Fundamental Perceptual Characterization of an Integrated Tactile Display with Electrovibration and Electrical Stimuli

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1 micromachines Letter Fundamental Perceptual Characterization of an Integrated Tactile Display with Electrovibration and Electrical Stimuli 3, 1 2 , Hiroyuki Kajimoto and Hiroki Ishizuka * Seiya Komurasaki 1 Division of Intelligent Mechanical Systems Engineering, Graduate School of Engineering, Kagawa University, 2217-20 Hayashi-cho, Takamatsu, Kagawa 761-0396, Japan; [email protected] 2 Department of Informatics, University of Electro-Communications, 1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan; [email protected] 3 Division of Bioengineering, Department of Mechanical Science and Bioengineering, Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan * + 81-6-6850-6500 Correspondence: [email protected]; Tel.:    Received: 4 April 2019; Accepted: 30 April 2019; Published: 3 May 2019 Tactile displays have been widely studied for many decades. Although multiple tactile Abstract: ff ective to improve the quality of the presented tactile sensation, most tactile stimuli are more e displays provide a single tactile stimulus. An integrated tactile display with electrovibration and electrical stimuli is proposed herein. It is expected that vibrational friction, pressure and vibration can be presented at the same time through the tactile display. Also, these stimuli only require electrodes for stimulation. Therefore, the tactile display can be easily miniaturized and densely arrayed on a substrate. In this study, a tactile display is designed and fabricated using the micro-fabrication process. Furthermore, the display is evaluated. First, the relationship between a single stimulus and the perception is investigated. The electrovibration and electrical stimuli have a frequency dependence on perception. Second, whether the multiple stimuli with the electrovibration and electrical stimuli are perceivable by the subjects is also evaluated. The results indicate that the multiple tactile stimuli are perceivable by the subjects. Also, the possibility that the electrovibration and electrical stimuli ff a ect each other is confirmed. Keywords: tactile display; electrode array; electrical stimulus; electrovibration stimulus; multiple tactile stimuli 1. Introduction Tactile displays provide a tactile stimulus to users and have been widely studied for several decades to improve the quality of a presented tactile sensation and their applications. Tactile displays stimulate tactile mechanoreceptors, which perceive a tactile stimulus through the skin and provide tactile sensation to users. Tactile displays can provide not only a vibrational stimulus for simple tactile feedback [1,2] but also realistic tactile sensation for a virtual surface with detailed signal control [3,4]. According to the stimulus principles, tactile displays can be categorized into mechanical tactile displays and electrical tactile displays. Most mechanical tactile displays provide a tactile stimulus such as displacement or vibration with actuators. For example, L é vesque et al. developed a tactile display with an array of piezo actuators [ 5 ]. They experimentally confirmed that the developed tactile display was able to provide virtual braille dot patterns with the lateral displacement of actuators. Zhang and Follmer combined a linear actuator and an electrostatic actuator [ 6 ]. They densely arrayed them to develop a high spatial resolution tactile shape display. Kosemura et al. developed a micro-fabricated tactile display with an array of piezo Micromachines 2019 , micromachines , 301; doi:10.3390 / mi10050301 www.mdpi.com / journal / 10

2 Micromachines 2019 10 , 301 2 of 12 , 7 ]. They experimentally investigated the relationship between the vibration condition of actuators [ actuators and the presented virtual surface through the tactile display. Zhao et al. developed a compact 8 ]. Singh et tactile display with an array of shape-memory alloy wires to present braille dot patterns [ al. developed a portable tactile display with encapsulated magnetic fluid, which has the characteristic ]. Hoshi et al. applied ultrasonic radiation to tactile of being attracted toward the magnetic field [ 9 ]. The developed tactile display consisted of an array of ultrasound transducers and was 10 feedback [ able to provide acoustic radiation pressure without any contact. In recent years, Hasegawa and Shinoda 11 ]. Strong proposed a method to provide vibrational sensation to users with ultrasonic radiation [ and Troxel developed an electrovibration tactile display based on frictional force modulation with an electrostatic force between a sliding finger pad and an electrode [ ]. The electrovibration tactile 12 display can provide vibrational friction induced by the modulated frictional force. The electrovibration tactile display only requires an electrode and an insulator layer. Therefore, the devices are thin and not bulky, as compared with the devices based on other principles. Bau et al. integrated an electrovibration tactile display into a touchscreen to provide tactile feedback through the touchscreen [ 13 ]. They also proposed a method to add the electrovibration feedback to any object [14]. Electrical tactile displays stimulate tactile mechanoreceptors inside the skin using current stimulation. These displays can provide tactile sensation such as vibration and pressure [ ]. In 15 comparison with mechanical tactile displays, electrical tactile displays have a simple structure because they require only electrodes for stimulation. Kaczmarek and Haase developed an electrical tactile display and investigated the relationship between voltage waveforms and the perception of stimulus patterns [ 16 ]. Kajimoto developed an electrical tactile display consisting of an array of almost 1500 electrodes to stimulate the whole palm [ 17 ]. Kitamura et al. developed a needle-type electrical tactile display for low-voltage stimulation [18]. The mentioned mechanical and electrical tactile displays can provide only a single tactile stimulus. It is known that human beings perceive tactile sensation by integrating signals from tactile mechanoreceptors [ ]. A combination of tactile stimuli is e ff ective to improve the quality of the 19 perceived tactile sensation, because many mechanoreceptors inside the skin can be stimulated at the same time with multiple tactile stimuli. For this purpose, the realization of a tactile display that can provide multiple tactile stimuli is required to