# Dielectric Elastomer Actuator for Soft Robotics Applications and Challenges
## 10 Future Challenges for Practical Robotics  
Various applications of DEAs have been introduced in this paper. However, challenges still remain, such as achieving low driving voltage, high-stress output, large displacement, and enhancing the breakdown strength. First, the high voltage used in DEAs is undesirable in wearable applications and enforces the use of an external electrical component. To decrease the voltage level, the dielectric elastomer can be either multilayered or processed to thin films [149; 150; 151]. Also, increasing the material's dielectric constant (permittivity) is being researched as well [152; 153]. Another approach is the fabrication of a high voltage controller (or an amplifier) that is small and consumes low power to eliminate the hindrance in working voltage generation [154; 155].  
The production of high-stress output is challenging for dielectric elastomer actuators with a simple film structure, due to their soft material properties. This limitation can be overcome by stacking [56; 57; 58; 59; 60; 61; 62; 63; 64] or rolling [1; 35; 36; 64; 80; 81; 82] the dielectric elastomer. Its output can be amplified further by using them in bundles [44]. In addition, by adopting a biased spring [71; 72; 73; 74; 75; 76; 77; 78] or a bi-stable structure [73] that can store elastic energy, larger stress can be output.  
To achieve large displacement, artificial muscles based on dielectric elastomers are being investigated. Artificial muscles are designed to mimic human muscle characteristics. A typical human muscle displays a maximum of 40% strain in the axial direction [13; 19]. However, such large deformation is not attained with just long linear dielectric elastomer actuators. In order to tackle this problem, research focusing on the topology of DEAs, such as stack [56; 57] or conical actuators [72], needs to be performed. Another approach is the IPN (interpenetrating polymer network) method [156; 157]. Since the IPN processing maintains the pre-strain induced in the dielectric elastomer, it is assumed to increase the actuator performance, while sustaining a simpler structure without an additional rigid frame.  
Unexpected breakdown phenomena exist for DEAs. Such phenomena are derived from various factors, such as the material properties of DEs, the compliant electrode's properties, and surface condition. This uncertainty must be solved to guarantee a stable actuation system. As of now, methods of increasing the breakdown strength through prestretching, liquid dielectric immersion [158; 159; 160], or predicting the material's breakdown strength are being researched [160; 161].  
Working principle, characteristics and tunable optical applications, J.-H.Y.; modeling, S.M.J.; artificial muscle, J.P.; locomotion, H.K.; gripper, G.H.; wearable applications, K.H.; draft composition and supervision, K.-U.K. All authors have read and agreed to the published version of the manuscript.  
This research has been supported by the R&D Program (No.2019 RIA2C2006362) of National Research Foundation (NRF), Korea (No. 01190510) and the Alchemist Project (No. 20007014) of Ministry of Trade, Industry and Energy.  
The authors declare no conflict of interest.