Examination of factors to improve the performance of dielectric elastomer transducers and their applications

Actuators, sensors, and generators using dielectric elastomers (DEs) are inexpensive and light, and can be easily to structured, multilayer-able, and very efficient. They are ideal for an eco-energy society. In the latest technology, an only 0.15 g DE can lift an 8 kg weight by 1 mm or more in just 88 ms. The near future, it can be applied to efficient drive systems of humanoid robots, systems that assist in driving the motors of electric vehicles, and various industrial machinery. It is highly likely that very thin and miniaturized DE sensors would also support the driving of motors. In addition, DE generators, which can be applied to various external forces, have attracted significant attention as a renewable energy source. In this paper, we discuss the R&D status of DEs using mainly commercially available elastomer materials, give examples of issues, and discuss and their potential applications, and usefulness. The excellent performance of the DEs mentioned above is largely due to their carbon-based electrodes. In this study, various carbon materials (including carbon grease, carbon black, MWCNT, and SWCNT) and their DE performances were compared.


Introduction
An electroactive polymer (EAP) was developed as an actuator that can electrically control actual muscle movement.As they are made of flexible materials, they are also called 'soft actuators'.Various types of EAPs are currently being developed.EAPs can generally be classified into three categories: (1) wet (ionic) types [1][2][3][4][5][6], (2) dry (electric field responsive) types [7][8][9][10], and (3) other types [11][12][13][14][15][16]: Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence.Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
1) Wet types use the electrically driven mass transport of ions or electrically charged species to change their shape (or vice versa).They can be driven by low voltages.However, they are relatively slow, limited in size, and energy inefficient.They operate best over a narrow range of temperatures, and must often be kept moist.Examples of wet types are given below.
(a) Ion polymer metal composite materials: these materials are driven by ion migration and water molecules within a polymer film [1,2].(b) Conductive polymers (CPs): CPs use a driving force to move ions by applying a voltage between the CPs [3].(c) Ion polymer gels: these are driven by the movement of ions caused by chemical changes in ions (such as pH changes) [4,5].(d) Carbon nanotube (CNT) actuators: these actuators use the mechanism of quantum chemical-based expansion due to electrochemical double-layer charging, so they do not require ion intercalation [6].2) Dry types use an electric field to affect the shape change by acting directly on charges within the elastomers (or vice versa).Dry types can be fast, efficient, and insensitive to temperature fluctuations.These devices can operate at relatively high voltages and low currents.
(a) Dielectric elastomers (DEs): they are driven by the Coulomb force generated by sandwiching an elastomer between compliant electrodes and applying a high voltage to it [7,8].(b) Piezo polymers: they use the piezoelectric phenomenon [9,10].3) Other types do not require electricity in order to drive, but they can be driven like muscles using air, light, heat, magnetism and so on.These can be classified as EAPs in this paper [11][12][13][14][15][16].
In this paper, in consideration of the papers mentioned above, electrodes (including carbon grease, carbon blacks, and CNTs), elastomer materials, DEs' durability, circuits, operating conditions, and other related issues are described and discussed.Moreover, the latest R&D trends of the DE actuator (DEA), DE generator (DEG), and DE sensor (DES) and their future potential are discussed.

R&D of DEAs
DE's driving principle, materials, circuits to drive them, durability, application, etc. of DEs are discussed.

