We developed a photopatternable ionic gel to make soft microactuators driven at low voltage under atmosphere. A prototype of microactuator using the p... Microactuators - Actuators - Polymer gels - Protection switching - Electrodes - Atmosphere - Low voltage - Prototypes - Thermal conductivity - Cathodes - microactuators - position control - real-time systems - photopatternable ionic gel - soft microactuators - real-time position control
Development of microactuators using photopatternable ionic gel Shutaro Saito', Yuichi Kato2 Hisashi Kokubo2 Masayoshi Watanabe2 and Sholi Maruo" 3
1. Department of Mechanical Engineering, Graduate School of Engineering, Yokohama National University 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan 2. Department of Chemical Engineering, Graduate School of Engineering, Yokohama National University 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan 3. PRESTO, Japan Science and Technology Agency, Sanbancho Building, 3-5, Chiyoda-ku, Tokyo 102-0075, Japan Abstract: We developed a photopatternable ionic gel to make soft microactuators driven at low voltage under atmosphere. the A of microactuator using prototype photopatternable ionic gel was produced and driven by applying voltage of below ±1.5V under atmosphere. The driving performance of the microactuator was examined experimentally. The maximum displacement of the actuator was proportional to input voltage. It was demonstrated real-time displacement of the actuator was proportional to the integration value of input current. This feature is useful for precise real-time position control of the actuator. 1. INTRODUCTION
Recently, soft actuators using electroactive polymers (EAPs) have attracted much attention because of their unique features such as light-weight, flexibility, and low energy [1, 2]. In general, EAPs are classified into ionic EAPs and electronic EAPs. Ionic EAPs can be driven at low voltage, but limited to the driving in solution due to the evaporation problem . On the other hand, electronic EAPs can be driven under atmosphere. However, high voltage on the order of several kV is required to drive them . The high voltage is not suitable for specific application such as portable devices and medical devices. Recently, as a promising material, ionic gel are proposed and developed for the development of microactuators driven at low voltage under atmosphere [5, 6]. Ionic gel is a conductive polymer that contains ionic liquid in a polymer matrix. Since ionic liquid consists entirely of cations and anions, it has unique features: low melting points, negligible volatility, non-flammability, thermal and chemical stability, and high ionic conductivity. Owing to the unique features, ionic gel containingz ionic liquid can be utilized for the
paper, we developed a photopatternable ionic gel as alternative material for ionic gel actuators. The use of photopatternable ionic gel makes possible to produce 3-D microactuators by using 3-D microfabrication techniques such as single-photon microstereolithography [6, 7] and two-photon microfabrication [8, 9]. In addition, photolithographic approach with the photopatternable ionic gel offers mass production of microactuators. In this paper, we made a prototype of microactuators with the photopatternable ionic gel and electrodes containing carbon materials. The driving performance of the actuator was evaluated. As a result, we founded that the integration value of injected current was useful for real-time displacement control of the actuators. 2. THE ACTUATION MECHANISM OF IONIC GEL ACTUATORS
Ionic gel actuators are composed of ionic gel layer and two electrode layers, and driven by applying voltage to the electrodes of these both ends. Figure 1 shows schematic of a typical ionic gel actuator. Although the actuation mechanism of the ionic gel actuator has not been made clear
yet, the difference in the electric-double-layer capacity between the anodic and cathodic interfaces is one of the dominant driving force. In the ionic gel membrane, anions Electrical double layer
evaporation problem unlike conventional ionic EAPs. Ionic gel Electrodes Since ionic gel developed previously uses thermoplastic polymers as polymer matrix, they need heating and molding processes or press process to fabricate the actuator [5, 6].(a(b Figure 1 Actuation mechanism of ionic gel actuator (a) before voltage is This restricts to make sophisticated three-dimensional (3-DJ) applied, (b) after voltage is applied. microactuators using ionic gel. On the other hand, in this
and cations are randomly distributed before applying voltage (Figure 1 (a)). When voltage is applied, anions and cations move to cathode and anode, respectively, and electric-double-layer is generated at each interface between the ionic gel and the electrodes (Figure 1 (b)). The difference between anodic and cathodic electro-double-layer capacity causes to bend the cantilever-type actuator to cathode. 3. DEVELOPMENT OF A PHOTOPATTERNABLE IONIC GEL We examined the compatibility of an ionic liquid and polymer matrix before and after ultraviolet exposure. In our experiments,1-ethyl-3-methylimidazoliumbis(trifluorometha nesulfonyl)imide (EMITFSI) was used as an ionic liquid. The ionic liquid is mixed with an acrylate-type photopolymer. As a result, the acrylate-type photopolymer can be mixed without phase separation as shown in Fig. 2(a). Then, the mixture was irradiated with ultraviolet (UV) light whose wavelength is 365nm. Figure 2(b) shows the mixture after UV irradiation. It was found that the mixture was changed into an ionic gel without phase separation. The ionic gel represents pale yellow and has elasticity sufficient for the production for microactuators. From the above preliminary experiments, we succeeded in developing a photopatternable ionic gel by using the acrylate-type photopolymer as a polymer matrix.
