Human Body Constituted Triboelectric Nanogenerators as Energy

May 22, 2018 - For example, an eye-motion-based(14) TENG can trigger a wireless hands-free typing system. There are many protocols(3,15−19) derived ...
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Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

Human Body Constituted Triboelectric Nanogenerators as Energy Harvesters, Code Transmitters, and Motion Sensors Renyun Zhang,*,† Magnus Hummelgård,† Jonas Ö rtegren,† Martin Olsen,† Henrik Andersson,‡ Ya Yang,§ and Håkan Olin† †

Department of Natural Sciences, Mid Sweden University, Holmgatan 10, SE-85170 Sundsvall, Sweden Department of Electronics Design, Mid Sweden University, Holmgatan 10, SE-85170 Sundsvall, Sweden § CAS Center for Excellence in Nanoscience, Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 100083, P. R. China ‡

S Supporting Information *

ABSTRACT: Human skin is a dielectric material that can be used as a triboelectric material for harvesting energy from body motions. The output power of such a human skin-based triboelectric nanogenerator (TENG) is relatively low. Here, we assembled high-output human body constituted TENGs (H-TENGs) by taking advantage of the unique electrical properties of the human body, such as high skin impedance, low tissue resistance, body capacitance, and conductivity. The output of a H-TENG can reach 30 W/m2, which is enough to drive small electronic devices, such as a timer or a calculator. The unique feature of the H-TENG is that it can perform the four fundamental modes of TENGs, which has not been reported elsewhere. Such a feature allows the H-TENG to act as a code transmitter to send light and electrical signals, such as Morse code. H-TENGs also benefit the development of high-performance, selfpowered body motion sensors. Our findings suggest new strategies for harvesting energy from human body motions, as well as new types of motion sensors and signal senders. KEYWORDS: human body, triboelectric nanogenerators, energy harvesting, code transmitters, motion sensors

1. INTRODUCTION Triboelectric nanogenerators (TENGs) have attracted increasing interest since they were invented in 2012 by Wang’s group.1 Such nanogenerators2−4 can harvest energy from mechanical motions,3 wind,5,6 sound,7 and water waves.8 The output power of a TENG can reach above 1200 W/m2,9 and the conversion efficiency has reached approximately 85%.10 TENG is also an efficient way to harvest energy from body motions such as walking11 and arm12 and hand movements,13 etc. Besides energy harvesting, TENG could also be used in a self-powered mechnosensational communication system. For example, an eye-motion-based14 TENG can trigger a wireless hands-free typing system. There are many protocols3,15−19 derived from the four fundamental modes9 to fabricate TENGs for different applications, for example, using synthetic polymers, such as polytetrafluoroethylene (PTFE),20,21 fluorinated ethylene propylene (FEP),22,23 and Kapton films,6,24 as the triboelectric layer. Beside those synthetic polymers, human skin has also been used in a few TENGs13,25,26 owing to the triboelectric properties of skin. The output power of such human-skin-based TENGs is usually in the range of several to hundreds of milliwatts by harvesting the charges generated on the triboelectric materials. Our recent study27 indicated that charges generated on the human body can also be harvested using human-body-based triboelectric nanogenerators (H-TENGs) owing to the unique © XXXX American Chemical Society

electrical properties of the human body. Such H-TENGs can have an output power up to 3.3 W/m2, which can drive 377 LEDs and a timer. However, such output power is still low compared with that of other TENGs. Here, we report a new type of H-TENG that harvests both the charges generated on the human body and on the skin contacting the triboelectric material. The H-TENGs have an output power up to 30 W/m2, which is almost 10 times higher than that of the previous report. Our results provide scientific insights into the functions of the human body that can promote further improvement of TENGs for harvesting energy from motion or for application in self-powered electronics.

2. EXPERIMENTAL SECTION 2.1. Materials. PTFE (0.1 mm) film was purchased from Hightech-flon and was cut into 5 × 5 cm2 pieces. The pieces were immersed in absolute ethanol, followed by sputtering of a 60 nm thick Au:Pd layer (80% Au, 20% Pd) using a Q150T ES sample preparation system (Quorum). 2.2. Assembly of H-TENG. A 0.5 mm wide copper tape was attached on the Au:Pd sputtered surface and connected with a crocodile clip. Then, the side was covered by packing tape to protect the Au:Pd layer. The uncoated side was polished with 4000 grit SiC paper for 1 min. The assembled unit was then fixed on a table or on Received: April 25, 2018 Accepted: May 22, 2018 Published: May 22, 2018 A

