# Dielectric Elastomer Actuator for Soft Robotics Applications and Challenges
## 5 Soft Robot Hand and Gripper
In a stiff state with closed configuration, the gripper with LMPA can lift an 11-g weight while the mass of the active parts of the gripper is less than 2 g. Another study on a DE gripper with tunable stiffness is based on shape memory polymer (SMP) electrodes [92]. Carbon-infused polyurethane SMP is applied as compliant electrodes of a DEMES-structured gripper for variable stiffness modulation. The SMP electrodes have relatively high stiffness in a cold state but exhibit lower stiffness characteristics in a hot state. Therefore, the gripper is actuated in the hot state, and holds up an object in the cold state, where the gripper has higher stiffness. The results demonstrate that the DEMES gripper with SMP electrodescan lift a 30-g weight with a two-fingered gripper. Here, the designed finger has a weight of 0.48 g and an area of \(71\times 33\) mm, with \(1.4\times 1.1\) pre-stretched PDMS (Sylgard 186) material for the DE actuator. Figure 8: DE gripper with other enhancing strategy: (**a**) DE gripper with stiffness modulation of LMPA [91]; (**b**) DE gripper with stiffness modulation of electrostatic chucking [93]; (**c**) multipurposed electro-adhesion pattern for DE actuation [94]; (**d**) Grasping objects by using DE gripper with electro-adhesion [94]. Figure 7: DE gripper based on dielectric elastomer minimum energy structure (DEMES): (**a**) Fabrication and working mechanism of DEMES [84]; (**b**) Grasping objects using DEMES gripper [84]; (**c**) multi-segment DEMES gripper [86]; (**d**) multi-segment DEMES gripper with arch flexure [88]. As an alternative to Joule-heating, electrostatic chucking could be one of the working mechanisms of a variable stiffness DE gripper [93]. As shown in Figure 8b, with no voltage applied to chucking electrodes, the gripper has a lower stiffness and it can be freely bent by a DEA. However, applying a voltage to chucking electrodes generates friction between DEA units, resulting in increased stiffness. The experimental results show that a 10.2-g weight could be supported by the two-fingered gripper with six layers of chucking units in one finger. The two-finger gripper has a 0.6-g weight and 10 \(\times\) 20 mm areal dimension, when a PVDF terpolymer with a thickness of 10.6 um was used as the DE actuator. Other than variable stiffness methods, using adhesion with DE actuation is an effective strategy to reinforce grasping capability. A DE gripper with electro-adhesion, proposed by [PERSON] et al., has highly improved performance due to the intimate contact between the gripper and objects [94; 95]. The principle of electroadhesion is illustrated in Figure 8c. Applying voltage to the alternative electrode pattern generates fringe electric fields, resulting in induced electric charges on the surface of target objects. The induced electric charge and voltage applied to an electrode pattern produce an attractive force between the gripper and a target object. In this research, the electro-adhesion pattern also has a role as compliant electrodes of the DEA for bending motion, as described in Figure 8c. This integrated DE gripper with electro-adhesion can generate a lifting force of 16 N, while the gripper mass is just 1.5 g and the areal dimension of one finger is less than \(57\times 47\) mm, as shown in Figure 8d. Such extraordinary performance of the DE gripper was achieved due to the intimate contact generated by electro-adhesion. In recent researches, novel configurations of a DE gripper have been suggested as new design strategies. One of the innovative configurations is a fiber-embedded DE gripper [96]. This design utilizes fibers to restrict DE actuation in one direction, and to induce bending in the perpendicular direction of the fibers. Therefore, a gripper with desirable bending direction, depending on target objects, can be fabricated. As another mechanism of gripping technology, DE spring-roll bending actuators, as mentioned in Section 4, are able to constitute a gripper system [97].