# Dielectric Elastomer Actuator for Soft Robotics Applications and Challenges
## 7 Wearable User Interface  
Making a wearable robot is one of the most important research issues, not only in the bio-mimicking robotics area, but also in the conventional robotics or haptics area [9]. There are four guidelines to consider when designing a wearable interface: form factor, weight, impairment, and comfort [118]. The wearable interface should be small, compact, and lightweight compared to the body part where the interface will be attached. Also, the wearable interface should not interfere with the wearer's movement, while maintaining comfort. Thus, there are many advantages for using DEAs for wearable interfaces, such as their small size, flexibility, softness, and customizability [119; 120; 121; 122; 123; 124; 125; 126; 127; 128; 129; 130; 131]. Portable force feedback devices for surgical simulators have been developed using DEAs. The first portable force feedback device was suggested by [PERSON] et al. in 2004 [119]. Chain-like actuators made from planar DEAs are attached to the backside of the index finger to give force feedback. They are made of VHB 4910 pre-stretched to 0.109 mm thickness as dielectric elastomer films and a 250 nm thickness of beaten gold as the compliant electrode. The device performs a maximum force of 0.7 N at 4.5 kV under an initial pre-strain force of 4 N, and a maximum elongation of 10.2% at 4.5 kV under a load of 230 g. To enhance the output force of DEAs, multi-layered, multi-element, or multi-layer-element configurations were suggested. One way to improve output force is to use rolled DEAs [120]. The rolled DEAs have a compact structure and enhanced energy density, compared to planar DEAs. They are composed of 30-40 layers of VHB 4910, are 12 mm in diameter, and 45 mm in height. The portable force feedback device using rolled DEAs exerts a maximum force of 7.2 N and displays a maximum elongation of 11% at 3.5 kV. Tactile displays using DEAs have been researched for many years. By using DEAs, the disadvantages of conventional tactile displays, such as difficulty in miniaturization and fabrication,  
Figure 11: DEA driven swimming robot: (**a**) Fish robot [115]; (**b**) Frog robot [117]. low power density, high cost, and rigidity, can be overcome [121]. The first tactile display using DEAs was suggested by [PERSON] et al. in 2008 [121]. Their system is composed of \(4\times 5\) arrays of actuators as shown in Figure 12b. To be completely soft, each actuator is composed of a polymer, especially Nusil CF-2186, with the carbon powder solution as a compliant electrode, a 2 mm diameter and 0.002 g weight. The active layer consists of eight 200 \(\upmu\)m thick layers. The actuator demonstrated an output force of over 13.6 mN at 3.5 kV and 0.1 Hz, which is equivalent to a force-weight ratio of 6.8 N/g. Pre-straining DE materials could be one solution in enhancing the energy density of DEAs. However, a rigid frame is needed to maintain the pre-strain. To solve this issue, a method of embedding nylon fibers into the DEA was suggested [122]. The insertion of nylon does not deteriorate the breakdown voltage nor operation of a DEA due to the nylon's insulation property and low bending stiffness. A nylon-embedded module is attached to the forearm and conveys the shear force to the wearer. As another structure of DE tactile display, hydrostatically-coupled DEAs (HC-DEA) were suggested by [PERSON] et al. in 2010 [123]. The configuration of HC-DEAs for a tactile interface is shown in Figure 12c. The bubble-like form of HC-DEA is due to silicone-oil placed between the active membrane and the passive membrane. Since the active membrane is electrically connected and the passive membrane is the end-effector, the silicone oil's placement ensures electrical safety [124]. Both active and passive membranes consist of bi-axially 400% pre-stretched VHB 4910. Carbon grease is used as a compliant electrode to the active membrane. This actuator is 10 mm in radius and 7 mm in height. In this research, the maximum force is 0.6 N at a working voltage of 4.5 kV.