# A Haptic Feedback Actuator Suitable for the Soft Wearable Device
## 1 Introduction  
As an important way for humans to perceive the outside world, tactile sensation is essential to perform many precision operations in daily life, and it is also increasingly applied in human-computer interaction devices [1] to enhance the sense of reality and immersion. For many teleoperation tasks, people often need to control a machine to perform precise operations, in which perfect coordination of visual, auditory, and especially tactile feelings is required. To improve significantly the effectiveness of remote operations and the safety of operators, a haptic feedback device capable of reproducing tactile sensations in real time while the controlled robotic hand touches a remote object is urgently needed.  
Traditional haptic feedback devices are mainly composed of rigid structures [2, 3], which provide force feedback to the operators through a steering gear [4, 5]. These devices are usually bulky, not portable, and have poor compatibility with the human body. To overcome these shortcomings, an increasing number of haptic feedback actuators made of soft materials [6, 7, 8] have been created in recent years. They are small in size, friendly with humans, and provide convenient conditions for the emergence of wearable haptic feedback devices [9, 10, 11, 12, 13, 14, 15]. To create a good tactile sensation, the soft actuator shouldrespond rapidly, output a large displacement and force, and be safe and flexible to be integrated with a glove.  
[PERSON] et al. [16] designed an electro-pneumatic haptic feedback actuator placed at the end of a human finger for virtual reality (VR) systems. The gas in the actuator was squeezed by the Maxwell force between two electrodes, and correspondingly, a displacement was produced. However, the actuator only provided a maximum displacement of 0.125 mm. [PERSON] et al. [17] proposed a photothermally driven refreshable micro-actuator based on graphene-oxide-doped paraffin. The output of the actuator was achieved by the volume change during the phase change of the material. The actuator output a displacement of 1.76 mm in a circular area with final diameter of 10 mm, but the process required approximately 150 s. [PERSON] et al. [18] created a thermo-pneumatic actuator applied to a braille dot display module. The working fluid in the actuator was heated to vaporize, and the pressure in the sealed cavity was increased to achieve actuation. The actuator produced a maximum deflection of 1.37 mm at 100 s. Although very little power was required, the response time was still very long. [PERSON] et al. [19] proposed a liquid-coupled actuator based on a dielectric elastomer. The actuation was realized by applying voltages to one side of the dielectric elastomer, which produced the maximum displacement change (0.35 mm) on the other side. [PERSON] et al. [20] provided an electroactive polymer actuator made of commercially available silicone tubing. The silicone tube became mechanically anisotropic by applying asymmetric pretension in the circumferential and axial directions, which caused the actuator to generate an axial strain under an electric field. However, it is difficult to avoid the use of rigid frames for the dielectric elastomer, which undergoes little deformation, thus limiting the application of the soft actuator.  
In this paper, an electro-hydraulic actuator is proposed for haptic feedback. Compared with other dielectric elastomer (DE) actuators [21; 22], this actuator is entirely soft without rigid frames. In addition, the actuator has a hydraulically amplified self-healing ability [23; 24; 25], and thus, it is more suitable to resist a high voltage. The actuator is very flexible and safe, and it is a good fit for wearable devices owing to its well-designed structure and advanced preparation technology.