Background of DEAs
DEAs can operate based on simple principles.The elastomer was sandwiched between thin compliant electrodes.When a voltage is applied across the compliant electrodes, the elastomer shrinks in thickness and expands in area.Thus, when a voltage is applied, pressure can be efficiently generated by an electrode.The pressure p is expressed as follows: where, ε r and ε o are the dielectric constant of the free space and the relative dielectric constant of the elastomer, respectively.
Where E is the applied electric field, V is applied, and d is the thickness of the elastomer.
To enhance this pressure, the selections of elastomers types, electrode materials, and those synthesis methods are important factors.
From these studies, the factors that affect the performance of DEs include the Young's modulus, electric field breakdown strength, pressure generated by the Coulomb force, dielectric constant, and thickness of the film.Thus, DE elastomers, which can obtain a large deformation value, have an electric field breakdown strength (pressure generated by Coulomb force), Young's modulus, or a dielectric constant value.However, even if these parameters are increased, the DEs do not appear to transform significantly.In other words, the elastomer film does not deform [18] as long as there is no greater pressure owing to the larger Coulomb force (in the thickness direction).This is because when a voltage is applied to the DEA, the film is pushed by the Coulomb force and extends in the horizontal direction, but at the same time the film hardens.The slope of the SS curve of a film of DE that has a small or gentle curve may not become stiff quickly and may be transformed further.In other words, a large deformation may be obtained with a relatively small pressure.In accordance with this idea, the above parameters were considered, using the actual SS curve data and the dynamic viscoelasticity data of elastomers [18].Silicone, acrylic (including the modified version), and HNBR were used to measure the SS curves and dynamic viscoelasticity (figures 1 and 2).The hardness of the films was the hardest in silicone, followed by HNBR and acrylic.As shown in figure 1, the silicone film requires a larger force to deform because the film is harder than acrylic and HNBR.In other words, because of the large force, the film stretches, but it becomes faster because of the large compressive pressure, resulting in less deformation.From figure 2, it can be seen that silicon has fewer dynamic viscoelasticity effects than acrylic.As a result, the viscoelasticity of the elastomer is low; therefore, when the voltage is applied, the film becomes faster and stiffer, and the growth is reduced.Conversely, because the viscoelasticity of acrylic is larger than that of silicone, it is possible to make a larger deformation, which may be more dependent on the tensile speed.Interestingly, the slower the acrylic pulling speed, the greater is the maximum stress; thus, a larger deformation can be obtained.This is because of its large viscoelasticity (see figure 2).In the modified version of the acrylic, it was a 500 mm min −1 , and gained greater elongation than other cases.This was because the distortion of the film was removed.As a result, it is thought that the effects of large viscoelasticity Note: when the elastomer was created (e.g.cast), it was slightly distorted by solidification.By removing the distortion of the 3M film, it became more uniform, resulting in an increase in deformation at 300 mm min −1 , and 500 mm min −1 [18].appeared more.Figure 1 shows the results of the SS curves of the silicone (ELASTOSIL FILM 2030 250: shown by the red line), the HNBR ver.3 (see the blue line), the acrylic material made in the United States (3M/4905: shown in orange), and the film that corrected the distortion of the US-made acrylic (green line).For each material, the SS curves were measured at speeds of 100 mm min −1 , 300 mm min −1, and 500 mm min −1 , as shown in figure 1.It is noted that by a certain method, the films exhibited a larger elongation at 500 mm min −1 and 300 mm min −1 by removing the strain of 3M.
Table 1 uses the above film to create a circular DAE with a diameter of 40 mm and compares the elongations.The electrode material used was a single-wall CNT (SWCNT/ZEONANO ® -SG101, ZEON Corporation), and the voltage used for the DEA was 3000 V [36].Although the thickness of each film was not constant, the elongation results supported this hypothesis.In this experiment, the films were pre-stretched in advance.The amount of this pre-stretch used was 50% (a relatively smaller size), based on the SS curve and dynamic viscoelasticity results of the comparison films.This is because that the silicone was relatively hard, so 50% was determined as the amount of pre-stretch.Therefore, in the standard of amount of pre-stretch for the experiment, the degree of pre-stretch for elastomers used was set to 50%.There are various discussions on a pre-stretch [20,22,30,32,36,53,54].However, if the pre-stretch was not applied, the film could not be restored after driving several times, and it would be unlikely that the elongation of the DE when a voltage was applied would be large [36].
It could be considered that the dynamic viscoelasticity of the material and the pre-stretch were similar to the relationship between the potential energy of a spring (i.e.molecular resistance) and its extension [36].In other words, when the degree of pre-stretch is increased, its behavior is analogous to that of the rising of the potential energy of the spring.Therefore, the nature of the dynamic viscoelasticity is gradually reduced, and the film gradually becomes harder, making it harder to elongate.However, when the degree of pre-stretch was small, the nature of the spring was also considerably small, and the nature of the dynamic viscoelasticity could appear too strong, making it difficult for a DE to attain a very good performance.This situation is the so-called the "random coil-shaped".That is, it is a formless situation (random coil shaped) in a situation where no stress is applied, so it would be difficult to move the unit freely (see the later part of this section).For the model that expresses a stress/easing mechanism of dynamic viscoelasticity, there is a Maxwell model that connects the spring (elastic response) and the dashpot (viscous response) in series [37].These phenomena may be possibly explained using this model.
As a method of softening the elastomer film, it is possible to increase the amount of crosslinking agent by cutting one of the double bonds or performing both and adjusting the hardness [17,18].Figure 3 shows the SS curve of HNBR ver.1, HNBR ver.3, and HBR ver. 6. HNBR Ver.1 is quite difficult because it is not processed above.In HNBR ver.3, the amount of crosslinking agent has been reduced to 2%, and double bonds were   reduced to 1%, which resulted in more softness than HNBR, and it has changed 220% as a circular DEA (see table 1).HNBR ver.6 sets the amount of crosslinking agent to 1%, and double bonds, the same as HNBR ver.3, were 1%.The film for a DE was too soft to work.When adjusting these factors, it is necessary to be careful when adjusting the amount of double bonds.Figure 3 shows the HNBR tensile test results [35].
Here, the above behavior will be explained in more detail as follows: An elastomer elongates when pulled, and returns instantly when it is released.An elastomer is a polymer with a mesh structure and dynamic viscoelasticity.That is, the unit (segment) that makes up the chain can be moved freely, and it shows a formless situation (random coil-shaped) when no stress is applied.It also has the following characteristics: Note: from a rheological point of view, rubber is seemingly a solid, but at the molecular level, it can be considered as a gas that does not flow.
In other words, it should be considered that as a solid that does not flow.Thus, the polymer was elastic against instantaneous changes.
From the points mentioned above, an elastomer could be assumed as an ideal elastic body; therefore, it could also be assumed that energy could be stored before and after deformation.In other words, when the elastomer material is pulled, the work caused by the external deformation is stored inside the object.This is similar to pressing the piston of a container containing the ideal gas.When the elastomer was not stretched, the molecule could move freely, and the molecular chains between the crosslinks also moved around freely.However, as the molecular chains elongate, they become more difficult to move.This difficulty of movement is considered the same phenomenon as a 'decrease in entropy.' In other words, when the force of moving around more freely is released, it is a source of elastic recovery.This is considered the same as the resistance of gas molecules that have lost space to move when the ideal gas inserted in the container is pushed with a piston.Here, the equation of the state of the ideal gas is governed by the law of thermodynamics, the pressure is proportional to the reverse number of volumes, and the pressure increases as the piston is pushed in.Because the elastic origin is considered the same here, it is assumed that the proportional relationship between the force and the elongation when pulling the elastomer is exactly the same.In other words, pulling an elastomer elongates according to its power, and as a result, the following relationship formula is established: where, v m is the total number of molecular chains contained in a unit volume, k b is the Boltzmann constant, T is the temperature, R is the gas constant, and λ is the length of the change in the elastomer.Therefore This can be deformed as shown above.Here, ρ is the density, and M is the molecular weight of the high molecular chain between the mesh and the mesh structures, that is, a polymer has three-dimensional (3D) mesh structures by bridges.From the above formular, it can be seen that the molecular weight between the mesh sheets is small; that is, the increased degree of crosslinking increases the stress.Based on this formula, the above phenomenon can be explained.
To increase the dielectric constant of the film (see equation ( 1)), Skov, Sikulskyi, Liebscher, and others have added barium titanate, copper calcium titanate, titanium dioxide, etc [23-25, 36, 54].The dielectric constant can be increased by adding them.However, as a result, the film becomes stiff, and the performance of DE often decreases.In addition, the electric field breakdown strength, decreases because of the poor compatibility of rigid particles with the matrix, resulting in many defects.However, when conductive polyaniline (PANI), which is an organic CP, is wrapped by an insulating polymer and then added to the silicone matrix as a high-dielectric-constant filler, a larger elongation may be obtained [54].However, their lifetime is not well understood yet [36].
Thermoplastic polyurethane (TPU)/polyethylene glycol (PEG) composites, they can be prepared using a solution blending method.However, the ionic conductivity of PEG also deteriorates the electrical insulation properties of the composites.Furthermore, the electric field breakdown strength was significantly reduced by the addition of PEG [54].In the case of 3M VHB acrylate elastomers, a large pre-stretch may be possible by constructing an interpenetrating network structure.However, increasing the degree of pre-stretching is likely to shorten the lifetime [36].
Opris reported on that one-step process toward selfhealable, silicone-based elastomers with large and tunable permittivity [55].Tugui et al claimed also that silicone elastomers have been shown to be well-suited for dielectric elastomer actuators (DEA), mainly due to their unique combination of properties such as, high elasticity and reliability, fast electromechanical response, and high thermal stability [56].

