# Dielectric Elastomer Actuator for Soft Robotics Applications and Challenges
## 7 Wearable User Interface
To improve the output force of the HC-DEA, adopting a stacked active membrane was introduced [125]. The active membrane is made of three layers of bi-axially 300% pre-stretched VHB 4910 and carbon grease as a compliant electrode. The thickness of the passive membrane is also matched to maintain the shape of the actuator. As a result, the maximum force is measured as 0.7 N under the working voltage of 4.0 kV. By using stacked HC-DEAs and increasing the thickness of the passive membrane, not only was the output force enhanced, but the working voltage was also decreased. Lower sustainability against pressurized contact is also a limitation of DEAs used as a wearable interface [126]. As shown in Figure 12d, a perforated protecting polymer is formed on the actuator to prevent the actuator from being initially pressurized. This structure can be utilized to create a  
Figure 12: Wearable interfaces using DEAs: (**a**) force feedback system using roll DEA [120]; (**b**) soft tactile display using DEA [121]; (**c**) tactile display using HC-DEAs [124]; (**d**) stacked DEA with polymeric frame to maintain curvature [126]. glove-type tactile display, that can give embossed feedback at fingertips, or a patched display that can give vibro-tactile feedback to the forearm. This actuator consists of six stacks of 100 \(\upmu\)m Elastosil P7670 as dielectric elastomers, and silver nanowires (AgNWs) as compliant electrodes. The diameter of the active area is 15 mm. The maximum force is reported as 255 mN at 4 kV and the resonant frequency is 191 Hz, while the maximum vertical protrusion is 650 \(\upmu\)m under an unloading condition and at 4 kV and 1 Hz. Research about the development of DE micro-actuators, which concentrate force on a localized area, was introduced in 2020 [127]. The DE micro-actuators are fabricated through photolithography, and the dimension of each micro-actuator is 200 \(\upmu\)m \(\times\) 7 mm \(\times\) 400 \(\upmu\)m. They are made of Sylgard 184 as the dielectric elastomer and AgNWs as compliant electrodes. With these DE micro-actuators, the tactile display is organized into an array, generating more than 30 times the force of the forearm vibro-tactile displacement perception threshold at 20 MV/m. There are also other possible wearable interface applications of DEAs. An active compression bandage using DEAs was suggested. It can be taken on and off easily, fits different calf sizes, and is provided in small size modules [128]. Also, the compression magnitude of the bandage can be modulated by changing the initial pre-stretch ratio or working voltage. A hand tremor suppression using DEAs was also introduced [129]. Undesired motions by Parkinsonian tremors have faster frequency (more than 3 Hz) than voluntary motion (0-2 Hz). By creating the motion which has that frequency of tremor, the movement can be canceled out. DEAs are attached to the wrist to reduce tremors in the flexion and extension direction. In addition to presenting these possibilities, researches on controller design and parameter design are being conducted [130,131]. Recently, research applying textile DEAs on a wearable interface has been conducted. In particular, replacing compliant electrodes with stretchable and conductive textiles have been investigated [132]. Wearable interfaces represent one of the recently emerging applications of DEAs. However, there are several challenges in the use of DEAs as actuators embedded into wearable devices. For example, reducing driving voltage is an issue to be solved with a high priority. High operating voltage often causes additional cost in terms of electronics as well as insulation. In addition, the insulation layers lead to output force degradation. Therefore, for low driving voltage DEA devices, we need to consider various factors including structural design, electronics cost, insulation material, as well as new DEA material with a high dielectric constant. In order to use DEA for haptic interfaces, resonant frequency needs to be considered for the efficient delivery of tactile cues to users.