Fabrication of Soft and Wearable Electrostatic Generator Based on Streaming Electrification

Electrostatic actuators, which are characterized by low weight, power consumption, noise, and remaining heat, have been studied widely and are expected to find use in wearable devices. However, they require high‐voltage control and safety circuits, which may increase their bulk. Herein, a soft electrostatic generator is proposed for use in wearable devices. Specifically, streaming electrification is used, wherein electric charge is generated through the interactions between a fluid and a solid material (the developed system is named a “streaming electrification generator”). The contact area with the insulating fluid is increased by using a porous material, and the amount of charge generated is increased to the level needed to drive electrostatic actuators using materials with different dielectric characteristics. Moreover, the generator is made of soft materials and therefore can adapt to the shape of the human body and not interfere with its movement. Finally, it is ideal for wearables because it does not use materials that are harmful to the human body. Thus, the developed system, which is made of soft materials, is a novel power generator and should be suitable for electrostatic actuators used in wearable devices.


Introduction
Due to the recent improvements in the fabrication processes and characteristics of materials, electrostatic actuators [1] are being studied widely. They exhibit several desirable features such as light weight, flexibility, high responsiveness, low power consumption, high power density, low-noise operation, and low remaining heat. By ensuring a narrow electrode gap to generate a high electric field, actuators with various mechanisms have been developed, such as dielectric elastomer actuators, [2,3] electrohydrodynamic (EHD) fluid actuators, [4,5] and electrostatic adhesive actuators. [6,7] Not only are such actuators being used in microelectromechanical system devices due to the scale effect, but the recent development of prototyping technology has also led to applications such as macroscale artificial muscles. Furthermore, the use of wearable devices is likely to increase with improvements in their safety. [8,9] However, powering wearable devices, that is, developing suitable energy sources for these devices, remains a problem. Because a high voltage is required to drive these devices, ordinary batteries cannot be used as is, and a voltage amplifier is required. A compact voltage amplification system can be constructed. However, in the case of wearable devices, an additional safety device would be required to suppress the overcurrent in the case of dielectric breakdown. Therefore, the power supply system for electrostatic actuators is likely to be bulky. It is also possible to drive electrostatic actuators with a low-voltage input by adjusting the properties and film thickness of the material used for actuation. [10] However, a complex and sophisticated manufacturing process, such as one that allows for the stacking of layers, would be required to ensure a stable output. Thus, there is an urgent need to develop a simple and wearable voltage generator that can drive electrostatic actuators.
Triboelectric nanogenerators (TENGs) are electrostatic generators that have attracted much attention in recent years. [11][12][13][14] TENGs produce electrical energy by inducing an electric charge through friction or other physical interactions. Researchers have proposed a TENG that generates electric charge based on the passage of a solid/liquid through a liquid membrane. [15] When 40 μL of water was passed through the membrane, the TENG outputted a voltage of 33 mV and a current of 0.85 nA. TENGs generally produce electricity based on the charge generated when the materials in sheet form come into contact with each other. Although sheet materials are usually lightweight and flexible, they are not stretchable or foldable and are thus unsuitable for use in wearable devices, which must not interfere with human movement. In addition, because TENGs output an AC signal, an AC-to-DC DOI: 10.1002/aisy.202100131 Electrostatic actuators, which are characterized by low weight, power consumption, noise, and remaining heat, have been studied widely and are expected to find use in wearable devices. However, they require high-voltage control and safety circuits, which may increase their bulk. Herein, a soft electrostatic generator is proposed for use in wearable devices. Specifically, streaming electrification is used, wherein electric charge is generated through the interactions between a fluid and a solid material (the developed system is named a "streaming electrification generator"). The contact area with the insulating fluid is increased by using a porous material, and the amount of charge generated is increased to the level needed to drive electrostatic actuators using materials with different dielectric characteristics. Moreover, the generator is made of soft materials and therefore can adapt to the shape of the human body and not interfere with its movement. Finally, it is ideal for wearables because it does not use materials that are harmful to the human body. Thus, the developed system, which is made of soft materials, is a novel power generator and should be suitable for electrostatic actuators used in wearable devices.
converter must be added usually to drive the actuators or charge the capacitor.