Electrodes of DEAs
DE electrodes include thin metal foil, liquid metal, carbon grease, carbon black, multi-wall CNT (MWCNT), and Note: the selected SWCNT were further separated the above SWCNT in a fixed angle rotor-type ultracentrifuge.As a result, SWCNTs with small diameters were selected.SWCNT [17, 29, 31, 34, 35, 41-43, 50, 52].Liquid metals and carbon grease must be carefully controlled to prevent liquid leakage when driving DEA [18,31,36,43,50].Table 2 shows the weight that can be lifted with a 5 mm stroke using different electrodes.A DEA having a diameter of 8 cm was used in this experiment [41].The elastomer used was acrylic (3M/4905), and its weight was 0.1 g.The amount of this pre-stretch used was 50%.The SWCNT used in this experiment were distributed in an aqueous sodium cholate solution and was distributed using ultrasonic homogenizers.To create an electrode, the hand spray was newly developed, and the SWCNT were sprayed evenly to form 50 µm electrodes.As a result, the thickness of electrode was thin, so the drag force for the elongation of the film was small, and DE was easily to elongated.As shown in table 2, highly conductive materials can generate greater force.As the DEA was extended, the density of the electrode material decreased.However, a cylindrical-shaped material, such as a CNT, is likely to be in contact with the material, even when it is stretched, so it can make a larger change.Because the selected SWCNT is a CNT with a thinner diameter, it is considered that the SWCNT density in the electrode is higher than that of the above SWCNT, so it is highly likely that somewhere is in contact, even if there is a large elongation.The above mechanism can be explained using a carbon black electrode as the example [36].Figure 4 shows the photograph of a DEA using the carbon black as an electrode: (a) is the power supply OFF and (b) is ON.As shown in the schematic diagram in figure 5, the electrodes were not pulled because the power was off.Therefore, the light dose not penetrate.In the drive state, the electrodes are pulled, and a certain amount of carbon black particles is pulled away, so that the light can penetrate through those parts.However, because all particles do not leave, they work as DEA.In this DEA, 3M/4905 is used, and the thickness of the electrode is quite thin at 40 µm.A transparent electrode is made by creating a fairly thin electrode (30 µm) using a selected SWCNT.In this case, the CNTs are well dispersed on the elastomer, and the amount is sparse; therefore, it is difficult to see the CNT by the human eye.Figure 6 shows a DEA using a transparent electrode [41].
As explained above, CNTs have a very fine cylindrical shape; they form a mesh (or network) when dispersed well.Therefore, even if the needle pierces it, it is pierced only a small hole somewhere in the polymer inside the CNT mesh, which has almost no effect.This is the same as stabbing a balloon with needle.If the CNT electrode is stabbed, only the inner polymer of the CNT net is burned, and there is no effect on the other parts.

