Review of Dielectric Elastomer Actuators and Their Applications in Soft Robots

Abstract

Robots play an increasingly important role in industrial production and daily life. Traditional robots are mostly made of hard materials, such as metal and plastic. Although rigidity of the material enables the traditional robot to perform tasks that are difficult for humans, for instance, carrying heavy objects, it also limits its adaptability. In nature, animals and plants are mostly made of soft materials, which make them have very high compliance and realize a variety of unimaginable actions. Similar to natural creatures, soft robots are usually composed of soft stimuli-responsive materials, which make them have good adaptability and compliance, and can achieve large deformation.Triggered by external stimuli, such as electric fields, magnetic fields, heat, and light, these soft stimuli-responsive materials can deform in a specific direction. The soft stimuli-responsive materials commonly used in soft robots include electroactive polymers (EAP), [1][2][3] hydrogels, [4][5][6][7] magnetic materials, [8][9][10] shape memory polymers (SMP), [11][12][13] shape memory alloys (SMA), [14,15] liquid crystal elastomers (LCE), [16][17][18] and so on. In addition, other actuation methods, such as pneumatic inflation and [19] electrostatic, [20,21] are also widely used in soft robots. To have an overview of different actuation methods, their performances with the maximum values are shown in Table 1. Although these maximum values were obtained from different actuators for each actuation method, they still demonstrate the potential upper limit.Dielectric elastomer (DE) is a typical EAP, which can generate significant deformation under the action of an external electric field. Thanks to its characteristics, such as large deformation, [22,23] fast response, [24,25] low elastic modulus, [2] high energy density, [26,27] light weight, [28,29] and low cost, [28] DE has the reputation of artificial muscles and has become one of the most promising materials in soft robots. [30][31][32][33][34] In this Review, we concentrate on the recent progresses of dielectric elastomer actuators (DEAs) and their application in soft robots. In Section 2, we first introduce the working principle of DEAs, followed by the DE materials and compliant electrodes. Then, various DEAs with different designs are introduced. In Section 3, the application of DEAs in soft robots is divided into seven categories. Some recent highlights are described and discussed in detail, especially those with biological inspiration. We summarize some of the challenges in Section 4 and this is followed with the conclusion in Section 5. Dielectric Elastomer Actuators Working PrincipleGenerally, the structure of a DEA is similar to a sandwich, consisting of a DE membrane, where both surfaces are coated with compliant electrodes, as shown in Figure 1a. Once the compliant electrodes are subjected to voltage, an electric field will be generated in the membrane. The induced Maxwell stress causes the membrane to expand its area and contract its thickness. Based on the mechanism of ...

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