# Dielectric Elastomer Actuator for Soft Robotics Applications and Challenges
## 4 Artificial Muscle Design and Fabrication
### Linear Contraction DEA  
A multi-stack actuator is fabricated by stacking multiple electrodes and elastomers in order to increase the Maxwell stress, and consequently its performance. A multi-stack actuator is shown in Figure 5a. It has been reported that over 100% of strain is obtained by the actuation of more than 100 layers using this configuration. The frequency response test confirmed that it has a resonant frequency at about 200 Hz [56; 57]. In addition, an actuator with a maximum strain of up to \(-10\%\) at 4.1 kV that can withstand 20.74 N has been studied through the development of an optimized design and fabrication process [58; 59]. On the other hand, as the number of layers increase, fabrication becomes more difficult. To solve this problem, a configuration of folding one DEA in half, so that the effect of two layers can be achieved with one electrode pair was proposed [60; 61; 62]. This actuator can create a strain of \(-7.5\%\) at a maximum of 3.5 kV under compressive stress with a 100g load. Currently, a remarkable strain of up to 24% was achieved by using a UV curable acrylic elastomer [63]. Moreover, a commercially available multi-stack actuator has come into the market [64]. This actuator produces 3.3% of strain at 1.2 kV and has low power consumption as well as a high life cycle. Another fabrication method that simplifies the process with monolithic fabrication was proposed as well. Helical and folded monolithic actuator configurations are described below.  
A helical actuator is a structure in which a compliant electrode is inserted between two identical pitch helical elastomers. The electrode pair in the axial direction expands in the radial direction due to the Maxwell stress, creating a contraction strain in the axial direction as shown in Figure 5b. A helical actuator with a strain of \(-5\%\) has been reported for an applied electric field of 14 V\(\mu\)m\({}^{-1}\)[65; 66]. The  
Figure 5: Schematics of Linear Contraction DEA. Transparent: Elastomers, Black: Electrodes (**a**) Multi-stack actuator; (**b**) Helical actuator; (**c**) Folded actuator; (**d**) Twisted actuator.  
helical actuator configuration can reduce fabrication time more than a multi-stack actuator configuration because of the reduced number of elastomers. However, a limit to elastomer thickness exists due to the blade fabrication method.  
A folded actuator is similar to multi-stack actuator, but the complexity and time of fabrication is reduced efficiently by folding a single long DEA multiple times, as shown in Figure 5c. A rectangular folded actuator with a contraction strain of \(-16\%\) is reported for an applied electric field of 12 V\(\upmu\)m\({}^{-1}\)[67; 68]. There are also examples of such actuators in hand rehabilitation devices and robotic eyeball moving equipment [69].  
Besides helical and folded actuator configurations, there is a twisted actuator, which is modified from a multi-stack actuator, as shown in Figure 5d. This configuration is made by twisting two cylindrical multi-stack actuators. The two spiral actuators contract to create strain as the helix angle changes. This actuator configuration solves the vulnerability issue of the multi-stack actuator. The twisted actuator is more stable to external force. An actuator with a maximum of 13.7 Wkg\({}^{-1}\) of specific power with a 300 g load and \(-5.2\%\) contraction strain without load has been reported [70].