# Dielectric Elastomer Actuator for Soft Robotics Applications and Challenges
## 6 Locomotion: Mobility Design
### Insect-Like Walking Robot  
The world's first polymer-actuator driven legged robot, FLEX1, was built by biomimicking a cockroach [99]. The insect-like robot has six legs. Each leg has two degrees of freedom and is actuated by a bowtie shaped DEA and a spring. For each joint, the spring acts as an antagonistic component of the actuator. The robot weighs 667 g, and its walking speed is in the order of centimeters per hour. One characteristic of this robot is that it is operated by onboard batteries, power conversion systems, and a control board. However, the poor uniformity and degradation of the actuator result in slow locomotion, and the supporting frame reduces the power density of the actuator.  
Later, FLEX2 was built by the same research group, which is shown in Figure 9a [8]. Instead of a bowtie shaped actuator, FLEX2 used a spring-rolled actuator, which shows better performance in every aspect. The speed of the robot is 3.5 cm/s, which is a huge improvement, and its lifetime increased significantly.  
Using the same spring-rolled actuator, Skitter, a six-legged robot, was developed [100]. Compared to the former, this robot has only one degree of freedom for each leg. The size of the robot is 178 mm long and 127 mm wide, and exhibits a locomotion speed of 7 cm/s. In MERbot, which is shown in Figure 9b, a 2-DOF spring-rolled actuator is used [80]. Here, the robot's legs are composed of only spring-rolled DEAs, without rigid linkage. For each leg, 20 layers of acrylic film (VHB 4910) are stacked in a roll. In this robot, a speed of 13.6 cm/s is obtained at 7 Hz.  
A quadruped robot driven by a multi-stack DEA was created in 2014 [62]. This robot adopted a slider-crank mechanism to perform rotational movement at each joint. A multi-stack DEA works as a slider in the mechanism. Later, S-Hex 1 was introduced using a double cone DEA from the same research group [101]. The actuator module consists of two cone-shaped DE elastomers. For the DE elastomer, acrylonitrile-butadiene rubber (NBR) based elastomer is used with 160-\(\upmu\)m thickness. Two independent electrodes are patterned on both sides of the elastomer. This allows for 3-DOF movement of the actuator: y-translation, z-translation, and x-rotation. The robot consists of six actuator modules in a planar configuration, and one leg is attached to each actuator module's center. The entire dimensions of the robot are \(191\times 100\times 115\) mm. Using z-translation and x-rotation, the robot could show a cockroach-like tripod gait with a speed of 4 mm/s. As shown in Figure 9c, S-Hex2, which is an enhanced version of S-Hex1, is built using a 5-DOF actuator module [102]. In this robot, Silicone material (Elastosil P7670, Nusil CF2-2186 mixed with a 4:1 weight ratio) is used as the DE elastomer for increasing the tearing strength. By attaching the leg to both sides of the actuator module, the robot could walk by using only three actuator modules with a tripod gait. The robot's dimensions are \(150\times 54\times 55\) mm, and a speed of 52 mm/s is obtained at 7 Hz, which is about 0.35 body-length per second.