DOI: 10.1021/acsaem.8b00667 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials

Figure 1. Performance of the H-TENG. (a) Schematic drawing of the circuit of the H-TENG. The function of the body is discussed in the section below. (b) SEM image of the PTFE film, scale bar = 1 μm. (c) SEM image of human palm skin, scale bar = 1 μm. (d) The open-circuit voltage measured on the H-TENG. (e) The short-circuit current measured on the H-TENG. (f) The measured current and voltages vs different load resistances. (g) The output power per square meter of the H-TENG vs different load resistances. The value here is calculated using W = I2R, where the value of the current is the average of the 5 highest currents measured within 20 s but not the highest value. (h) Charging of a 0.47 μF capacitor by the H-TENG at a hand patting frequency of 4 Hz. The figure shows the voltage and the corresponding charges on the capacitor. (i) Photograph of the H-TENG, showing the 5 × 5 cm2 PTFE/(Au:Pd) film and the connections. The red cable is connected to the left hand of the person who is performing the experiment. (j) Driving a timer with the H-TENG. (k) Running a calculator to do simple calculations [(5 + 2) × 7 = 49] by patting a 50 cm2 PTFE/(Au:Pd) film.

approximately 30 μA. Such behavior is believed to be due to the impedance change of the skin at voltages below or over the breakdown voltage. The output power of the H-TENG was measured with loads from 20 KΩ to 500 MΩ. Figure 1f shows the voltages measured upon loading and when current is passed through the loads. The output voltage increases with increasing load resistance up to 50 MΩ and then stabilizes at approximately 430 V at higher resistance. The current decreases significantly from 95 to 16 μA when the load resistance is increased from 20 KΩ to 500 MΩ. The output power was calculated using P = I2R, and the values are shown in Figure 1g. It is worth noting that the currents used for the calculations are the mean values of the five highest measured values at each load. That said, the output powers presented here are not the maximum values from the experiments, though the output power reaches 30 W/m2, which is 60 times that of the first human-skin-based TENG26 and 8.3 times the latest achievement.29 Such a high output can be a promising power supply to some electronics. We charged a 0.47 μF capacitor using the output from patting a 5 × 5 cm2 PTFE film at a frequency of 7 Hz. The voltage on the capacitor increases to 41 V in 15 s, which equals 1.28 μC/s (Figure 1h). Such a number is close to the output from the H-TENG (1.33 μC/s), calculated from the charges generated on the body (see calculation in the Supporting Information).

trousers. A rectifier (Vishay) was used to convert ac to dc; a PXI-4132 Source Measure Unit card (National Instruments) and a multimeter controlled by the LabView program were used to take electrical measurements. 2.3. Measurements and Characterizations. A surface dc voltmeter (Model SVM2, Alphalab) was used to measure the surface voltage of a hand and PTFE. The surface voltage was then converted to surface charge density according to the instructions for the voltmeter. The surface structure of PTFE was imaged using a Maia3 (Tescan) microscope.

3. RESULTS AND DISCUSSION 3.1. Energy Harvesting. 3.1.1. Electrical Outputs of HTENG. The schematic diagram of the H-TENG is shown in Figure 1a, where the right hand is used to pat the PTFE film and the left hand is connected to a rectifier. The human body is emphasized here since it plays different roles depending on the applicable mechanisms. SEM images of the PTFE film and palm skin are shown in Figure 1b,c. The results of electrical measurements indicate that the open-circuit voltage (VOC) measured upon patting a 5 × 5 cm2 square piece of PTFE film with the hand was between 460 and 520 V (Figure 1d). Such high voltages are very important for the performance of the HTENG, as they are around the resistance dc breakdown voltage of human skin, which is 500 V.28 The short-circuit current (ISC) was observed to vary in a large range from 2 to over 120 μA (Figure 1e). The most frequently measured current was B

