# Review of Soft Actuators Controlled with Electrical Stimuli: IPMC, DEAP, and MRE
## 2 Recent Progress in Development and Applications of Soft Actuators
### Magnetorheological Elastomers (MRE)
#### 2.3.2 Actual Trends in the Application of MRE in Soft Robotics  
The magnetorheological elastomers respond to the external magnetic field with the change of material properties and deformation, which has contributed over the last 20 years to the construction of devices that are widely used in industry and research [2,4,113]. Firstly, we consider the different response types of the MRE material under a magnetic field. The most popular property used for MRE applications is to change its mechanical stiffness by turning on/off the magnetic field. This phenomenon is used in the construction of passive and active vibration control systems [113]. However, various MRE responses to the magnetic field have recently become increasingly popular. The magnetic field can ensure that the MRE deforms linearly or radially. It is applicable in building valves, pumps, and robotics systems [2,4]. In the following sections, we will discuss the novel applications of MREs in these fields. One of the most important applications of MREs is the vibration systems [113]. The property of variable stiffness under a magnetic field allows the construction of active vibration absorbers or insulators. The principle of the operation of absorber and insulator is presented in Figure 9. The construction of both device types has similar elements, however, their definitions are different. The main goal of these devices is to attenuate the vibrations. Firstly, the passive solutions were applied, which have constant frequency characteristics. This means that the operation can be desired for a specific frequency. The application of varying stiffness allowed extending the working range. This type of MRE application has been widely known for the last 20 years. In recent times, the new applications of absorbers are found in works [114, 115, 116]. The main feature of these applications is working in a high-frequency zone [114], extending with control systems based on the Hilbert-Huang transformation [115], designing torsional absorber for power train system [116], tuning to variable-frequency excitation [117, 118]. The example of a vibration absorber is presented in Figure 10, which was presented in work [114]. Furthermore, the applications of insulators are also still developing as in [119]. In these devices, the new direction of research is designing advanced controllers like fuzzy PID [119], integrating the MRE insulator in new extended mechanical structures [120]. The example of an insulator is presented in Figure 11. Additionally, the MRE's universality allows the building of custom MREs devices as in works [121, 122, 123, 124], where the unique features of MREs allow to solve advanced problems from vibration control. From the authors' point of view, the vibration control systems are well-analyzed with multiple applications. Figure 10: The example of a vibration absorber designed for working in the high-frequency range (source: [114]). Figure 9: The schema of typical vibration control systems. The vibration absorber (**a**). The vibration isolator (**b**). Alternatives to the vibration control systems presented in the previous paragraph are the MRE devices actuated to produce radial stretch, linear movement, or custom shape bending. In the group of devices using the radial stretch is the proportional valve presented in [2]. The linear movement can be applied in haptic devices to create varying stiffness buttons [2]. In this area, the recent developments are the application of MREs in robotics. The interesting applications are especially for grippers [125; 126; 127; 128; 129]. In the study [128], MRE and magnetorheological fluid (MRF) materials are used to prepare a universal gripper that enables catching various objects. In [130], the grippers with soft skin are prepared to allow for fitting the gripper finger to hold an object. In [126], the preliminary concept of the soft gripper is examined with fully soft fingers.