Charge injection into an insulating fluid under a high electric field is the basis of the EHD phenomenon, which is used to induce a pressure in the case of fluid actuators. [4,5] During the phenomenon of EHD, fluid flow is generated by exposing an insulating fluid to a high electric field. The EHD phenomenon is governed by two mechanisms: conduction pumping and injection pumping. [16] Conduction pumping occurs at a relatively low electric field, and the flow is generated by the dielectric constant gradient created by the electric double layer near the electrodes. In contrast, injection pumping occurs under a high electric field when charge is injected into the fluid molecules from the electrodes. The charged molecules move under the electric field due to the resulting Coulombic force, and the exchange of momentum between the charged molecules and the fluid molecules induces a fluid flow. Power generation based on EHD has also been proposed. A high-voltage power supply is used to inject charge into an insulating fluid, which is then collected by a collector electrode to generate electricity. [17] However, this power generation method is inefficient because the electrical energy is produced by injecting and collecting electric charges using electrical energy. Fluids suitable for use in EHD are still being explored. However, it has been proposed that fluorinated fluids, which are negatively biased, may be suitable for use for EHD pumping. [4] Based on these facts, we attempted to develop a charge generator by injecting charge into a fluorinated fluid that exhibits physical interactions such as friction.
Herein, we propose a charge generation system based on streaming electrification, which involves the interactions between an insulating fluid and a porous material. We have named this system a "streaming electrification generator" (SEG). We succeeded in generating and collecting electric charge based on the interactions between an insulating fluid and a porous material with significantly different dielectric characteristics. Using the porous material as the charged part and by increasing its contact area with the fluid, the output performance of the SEG could be improved to the point where it could drive an electrical device. In addition, we were able to further improve the performance of the generator by investigating the relationship between the design of the generator/the materials used in it and the amount of charge generated. Furthermore, we were able to drive electrostatic actuators (a dielectric elastomer actuator and a leaf electroscope) using the SEG. In addition, for one of the electrostatic actuators, namely, the leaf electroscope, we used a physical model that could predict the amount of charge generated based on the magnitude of the angle of displacement of the leaves of the electroscope. Finally, we succeeded in developing a wearable generator based on the mechanism of the SEG with the aim of harvesting energy from daily activities such as stepping and the bending and grasping of the arm.

Design of SEG
We focused on streaming electrification ( Figure 1A) to inject electric charge into the insulating fluid; this mechanism is similar to EHD pumping and EHD power generation. Streaming electrification is the generation of charge through contact between an insulating fluid and a dielectric material. In the case of a low-conductivity liquid, gas, or powder, this phenomenon can lead to the generation of a large amount of charge, which may cause an explosion or accident. [18] This study examined whether it is possible to generate and collect electric charge based on the streaming electrification of a fluorinated liquid, as is the case for the insulating liquids used in EHD devices. The materials used were selected based on their charge characteristics, [19] as shown in Figure 1B. Because fluorinated materials tend to be negatively biased, they should interact with positively biased materials. In addition, to enhance the charge generation performance by increasing the probability of contact, we used a porous material. Figure 1C shows a conceptual diagram of the SEG. The SEG was filled with an insulating liquid. The charged part was a sponge packed in a case fabricated using a 3D printer (AGILISTA-3200, KEYENCE). Figure 1D shows the charge generation mechanism of the SEG. A fluorinated liquid flows through the sponge material and generates an electric charge upon contact. This charge is collected from the liquid by the collector electrode in the form of electrical energy. Figure 1E shows a conceptual diagram of the measurement system. The liquid pumped from the pump was passed through the charged part and came into contact with the charge collection part. The collector electrode was connected to a Coulomb meter (NK-1002A, Kasuga Denki) to measure the amount of charge generated. A photograph of the experimental system, which consists of a pump, charged part, and charge collection part, is shown in Figure S1, Supporting Information. The proposed system has the following advantages: 1) it is stretchable and foldable; 2) it is simple to manufacture; 3) it is lightweight; 4) it is made of materials that are not harmful to humans; 5) it has no driving parts; 6) it has a constant-current output; and 7) it can be used to transform any movement into electrical energy if it can be converted into liquid flow.