improve the quality of the presented tactile sensation through the tactile display. However, only a few groups have dealt with tactile displays for multiple tactile stimuli. Yem and Kajimoto integrated an electrical tactile display and a vibrator to develop a 3 ]. Pyo et al. developed an integrated tactile display with an electrode for the wearable tactile display [ electrovibration stimulus and an electrostatic vibrator [ 20 ]. Ryu et al. integrated an electrovibration tactile display and vibrators [ 21 ]. They revealed the relationship between the multiple tactile stimuli and perception. A tactile display with magnetic actuators and a Peltier element was developed in [ 22 ]. The tactile display was able to provide both vibration and thermal stimulus. We previously developed an integrated tactile display with an electrovibration tactile display and a Peltier element 23 ]. The problem with the mentioned tactile displays as the application of a flexible tactile display [ is their bulkiness, due to the use of mechanical actuators in such displays. The bulkiness of a tactile display makes it di ffi cult to be integrated into other devices and restricts its application area. In this study, we propose a novel integrated tactile display based on the principles of the electrovibration and electrical stimuli, as shown in Figure 1. The electrovibration and electrical stimuli have di ff erent characteristics. Therefore, vibrational friction, pressure and vibration can be provided to users at the same time by combining these principles. Additionally, as stated earlier, both electrovibration and electrical stimuli only require small electrodes for stimulation. The integrated tactile display can be easily miniaturized, made thin and has the advantage of easy implementation. Therefore, it can be expected that the integrated tactile display is able to be integrated into other information devices and widens the application of the tactile feedback. Also, the stimuli resolution of the integrated tactile display can be easily increased by arraying the electrodes densely. This also contributes to the improvement in the quality of the presented tactile sensation. We previously

3 Micromachines 10 , 2019 , 301 3 of 12 3 of 13 Micromachines , x 10 , 2019 10 , x 3 of 13 Micromachines 2019 , developed an integrated tactile disp lay with electrovibration and electrical stimuli [24]. In that tactile developed an integrated tactile display with electrovibration and electrical stimuli [ 24 ]. In that tactile developed an integrated tactile disp lay with electrovibration and electrical stimuli [24]. In that tactile display, the electrovibration stimulus was applied through a sheet-type slider as frictional force and display, the electrovibration stimulus was applied through a sheet-type slider as frictional force and display, the electrovibration stimulus was applied through a sheet-type slider as frictional force and the application was limited to wearable uses. The tactile display proposed in this study can directly the application was limited to wearable uses. The tactile display proposed in this study can directly the application was limited to wearable uses. The tactile display proposed in this study can directly provide electrovibration and electrical stimuli to th e finger pad through an array of flat electrodes provide electrovibration and electrical stimuli to the finger pad through an array of flat electrodes e finger pad through an array of flat electrodes provide electrovibration and electrical stimuli to th and is preferred for applications such as touchscreen s. In this study, we designed and fabricated the and is preferred for applications such as touchscreens. In this study, we designed and fabricated the s. In this study, we designed and fabricated the and is preferred for applications such as touchscreen integrated tactile display using the micro-fabrication process. We conducted a sensory experiment to integrated tactile display using the micro-fabrication process. We conducted a sensory experiment integrated tactile display using the micro-fabrication process. We conducted a sensory experiment to characterize each stimulus with subjects. Finally, her the multiple tactile we also evaluated whet to characterize each stimulus with subjects. Finally, we also evaluated whether the multiple tactile her the multiple tactile we also evaluated whet characterize each stimulus with subjects. Finally, stimuli were able to be perceived by subjects be cause the combination of these tactile stimuli has stimuli were able to be perceived by subjects because the combination of these tactile stimuli has never cause the combination of these tactile stimuli has stimuli were able to be perceived by subjects be never been investigated. been investigated. never been investigated. ctile display can provide both electrovibration Concept of the proposed tactile display. The ta Figure 1. Figure 1. ctile display can provide both electrovibration Concept of the proposed tactile display. The ta electrodes. In this study, we revealed that the stimulus and electrical stimulus with an array of Concept of the proposed tactile display. The tactile display can provide both electrovibration Figure 1. electrodes. In this study, we revealed that the stimulus and electrical stimulus with an array of multiple stimuli were able to be perceived by the subjects. stimulus and electrical stimulus with an array of electrodes. In this study, we revealed that the multiple multiple stimuli were able to be perceived by the subjects. stimuli were able to be perceived by the subjects. 2. Principle and Design 2. Principle and Design 2. Principle and Design Figure 2 shows the working principle and the structure of the integrated tactile display. Figure 2 shows the working principle and the structure of the integrated tactile display. Electrodes structure of the integrated tactile display. Figure 2 shows the working principle and the Electrodes for the electrovibration and electrical stimuli were formed on a substrate, as shown in for the electrovibration and electrical stimuli were formed on a substrate, as shown in Figure 2 (left side). stimuli were formed on a substrate, as shown in Electrodes for the electrovibration and electrical Figure 2 (left side). Figure 2 (left side). Principle and structure of the proposed tactile display. Electrodes for electrical stimulus are Figure 2. Figure 2. Principle and structure of the proposed tactile display. Electrodes for electrical stimulus Figure 2. Principle and structure of the proposed tactile display. Electrodes for electrical stimulus are ile display. The electrodes for the electrovibration stimulus were located on the surface of the tact are located on the surface of the tactile display. The electrodes for the electrovibration stimulus were located on the surface of the tact ile display. The electrodes for the electrovibration stimulus were formed under the electrodes for the electrical stimulus. formed under the electrodes for the electrical stimulus. for the electrical stimulus. formed under the electrodes