Circuit of a DEA
A high voltage is required to drive the DEA, and the normal voltage must be made to a high voltage using the Cock Croft Walton Circuit (booster circuit) [17,48].Figure 7 shows an example of a booster circuit that drives a sheet-type DEA vibrator in a game [17].Most of the circuits shown in the figure are equalizer circuits for controlling the audio circuit and sound built into headphones.The circuit that drives the DEA is surrounded by red and is 0.9 mm × 15 mm in size.The circuit size was small because the current was very small.This booster boosts from 5 V to 1,200 V. Three silicone sheet-type DEAs (3 mm × 25 mm) were driven using the boost voltage.

Voltage level used in DEA
A high voltage (approximately 1,000 V to 3,000 V) is required to drive the DEA.However, the DEA drive voltage is closely related to the elastomer film thickness.If a low voltage is required, it is better to reduce the film thickness [20,30,36].For example, the voltage that drives a 4 cm diameter diaphragm-DEA produced using an acrylic film (3M/4905) with a thickness of 35 µm is 2 V [36].
Another factor that determines the DEA voltage is the electric field breakdown strength of the film [2].The larger the value, the larger the voltage.Thus, it depends on the type of film used, but in general, a larger force and elongation can be expected using a larger voltage.However, as described above, if the film is harder, a high voltage can be applied, but elongation cannot be expected.

Lifetime of a DEA
In general, it is said that the life of DE is short, but it has been reported that if used at a much lower level of DEA's maximum performance, its life would be greatly extended [36,51].Acrylic (3M/4905) and DEA using carbon black electrodes were transformed by deforming approximately 3%-5%, and the drive cycle achieved 10 million times at max [36,51].Recently, using a 0.15 g acrylic DEA, it has become possible to lift an 8 kg weight by 1 mm or more in 88 ms (see figure 8) [17].In addition, the DEA electrode used SWCNTs.If this system was driven with a low output, it would be possible to achieve a high durability.
DEs are also vulnerable to humidity.This is particularly noticeable when the DEA usage temperature is high.Initially, it was pointed out that acrylic materials were vulnerable to moisture, but it was found that silicon was also vulnerable [55].Silicone materials are somewhat disappointing because the temperature range is wide, and they are expected to generate waves.However, this can be managed by placing the DE in a moisture-resistant polymer bag, controlling the pressure in the installation location of the DE, adding silica gel, enclosing it in nitrogen, or performing all these methods.Thus, the durability can be greatly extended [36].Albuquerque and Shea also experimented, using a soft silicone as the thin encapsulation layer encapsulated, and reported that the lifetime increased by up to 140 times, even at 85 • C and 85% RH [30].Nevertheless, there is a difference in the lifetime depending on the electrode materials [36,42].SWCNT appeared to be the best [36].
Furthermore, if multiple DEA systems are prepared and the same of DEs are suddenly damaged, it can continue to drive by using the other DEs, and it will be possible to avoid the risk of not being able to drive at all [36,57].Recently, attempts have been made to create DEAs using 3D printers (including Laser Structuring), a micro atmospheric plasma jets, or a pressure plasma jets [24,26,52].However, from the perspective of improving the durability and performance, the benefits are not clear when compared to conventional methods.

