TENG, c) charge output of HBDC-TENG, d) output current and peak power of HBDC-TENG as a function of load resistance.Voltage output for HBDC-TENG using e) different basketball materials, f) different ground materials, g) different shoe materials, h) different heights, and i) different humidity levels.

Figure 3 .
Figure 3. Directing LEDs lighting and Capacitor charging performance of the HBDC-TENG.a) Stability result of HBDC-TENG measured at 50% humidity.b) Demonstration of playing basketball to directly light up 240 LEDs.c) Equivalent circuit diagram management unit for charging capacitor.d) Charging curve of the HBDC-TENG device using a 100 μF capacitor with power management unit.e) Charging curves for 100, 220, and 330 μF capacitors.

Figure 4 .
Figure 4. Output waveforms of HBDC-TENG under different movements and machine learning-based basketball player movement identification.Output voltages of HBDC-TENG when the player is a) dribbling, b) cross-leg dribbling, c) walking, d) dribbling breakthrough, and e) stepping back, which is f) summarized for a clearer demonstration.g) Prediction accuracy of LSTM algorithm.h) Confusion matrix for different movements.i) 3D PCA for the movement signal dataset recorded by HBDC-TENG.

Figure 5 .
Figure5.Application of HBDC-TENG in wireless human motion and environment monitoring.a) Schematic of the wearable electronic device carried by the athlete.b) A photograph of the wearable physiological motion sensor connected to the arm to measure the alternating current through the body and display motion information on the screen.c) A photograph of the wearable wireless sensing system attached to the lower leg, which collects energy via the HBDC-TENG and transmits the sensing information to a mobile phone.One terminal of the wireless sensing system is connected to the skin using electrodes, while the other terminal is grounded using a conductive strip adhered to the shoe sole.d) A photograph of the information transmitted wirelessly from the wearable wireless sensing system to the mobile phone.Historical e) temperature and humidity and f) angle between lower leg and ground.