4 Micromachines 2019 4 of 12 , 10 , 301 On the surface of the tactile display, electrodes for the electrical stimulus were provided. Normally, two electrodes are used for electrical stimulation. One electrode is connected to a high voltage and other electrode is grounded. When a user touches the electrodes, the electrodes are electrically connected because of the electrical conductivity of the contacting skin (Figure 2, upper right). Then, current flows through the skin and stimulates tactile mechanoreceptors such as Merkel’s disks and Meissner ’s capsules, which are located near the surface of the skin, and the user can perceive tactile sensation such as vibration and pressure. The intensity and frequency of the electrical stimulus can be controlled by controlling the applied current waveform. The electrodes for the electrovibration stimulus are formed under the electrodes for the electrical stimulus. The principle of the electrovibration is shown in Figure 2 (bottom right). Electrovibration requires an insulator and electrodes for the stimulation and is based on an electrostatic force generated by the polarization of skin. The polarized skin and electrodes attract each other, with Coulomb’s law and an electrostatic force applied to the skin. The electrostatic force is not strong enough for users to perceive the attractive force. However, the electrostatic force is strong enough to modulate the resulting frictional force to a sliding finger pad. Without voltage, when a user slides a finger pad on the surface of the tactile display, no external force is applied to the finger pad. Then, the user perceives a smooth surface. On the other hand, with voltage applied to electrodes, the electrodes are charged positively and the sliding finger pad is charged negatively because of the dielectric polarization of the separating insulator layer. As a result, an electrostatic force is applied to the sliding finger pad. The finger pad is attracted toward the electrodes and the resulting frictional force to the finger pad is increased. The electrostatic force and resulting frictional force are expressed as follows [25]: ) ( 2 ′ ( ) V t A εε 0 (1) = F 2 d ) ( 2 ′ ( ) V t εε A 0 ′ μ ( F + N F μ ( (2) = + N ) ) = 2 d where F ε is the relative permeability of the stratum corneum, is the electrostatic force of the finger pad, ′ ( is the overlap area between the finger pad and the electrode, V A t ) is ε is the vacuum permeability, 0 ′ the applied voltage across the stratum corneum, F d is the is the thickness of the stratum corneum, resulting frictional force, μ is the frictional coe ffi cient and N is the normal force toward the surface. The equations consider the e ff ects of the skin condition, the contacting condition and the device on the electrostatic force and resulting frictional force, because the voltage across the stratum corneum represents the resulting voltage modulated by the mentioned e ects. We consider that the equations ff well represent the actual condition. Normally, a periodic voltage is applied to the electrovibration tactile display. This results in a periodic change in the resulting frictional force. The entire finger pad is periodically vibrated and tactile mechanoreceptors such as Pacinian corpuscles, which are located deep within the finger pad, are stimulated. Thus, the user can perceive vibrational friction through the electrovibration tactile display. Equations (1) and (2) indicate that the intensity and frequency of the electrovibration stimulus can be controlled by the applied voltage waveform. The user can perceive both electrical and electrovibration stimuli, by sliding the finger pad on the proposed tactile display. By controlling the current to the electrodes for the electrical stimulus and the voltage to the electrodes for the electrovibration stimulus, the intensity and frequency of the provided multiple stimuli can be controlled. Figure 3 shows the design of the proposed tactile display. The width and length of electrodes for each stimulus were 0.9 mm and 18 mm, respectively. The electrode designs were determined on the basis of the results of related studies, considering the spatial stimulation [ 26 , 27 ]. A pad for electrical connection was formed at the end of the electrode. The area of the pad was 1.5 mm × 1.5 mm. The separation distance between electrodes for each stimulus was 1.1 mm. The electrodes for the electrovibration stimulus and the electrodes for electrical stimulation were alternately arranged. The

5 Micromachines , , 301 5 of 12 2019 10 m. A total of 15 electrodes for each stimulus were μ thickness of the separating insulator layer was 4 formed on a glass substrate. , 10 , x 5 Micromachines 13 2019 of Figure 3. De sign of the tactile display. The width and length of electrodes for each stimulus are 0.9 Design of the tactile display. The width and length of electrodes for each stimulus are 0.9 mm Figure 3. mm and 18 mm, respectively. The separation distance between the electrodes for each stimulus is 1.1 and 18 mm, respectively. The separation distance between the electrodes for each stimulus is 1.1 mm. Fifteen . te mm. electrodes for each stimulus are arranged on a substra Fifteen electrodes for each stimulus are arranged on a substrate. 