First, we investigated the relationship between the amount of charge generated and the presence of a porous material. Figure 1F shows the results. When the sponge was not inserted into the charged part, no charge was collected. In addition, when the charged part was replaced with a tube, no charge was collected. Melamine foam and urethane foam were tested as the sponge materials. Figure S2, Supporting Information, shows the sponge packed in the charged part. We observed charge generation when the melamine sponge was used as the charged part. In contrast, the use of the urethane sponge did not result in charge collection. This result is consistent with the choice of the fluorinated liquid; the charge properties of melamine are more different from those of urethane than those of the liquid. During these experiments, the liquid used was Novec 7300 (3M), the collector electrode used was made of Cu, and the volume ratio of the sponge case and the sponge was 1:1.
The next step was selecting the liquid to use. Figure 1G shows the amounts of charge generated when various fluorinated liquids and alcohols were used. The highest amount of charge was collected in the case of Novec 7300. Alcohols such as ethylene glycol resulted in the collection of smaller amounts of charge than dielectric liquids because of the differences in their conductivities. The conductivity of ethylene glycol, which is generally considered a conductive liquid, is %10 8 pS m À1 , whereas that of the Novec liquids, which are insulating liquids, is %10 pS m À1 . Liquids with high conductivity cannot be charged readily as they are antistatic agents and form a conducting film on their surface, which promotes the flow of electric charge. In other words, insulating liquids are more suitable for use in charge generation by streaming electrification. With respect to the dielectric liquids used, the amount of charge generated was higher in the case of Novec 7300 than for Novec 7100. The viscosities of Novec 7100 and Novec 7300 are 5.8 Â 10 À4 and 12 Â 10 À4 Pa s, respectively. Therefore, Novec 7300, which has a higher viscosity, would interact more with the sponge surface. The pressure loss in the charged part depends on the viscosity and sponge-filling rate of the liquid used. During these experiments, the sponge material used was melamine foam, the collector electrode was made of Cu, and the volume ratio was 1:1. Figure 1H shows the amounts of charge generated for different collector electrode materials. Cu, Al, and Pb were used for the collector electrode. The volume resistivities of these materials are Figure 1. A) Principle of streaming electrification. An insulating fluid is charged to different polarity when it comes into contact with a pipe. B) Charged row. [21] Materials listed will be either positively or negatively charged. Materials on the left are more likely to be negatively charged, while those on the right are more likely to be positively charged. The further apart the materials are on the list, the more strongly they are charged when interacting. C) Schematic of the SEG. The SEG consists of a pump, charged part, and collector. The system is filled with an insulating liquid, which is pumped to the charged part to generate an electric charge. The electric charge is collected by the collector. D) Mechanism in charged part. By using a sponge, we aimed to increase the amount of charge generated and collected by increasing the contact area. E) Schematic of measurement system. The amount of charge collected by the collector is measured with a Coulomb meter. F) Effect of type of sponge used on amount of electric charge collected. We succeeded in generating electric charge using a sponge as the charged part. Because a fluorinated liquid was used, a melamine sponge, whose charge properties are very different from those of the liquid, generated the most charge. G) Amounts of charge generated for different liquids. Alcohols, which have high electrical conductivities and act as antistatic agents, barely generated any charge. H) Amounts of charge generated for different collector materials. The lower the volume resistivity of the material used, the more charge was collected.
www.advancedsciencenews.com www.advintellsyst.com 1.55 Â 10 À8 , 2.52 Â 10 À8 , and 19.2 Â 10 À8 Ω m, respectively. Thus, materials with a lower volume resistivity would collect more charge. During these experiments, the sponge material used was melamine foam, the liquid used was Novec 7300, and the volume ratio was 1:1. The previously described results show that Novec 7300 resulted in the collection of the largest amount of charge in the melamine sponge after coming in contact with the Cu electrode. Therefore, these materials were used in the subsequent experiments. Next, the design of each module was evaluated to improve the output performance. Figure 2A shows the amount of charge collected for a pump output flow rate of 900 mL min À1 , and Figure 2B shows the result for a flow rate of 400 mL min À1 . When the flow rate was 900 mL min À1 , the amount of charge collected was higher. This is because the time of contact between the liquid and the sponge was higher at this flow rate. Therefore, the flow rate significantly affects the charge generated in the SEG system.