Application examples of DEA
A DEA can use several thousand volts of high pressure for driving voltage.Therefore, because it can be driven by a very small current, it can reduce the losses caused by mechanical sliding, electrodes, wiring, and driving circuits.As a result, there is a great possibility that high-efficiency actuators will be realized (17,36).In addition, the DEA itself is light, and the amount of heat generated is low; thus the electrode and wiring can be thinner and finer, and the driving system can be reduced.
The types of DEA that have been developed so far include roll-type, sheet-type, diaphragm-type, small insect-shaped robot, snake-shaped robot, swimming robot, wall-climbing robot, flapping robot, jumping robot, and so on.These devices have only a small output in many cases [17, 33-36, 44, 49, 53].Lu et al claimed that the actuator could be independent of the length ratio, making it possible to mimic natural muscle with a slender shape [58].Wingert et al showed a concept in a simple experimental prototype of a 6-DOF binary manipulator [59].Conn and Rossiter developed a cone DEA, aiming for integrated 6-DOF actuation with muscle-like performance from a single structure [60].The origami-inspired active structure can be folded into complex 3D shapes and are possibly more lightweight and versatile than traditional actuation methods.McGough et al attempted finite element analysis and experimental verification of a folding actuator made of unimorph actuators [61].Chen et al aimed to develop an aircraft robot with passively soft artificial muscles having an open-loop (driven by a predetermined signal without feedback), in order to demonstrate stable climb flight and closed-loop hover flight [62].Gupta et al reviewed applications of DEAs to soft robots, including robotic grippers, terrestrial robots, underwater robots, aerial robots and humanoid robots [63].Examples of interesting applications include devices remotely control the angle of the mirror of the astronomical reflective telescope in orbit using a DEA [33], and DEA devices [44] for controlling airplanes that explore Mars' surfaces.Such a device must be lighter in order to carry a rocket.The DEA is a very promising device in this respect.In addition, DEAs are available as speakers [36].
As mentioned above, using a 0.15 g DEA, it is possible to lift an 8 kg weight by 1 mm or more in 88 ms (see figure 8) [17].
In addition, the actual weight of the acrylic was 0.15 g.However, after operating it a few times, it tended to break, so the lower part of the diaphragm (see figure 8(b)) was reinforced with acrylic material.
The system shown in figure 9 is more practical than an ordinary motor drive system for driving a robot or prosthetic hands/fingers (36).
This system is the same as the tendon system of human hand, and the robotic finger is driven by wires.The two diaphragms (see figure 9(a)) were connected to the tip of the finger with each wire, and the fingers could be bent or stretched by alternately moving each diaphragm of the DEA up and down.For reference, the diaphragm stroke was 1.5 mm.With such small movements, the finger can be driven.As mentioned above, the DEA's strokes, which can lift 8 kg, are approximately 1 mm, but the force and elongation are inversely proportional, so it can be applied to robots and prosthetic hands sufficiently by reducing the weight that can be lifted.To improve the efficiency of rehabilitation, Carpi et al investigated 'active' orthoses with devices that could electrically control mechanical stiffness [64].Calabrese et al showed the potential of an electroactive bandage made of two prestretched layers of electroactive acrylic elastomer sandwiched between two insulating layers of passive silicone elastomer [65].Calabrese et al also showed a smart robot structure that exploits anisotropic friction to achieve stick-slip locomotion [66].

R&D of DEGs
A DEG is a power generation system that uses an external force on the DE to change its shape.The driving mode of DE power generation is the opposite of that of the actuator.Thus, it is a power generation system that uses an external force on the DEG to change the shape of the DE, and uses the increase in static energy, generated by the DE [17].The structures of the DEA and DEG are basically the same.

Background of DEGs
In the power generation mode, the DE is considered to be a variable capacitor whose volume is unchanged.When the elastomer film is extended (high capacity), charges appear on the surface of a film.When this film shrinks (low capacity), the elasticity of the elastomer works in opposition to the electric field pressure, thereby increasing the electric energy.Again, this is because the volume does not change, even if the DEG is deformed.Microscopically, the electric charges are pushed out toward the electrode side owing to the elastic force of the film, (the thickness of the elastomer film increases owing to the shrinking state), and the distance between each electric charge is shorter (at a reduction of the planar region in a contracted state).Such changes in charge increase the voltage difference and, as a result, the amount of electrostatic energy increases (see figure 10) [38].
The electrostatic capacity C of the elastomer film can be described as follows: where ε 0 is the dielectric constant of free space, ε is the dielectric constant of the polymer film, A is the active polymer area (area of a variable condenser), and t and b are the thickness and volume of the polymer, respectively.Because an elastomer is essentially the same as rubber, even if it elongates with an external force or returns to its original length by removing its external force, the volume of the elastomer is unchanged, meaning that At = b = constant.The power generation E of the DE per cycle of extension and contraction is related to the change in the electrostatic capacity of the DE and is represented as follows: where, C 1 and C 2 are the electrostatic capacities of the elastomer film in the stretched and contracted states, respectively; and V b is the bias voltage.
Considering the changes in voltage here, the charge Q of the elastomer membrane is considered to be constant over a short period and in a basic circuit.Because V = Q/C, the voltages of the extension and contraction states can be expressed as V 1 and V 2, respectively, and the following equation is obtained, where C 2 < C 1 , the contracted voltage is higher than the stretched voltage, which corresponds to the energy argument noted above.A higher voltage can be measured and compared to predictions based on the theory developed by Chiba et al [38][39][40], and experimental data based on high-impedance measurements were in excellent agreement with their predictions [40].When conductivity is assumed to be preserved in the range of electric charging, Q remains constant.