3. Fabrication Process 3. Fabrication Process Figure 4 shows the fabrication process of the integrated tactile display. A glass plate was dipped Figure 4 shows the fabrication process of the integrated tactile display. A glass plate was dipped plasma into hydrolysis with sulfuric acid for 5 min to remove any remaining organic matter. O 2 remaining organic matter. O into hydrolysis with sulfuric acid for 5 min to remove any plasma 2 treatment was given for 10 min to strongly adhere Cr to the glass plate. Cr was deposited on the treatment was for 10 min to strongly adhere Cr to the glass plate. Cr was deposited on the glass given glass plate for 7 min, as shown Figure 4a. The resulting thickness of the Cr layer was 100 nm. A 4a plate for 7 min, as shown Fig ure . The resulting thickness of the Cr layer was 100 nm. A positive positive photoresist layer was formed on the Cr layer with spin coating at 3000 rpm. The photoresist photoresist layer was formed on the Cr layer with spin coating at 3000 rpm. The photoresist was was exposed to UV light with a photo mask. The exposed photoresist was selectively dissolved with a d with exposed to UV light with a photo mask. The exposed photoresist was selectively dissolve a photoresist developer, as shown in Figure 4b. The bare part of the Cr layer was dissolved with a . The bare part of the Cr layer was dissolved with a Cr ure 4b photoresist developer, as shown in Fig Cr etching solution to form an electrode pattern for the electrovibration stimulus. The remaining etching solution to form an electrode pattern for the electrovibration stimulus. The remaining photoresist was dissolved through hydrolysis with sulfuric acid, as shown in Figure 4c. SiO was 2 2 through photoresist was dissolved was hydrolysis with sulfuric acid , as shown in Fig ure 4c . SiO deposited on the glass plate for 400 min to form an insulator layer, as shown in Figure 4d. The resulting . The 4d ure deposited on the glass plate for 400 min to form an insulator layer, as shown in Fig thickness of the insulator layer was almost 4 μ m. Cr was deposited again and the resulting thickness almost 4  m. Cr was deposited again and the resulting resulting thickness of the insulator layer was of the Cr layer was 100 nm, as shown in Figure 4e. The photoresist layer was formed on the Cr layer ure . The photoresist layer was formed on the thickness of the Cr layer was 100 nm, as shown in Fig 4e through spin coating. The patterned photoresist was obtained with UV light exposure and a photoresist a spin through Cr layer coating. The patterned photoresist was obtained with UV light exposure and developer, as shown in Figure 4f. Finally, the Cr layer was etched with a Cr etching solution to form an Cr etching photoresist developer, as shown in Fig ure 4f . Finally, the Cr layer was etch ed with a electrode pattern for the electrical stimulation, and the remaining photoresist was removed through , and the remaining photoresist was solution to form an electrode pattern for the electrical stimulation hydrolysis with sulfuric acid, as shown in Figure 4g. Figure 5a is a photograph of the fabricated tactile ure photograph of through 4g. Figure 5a is a , as shown in Fig acid hydrolysis with sulfuric removed display. Figure 5b shows the measured thickness of the insulator layer with a stylus profiler. The display. with a shows the measured thickness of the insulator layer 5b the fabricated tactile Figure μ insulator layer with a thickness of almost 4 m was formed on the glass substrate. with a thickness of stylus profiler  4 almost m was formed on the glass substrate. insulator layer . The

6 Micromachines , 6 of 12 2019 10 , 301 2019 10 6 of 13 , Micromachines , x , x 10 , 2019 6 of 13 Micromachines ) photoresist patterning; b ) Cr layer deposition; ( Fabrication process of the tactile display. ( Figure 4. a ) Cr layer deposition; ( b ) photoresist patterning; Fabrication process of the tactile display. ( a Figure 4. b a Fabrication process of the tactile display. ( Figure 4. ) Cr layer deposition; ( ) photoresist patterning; g ) photoresist patterning; ( f deposition; ( e 2 ) Cr etching. ) Cr layer deposition; ( d ) Cr etching; ( c ( ) SiO deposition; ( ) SiO d ) Cr etching; ( c ( ) Cr etching. g ) photoresist patterning; ( f ) Cr layer deposition; ( e e ) photoresist patterning; ( ) Cr etching. g f ) Cr layer deposition; ( deposition; ( 2 ) Cr etching; ( ) SiO d c ( 2 ) a ( ) ( a b ( ) ( b ) Fabrication results. ( a ) The measured Figure 5. b ) Photograph of the fabricated tactile display. ( ) Photograph of the fabricated tactile display. ( a Fabrication results. ( b Figure 5. ) The measured ) Photograph of the fabricated tactile display. ( b ) The measured a Fabrication results. ( Figure 5. thickness of the insulator layer. thickness of the insulator layer. thickness of the insulator layer. 4. Experimental Procedure 4. Experimental Procedure 4. Experimental Procedure To characterize the fabricated tactile display, we conducted sensory experiments with one To characterize the fabricated tactile display, we conducted sensory experiments with one female To characterize the fabricated tactile display, we conducted sensory experiments with one female and eight male subjects (average age: 22.1 years, SD: 1.0). This experiment was approved by and eight male subjects (average age: 22.1 years, SD: 1.0). This experiment was approved by the (average age: 22.1 years, SD: 1.0). This experiment was approved by female and eight male subjects the Research Ethics Committee of Kagawa University (30-005). Research Ethics Committee of Kagawa University (30-005). Kagawa University (30-005). the Research Ethics Committee of

7 Micromachines 10 , 301 7 of 12 , 2019 , 10 , x 7 of 13 Micromachines 2019 4.1. Experimental Setup 4.1. Experimental Setup Figure 6a shows the schematic illustration of the experimental setup. The experimental Figure 6a shows the schematic illustration of the experimental setup. The experimental setup setup consisted of a laptop computer (Surface Book Pro, Microsoft Corp., Redomond, WA, USA), ok Pro, Microsoft Corp., Redomond, WA, USA), consisted of a laptop computer (Surface Bo microcontrollers (mbed LPC 1768, ARM Ltd., Cambridge, UK), a high current supply for the electrical e, UK), a high current supply for the electrical microcontrollers (mbed LPC 1768, ARM Ltd., Cambridg stimulus (MHV 12-300S10P, Bellnix Co., Ltd., Saitama, Japan), a high voltage power supply for stimulus (MHV 12-300S10P, Bellnix Co., Ltd, Saitama, Japan), a high voltage power supply for the the electrovibration stimulus (MHV 12-1.0k2000P, Bellnix Co., Ltd., Saitama, Japan), an additional llnix Co., Ltd., Saitama, Japan), an additional electrovibration stimulus (MHV 12-1.0k2000P, Be keyboard (KB212-B, Dell Inc., Round Rock, TX, USA), a hand tracking system (LEAP MOTION, LEAP keyboard (KB212-B, Dell Inc., Round Rock, TX, US A), a hand tracking system (LEAP MOTION, LEAP MOTION Inc., San Francisco, CA, USA) and the fabricated tactile display. An actual photograph of the MOTION Inc., San Francisco, CA, USA) and the fabr icated tactile display. An actual photograph of experimental setup is shown in Figure 6b. The values of the current for the electrical stimulus and the experimental setup is shown in Figure 6b. The va lues of the current for the electrical stimulus and the voltage for the electrovibration stimulus were changeable through a