# Flexible Vibrotactile Actuator Based on Dielectric Elastomer for Smart Handheld Devices
## 2 Design of Vibrotactile Actuator Based on Dielectric Elastomer  
### Structure of Proposed Vibrotactile Actuator and Its Operating Principles  
The proposed flexible vibrotactile actuator consists of an upper layer, an adhesive tape layer, a dielectric layer, and a lower layer, as shown in Figure 1a. The upper layer, which is made of polyimide film, is composed of a moving plate and four springs. The copper electrode is laminated to the bottom surface of the moving plate in the upper layer (Figure 1b). Another electrode is laminated to the top surface of the lower layer. The edges of the upper layer are bonded to the edges of the lower layer using a double-sided adhesive tape (we called it the adhesive tape layer). The dielectric layer is sandwiched between the upper and lower layers. The sizes of the upper and lower layers are 36 mm (L) \(\times\) 36 mm (W) \(\times\) 100 \(\upmu\)m (T), and the sizes of the moving plate and dielectric layer are 23 mm \(\times\) 23 mm \(\times\) 100 \(\upmu\)m and 23 mm \(\times\) 23 mm \(\times\) 450 \(\upmu\)m, respectively. The overall size of the fabricated vibrotactile actuator is 36 mm \(\times\) 36 mm \(\times\) 650 \(\upmu\)m. The proposed vibrotactile actuator is highly flexible and bendable enough to be attached to flexible and foldable devices as shown in Figure 1c. Figure 2 shows a cross-sectional view of the proposed vibrotactile actuator. As previously mentioned, the proposed vibrotactile actuator is composed of the upper layer, the adhesive tape layer, the dielectric layer, and the lower layer. In this structure, the dielectric layer is compressed along the thickness direction and is stretched along the horizontal direction when a voltage is applied to the actuator. The dielectric layer tends to bend under the applied voltage without stretching along its horizontal direction because both ends of the dielectric layer are fixed. The amount of bend increases with increasing voltage. However, there is no space available to allow this type of bending in the proposed actuator because of the sandwich structure, as shown in Figure 2b,c. Therefore, the length of the dielectric layer can be decreased or empty space can be created between the dielectric layer and the upper layer to allow bending. However, the decreased length makes the area that can create vibration narrow, and furthermore, having a thicker actuator due to the empty space not only makes the proposed actuator bulky but also makes its applying voltage higher. To avoid these issues, we fabricated a dielectric layer whose surface has a bumpy shape that allows actuator motion and therefore maximizes the haptic effect. Figure 2d,e show the operating principle of the proposed actuator. Figure 2d shows the initial state without an applied voltage, and Figure 2e shows the state after applying voltage. When a voltage is applied to the proposed actuator, an electrostatic force is generated between the two parallel electrodes. This generated electrostatic force attracts the moving plate in the upper layer, causing it to move downward, compressing the bumpy-shaped dielectric layer. As a result, the dielectric layer is stretched along the horizontal  
Figure 1: Proposed flexible vibrotactile actuator based on dielectric elastomer: (**a**) overall structure of the proposed actuator and assembled vibrotactile actuator; (**b**) bottom surface of the upper layer; (**c**) proposed actuator attached to a foldable smartphone. Figure 2: (**a**) Fabricated vibrotactile actuator; operating principles of the proposed actuator based on (**b**,**c**) a flat-shaped dielectric layer or (**d**,**e**) a bumpy-shaped dielectric layer. direction and is compressed in the vertical direction [19]. Because of this effect, the ridges in the bumpy-shaped dielectric layer are stretched and compressed, resulting in a gently sloped shape (Figure 2e). As soon as the applied voltage is removed, the dielectric layer and the moving plate are returned to their initial shape by the four springs in the upper layer and by the elasticity of the dielectric layer.