# Flexible Vibrotactile Actuator Based on Dielectric Elastomer for Smart Handheld Devices
## 1 Introduction  
Considering their abundant capabilities, smart devices have become an integral part of most people's lives. Smart devices can provide absorbing and interesting content to users through multisensory channels (visual, auditory, and haptic). In particular, in the case of smart handheld devices, because the size of the visual display is not large enough to provide an immersive sensation to users, haptic information coupled with visual information enables users to delicately and immersively manipulate target objects. Haptic information is mainly created by small and lightweight vibrotactic actuators already embedded in smart handheld devices [1, 2, 3, 4]. Recently, smart handheld devices have evolved into fully flexible types that maximize their usability and availability [5, 6, 7]. The currently available rigid vibrotactile actuators are limited from creating vibrotactile sensation in fully flexible devices, such devices having the ability to show diverse shapes. Therefore, it is necessary to consider flexible vibrotactile actuators that can change shape to create improved haptic sensations in fully flexible devices. One of the most attractive methods for fabricating flexible vibrotactile actuators is the use of electroactive polymers (EAPs) [8, 9, 10, 11, 12, 13, 14]. Among EAPs, dielectric elastomers have many benefits, such as large displacement, moderate bandwidth, low power consumption, and low mechanical impedance compared to other electroactive polymers (ionic polymer metal composites, piezoelectric polymers, etc.) [15]. Many studies have been conducted on the fabrication of flexible actuators using dielectric elastomers [11, 12, 13, 14]. [PERSON] et al. proposed a dielectric elastomer-based flexible tactile display to create multiple mechanicalstimuli [11]. [PERSON] et al. suggested a soft wearable tactile display based on a hydrostatically coupled dielectric elastomer [12]. [PERSON] et al. fabricated a transparent and integrable surface texture change device based on a dielectric elastomer for localized tactile feedback [13]. [PERSON] et al. developed a wearable soft tactile actuator based on a dielectric elastomer (acrylic elastomer and Wacker Elastosil) [14]. Among the many dielectric elastomers available, polydimethylsiloxane (PDMS) is one of the most attractive materials because it can create a strong force to stimulate human mechanoreceptors. Other reasons are that it is durable, resilient, and moldable. [PERSON] et al. proposed a flexible vibrotactile actuator based on a pyramidal microstructured PDMS layer [16]. [PERSON] et al. presented a transparent and flexible visuo-haptic interface using PDMS and a polymer waveguide [17]. [PERSON] et al. developed a PDMS-based soft haptic actuator with thickness of 18 \(\upmu\)m [18]. The magnitude of the vibrational force is a very important factor to create a variety of haptic sensations. If there is a huge force difference between absolute threshold and maximum force at a specific frequency, the user can feel the various haptic sensations. However, although these actuators can provide human-perceivable vibrotactile information, their vibrational force is not sufficiently strong to create a variety of haptic sensations. Here, we present a PDMS-based soft vibrotactile actuator that creates a strong vibrational force to express a variety of haptic sensations by simply integrating an upper layer, an adhesive tape layer, a bumpy-shaped dielectric layer, and a lower layer. We then optimize the actuator by investigating the mechanical/dielectric properties of various prepared dielectric layer samples. The dielectric layer is sandwiched between the upper and lower layers. If we apply a voltage across the upper and lower layers, the dielectric layer is compressed in the thickness direction and is stretched in the longitudinal direction. However, because the dielectric layer is tightly encapsulated by the upper and lower layers, there is no space available that allows it to stretch.