DOI: 10.1021/acsaem.8b00667 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials We further used the H-TENG (Figure 1i) to drive a timer (Figure 1j) and a calculator (Figure 1k). The timer shows accurate counts of seconds compared to another timer driven by a battery. When a calculator is connected, simple calculations, such as (5 + 2) × 7 = 49, can be performed. All the electronics are connected to the H-TENG through a rectifier (Figure 1a). Videos of all the tests are given in the Supporting Information Videos V1 and V2. An issue that should be discussed is the safety of the HTENG because of the charge flow through the body. In our experiment, we used a commercial PTFE film that has been used in many products. Many people have contacted the products while no reports indicate risks of health due to contact electrification. The maximum current flow through the body is about 120 μA, which is not harmful to the human body. In fact, charge flow is a common phenomenon in our daily life, especially when we touch a metal in a dry winter climate. There are also no reports that indicate the risks of health due to this charge flow. Thus, we believe the current flow in our experiment is safe. However, if one uses designed materials that can generate a current in the range of a milliampere, it is necessary to study the risks. 3.1.2. Four-in-One Nanogenerator. There are four fundamental modes of a TENG:30 vertical contact (VC) or contact separation (CS) mode, single electrode mode (SE), lateral sliding mode (LS), and freestanding triboelectric-layer mode (FT). One, and only one, of these four modes is used in most of the reported TENGs, though modification to apply two modes is possible. However, none of them can perform all four modes because the mechanical motions in the TENGs are either vertical or lateral. To perform all four modes in one TENG, three-dimensional freedom of motion is needed, which can be achieved by hand movements. We designed a H-TENG with a specific circuit to perform the four TENG modes, as shown in Figure 2a. A PTFE film was coated with two (Au:Pd) electrodes separated by a 3 mm gap. Four switches were set to adjust the circuit for different modes. To simplify the description, the open and closed states of the switch are represented by 0 and 1, and hand patting and sliding on the PTFE/(Au:Pd) film are denoted P and S. P1100 means hand patting on the PTFE/(Au:Pd) film while switches A and B are closed and C and D are open, which is the VC mode. P0101 is the SE mode; S1100 is the LS mode, and S0010 is the FT mode. Figure 2b−i shows the open-circuit voltages and short-circuit currents of the H-TENG in each mode. By simply applying P = UI to estimate the output (Figure 2d) of the four H-TENG modes, we obtained the order VC > LS > FT > SE. This shows that the participation of the human body as part of the circuit, as in the VC and LS modes, generates a higher output power. The order of the LS and FT modes is different from the figure-of-merit (FOM) reported by Zi et al.,31 which may be due to a unique function of the human body and requires further study. 3.2. Code Transmission. The participation of the human body brings new features to the H-TENG not achievable by other types of TENGs. We constructed electrical and/or light code transmitters to convert text to Morse code by taking advantage of these new features. The first code transmitter for sending our electrical and light singles was assembled by using different movements of the hand contacting the PTFE films. Figure 3a shows the circuit of the code transmitter, which is similar to that in Figure 2a but without switches. A pat of the hand leads to short-term contact

Figure 2. Applying the four fundamental modes in one H-TENG. (a) Circuit of the H-TENG for performing the four fundamental modes. The hand motion is presented by P (patting) and S (sliding), and the states of the four switches A, B, C, and D are presented by 0 (open) or 1 (closed). P0101: single electrode (SE) mode. P1100: vertical contact (VC) or contact separation (CS) mode. S0010: freestanding triboelectric-layer (FT) mode. S1100: lateral sliding (LS) mode. (b) Short-circuit current measured in the different modes. (c) Open-circuit voltage measured in the different modes. (d) Output power of the different modes using P = UI.

Figure 3. Coder transmitters based on H-TENGs. (a) Circuit of the code transmitter that sends out electrical and light signals. (b) Photograph of the H-TENG-based code transmitter for sending out light signals, and the electrical Morse code for SOS transmitted using the H-TENG. (c) Circuit of the code transmitter that sends out light signals. (d) Light Morse code transmitted using the H-TENG.

with the PTFE film, giving a short spike in current, which represents the “dot” of Morse code. A slide of the hand leads to a longer contact time with the PTFE film, giving a longer spike in current, which represents the “dash”. The duration of the dash should be 3 times that of the dot according to Morse code. Such a requirement can be achieved by controlling the patting C

DOI: 10.1021/acsaem.8b00667 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials

Figure 4. H-TENG-based self-powered active sensors. (a) Open-circuit voltages measured upon 1−4 finger taps vs the effective contact area of the fingers and the PTFE film. The insets show the measured voltage over time. (b) Open-circuit voltages measured by pressing a hand on the PTFE film. The inset shows a photograph of the performance. (c) Open-circuit voltages measured by rubbing a hand on the PTFE film. The inset shows a photograph of the performance.