DEG circuit
As an example of a circuit design, Mckay et al demonstrated the possibility of a DEG in which the electric circuit elements in the film were installed [45].Anderson et al introduced soft DE electronics technology that can replace diodes in the DEG priming circuit [46].To improve the control of the DEG charge state, the diode can be replaced with active switches between the DEG and voltage source and load [45,46].Kussel et al investigated a passive DEG power generation system that used only diodes at the generator level [47].

Important points of design of a power generation circuit.
A DEG power generation control circuit must be charged at the most extended state of DE, and it is also important to discharge when the extension state is set free.It is possible to use a mechanical switch to switch between charging and discharging statuses.However, it is better that the mechanical switch uses a semiconductor switch with diodes in the long-term power generation system because of the short contact life.Specifically, at the point where the DE is the most extended, a voltage should be applied to the DE and the charging state (initial charging stage).When the extension of the DE is set free and the voltage charged to the DE is the highest, discharge should be performed immediately.It is important to perform charging and discharging according to the extension state of the DE.At the optimal timing, if a charge or discharge was not performed, the power generation efficiency decreased significantly.One method for adjusting the charge and discharge timing is a displacement sensor or voltage monitoring circuit.However, it is necessary to select a method that can operate with less power consumption.The voltage used for a DEG becomes a high-voltage such as several thousand volts, the problem is there are only a few devices can be used.In the future, with the spread of DEGs, the development of dedicated semiconductors etc. will proceeds, and it will become possible to develop a simpler and more efficient power generation control circuit.

Circuits to charge to secondary batteries.
To store the electricity generated from DEGs to the secondary battery, a stable DC voltage that matches the specified voltage of the secondary battery is required.DEGs cannot be stored in a secondary battery because the power generation energy is intermittent at high pressures.Figure 11 shows an example of a DEG power generation system that can be stored.In this circuit, a capacitor that can efficiently accumulate electric energy generated intermittently with a short charging time is used.In addition, the electric energy stored in the capacitor can be used directly; however, by supplying it to a storage circuit using a lithium-ion battery, more electric energy can be stored.
Based on the above, a DEG device was created using two circular DE cartridges with a diameter of 8 cm, and 33.6 mJ of energy was obtained.SWCNTs were used as the electrodes of this device.The electricity generated by this DEG can be charged to a secondary battery [17].