To elucidate the effect of the size and density of the sponge on the amount of charge generated, we prepared nine different sponges and tested them ( Figure S3, Supporting Information). Sponges with three different lengths (4, 2, and 1 cm) and compression ratios (100%, 50%, and 25%, respectively) were used. The compression ratio, C, is defined in Equation (1), where V sc is the volume of the sponge case and V s is the volume of the sponge Thus, a volume ratio of 1:1 in the previous experiments represents a compression ratio of 100%. www.advancedsciencenews.com www.advintellsyst.com Figure 2A,B shows the effects of varying the length and compression ratio of the sponge used in the charged part. The maximum output was obtained when the length was 2 cm and the compression ratio was 100%. The maximum output corresponding to the lengths of 4 and 2 cm was obtained for the compression ratio of 100%, while the maximum output when the length was 1 cm was obtained for the compression ratio of 50%. The extent of contact of the liquid with the sponge was the highest when the sponge length was 4 cm and the compression ratio was 25%. However, the output corresponding to these conditions was not the highest. There are two possible reasons for this: 1) an excessively high compression ratio probably results in high pressure on the liquid in the sponge, resulting in a lower flow rate. This, in turn, reduces contact and therefore the amount of charge generated. 2) Charge relaxation occurs in the charged part. As shown in Figure S4, Supporting Information, charge relaxation occurs due to charge recombination when the polarization of the sponge is the opposite of that of the liquid.
The amount of charge collected increased linearly during each experiment. Assuming that the amount of liquid passing through the sponge remains constant over time because of the constant flow pump rate, it can be concluded that the system remained stable over time, with no decrease in charge generated due to the degradation of the electrode or the denaturation of the liquid.
For the proposed SEG system to be suitable for use in actual applications, its output should remain stable even when the external environment changes. Thus, the system was tested at a different location, and the amount of charge generated was measured (Table S1, Supporting Information). Figure 2C shows the averages of the current values for three experiments performed under different conditions. The current values were obtained by time-differentiating the charge values. The most significant variance, which was %À22 AE 3.0 nA, was obtained for a sponge length of 2 cm and compression ratio of 100%. The generator system exhibited stable performance in the external environment. Generally, when the humidity is high, water condenses on the material surface, resulting in an increase in conductivity and decrease in the amount of charge generated. In the proposed system, however, the charged part is enclosed within the insulating liquid. This is the reason the system remained stable in the external environment.
We connected an external load resistor to the SEG to form an electrical circuit for evaluation. Figure 2D shows the experimental setup used for the measurements and Figure 2E shows the output current and voltage characteristics when the external resistance was changed. It can be observed that a constant charge was supplied continuously to the load due to charge generation and collection because of streaming electrification. Thus, the SEG exhibits electrical properties similar to those of general electrostatic generators. Electrostatic actuators are capacitor-type actuators whose mechanism involves applying a high voltage and low current to a dielectric. Therefore, they are compatible with constant-current sources. Hence, the developed system is suitable for driving electrostatic actuators. FigureS5, Supporting Information, shows the results of the experiment to confirm the durability of the system. The SEG was directly connected to the digital multimeter to measure current and driven for 1800 s. The output current did not change much before the liquid volatilized. The amount of liquid decreased after 1800 s because the insulating liquid Novec7300 was highly volatile, and the output current became unstable. In the measurement experiment, we conducted the experiment in an open system to prevent a vacuum from forming due to the characteristics of the constant flow pump. In the actual applications, it will be a sealed system. Therefore, no volatilization of the liquid will occur, and the system will be durable.

