# Actuation Mechanisms and Applications for Soft Robots: A Comprehensive Review
## 2 Actuation Materials and Mechanisms for Soft Robots
### Shape Memory Alloy (SMA)
#### 2.1.2 SMA Wire  
The general design concept of SMA wire driving is to install an SMA wire into the main body of a soft robot. By applying the electric current, the SMA wire deforms, bending the main body and generating sliding frictions. [PERSON] et al. designed a bionic inchworm robot (shown in Figure 4a) [80]. As shown in Figure 4b, when the temperature of the SMA wire exceeds its phase transition temperature, the SMA wire returns to its initial shape. When the temperature is reduced, the wire is under relaxation. In Figure 4c, during the 2 nd time interval, the SMA wire starts to deform and arch up (at the high state), directly driving the rear foot to move forwards. At the same time, the front foot moves back. Then, in the 4 th time interval, the SMA wire relaxes and recovers to its original shape, the front and rear legs spread out, and the robot advances to a certain distance. [PERSON] et al. developed a crawling soft robot to mimic the worm movement [81]. The main body of the robot is a thin tetragonal composite plate with two pairs of SMA wires placed on the four sides of the robot. Periodic currents are applied to deform the SMA wire, enabling the robot to turn and crawl. In a single motion cycle, the moving distance of the robot is 54 mm, which is about one-third of its own body length, and the maximum linear speed can reach 3.6 mm/s. Inspired by the movement of caterpillars, [PERSON] et al. designed a rolling soft robot using silicone materials and SMA wires [82]. The minimum weight of the robot is only 5 g, and the SMA wires installed on both sides of the robot can provide a driving force of 10 mN. Under excitation, the SMA wire deforms and makes the robot curl into a wheel shape, and the rolling speed can reach 0.5 m/s. From the manufacturing perspective, the spring form is more complicated than the wire form. From the structural perspective, the wire form can be flexibly bent to connect two kinematic joints, whereas the spring form needs to be fixed in a specific position and  
Figure 4: (**a**) An SMA wire robot [80]. (**b**) The movement mechanism. (**c**) The moving states of each robot’s part in the time intervals. can only be extended. From the perspective of the bearing capacity, the spring form can produce a greater output force, and the spring form-enabled soft robot can bear a higher load. Therefore, in comparison with the spring form, the wire form can be employed to design a versatile soft robot to play the role of a hand, a body, and a leg to grasp, stretch and move objects [15; 83; 84]. In addition to spring and wire forms, SMA can also be made into other forms. The research in [85] is a bell-shaped jellyfish robot, which body is composed of eight SMA compound actuators. The deformation of SMA is controlled by voltage changes to simulate the swimming motion of a jellyfish. During 14 cyclic movements, the average thrust and power consumption of the jellyfish robot are 0.0039 N and 16.74 W, respectively, and the maximum speed reaches 5.42 cm/s. RoBeetle, designed by [PERSON] et al., is powered by the catalytic combustion of methanol fuel with high specific energy [86]. Its four legs are made of SMA actuators. Methanol vapor flows out of the container and reaches the surface of the SMA legs. The SMA shrinks when the catalytic surface is exposed to fuel and oxygen, and expands when the fuel flow stops. The robot exhibits a maximum average stride of 2.83 mm, and a maximum average speed of 0.76 mm/s. It can carry a maximum payload of 230 mg (about 2.61 times the weight of the prototype). As a flexible material that can be deformed and used in widespread applications, SMA still has some technical limitations, no matter whether the form is spring, wire, or other. Firstly, in the repeated phase-changing process, SMA exhibits functional fatigue, which will greatly shorten the service life of the SMA actuator and reduce the driving efficiency [87]. Secondly, most SMA actuators have hysteresis.