Performance of DEG systems and application examples of the DEG
First, as examples of improvements on DEG system performances, basic research has been conducted on the use of external forces (mechanical energy) and the efficient generation of electricity (converting it into electricity).In these examples, Koh et al examined the maximum energy conversion cycle considering various failure models, such as electrical breakdown, electromechanical instability, and loss of tension [67].Huang et al claimed that some improvement in energy density with power density using equi-biaxial stretching was achieved [68].A reduced dynamic model for inflating a circular diaphragm-DEG (ICD-DEG) that features one kinematic degree of freedom, which accounts for DE viscoelasticity, was also investigated [69].The performance of  a DEG formed by a hyper-electro-elastic annular film deforming non-homogeneously out-of-plane was investigated with parameters adjusted to simulate the behavior of materials such as an acrylic elastomer and natural rubber [70].Moretti et al also researched parallelogram-shaped (PS) DEG and argued that they could be used effectively as rotating and linear transducers [71].Brochu et al claimed that they showed the capability of generating approximately 4 mJ per cycle in a single layer device with an active elastomer volume of 0.57 cm 3 and a maximum energy conversion efficiency of over 55%, using a wind power system based on a DEG [72].Zhou et al pointed out that material viscoelasticity has a significant influence on the electromechanical coupling and dynamic performance [73].It is known that the timedependent response of materials influences the performance of DEG and DEA and must be taken into account [36].Bortot et al also focused on the viscoelastic behavior of polymers and the change in dielectric constant with strain [74].Lu et al reviewed the viscous effect and the vibration of the DE [75].In section 2.6, the lifetime of DEA is discussed.In case of DEG, Han et al claimed that they achieved more than 100 000 cycles of mechanical deformation between 0 and 100% area strain, using a four-layer 3M acrylic film to create a circular diaphragm DEG [77].They also argued that the introduction of a DE binder between acrylic films showed a strong adhesion effect between the accumulated DEGs, enabling longterm operational stability.For reference, the typical wave of the ocean is approximately 0.2 Hz, the number of DEs driving in one year is approximately 6.3 million.As mentioned in section 2.6, if it was used at a level that was far below the maximum performance, it could possibly be driven approximately 10 million times.Therefore, a wave power generation system seems to be functionally practical when the maximum performance is sufficiently reduced in terms of durability [38].
Similar to DEAs, the factors that improve the performance of DEG include multi-layered structures, shapes (sheet, diaphragm, drape, etc), and types of elastomers and electrode materials [17,36,66,69,70].The impact of differences in the types of electrodes used seems to be a major factor in improving the power generation performance (see table 3).Using a drape-type DEG having a height of 120 mm and diameter of 260 mm, the amount of power generation was compared, as shown in figure 12.The DEG was pulled approximately 60 mm, and the generated power is summarized in table 3.An acrylic film was used, and its weight was 4.6 g.The amount of this pre-stretch used was 50%.The electrode materials used for comparison were carbon black, MWCNT, SWCNST, and selected SWCNT.
As an example of these power generation methods, fluid/wind power generation (see figure 13) and the wave generation (see figure 14) are shown below.
In fluid power containing wind power, the wings are moved up and down using Kalman turbulence, and the force created from the upward and downward movement is transmitted to the DEG and drives it.In this system, even if the flow of water is quite loose (approximately 0.5 m s −1 ), it is considered that wings can be driven sufficiently [36].Therefore, it is estimated that this generator is possible in various places, so it has a large value for renewable business and prevention of crime in places without solar cells [36].
The upper and lower movements of the waves were transmitted to the DEG, using a system that incorporated a diaphragm or drape-shaped DEG in the buoy, and power could be generated.This generator can generate electricity even with small waves, so it is thought to be available on ports, sea sides of waving blocks, or see shores, using a system that incorporates a diaphragm-type or drape-shaped DEG in the buoy [17,36,[38][39][40].
The data shown in figure 15 were obtained from a power generation experiment conducted at the tip of Izu Peninsula (Shizuoka Prefecture, Japan).When the height of the waves was 4 cm, 52 mW of electricity was generated.The maximum  instantaneous power of 131 mW was obtained at a wave height of 17 cm.However, the average energy per second was 27 mW because the wave cycle was 5 s.Interestingly, in the case of a small-amplitude wave with a height of approximately 4 cm, the cycle is a very fast at 1 s.It is important to note that 52 mW of electricity can be generated.This is approximately twice the average.These results prove that DEG generation with a rapid wave cycle of small-amplitude waves can provide more power.In this experiment, a buoy was equipped with a hydrogen generator next to the power generation buoy, and hydrogen was successfully generated using the power generated by the DEG (see figure 14).
Oscillating water column (OWC) wave energy generators are promising wave energy converters.The efficiency of the OWC device peaks during a resonated wave cycle [78].However, the efficiency was significantly reduced during other waves.Two possible ways to increase the power generation efficiency of an OWS can be considered: (a) By placing DEGs around an OWC, waves can be processed during the bad period of the OWS.As described above, the wave cycle that can be generated by the DEG is very wide.(b) DEGs can be placed instead of the compressor and electromagnetic motor used in the OWC, and air compressed by the waves transforms DEs to generate electricity.Using this idea, it was done by incorporating DEGs into the OWC.[36,69,73,77], as shown in figure 16.
In this system, when the waves arrive, there is an air section at the bottom of the DEG, and the energy of the waves compresses air upward.As a result, the DEG is deformed, and the power is generated (see figure 16(b)).

R&D of DESs
Because the DEA's change and extension are theoretically proportional relationships, they can be used as load or stretch sensors.To date, a load sensor has been created using a diaphragm-type [18,79,80], a complex laminated sheet type, etc [82].However, nowadays, a simple shape-sheet DES is possible, using HNBR ver.3 and HNBR ver.6.In HNBR ver.3, the measurement load range is 39.23 N-1176.80N and in HNBR ver.6, 0.01 N-196.13N is possible [35].The diameters of these sensors were 20 mm, thick 200 µm, and an SWCNT was used as the electrode (see figure 17(a)).As shown above, HNBR ver.6 is a material that is much softer than HNBR ver.3.In addition, the supporting material (H05-100), which is slightly harder than HNBR ver.6 is located under the ver.6 (see figure 17(b)), the load given from above is uniformly given, without any damage, so even less pressure can be measured.The stretch sensor also created a rectangular sheet sensor using HNBR ver.6 (see figure 17(c)).The dimensions were 10 mm × 20 mm, the thickness was 200 µm, and the electrode material used was SWCNT.It turned out that this sensor could be extended by approximately 400%.These pressure sensors and stretch sensors (as position sensors) were fabricated to   remotely control the robot fingers.In the diaphragm type, attaching it to the finger was quite clunky and it was a bit unreasonable in size, but this sensor was a smart and accurate sensor [35].
These small and very thin sensors are likely to be useful for attaching them to the robot's finger and knowing the pressure and location information while driving.Remote control can also be used as a system, in which the information is directly transmitted to the operators when the fingers of the robot detect the pressure and position.