Driving of Devices and Performance Evaluation
As shown in Figure 3A, a leaf electroscope is an electrostatic actuator. Initially, its leaves are closed, as shown in Figure 3A(i). However, when a charge is applied to the top plate, a charge of the same polarity is sent to the metal leaves connected to the top plate (see Figure 3A(ii)). As a result, the electrical equilibrium of the metal leaves is broken, and the leaves are biased to the same type of charge, resulting in a repulsive force, which causes the leaves to open. Figure 3A shows the mechanism by which a negative charge is introduced into the leaf electroscope. In general, leaf electroscopes are driven using an induced charge. However, here, the leaf detector was driven by directly feeding it the charge generated by the SEG. Thus, the movement of the leaves of the electroscope was reflective of the movement of the generated charge. This experiment confirmed that the SEG can indeed drive electrostatic actuators, which can allow one to visualize the generated charge, as shown in Figure 3B. Figure 3C and Video S1, Supporting Information, show the actual operation of the leaf electroscope. Figure 3C(i) shows the results when a pump with an output flow rate of 900 mL min À1 was used and Figure 3C(ii) shows the results when a pump with an output flow rate of 400 mL min À1 was used. The charged part was 2 cm long and had a compression ratio of 100%. The electric charge generated in the charged part is sent through the Cu electrode to the top plate of the leaf electroscope. Because a negative charge is generated, the negative charge held by the top plate is pushed to the leaves. As a result, the negative charge on the leaves increases, and the leaves spread by repelling each other. When the system is in operation, the leaves remain open because charge is continuously sent to the top plate. Figure 3D shows the results of image processing performed to determine the time variations in the angle of the leaves; the image in Figure 3D(i) shows the results obtained using a pump output flow rate of 900 mL min À1 and that in Figure 3D(ii) shows the results obtained using a pump output flow rate of 400 mL min À1 . When a pump with a flow rate of 900 mL min À1 was used, the angle plateaued at 90.0 in 6 s. In contrast, when a pump with a flow rate of 400 mL min À1 was used, the angle plateaued at 39.5 in 110 s. The average angular velocities were 15 and 0.36 s À1 for the 900 and 400 mL min À1 flow rate pumps, respectively. Moreover, the termination angles were similar when the leaf electroscope was connected to a high-voltage system (HEOPT-20B10, Matsusada Precision Inc.) and voltages of 3 and 0.5 kV were applied. Thus, it was determined that the SEG could output the same voltage under the open circuit condition. Figure 3E shows the results when a Coulomb meter was used to measure the amount of www.advancedsciencenews.com www.advintellsyst.com charge collected under the same conditions. Here, the flow rate of 400 mL min À1 resulted in the leaves opening at an angle of 20.3 for 6 s while in the case of the flow rate of 900 mL min À1 , the angle was 90 . As shown in Figure 3E, the amount of charge at this time was À90 nC for the flow rate of 900 mL min À1 and À20 nC for the flow rate of 400 mL min À1 . On dividing these values by the corresponding angle, we obtained a value of %À1 nC deg À1 in both cases. To estimate www.advancedsciencenews.com www.advintellsyst.com the charge density from the leaf angle, we modeled the leaves as shown in Figure 3F and derived the analytical solution. The relationship between the charge density, ρ, and the leaf angle, θ, can be expressed as in Equation (2) [20] ρ ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi where ε is the permittivity of air, g is the acceleration due to gravity, m is the leaf mass, h is the leaf length, and O is the leaf junction point. Figure 3G shows the relationship between the charge and the leaf angle. The blue plot is the analytical solution obtained from Equation (2), the orange plot is the relationship between the angle and the amount of charge as determined from the results given in Figure 3D,E, and the gray plot is the amount of charge collected by placing the Coulomb meter in contact with an open leaf. The blue and orange plots are similar in that the amount of charge collected increases with the leaf angle. We plan to develop a method to determine even smaller amounts of charge with reasonable accuracy using a leaf electroscope through open circuit measurements.
The SEG can also drive other electrostatic actuators and lightemitting devices. Figure 3H shows the driving mechanism of a dielectric elastomer actuator (DEA) and the charge flow from the SEG. The DEA was fabricated by sandwiching and bonding a thin film of a dielectric elastomer material with a highly elastic electrode. Figure 3I shows how the SEG could drive the DEA. When the area of the DEA was measured, it had increased by 19.7% from 0.835 to 1.000 cm 2 . The electrode area of the DEA remained at its maximum as the liquid continued to flow through the pump. When the injection of the liquid was stopped, it immediately shrank and returned to its original value (Video S2, Supporting Information). Figure S6, Supporting Information, shows a fluorescent lamp (FL9114018, Panasonic) lit using the SEG (Video S3, Supporting Information). An SEG with a length of 2 cm and compression ratio of 100% was used with these devices.

