# Review of the Research Progress in Soft Robots
## 2 Driven Methods and Materials for Soft Robots
### Smart Materials and Their Driven Methods
#### 2.2.2 Electroactive Polymer (EAP)  
EAP is a new type of polymeric flexible smart material that can change in a large scale or shape under the excitation of an electric field [43]. According to its actuation mechanism, it can be divided into electronic and ionic EAP. The advantages of EAP materials are fast response, lighter material mass, higher driven efficiency, and stronger impact resistance; the disadvantages of EAP are high driven voltage for electronic EAP, ionic EAP requires a wet environment, and poorer load-bearing capacity and stability.  
Electronic EAP is driven by a high voltage electric field to generate electromigration-induced stress, which can convert electrical energy into mechanical energy directly. The dielectric elastomer (DE) is a typical electronic EAP with a high dielectric constant among them. The Maxwell stress will be generated immediately when a voltage is applied to the upper and lower sides of the elastomer film surface. The Maxwell stress compresses and causes the elastomer film to deform, thus converting electrical energy into mechanical energy [44]. Shown in Figure 4a, the National University of Singapore developed a soft planar dielectric elastomer actuator (DEA). This soft freestanding actuator relied on compression springs for independence to enable linear actuation and steering while maintaining the same dimensions, weight, and structural complexity [45]. Shown in Figure 4b, [PERSON] et al. took advantage of the fact that DEAs did not require additional elastic elements to achieve resonant actuation. They developed a bioinspired flapping wing mechanism (FWM) that  
Figure 3: (**a**) Concept prototype of the bionic robot [31]. (**b**) Self-folding paper bird [33]. (**c**) Bionic-inchworm micro-robot [34]. (**d**) A soft robotic jellyfish [35].  
used a double cone DEA (DCDEA) with a power density comparable to that of an insect muscle for actuation [46]. Professor [PERSON]'s team produced a kind of artificial muscle with a dielectric elastomer spring structure by using the process of the spring structure forming the structural support layer, pre-stretching dielectric elastomer film forming the driving layer, and carbon paste forming the conductive layer [47]. Shown in Figure 4c, Professor [PERSON]'s team aligned the direction of DEA in extension with the robot motion movement and developed a soft robot consisting of a unidirectional DEA, a compliant arched robot body, and one-way bearing wheels, which enabled efficient energy conversion [48].  
Ionic EPA is based on electrochemistry, which converts electrical energy to mechanical energy by using chemical energy as a transition. The Ionic Conducting Polymer (ICP) is a typical ionic type of EPA that consists of a high molecular weight polymer matrix, alkali metal salts, and inorganic fillers [49]. [PERSON] et al. of the Intelligent Systems Institute in Japan used Ionic Polymer-Metal Composite (IPMC) as its actuator to make the locomotion of a snake-like underwater robot. The snake-like underwater robot could move by changing the driven voltages of each Ionic Conducting Polymer Film (ICPF) actuator [50]. Shown in Figure 4d, [PERSON] et al. developed a high performance IPMC actuator and combined it with expendable lift-scissor structures to design an earthworm-like soft robot [51]. [PERSON] et al. proposed an ionic electroactive polymer (PAST-iEAPs) with excellent tensile, electrochemical, and electromechanical properties, as well as good self-healing ability, which could be well used in the future for the manufacture of soft robots and soft actuators [52].