control system on the laptop. changeable through a control system on the laptop. the voltage for the electrovibration stimulus were decreased by pressing assigned buttons on the keyboard. A control signal / The values were increased The values were increased/decreased by pressing as signed buttons on the keyboard. A control signal was sent to the microcontroller according to the control system and the microcontroller controlled the was sent to the microcontroller according to the control system and the microcontroller controlled voltage or current waveform according to the signal. The value of the peak voltage provided by the the voltage or current waveform according to the signal. The value of the peak voltage provided by voltage power supply for the electrovibration stimulus was varied from 0 V to 600 V. The average imulus was varied from 0 V to 600 V. The average the voltage power supply for the electrovibration st decrease was 2.945 V / / voltage increase pressing. Furthermore, the value of the peak current provided lue of the peak current provided voltage increase/decrease was 2.94 5 V/pressing. Furthermore, the va by the current supply for the electrical stimulus was varied from 0 mA to 5 mA. The average current s varied from 0 mA to 5 mA. The average current by the current supply for the electrical stimulus wa pressing. Before the experiments, we instructed the subjects to slide / increase decrease was 0.025 mA / experiments, we instructed the subjects to slide increase/decrease was 0.025 mA/pressing. Before the / s using the hand tracking device. The average room their dominant finger pad with a speed of 50 mm using the hand tracking device. The average room their dominant finger pad with a speed of 50 mm/s ◦ temperature and humidity were 21.8 C and 36.5%, respectively. Figure 6c shows a photograph of the ° temperature and humidity were 21.8 6c shows a photograph of the C and 36.5%, respectively. Figure actual experiment. actual experiment. a ) Schematic illustration of the experimental setup. ( b ) Actual photograph of the ( Figure 6. ) Schematic illustration of the experimental setup. ( ( b Figure 6. a ) Actual photograph of the experimental ) Electrode connection for the ) Actual photograph of the experiment. ( c experimental setup. ( d c setup. ( ) Electrode connection for the electrical stimulus d ) Actual photograph of the experiment. ( ovibration stimulus evaluation. ) Electrode connection for the electr e electrical stimulus evaluation. ( e ) Electrode connection for the electrovibration stimulus evaluation. ( f ) Electrode evaluation. ( f ) Electrode connection for the multiple stimuli evaluation. ( connection for the multiple stimuli evaluation. 4.2. Evaluation for a Single Tactile Stimulus e display was able to successfully provide the First, we evaluated whether the fabricated tactil igned electrodes. We experimentally evaluated the electrovibration and electrical stimuli with the des

8 Micromachines 2019 10 , 301 8 of 12 , 4.2. Evaluation for a Single Tactile Stimulus First, we evaluated whether the fabricated tactile display was able to successfully provide the electrovibration and electrical stimuli with the designed electrodes. We experimentally evaluated the minimal values of the voltage for the electrovibration stimulus and the current for the electrical stimulus to reveal the minimal stimulus conditions and the trends of the perception with the fabricated tactile display. In this experiment, the subjects slid their dominant finger pad on the tactile display from side to side and adjusted the value of the voltage for the electrovibration stimulus by pressing a button. We asked them to note the minimal value of the voltage where they were able to perceive the electrovibration stimulus. One electrode for the electrovibration stimulus was connected to the voltage power supply, as shown in Figure 6d. The voltage waveform was square and the duty cycle of the voltage was 20%. The perceivable duty cycle was selected from the related study [ ]. The frequency 23 of the voltage was selected from 5 Hz, 20 Hz, 80 Hz, 160 Hz and 320 Hz. Each frequency was evaluated twice and a total of 10 trials were conducted. After the evaluation for the electrovibration stimulus, we also conducted trials for the evaluation of the electrical stimulus. In this experiment, the subjects slid their dominant finger pad on the tactile display from side to side and adjusted the value of the current for the electrical stimulus by pressing a button. We asked them to note the minimal value of the current where they were able to perceive the electrical stimulus. In this experiment, two electrodes for the electrical stimulus were connected to the current supply, as shown in Figure 6e. The pulse width of the current was fixed at 200 μ s. We determined the perceivable pulse width from the related study [ 28 ]. The frequency of the current was selected from 5 Hz, 20 Hz, 80 Hz, 160 Hz and 320 Hz. Each frequency was evaluated twice and a total of 10 trials were conducted. 4.3. Evaluation for Multiple Tactile Stimuli Next, we evaluated whether the multiple tactile stimuli with electrovibration and electrical stimuli were perceivable by the subjects. In this experiment, we evaluated the minimal value of the current for the electrical stimulus with the electrovibration stimulus applied, to confirm that the electrical stimulus was successfully added to the electrovibration stimulus. Before the experiment, the subjects experienced the electrical stimulus to distinguish it from the electrovibration stimulus. In this experiment, the electrovibration stimulus was provided as a base tactile stimulus. The voltage waveform for the electrovibration stimulus was square and the duty cycle was 20%. The peak voltage where all subjects were able to perceive the electrovibration stimulus under the selected frequencies was approximately 330 V. The subjects slid their dominant finger pad on the tactile display from side to side and adjusted the value of the current for the electrical stimulation. We asked them to note the minimal value of the current where they were able to perceive the electrical stimulus. In this experiment, two electrodes for the electrical stimulus were connected to the current supply for the electrical stimulus, and one electrode for the electrovibration stimulus between the electrodes for the electrical stimulus was connected to the voltage power supply, as shown in Figure 6f. The pulse width μ s. The frequencies of each stimulus were 20 Hz (low), 80 Hz (middle) of the current was fixed at 200 and 320 Hz (high). We evaluated the combination of three frequencies for the electrovibration stimulus and three frequencies for the electrical stimulus. Each combination was evaluated twice. A total of 18 trials were conducted. 5. Experimental Results 5.1. Evaluation for a Single Tactile Stimulus Figures 7 and 8 show the experimental results for electrovibration and electrical stimuli, respectively. The average values were plotted. A subject was not able to perceive the electrovibration stimulus with a frequency of 5 Hz in a trial. Thus, the number of the obtained threshold voltages for the electrovibration stimulus with a frequency of 5 Hz was 17. The minimal values of the voltage for the electrovibration stimulus was decreased by increasing the frequency of the applied voltage. The values were not largely

