# Review of the Research Progress in Soft Robots
## 2 Driven Methods and Materials for Soft Robots
### Fluid-Driven Methods and Their Materials  
The fluid-driven soft robots are mainly to control the change of the fluid volume in each channel and produce the change of pressure intensity under the pressure force and constraint, thereby driving the transformation of different parts for soft robots such as contraction, expansion, and bending, which helps the soft robots to interact with the external environment [18]. Gas media is widely used in soft robots because of its light weight, being widely available, and not having pollution. Under the inspiration of the elephant trunk, Professor [PERSON]'s team investigated the deformation parameters of pneumatic artificial muscles (PAMs) in axial, bending, and helical muscles and used PAMs to compose a soft tandem manipulator with high flexibility and adaptability [19]. Shown in Figure 1a, Dong Yi University in Korea proposed the wearable rehabilitation robot, DULEX. The pneumatic artificial muscle was fixed at DULEX's elbow, by the length of maximum contraction and relaxed to complete the movement [20]. Shown in Figure 1b, [PERSON] has designed a soft-legged untethered quadruped robot that requires only simple pneumatic circuitry to control the robot's movement [21]. Although there are a lot of research achievements on pneumatic-driven soft robots at present, the gas pressure and flow rate of the gas change non-linearly with time, which makes it impossible for the pneumatic soft robot to be accurately controlled [22]. Liquid medium is also used in soft robots because of its high power density and fast reaction. Shown in Figure 1c, [PERSON] designed a bionic jellyfish robot that was fluid-driven and verified the reasons for this new structure and dynamical model [23]. Harvard University invented smart thermally actuating textiles (STATs). STATs could be de-tethered from pneumatic tethers by internally engineered fluid to change in different environments, contributing to the development of compact wearable devices [24]. So far, there are relatively few studies on fluid-driven soft robots using liquid as the medium. The main reason is that liquid medium is incompressible, so the design process is relatively complicated due to the need for auxiliary devices. Liquid is also greatly affected by temperature, so the liquid-driven soft robots may not be able to be applied in high or low temperature environments. However, with the development of production technology, the liquid-driven method can be combined with other driven methods in the future design of soft robots to overcome the shortcomings of the liquid driven and amplify its advantages. In order to make fluid-driven soft robots move more flexibly than before, most of their substrates are made of soft, super-elastomeric materials such as silicone rubber. Silicone rubber was widely used in the early use of soft robots because it does not react with anything except strong alkali and hydrofluoric acid, as well as being insoluble in any solvent, and has non-toxic tasteless, stable chemical properties [25]. Shown in Figure 2a, [PERSON] et al. designed a biomimetic earthworm crawling robot with self-recuperating capability based on Ecoflex00-30 silicone, which can move on various angular planes, the robot achieves the self-recuperating function by inserting the self-recuperating silicone elastomer at key parts [26]. Shown in Figure 2b, [PERSON] et al. developed a soft robot that was made of silicone and driven by strings. It could grasp various objects by utilizing octopus-like behavior and climbing various columnar objects [27]. Shown in Figure 2c, inspired by the structure of a deep-sea snailfish, Zhejiang University has designed an untethered soft-bodied robot for deep-sea exploration by integrating electronic devices into silicone; the robot can be successfully launched at the deepest depth of 10,900 m, and can swim freely at 3224 m [28]. Figure 1: (a) DULEX [20]; (b) A soft-legged untethered quadruped robot [21]; (c) A bionic jellyfish [23]. Figure 2: (a) The diagram of earthworm body structure [26]. (b) TAOYAKA-S II [27].