## 2 Design of the Proposed ETB Module  
Figure 1a shows a schematic illustration of the proposed electro tactile beat (ETB) module consisting of an upper part, a connection part and a lower part. The connection part consisting of four connectors having 0.5 mm height connects the upper part to the lower part. Figure 1b shows the fabricated ETB module with dimension of 100 mm (H) \(\times\) 200 mm (W) \(\times\) 3.5 mm (T). Figure 2 shows the fabrication process of the upper part. An acrylic plate was prepared for making the upper part and it was carved and drilled using a layer cutter for assembling with the lower part. After that, silver nanowire was sprayed (thickness: 0.1 \(\upmu\)m) onto the bottom side of the plate to make an electrode (we call this electrode as an electrode 1). Finally, we could obtain the upper plate (whose thickness is 2 mm) with an electrode. Figure 3a shows the structure of the lower part consisting of three components; a dielectric layer, an active layer (electrodes 2 and 3) and a bottom plate layer. The dielectric layer (thickness: 100 \(\upmu\)m) and the bottom layer (thickness: 1 mm) are made of a polyimide film and a glass epoxy laminate, respectively. The dielectric layer is used for preventing electric short circuit between the electrodes 2 and 3. The active layer is sandwiched between the dielectric layer and the bottom layer. Two voltage inputs are applied to the electrodes 2 (green) and 3 (red). The electrodes 2 and 3 take the form of interdigitated pattern with a gap of 0.2 mm. Figure 3b shows the fabrication process of the active layer. An epoxy glass fiber plate was prepared and an aluminum mask was also prepared for sputtering electrodes onto the epoxy glass fiber plate. We put the aluminum mask on the epoxy glass fiber plate and then we sputtered copper on the epoxy glass fiber plate and the mask. By removing the mask, we obtained the active layer with patterned electrodes (electrodes 2 and 3). Figure 1: Fabricated electro tactile beat (ETB) module consisting of an upper part, a connection part and a lower part, (**a**) its overall structure, (**b**) the fabricated ETB module. Figure 3: Structure of the lower part and fabrication process of the active layer, (**a**) component of the lower part, (**b**) fabrication process of the active layer. Figure 2: Fabricated process of the upper part. Figure 4 shows the operating principle of the proposed ETB module. The electrode 1 in the upper part is used for a cathode and the other two electrodes (electrodes 2 and 3) in the lower part are used for anodes. Figure 4a shows the initial state (no voltage inputs) of the proposed ETB module. Let us consider a case where two alternating current (AC) sinusoidal input voltages with slightly different frequencies are applied to the electrodes 2 and 3, respectively as shown in Figure 4b. According to the applied input voltages, electrodes 2 and 3 generate two different electric fields. Since the two electrodes were covered by the dielectric layer and a gap between the electrodes 2 and 3 is too small, the two electric fields have direct influence on each other. That is, the two input voltages are combined to create new electric signals which make an interference voltage pattern as shown in Figure 4c. We define the new electric signal as electric beat. We can generate a variety of electric beats by adjusting the frequencies of the two input voltages. The electric beat makes electrostatic attraction force between the electrode 1 and the dielectric layer having electrodes 2 and 3 (Figure 4d) and the created electrostatic attraction force makes the upper plate and lower plate bend. If we provide zero voltage, the two layers are reshaped back to their original states. So, AC voltage input makes the two layers vibrate and finally a user can feel the vibration on the upper plate as shown in Figure 4e. We designed a portable ETB platform consisting of a control part, a circuit part and an operation part as shown in Figure 5a.