9 Micromachines , 301 10 , 2019 9 of 12 changed under high-frequency conditions. The obtained trend was similar to the results of related 26 25 studies [ ]. One considerable reason for this trend is the attenuation of the applied voltage induced , 25 ]. Another reason is the perception of human beings. Pacinian corpuscles, by the skin and devices [ 29 ]. which are the target of the electrovibration stimulus, are not sensitive to low-frequency vibration [ Micromachines 2019 9 of 13 , x 10 , Therefore, the subjects were not sensitive to the electrovibration stimulus, which was the vibrational friction, under low-frequency conditions. The minimal values were higher compared with the related stimulus, which was the vibrational friction, under low-frequency conditions. The minimal values 25 , 26 ]. In this study, the finger pads were not grounded because some subjects perceived pain studies [ were higher compared with the related studies [25, 26]. In this study, the finger pads were not in trial experiments for the electrical stimulus under the grounded state. The ungrounded state of grounded because some subjects perceived pain in trial experiments for the electrical stimulus under the finger pads caused an increase in the minimal values. Also, the ungrounded state of the finger the grounded state. The ungrounded state of the finger pads caused an increase in the minimal pads might increase the standard deviations. In our previous study, the standard deviations were values. Also, the ungrounded state of the finger pads might increase the standard deviations. In our 26 ]. The finger pads were grounded and the voltages between the lower than those of this study [ r than those of this study [26]. The finger pads previous study, the standard deviations were lowe subjects and the device were almost constant in the previous study. In this study, the voltages between were grounded and the voltages between the subjects and the device were almost constant in the the subjects and the device were not controlled. This resulted in high standard deviations. We also the subjects and the device were not controlled. previous study. In this study, the voltages between found that the di ff ected the threshold voltages ff erence in the skin conditions among the subjects a This resulted in high standard deviations. We also found that the difference in the skin conditions and the standard deviations. The minimal values of the current for the electrical stimulus were also among the subjects affected the threshold voltages and the standard deviations. The minimal values decreased by increasing the frequency. The trend was di erent from the related study, although the ff of the current for the electrical stimulus were al so decreased by increasing the frequency. The trend ]. In the related study, the minimal values of the current 18 finger pad was fixed in the related study [ was different from the related study, although the finger pad was fixed in the related study [18]. In were not largely changed by the frequency of the applied current. We considered that the sliding of the related study, the minimal values of the current were not largely changed by the frequency of the ected the trend of the minimal values of the current. The current was applied to the ff the finger pad a the sliding of the finger pad affected the trend of the minimal applied current. We considered that finger pad only when the pad was overlapped with electrodes. In this experiment, the sliding of the values of the current. The current was applied to the finger pad only when the pad was overlapped finger pad decreased the time when the finger pad and electrodes were contacted. Therefore, under with electrodes. In this experiment, the sliding of the finger pad decreased the time when the finger low-frequency conditions, the number of the electrical stimuli applied to the finger pad decreased and pad and electrodes were contacted. Therefore, un der low-frequency conditions, the number of the the subjects were not sensitive to the electrical stimulus. The standard deviations were relatively high. ased and the subjects were not sensitive to the electrical stimuli applied to the finger pad decre erent among the subjects [ ff The related study showed that the skin impedance conditions were di ]. 27 electrical stimulus. The standard deviations were re latively high. The related study showed that the The relatively high standard deviations in the experiment using a flat electrode tactile display were skin impedance conditions were different among the subjects [27]. The relatively high standard ]. In this experiment, the di ected ff erence in the skin conditions also a ff 18 shown in the related study [ were shown in the related study a flat electrode tactile display deviations in the experiment using the minimal values of the current and the standard deviations were increased. We considered that the conditions also affected the minimal values of the [18]. In this experiment, the difference in the skin electrical stimulus was not a stable stimulus and a method to stabilize the intensity of the electrical We considered that the electrical stimulus was current and the standard deviations were increased. stimulus was required. not a stable stimulus and a method to stabilize the intensity of the electrical stimulus was required. e electrovibration stim The relationship between the frequency of th ulus and the minimal Figure 7. The relationship between the frequency of the electrovibration stimulus and the minimal Figure 7. voltage. The minimal voltage decreased with a decr ease in the frequency. The trend was similar to voltage. The minimal voltage decreased with a decrease in the frequency. The trend was similar to the the reported trends. reported trends.

