# Actuation Mechanisms and Applications for Soft Robots: A Comprehensive Review
## 2 Actuation Materials and Mechanisms for Soft Robots
### Electroactive Polymer (EAP)
#### 2.3.4 Ionic Gel  
Ionic gel is a kind of reticular polymer that can expand in water and is affected by the pH and electric field after being put into aqueous solutions [142] (examples are illustrated in Figure 9). Through the interaction between the polymer and water, the shape and volume of ionic gel are able to change, thereby obtaining the functions of actuators or sensors. According to the formed gel network, gels are divided into two types: chemical gels and physical gels. While in chemical gels, the cross-linked gel network is formed by irreversible covalent bonds, resulting in gels with high elastic modulus and low strain tolerance. In physical gels, cross-linked networks are reversibly formed through non-covalent interactions, such as hydrogen bonding, \(\pi\) stacking, and electrostatic interactions. Since the non-covalent bonds of physical gels can be destroyed by the application of external heat and reformed upon cooling, the gels exhibit sol-gel transitions. At each temperature, the gels experience transition from a gel state to a sol state. However, the sol state is temporary and will return to the gel state after cooling [143]. A soft tactile sensor composed of ionic gel and elastomer was presented and the relationship between its impedance with current frequency was illustrated by [PERSON] et al. [144]. In future work, they will integrate the pressure sensor for soft robot design.  
Figure 9: (**a**) Laser-cutting device to fabricate ionic gel material [142]. (**b**) A fabricated biomimetic aqua-bot [142]. (**c**) A fabricated walking soft robot [142].  
In addition, there are some other polymers applied to make soft robotics. In reference [145], a tank-shape climbing robot (Tankbot) was designed using non-patterned flat and elastomer adhesive treads that operate as tracks. These palm-sized robots are lightweight (60-150 g), fast (up to 12 cm/s), and relatively energy efficient (efficiency up to 0.18). Driven by the motor in the robot, the elastomer tracks enable the Tankbot to climb over obstacles at 16 mm height. [PERSON] et al. demonstrated a kind of artificial muscle polymer called a twisted coil actuator (TCA) [146]. The actuator can provide opportunities for developing untethered soft robots or robotic hands due to its excellent bending performance and programmable motions. A single TCA weighs 0.3 g and has a maximum tensile force of 0.78 N (stress is 8.59 MPa). It can overcome a peak friction force of 0.4 N and achieve a free stroke of 48%. In [147], a rolling robot (210 g total weight) composed of 6 curved fluidic elastomer actuators was put forward. With the catalysed decomposition of hydrogen peroxide, the internal air pressure increases to 24.1 kPa, causing the elastomer to expand and deform. Under the alternate action of six fluidic elastomer actuators, the robot rolls forward. At each step, the robot rolls forward for approximately 40.2 mm.  
In general, an EAP-based soft robot exhibits the advantages of large deformation, high energy density, lightweight and compact structure, etc. However, the robot also has the following technical difficulties:  
1. It requires a high voltage above kilovolts with high power consumption;
2. It has a slow response time, which needs much time to deform;
3. Because of the softness of these polymers, their load-bearing capacities are less than other actuations.