Summary
From the review above, the following findings were obtained regarding elastomer materials, DEAs, electrode materials, DESs, DEG systems, and their step-down circuits: 1) Elastomer material and DEAs If the slope of the SS curve of the elastomer is low or gentle, the DEA can be better deformed, and removing the distortion of the elastomer film may result in a larger deformation.For films with high dynamic viscoelasticity, pre-stretching seems to contribute to deformation.In addition to these, appropriately reducing crosslinking agents and double bonds in the film contributes to improved deformability.Regarding film thickness, by making the film thinner, it becomes possible to drive at lower voltages.Regarding lifespan, it appears that using the DEA at levels well below its maximum performance will significantly extend lifespan.
2) Electrode materials By using a more conductive electrode material such as SWCNTs, the elongation or output of DEs can be further improved.

3) DES
By adopting an improved version (Ver.3) that softens HNBR, it has become possible to create a small and thin seashaped load sensor that can measure forces from 39.23 N to 1176.80 N. In addition, in the case of the improved version of HNBR with an even softer sheet (Ver.6), it has become a small and thin sheet-like load sensor that can measure from 0.01 N to 196.13 N. Using this HNBR (Ver.6) as a small and thin sheet stretch sensor, it has become possible to measure up to 400%.

4) DEG systems
Even when the river flow is slow (about 0.5 m s −1 ), the DEG can generate electricity using Karman vortices.Furthermore, in the case of a buoy equipped with a DEG, it is possible to generate electricity even at wave heights of only about 4 cm.

5)
Step-down circuit for DEG By devising a step-down circuit, it is possible to charge a secondary battery even with a small amount of power generated from a small DEG.These improvements in the DEA, DEG, and DES performance will accelerate the delivery of more durable devices that meet business needs.

Figure 1 .
Figure 1.The results of SS curves of the silicone (ELASTOSIL FILM 2030 250: shown in red), the HNBR ver.3 (shown in blue), the acrylic material made in the United States (3M/4905: shown in orange), and the film that corrected the distortion of the US-made acrylic (shown in green), (a) Measured at speeds of 100 mm min −1 , (b) 300 mm min −1 , and (c) 500 mm min −1 .Note: when the elastomer was created (e.g.cast), it was slightly distorted by solidification.By removing the distortion of the 3M film, it became more uniform, resulting in an increase in deformation at 300 mm min −1 , and 500 mm min −1[18].

Figure 2 .
Figure 2. The results of viscoelasticity tests; (a) storage and (b) loss modulus of various materials (the silicone shown red, the HNBR shown in blue, the 3M shown in orange and the improved version shown in green).
(a) When a tension is applied, it elongates with almost no loss of energy.(b) When stretched sufficiently, it shows high tensile strength and high elasticity.The dynamic viscoelasticity was lost at this point.(c) When the tension is excluded, it shrinks and returns to its original length.Dynamic viscoelasticity returns at the point.

Figure 4 .
Figure 4. Changes in light transmittance of a DEA using carbon black.(a) Power off, and (b) Power on.

Figure 5 .
Figure 5. On the elastomer, a state in which carbon black particles are lined up as an electrode and the state in which the elastomer is being elongating.

Figure 6 .
Figure 6.Transparent diaphragm DE (used acrylic as an elastomer).(a) Photo with the needle stuck transparent electrode, and (b) DE having a 4 cm diameter lifts a cylindrical weight of 16 g.

Figure 8 .
Figure 8. Diaphragm DEA with a weight of 8 kg.(a) 8 kg weight placed on the DEA on a test bench, and (b) diaphragm DEA.

Figure 9 .
Figure 9. Finger drive system using a diaphragm DE: (a) driving of fingers by DEA, and (b) bending the finger by adding the voltage.

Figure 10 .
Figure 10.The voltage wave form that occurs when the DE return to the original state from a contracted state: the length of generated voltage waveform is from about several mm-seconds to several tens of mm seconds in one impact in the case of a piezo element.One of the features of the DE is that the power generation time is long, about 150 ms to 200 ms.

Figure 11 .
Figure 11.Block diagram of DEG power generation system that can be stored using a capacitor.

Figure 13 .
Figure 13.A schematic diagram of fluid (wind) power generation system.

Figure 15 .
Figure 15.Wave form data obtained by wave power generation and electric power obtained accompanying it: (a) wave data, and (b) electrical power obtained from each wave corresponding to the energy.

Figure 16 .
Figure 16.OWC wave energy generator, (a) bottom of wave, and (b) top of wave.

Figure 17 .
Figure 17.The prototype of a DE load sensor and DE stretch sensor using HNBR.(a) Photograph of the DE load sensor, (b) A cross section of the DE load sensor, and (c) DE stretch sensor.

Table 1 .
Film types and their elongations.

Table 2 .
Types of electrodes and weights to be lifted.

Table 3 .
Difference in power obtained when changing electrode material.