pressure, and d0 is the maximum distance between the two triboelectric layers. In our finger tapping experiments, the pressure was kept relatively unchanged (approximately 1800 Pa, Supporting Information Figure S1), and thus, the VOC was expected to have a linear relationship with the contact area, which can be simply represented by the number of fingers. Figure 4a shows that the finger tapping was sensed by the H-TENG, and the measured voltages were found to be proportional to the contact area. The results of the ISC measurements are given in Supporting Information Figure S2. Hand pressing in our H-TENG can be recognized as a variation of the vertical contact mode. The VOC of such a mode is related to the effective size of the H-TENG and the pressure applied by the hand. Due to the unique geometry of the hand, higher pressure on the hand can create more contact area between the hand and the PTFE film. In addition, because of the nonflat surface of the hand, the area over which pressure is applied on the PTFE film is not easy to define. However, one can easily measure the VOC when pressure is added or removed (Figure 4b). The results of the ISC measurement are given in Supporting Information Figure S3. Friction between the skin and the PTFE film from hand sliding was also sensed using an H-TENG as an active sensor. Surprisingly, a similar pattern of voltage changes was observed upon forward or backward motion of the hand (Figure 4c). The reason for this is not clear, but a guess is that the tissue of the leg may have a shape change similar to that of a hand moving back and forth. The electrical signal generated by hand rubbing was found to depend on the speed of the hand movement (Supporting Information Figure S4), where the lowest speed that can generate a detectable signal with our setup was 5 cm/s.

and sliding speed of the hand. Figure 3b shows an example of code transmission using H-TENG to send out SOS, where the half peak duration is approximately 0.04 s for the dots and 0.12 s for the dashes. A video showing the light signal of this code transmitter is given in the Supporting Information Video V3. The shortcoming of the first example is that the duration of the light signal is relatively short, which may lead to misunderstanding. An effective way to distinguish the light signal is by shape instead of duration. We assembled another code transmitter prototype that takes advantage of finger movement. Figure 3c shows the circuit of the code transmitter, where the fingers act as switches to conduct two circuits to light different numbers of LEDs. The H-TENG here is based on the VC mode, and the rectifier is removed to simplify the system. Contact by the index finger lights several LEDs, which represents a dot, while contact by the middle finger lights more LEDs, which represents a dash. Figure 3d shows an example of transmission by the H-TENG from text to light codes that are more readable by receivers. A video of this example is given in the Supporting Information Video V4. This kind of code transmitter could help people to communicate at certain situations where sound is not an option. It could also help people with disabilities to communicate with each other when they could not use hand languages, e.g. when illumination is bad. 3.3. Self-Powered Body Motion Sensors. H-TENGs act as a natural human body motion sensor since the electrical signals are directly generated on the human body. Here, we show some simple cases to illustrate the application of HTENGs as active motion sensors. The PTFE/(Au:Pd) film was fixed on trousers by packing tape, while copper tape was attached on the (Au:Pd) side to conduct the current. Finger tapping, hand patting, sliding, and pressing were performed on the PTFE/(Au:Pd) film. Hand patting was already shown in the above results and thus is not shown in this part. The variation in the open-circuit voltage of a VC-mode selfpowered active sensor has a linear relationship with the size of the TENG or the pressure applied to the TENG, which can be predicted with the equation32 VOC,0 −VOC VOC,0

=

S p kd0

4. CONCLUSIONS In summary, high-output H-TENGs were developed with a power up to 30 W/m2 by taking advantage of the structure of skin and the electrical properties of the human body. Due to the degree of freedom of hand movements, a designed HTENG can perform all four fundamental modes of a TENG by patting or rubbing a hand on the other triboelectric film of the H-TENG. The simplicity of the H-TENGs makes them easy to install on any surface, such as clothes, tables, walls, or floors, for energy harvesting and motion sensing.

(1)

where VOC and VOC,0 are the voltage of the TENG with and without applied pressure, S is the effective area of the TENG, k is the elastic properties of the materials in the TENG, p is the D

DOI: 10.1021/acsaem.8b00667 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.8b00667. Calculation of the charges generated on the human body and figures with additional characterization data (PDF) Video V1 showing testing with the timer (MOV) Video V2 showing testing with the calculator (MOV) Video V3 showing Morse code (MOV) Video V4 showing light codes (MOV)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. ORCID

Renyun Zhang: 0000-0003-2873-7875 Ya Yang: 0000-0003-0168-2974 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. All authors discussed the results and commented on the manuscript. All authors have given approval to the final version of the manuscript. Funding

The authors acknowledge the financial support of J. Gust Richert Stiftelse, The Knowledge Foundation of Sweden, Energimyndigheten, European Regional Development Fund, and Länsstyrelsen Västernorrland. Notes

The authors declare no competing financial interest.

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REFERENCES

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