# Actuation Mechanisms and Applications for Soft Robots: A Comprehensive Review
## 2 Actuation Materials and Mechanisms for Soft Robots
### Fluid
#### 2.2.1 Pneumatic  
A pneumatic actuator is a practical and versatile pneumatic device. It can operate in an antagonistic way like human muscles, and only generate traction when powered by compressed air [90]. Pneumatic utilises air pressure to generate a driving force to actuate a soft robot. It demonstrates the characteristics of quick response speed, good safety, and strong flexibility [91]. In pneumatic robots, gases are pressurised into embedded channels and elastic cavities [60]. This method generates stress at the gas-solid interface, thereby causing the structure of the soft robot to deform. It is usually assumed that different pressure levels of the gases in the internal cavity of the robot propagate fast enough so that all the gases in the structure have a uniform pressure level [92].  
The control and movement mechanism of the pneumatic is shown in Figure 5. The soft robot in Figure 5 consists of an air pump, an air chamber, and two feet [93]. Figure 5 shows that the air pump inflates or deflates the air chamber for elongation or contraction. At the same time, the front and rear feet move to realise the movement of the robot alternately. Figure 5 describes that, during the above entire cycle, the air chamber completes elongation and contraction, and the two feet are anchored or moved collaboratively to realise the movement of the robot. In the 2 nd time interval, the pump inflates the chamber to elongate, driving the front foot moving forwards. In the 4 th time interval, the front foot is anchored, and then the pump deflates, making the chamber contract and pull the rear foot.  
In applications, [PERSON] et al. designed pneumatic devices to imitate worms to achieve a \(\Omega\)-shaped crawling motion [94]. The robot is composed of eight air chambers, which are in a sawtooth shape. When inflated, the main body of the robot swells and arches upward, and then returns to its original shape after deflation. Then, a crawling process is completed. The robot can shrink the length of its body down to 40 mm (the initial length is 98 mm), the maximum moving distance is 2 mm in one cycle (0.4 s), and the average speed reaches 2.2 mm/s. In terms of materials, [PERSON] et al. utilised the self-healing ability of Diels-Alder (DA) polymers to design soft-handling robots, soft grippers, and artificial muscles [95]. Under the air pressure of 25 kPa, each actuator produces a gripping force of 0.25-0.32 N and the robotic hand can grab objects up to 92.8 g. If the robot hand is damaged when grasping sharp objects, it can be healed automatically at a high temperature (80 \({}^{\circ}\)C) through the DA reaction of the material. This characteristic expands the application field of pneumatic robots. In [96; 97], some pneumatic actuation-based soft robots for pipeline-related applications were developed. By integrating a pneumatic device that has telescopic characteristics, a soft robot was designed to monitor and dredge pipelines. When it is possible to make pneumatic actuation-based soft robots smaller, the robots can be applied to medical treatments, such as treating vascular diseases. In addition, researchers leveraged the elongation characteristics of pneumatic actuators to mimic the process of biological navigation [98] and the movement of octopus tentacles [99].