# Dielectric Elastomer Actuator for Soft Robotics Applications and Challenges
## 5 Soft Robot Hand and Gripper  
The grasping of various-shaped objects is one of the most challenging issues in the robotics field. As a solution to this issue, DEAs attract a lot of attention from researchers. The inherent flexibility of DEA enables an intimate contact between an object and a gripper with a DEA, while making an efficient grasping configuration. Also, the lightweight and high energy density of a DEA are important characteristics in achieving a high weight ratio of gripper mass to grasping capability. Based on a simple working principle and large actuation strain of the DEA, different types of DE grippers have been developed with various configurations. The first emergence of DE grippers was in 2007 by [PERSON] et al., based on a dielectric elastomer minimum energy structure (DEMES) [84]. An out-of-plane configuration is generated by laminating a pre-stretched DE to a flexible plastic frame [85]. The tension of the pre-stretched DE contracts its own structure and releases elastic energy. Then, a part of the released energy is stored in the flexible plastic frame, resulting in bending, as shown in Figure 7a. When voltage is applied to the DEA, the tension of the DE decreases and the whole structure is opened enough to grasp a target object. The overall gripping situation is demonstrated in Figure 7b. To enhance the practicality of the DEMES gripper, a new design with multi-segment DEMES was presented in 2014 [86; 87]. As shown in Figure 7c, this design consists of repetitive DEMES segments with characteristics of a high aspect ratio. The high aspect ratio enables the gripper to wrap objects of various sizes, as described in Figure 7c. However, the grasping force of this structure is not enough to hold up an object. Since the material for the DE actuator is PDMS (Sylgard 186) with an original thickness of 70 um and a pre-stretched ratio of 1.3, it cannot generate high tension due to a low pre-stretch ratio. Although the gripping force is not sufficient, this design could be recognized as a milestone for developing DE grippers. Multi-segment DEMES with an arch-structured flexible frame was developed to achieve a higher grasping capability, as shown in Figure 7d [88]. The pre-stretched DE with the arch-structured frame can generate a higher bending moment than with a flat frame, due to the increased distance between the pre-stretched DE and neutral axis [89]. In addition, they used an acrylic elastomer, VHB 4910, as a material of the DE actuator with a high pre-stretch ratio of \(5\times 6\) times in length and width direction, respectively. Due to the highly pre-stretched materials, the actuator can generate an enhanced output force. Thanks to the reinforced DE fingers, this gripper can lift objects that are 8-9 times heavier than the gripper's weight of 7.5 g. Moreover, since the area of this gripper is \(103\times 40\) mm, it can hold up a practical-sized object such as a mandarin orange. As a further step, a 3D printing process was applied as a novel fabrication method of the DEMES gripper [90]. In this research, an FEM analysis of the 3D-printed DEMES gripper is investigated, while providing a systematic design strategy for a DEMES gripper. To enhance gripping capability, stiffness modulation, as well as DE actuation, a gripper system can be adopted to. In 2015, the first approach to combine a DE gripper with a stiffness modulation element was demonstrated by [PERSON]'s group in EPFL, using a low-melting-point alloy (LMPA) as shown in Figure 8a [91]. The gripper has a unimorph configuration to generate a bending actuation and the low-melting-point alloy is embedded in a passive layer of the unimorph structure. The stiffness is modulated by a phase change of LMPA, which can be melted by Joule-heating. In the solid state of LMPA, the gripper maintains its shape and has a high stiffness to hold a target object. However, when the phase of LMPA changes to a liquid state by Joule-heating, it can alter its open/closed configuration.