# Enhanced Design of a Soft Thin-Film Vibrotactile Actuator Based on PVC Gel
## 1 Introduction  
Due to the advances of computer hardware and material engineering, electroactive polymers (EAPs), which can be grouped into two categories of ionic and non-ionic EAPs, have recently been receiving a great deal of attention as a raw material for compact haptic actuators. In ionic EAPs, positive ions move toward the cathode under voltage input, and this movement makes the ionic EAP actuate. For making ionic EAP actuators, researchers have been studying hydrogels and ionic polymer metal composites (IPMCs). [PERSON] et al. developed an electrically controllable supramolecular hydrogel as an actuator from a self-assembling short peptide [1]. [PERSON] et al. investigated the effect of metal ion type on the movement of catechol-modified hydrogel actuators and revealed that the type of an ionprinted metal ion can be used to tune the rate and extent of actuation for catechol-containing hydrogel [2]. [PERSON] et al. proposed a soft bending actuator with graphene oxide-containing hydrogels to enhance its bending [3]. [PERSON] et al. presented an underwater soft actuator using Na-4-vinylbenzenesulfonate (Na-4-VBS) based homeostatic hydrogel [4]. [PERSON] et al. presented an actuator for controlling scissor-type grippers using IPMC [5]. [PERSON] et al. developed an ion-selective IPMC actuator based on crown ether [6]. Although the ionic EAP creates a large bending motion under relatively low input voltage (1-3 V), it is normally operated in wet conditions and it requires high electric power (high electric current) for operation. Another problem is that its actuation force is not sufficiently strong to stimulate human skin. Unlike ionic EAP, non-ionic EAP has a fast response time and can create stronger force [7]. A new vibrotactile tactile module (ViviTouchTM) based on a non-ionic EAP was developed in order to convey realistic haptic sensation to game users [8]. Since the performance of the non-ionic EAP is influenced by the ratio of pre-stretching, a new mechanism for stretching the non-ionic EAP was addressed [9; 10]. An arrayed tactile display with liquid coupling was presented using a non-ionic EAP [11]. Graphene or hydrogel was applied to transparent non-ionic EAPs tomake the actuator transparent [12; 13]. In the case of non-ionic EAP, it quickly expands in planar directions and compresses the thickness direction when voltage is applied to the electrodes, which are attached at both sides of the EAP. As soon as the applied voltage is removed, the non-ionic EAP quickly returns to its original shape. By this fast response behavior, non-ionic EAPs have been used for vibrotactile actuators. However, non-ionic EAPs require a pre-stretching or multi-stacking process to maximize force that is strong enough to stimulate human mechanoreceptors. Since the pre-stretching process for a non-ionic EAP makes the thickness of the actuator thin, it can be easily torn by small external force, even the high input voltage [14]. Another problem is that the non-ionic EAPs require stretchable electrodes. These problems cause low durability and/or a complex manufacturing process in fabrication of a haptic actuator based on non-ionic EAPs. In order to solve these problems, a non-ionic plasticized polyvinyl chloride gel (PVC gel) was previously introduced [15; 16]. The PVC gel shows electroactive and reconfigurable behavior with applied voltage. When we provide voltage input to the PVC gel, the PVC gel deforms toward an anode. As we remove a voltage input, it quickly returns to its initial shape by the elastic restoring force of the PVC gel, observing reversible deformation. Additionally, the PVC gel does not need stretchable electrodes for operation. Due to the characteristics of the PVC gel, it can serve as a material for a soft thin-film vibrotactile actuator. A contraction type soft actuator using poly vinyl chloride gel was developed [17]. Their work has excellent performance in contraction at low frequency.