10 Micromachines 2019 10 , 301 10 of 12 , 2019 , x 10 of 13 , Micromachines 10 , x 10 of 13 2019 Micromachines , 10 e electrical stimulus and the minimal current. Figure 8. The relationship between the frequency of th Figure 8. The relationship between the frequency of th e electrical stimulus and the minimal current. The relationship between the frequency of the electrical stimulus and the minimal current. Figure 8. The minimal current decreased with a decrease in the frequency. The minimal current was also The minimal current decreased with a decrease in the frequency. The minimal current was also The minimal current decreased with a decrease in the frequency. The minimal current was also a ected ff affected by the frequency. affected by the frequency. by the frequency. 5.2. Evaluation for Multiple Tactile Stimuli 5.2. Evaluation for Mu ltiple Tactile Stimuli ltiple Tactile Stimuli 5.2. Evaluation for Mu Figure 9 shows the experimental results for multiple tactile stimuli. The average values were Figure 9 shows the experimental results for mult iple tactile stimuli. The average values were Figure 9 shows the experimental results for mult iple tactile stimuli. The average values were plotted. The subjects successfully perceived the electrical stimulus even though the electrovibration trical stimulus even though the electrovibration plotted. The subjects successfully perceived the elec trical stimulus even though the electrovibration plotted. The subjects successfully perceived the elec stimulus was applied. The minimal values of the current were increased, compared with the values for stimulus was applied. The minimal values of the current were increased, compared with the values current were increased, compared with the values stimulus was applied. The minimal values of the the single electrical stimulus, as shown in Figure 8. These results indicate that the electrovibration for the single electrical stimulus, as shown in Figure 8. These results indicate that the electrovibration for the single electrical stimulus, as shown in Figure 8. These results indicate that the electrovibration stimulus masked the perception of the electrical stimulus and the subjects required larger currents ulus and the subjects required larger currents to stimulus masked the perception of the electrical stim stimulus masked the perception of the electrical stim ulus and the subjects required larger currents to to perceive the electrical stimulus. Also, the frequency of the electrovibration stimulus might have perceive the electrical stimulus. Also, the freque ncy of the electrovibration stimulus might have perceive the electrical stimulus. Also, the freque ncy of the electrovibration stimulus might have a ected the perception of the electrical stimulus, because the minimal current tended to be relatively ff because the minimal current tended to be relatively affected the perception of the electrical stimulus, because the minimal current tended to be relatively affected the perception of the electrical stimulus, low under the low-frequency electrovibration stimulus. For the single electrical stimulus, we confirmed low under the low-frequency electrovibration stimulus. For the single electrical stimulus, we low under the low-frequency electrovibration stimulus. For the single electrical stimulus, we the frequency dependence of the perception. However, in this experiment, it seemed that the e ect ff on. However, in this experiment, it seemed that confirmed the frequency dependence of the percepti confirmed the frequency dependence of the percepti on. However, in this experiment, it seemed that of the frequency of the current on the minimal values was smaller than that of the single electrical the effect of the frequency of the current on the minimal values was smaller than that of the single the effect of the frequency of the current on the minimal values was smaller than that of the single stimulus. Therefore, there is a possibility that the electrovibration stimulus and the electrical stimulus electrical stimulus. Therefore, there is a possibility that the electrovibration stimulus and the electrical electrical stimulus. Therefore, there is a possibility that the electrovibration stimulus and the electrical interact with each other. Sometimes, the subjects did not perceive the electrovibration stimulus when e subjects did not perceive the electrovibration stimulus interact with each other. Sometimes, th e subjects did not perceive the electrovibration stimulus interact with each other. Sometimes, th the intensity of the electrical stimulus was too high. The high intensity of the electrical stimulus also stimulus when the intensity of the electrical stimulus was too high. The high intensity of the electrical stimulus when the intensity of the electrical stimulus was too high. The high intensity of the electrical masked the electrovibration stimulus. The relationship between the intensities of two stimuli and the stimulus also masked the electrov ibration stimulus. The relationship between the intensities of two stimulus also masked the electrov ibration stimulus. The relationship between the intensities of two perception should be revealed in detail to utilize multiple tactile stimuli. Further investigations are tail to utilize multiple tactile stimuli. Further stimuli and the perception should be revealed in de tail to utilize multiple tactile stimuli. Further stimuli and the perception should be revealed in de required to characterize multiple tactile stimuli with the electrovibration stimulus and the electrical tactile stimuli with the electrovibration stimulus investigations are required to characterize multiple investigations are required to characterize multiple tactile stimuli with the electrovibration stimulus tactile stimulus. and the electrical tactile stimulus. and the electrical tactile stimulus. Experimental results for multiple tactile stimuli. Multiple tactile stimuli were perceived by Figure 9. Experimental results for multiple tactile stimuli. Multiple tactile stimuli were perceived by Figure 9. Experimental results for multiple tactile stimuli. Multiple tactile stimuli were perceived by Figure 9. the subjects. The frequency dependence of the percep tion on the electrical stimulus was smaller than the subjects. The frequency dependence of the percep tion on the electrical stimulus was smaller than the subjects. The frequency dependence of the perception on the electrical stimulus was smaller than that of the single electrical stimulus. that of the single electrical stimulus. that of the single electrical stimulus.

