# Actuation Mechanisms and Applications for Soft Robots: A Comprehensive Review
## 2 Actuation Materials and Mechanisms for Soft Robots
### Electric and Magnetic  
Electric can convert electrical energy into other forms, such as mechanical and magnetic, leading electrical soft robots to move. Electrical robots possess the ability to move and achieve functions under wireless control. An electric motor can be used as a direct actuator, as determined in [148; 149]. After receiving motion signals, the motor drives the grippers or limbs of robots directly, and, ultimately, the robots will climb trees or swim under water.  
[PERSON] et al. developed an inchworm robot capable of two-anchor motion that can crawl on flat and inclined surfaces [150]. It is a three-segment body connected with 3D-printed lever arms and two servo motors. This mobile robot is able to carry supplies for search and rescue missions. The speed of the robot can reach 2.54 cm/s, and it can crawl on the horizontal plane and the slope of 19\({}^{\circ}\) with a load of 0.45 kg. Another form is a wall-climbing robot designed based on electrostatic adhesion mechanisms [151]. When a high voltage of 3 kV is applied to the pad electrodes of the robot, each pad can generate a holding force of about 3.2 N on the glass surface, supporting the robot to complete straight-line crawling and turning. The maximum speed is 2 cm/s, and the maximum single turning angle is 30\({}^{\circ}\). By controlling the adhesion force of each foot, the robot can climb similarly to a real gecko. In [152; 153], electric motors were used to design electric-motor-driven soft grippers.  
A magnetic actuator can penetrate most materials and control the direction/scale of the magnetic field efficiently and accurately. In the magnetic field, the magnetic particles on the robot will deform as the magnetic field changes, such as elongation, contraction, bending, or other forms, driving the robot to move. This actuation has the advantage of being wirelessly controlled by a remote untethered magnetic field without physical contact.  
In [154], a modular and untethered robot named Limpet II (weight: 450 g; size: 250 \(\times\) 250 \(\times\) 140 mm) was developed. It utilises the hybrid electromagnetic module (EMM) as its core module and a 450 mAh 11.1 V Li-Po battery as power, allowing the capabilities of adhesion and locomotion. The maximum force achieved by the EMM is 1.8 N. With an actuation current of 1500 mA, a single module can lift objects with a mass below 184 g. As a sensor, it also can be used for inspecting and monitoring offshore structures and nearby conditions. [PERSON] et al. introduced a 54 mm long magnet worm-like robot actuatedby permanent magnetic patches [155]. The magnets are embedded in the robotic feet, and each adjacent magnet has opposite magnetisation directions. When the speed of the driving magnet reaches 150 mm/s or more, the robot has a stride of about 6 mm and an average speed of 38.8 mm/s. If the robot prepares to move, another permanent magnet will generate a moving magnetic field. [PERSON] and [PERSON] demonstrated an unconstrained full 3D-printed soft robot with a multi-modal motion capability involving linear crawling and turning locomotions [25]. It is very light (only 0.23 g), but has a carrying capacity ten times greater than its own weight. Affected by the external magnetic field, the robot changes its motion state and delivers medicines to a designated location at a speed of 3.1 mm/s.  
Electrical and magnetic actuators have the advantages of quick and accurate reactions, low cost, remote control, and easy maintenance. Nevertheless, the technical difficulty of the actuators is the same as their drivers, motors, or coils. Meanwhile, the actuators are not scalable when compared with SMA and EAP. Furthermore, electrical actuators exhibit limited flexibility and high rigidity while magnetic actuators need an external strong magnetic field.