# Actuation Mechanisms and Applications for Soft Robots: A Comprehensive Review
## 1 Introduction  
Smart material-enabled soft robots exhibit greater degrees of freedom and produce muscle-like behaviours [1]. Compared with a rigid robot, a soft robot demonstrates the advantages of reducing the complexity of the endchatronic design, minimising the requirement for algorithmic programming, and interacting with unknown environments in a flexible means. Soft robots have been extensively applied in broad prospects, such as industrial production, agriculture, biomedical science, disaster resilience, etc. [2; 3; 4]. For soft robots, some critical parts are made of smart materials, like polymers, rubbers, silicones, or other soft materials [5; 6; 7; 8; 9; 10]. The materials are used to design actuation mechanisms to drive soft robots to fulfill complex functions and adapt to changes in complex environments intelligently and autonomously [11; 12; 13; 14]. Some typical soft robots and their actuation mechanisms are illustrated in Figure 1, including shape memory alloy (SMA) (Figure 0(a),0(b)), fluid (Figure 0(c)-0(f)), electro-active polymer (EAP) (Figure 0(g)-0(i)), magnetic (Figure 0(j),k) and origami (Figure 0(j)). Almost all soft robots are based on the design principles of bionics [14]. By imitating the behaviours of various creatures in nature, such as reptiles, annelid, molluscs, and humans (bionic hands, arms, etc. ), different types of soft robots have been designed, like snake-shaped robots [27; 28], inchworm robots [29; 30], earthworm robots [31; 32], jellyfish robots [33; 34; 35], robotic hands [36; 37; 38; 39; 40], auxiliary gloves [41; 42; 43], etc. Such a bionic soft robot can achieve a single function, such as multi-gait crawling [44; 45; 46; 47], frictional crawling [48; 49; 50], jumping [51; 52], and rolling [53; 54], or it can achieve multiple functions, such as two modes of scrolling and crawling [55] and three modes of straight crawling, steering and rolling [56]. The combination of different actuation mechanisms can make a soft robot more adaptable in different terrain environments, such that it could be used to be underwater, on land, or in the low sky. Some previous reviews for soft robots (e.g., [2; 57]) have focused on the motion modes or the applications of soft robots. In some reviews [58; 59; 60; 61; 62], soft robots and their actuation mechanisms have been summarised. For instance, in [61], a survey of magnetically actuated bio/soft robots was conducted, and the future prospects of actuation mechanisms used to improve the performance of different types of soft robots have been discussed. In [62], the actuators of soft robots have been divided into three types, i.e., variable length tendon, fluidic actuation, and electro-active polymer (EAP). In the research, the sensing mechanisms, structures, and applications of soft robots have been introduced. Nevertheless, more comprehensive evaluations and discussions on the latest development of soft robots and their actuation mechanisms are highly imperative. For instance, some newly emerging types of actuations, like light [63; 64] and chemical [65], have not been discussed yet. To bridge the aforementioned knowledge gap, this paper is aimed to update the latest progress of actuation mechanisms and applications for soft robots. The objectives include:  
Figure 1: Various types of soft robots and their actuation mechanisms. (**a**) A centimetre-level untethered soft robot [15]. (**b**) A compliant robotic gripper [16]. (**c**) A spatial soft arm [17]. (**d**) An energy-efficient soft gripper [18]. (**e**) Bionic Handling Assistant (BHA) [19]. (**f**) A hydraulic robotic arm inspired by an octopus [20]. (**g**) A triboelectric nanogenerator-powered soft robot [21]. (**h**) A soft crawling robot motivated by a single twisted and coiled actuator (TCA) [22]. (**i**) A light-driven robot made of liquid crystal elastomer membrane [23]. (**j**) A novel inchworm robot driven by an electromagnetic oscillatory actuator (EOA) [24]. (**k**) A magnetically driven soft robot with multi-modal motion capability [25].