11 Micromachines 2019 10 , 301 11 of 12 , 6. Conclusions To develop a tactile display for multiple tactile stimuli, in this study we designed and fabricated an integrated tactile display using electrovibration and electrical stimuli with the micro-fabrication process. The width and length of the electrodes for each stimulus were 0.9 mm and 18 mm, respectively. The electrodes were densely arrayed on a glass substrate. In the first experiment, we evaluated the relationship between the single stimulus and the perception with the designed electrodes. The obtained trend for the electrovibration stimulus was similar to the previously reported trends. For the electrical stimulus, the trend was di ff erent from the reported results because of the sliding of the finger pad. We also evaluated whether the multiple stimuli with the electrovibration stimulus and the electrical stimulus were able to be perceived. The subjects successfully perceived the electrical stimulus with the electrovibration applied. Also, the experimental results indicated that the electrovibration stimulus a ff ected the perception of the electrical stimulus. In future studies, we will evaluate the relationship between the multiple tactile stimuli and the perception in detail, because the relationship between the multiple tactile stimuli and the perception is important to determine the applicable driving condition of the tactile display. In the experiments, the age range and gender of the subjects were not considered. The experimental results revealed the perception of males in their 20s to the tactile stimuli. In order to ff ects of the age range and gender of subjects in future consider real-world cases, we will evaluate the e ff work, because these factors may have e ects on skin characteristics. Also, we will develop methods to provide the electrovibration stimulus and electrical stimulus stably. Additionally, we will consider applications such as presenting a more realistic tactile sensation through the proposed tactile display by optimizing the voltage waveforms of both the electrovibration stimulus and the electrical stimulus. S.K. designed and fabricated the devices and conducted the experiments; H.K. prepared Author Contributions: the experimental setup; S.K. and H.I. analyzed the data; S.K. and H.I. wrote the paper. A part of this work was supported by JSPSKAKENHI, Grant Numbers 18H05010 and 17K12727. Funding: Acknowledgments: We thank to Fusao Shimokawa (Kagawa university) for his kind educational supports. Conflicts of Interest: The authors declare no conflict of interest. References 1. Fukumoto, M.; Sugimura, T. Active Click: Tactile Feedback for Touch Panels. In Proceedings of the 2001 CHI Conference on Human Factors in Computer Systems, Seattle, WA, USA, 31 March–5 April 2003; pp. 121–122. [CrossRef] Kang, J.; Lee, J.; Kim, H.; Cho, K.; Wang, S.; Ryu, J. Smooth vibrotactile flow generation using two piezoelectric 2. IEEE Trans. Haptics 2012 , 5 , 21–32. [CrossRef] actuators. 3. Yem, V.; Kajimoto, H. Wearable Tactile Device using Mechanical and Electrical Stimulation for Fingertip Interaction with Virtual World. In Proceedings of the 2017 IEEE Virtual Reality (VR), Los Angeles, CA, USA, 18–22 March 2017; pp. 99–104. [CrossRef] 4. Jiao, J.; Zhang, Y.; Wang, D.; Visell, Y.; Cao, D.; Guo, X.; Sun, X. Data-driven rendering of fabric textures on electrostatic tactile displays. In Proceedings of the 2018 IEEE Haptics Symposium, San Francisco, CA, USA, 25–28 March 2018; pp. 169–174. [CrossRef] L é vesque, V.; Pasquero, J.; Hayward, V.; Legault, M. Display of virtual braille dots by lateral skin deformation: 5. Feasibility study. ACM Trans. Appl. Percept. 2005 , 2 , 132–149. [CrossRef] 6. Zhang, K.; Follmer, S. Electrostatic Adhesive Brakes for High Spaital Resolution Refreshable 2.5D Tactile Shape Displays. In Proceedings of the 2018 IEEE Haptics Symposium, San Francisco, CA, USA, 25–28 March 2018; pp. 319–326. [CrossRef] 7. Kosemura, Y.; Ishikawa, H.; Watanabe, J.; Miki, N. Characterization of surfaces virtually created using MEMS tactile display. Jpn. J. Appl. Phys. 2014 , 53 , 06JM11. [CrossRef] 8. Zhao, F.; Fukuyama, K.; Sawada, H. Compact Braille display using SMA wire array. In Proceedings of the 18th IEEE International Symposium on Robot and Human Interactive Communication, Toyama, Japan, 27 September–2 October 2009; pp. 28–33. [CrossRef]

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