Wearable Applications
The wearability of the SEG was evaluated next. The system itself is stretchable and foldable because it consists of a liquid and a sponge; thus, it follows the movement of the human body without interfering with it. Moreover, the materials used are not harmful to the human body, making the SEG highly suitable for wearable devices. The SEG is also intended to function as an energy-harvesting device that generates electrical energy from the daily movements of the hands and feet. [21][22][23] Figure 4A shows the design concept for the aforementioned functionality. The liquid is transported to the sponge-filled area by human motion, and the liquid flowing out of the sponge is brought into contact with the electrode to collect the electric charge. The sponge is packed in the liquid outlet, and the liquid discharged from the sponge is brought into contact with the electrode. Figure 4B-D shows examples of the devices fabricated using the SEG, which generate electric charge based on grasping, stamping, and bending motions, respectively (Video S4-S6, Supporting Information). During grasping, liquid flow is generated by the crushing of a rubber ball that fits in the palm, thus passing the liquid through a sponge placed in the outlet. During stamping, the flow is generated by stepping on the pump with the foot. It is expected that, by incorporating this device into a shoe's insole, the user would be able to generate electrical energy without significantly affecting their daily life. Finally, during bending, the motion of the arm releases the liquid in the syringe by an attached mechanism. In this manner, by combining soft Figure 4. Use of the SEG in wearable devices. Using liquid charging, we succeeded in generating electric charge from body motion. A) Design concept. Wearer's movements cause liquid to be transported to the spongefilled area, and the liquid flowing out of the sponge comes in contact with electrodes, resulting in charge collection. B) Grasping type: grasping motion causes flow of liquid, which passes through the sponge to drive the leaf electroscope. C) Stamping type: the pump is operated by stepping on it with a foot, resulting in liquid flow. Because body weight is applied to the pump, the amount of force applied can be increased. D) Bending type: bending motion of an arm is used to push the liquid in a syringe. By combining three mechanisms, it should be possible to generate electrical energy from various locations on the human body using a device shaped like the body. and complex mechanisms, electrical energy can be collected from various parts of the human body using devices that follow the body's shape. The electrical signals of each device are shown in FigureS7, Supporting Information. It can be seen that stamping motion shows the largest output and the largest generated flow rate in the three motions.

Conclusion
In this study, based on the interactions between an insulating fluid (fluorinated liquid) and a porous dielectric (melamine sponge), we succeeded in generating and collecting charge. The overall charge generation process consists of two phases: charge generation and charge collection. During the charge generation phase, the fluid flow rate and sponge length/compression ratio affect the amount of charge generated. For example, a comparison of the results obtained for pump output flow rates of 900 and 400 mL min À1 showed that the amount of charge generated in the former case was up to six times higher. In addition, on examining the effects of the sponge length and compression ratio, we found that comparing the case of the length of 2 cm and the compression ratio of 100% and the case of length of 4 cm and the compression ratio of 25% showed that the amount of charge generated in the former case was up to two times higher.
Increasing the compression ratio of the sponge can increase the contact opportunities, but increasing the compression ratio of the sponge too much will reduce the flow rate. In addition, the relaxation of the electric charge must be considered, and it is difficult to estimate the appropriate contact opportunity with the sponge. We also want to reduce the size of the sponge because being wearable requires miniaturization, but if we make the tube thinner, the tube friction will become dominant, making it difficult to transport the liquid. This makes it difficult to improve the power output, and a possible trade-off relationship exists between the tube diameter and the power output.
As a proof of concept, only one sponge was used for charge generation in this study. However, it is expected that the output can be increased by modularizing the charged part and connecting several of them in series or parallel. For example, increasing the cross-sectional areas of the inlet and outlet to increase the contact area may result in an even higher output. In addition, we succeeded in driving a leaf electroscope without optimizing the material parameters, such as those related to their mixing and processing. In the future, it will be possible to increase the output by considering the charged row and adopting a new foaming material to the charged part. By developing our own foaming material, we can design a sponge with the optimum number of cells and pore density for the flow rate. In addition, it is expected that the output can be increased further by changing the surface morphology of the sponge to increase the contact area and by mixing the powder to improve the exchange of momentum upon contact.
The proposed SEG is made only of soft materials. It can thus be adapted to the shape of the human body such that it does not interfere with its movement. It is also highly suitable for use with wearable devices because it does not use any harmful materials. In addition, it is easy to fabricate and can collect electric charge under different conditions. Thus, this new power generation system should be ideal for the electrostatic actuators used in wearable devices, which must be lightweight and exhibit low power consumption, noise, and heat emission.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.