# Dielectric Elastomer Actuator for Soft Robotics Applications and Challenges
## 4 Artificial Muscle Design and Fabrication
### Linear Expansion DEA  
A cone actuator is a configuration in which DE material is inserted inside of a rigid frame and extended by pre-loading, as shown in Figure 6a. When the Maxwell stress is applied to the DE material, the stroke occurs in the direction of pre-loading by areal expansion. This actuator has the advantage of a relatively large energy density compared to other configurations because the pre-loading energy can be used. Cone actuators can be classified into single-cone actuators and double-cone actuators according to their shape. For the single-cone actuator, pre-loading can be applied by a hanging mass or using a compressed spring in the center. In the case of a hanging mass, a single-cone actuator composed of two layers of VHB 4910 acrylic elastomer and a mass of 200 g was reported to have a stroke of about 7 mm and an output force of 1.7 N [71; 72]. Also, a \(86\times 86\times 25\) mm sized spring single-cone actuator made of 50 \(\upmu\)m Elastosil had outstanding lifting performance, lifting 10 kg by 3 mm [73]. A double-cone actuator is composed of two antagonistic cone actuators and connected with a strut or magnetic repulsion in the center. The strut double-cone actuator is capable of vertical strokes as well as rotation and lateral strokes. Using a strut double-cone actuator made of VHB 4910, a maximum of 49.5\({}^{\circ}\) rotational stroke, 8.1 mm lateral stroke, and 11.3 mm vertical stroke were produced at 3 kV [74; 75]. Using this approach, up to six degrees of freedom can be achieved by patterning the electrodes [76] and eyeball movement devices were made by applying this advantage [77]. Recently, a method of using magnets instead of struts inside a double-cone actuator has been proposed. The magnetic double-cone actuator uses the principle that the stroke occurs up to the point where the magnetic repulsion force and the reaction force of the DE material are balancing. A maximum of 4.06 mm stroke at 60 V\(\upmu\)m\({}^{-1}\) electric field was reported by using 100 \(\upmu\)m Elastosil [78]. A tubular actuator consists of a DE tube with its insides and outsides covered with compliant electrodes as shown in Figure 6b. By contracting in the radial direction, the actuator performs linear expansion. One study shows a 4% strain for an electric field of 100 V\(\upmu\)m\({}^{-1}\) with this structure [79]. A tubular actuator consists of one layer in the radial direction, making it difficult to obtain high Maxwell stress. Therefore, an actuator with multiple layers is suggested to increase the force and strain. In addition, various tubular actuators with substructures have been fabricated. Additional structures were used to maintain the pre-strain of DE material, and to improve the performance. The representative examples are the tubular actuator with a compressed spring in the core, generating up to 14% strain in 7.5 kV [42]. A roll actuator is made by rolling a single long layer which has the effect of superposing Maxwell stress, like multi-stack actuators, for the same working voltage. The roll actuator is represented in Figure 6c. It is reported that more than 10% of strain was obtained with the basic configuration of the roll actuator [1]. Moreover, 2-DOF and 3-DOF roll actuators capable of bending or rotation as well as axial expansion motion were proposed by patterning two or four electrodes in the circumferential direction [80]. The roll actuator is mostly used as an artificial muscle among DE actuators. Currently, the theoretical and failure modeling of roll actuators has been conducted mostly to investigate and optimize performance [35; 36; 80]. Like the tubular actuator, the spring roll actuator has been reported by inserting a compressed spring in the core, and it shows a maximum 35% strain [44]. In addition, an interpenetrating polymer network that can compensate the residual stress of pre-strain has shown a 10% performance at 6 kV without a compressed spring [81]. Recently, many researches have reported using fiber to enhance performance.