4. PREPARATION OF THE IONIC GEL ACTUATOR To demonstrate the validity of the photopatternable ionic gel, we fabricate a cantilever-type actuator consists of three layers (ionic gel layer is sandwiched between two electrode layers) as shown in Fig. 3. In our experiments, we used carbon materials as electrodes. The electrode layer, containing carbon materials, was prepared as follows. A Poly(vinylidene EMITFSI, mixtures, containing fluoride/hexafluoropropylene) copolymer (P(VDF/HFP)), activated carbon (AC), and acetylene black (AB) by mass ratio 15 to 2 to 5 to 3, respectively, were mixed, and pressed at 130 degrees Celsius for a few minutes. The thickness of the electrode sheet was about 50ptm. An ionic gel layer, containing EMITFSI and acrylate-type photopolymer, was prepared as follows. EMITFSI and acrylate-type photopolymer were well mixed by mass ratio 7 to 4, respectively. The mixture was sandwiched between water-repellent films and irradiated with UV light whose wavelength is 365nm. The thickness of the ionic gel sheet was about 150ptm. Then, the ionic gel sheet was sandwiched with two carbon electrode sheets. The electrode sheets were tightly adhered to the ionic gel layer. Finally, the sandwiched sheet was cut off and its section was sandwiched between two cupper electrodes to drive the cantilever-type actuator. The driving section was 1 mm long and 6 mm wide. 5. EVALUATION OF DRIVING PERFORMANCE OF THE IONIc GEL ACTUATOR
(a) (b) Figure 2 Photopatternable ionic gel (a) before irradiating light, (b) after irradiating of UV light.
Ionic gel (150tm)
5.1. Experimental Setup Figure 4 shows experimental setup to drive the ionic gel actuator and measure the displacement of the actuator. The driving performance was examined by controlling the input voltage applied to two electrodes. The voltage and current were measured by using the digital multimeters (Aglient, 34401A and 34410A), and the displacement of ionic gel actuator was recorded by a laser displacement meter
Figure 4 Experimental setup for displacement measurement.
Figure 3 A prototype of the ionic gel microactuator
0 003 t- l-0* E0
X 0.02 0.01
0.5 1 input voltage (V)
Figure 5 The dependence of displacement on time at step voltage of 0.3V.
Figure 6 The dependence of displacement on voltage at 150 seconds after voltage is applied.
| 5.2. Step Response | The step response of the ionic gel actuator was measured. In this experiment, the time variation of the displacement of the actuator was measured at the position of 1mm from the tip of the actuator, when step voltage was applied. Figure 5 shows the displacement at step voltage of 0.3V as an example. In this experiment, as charge amount of the electric-double-layers was increased gradually with progress of time, the displacement of the actuator was also increased. It is also indicated that the displacement of the actuator reached the saturated value by charge saturation of the electric-double-layers. This phenomena is understand as follows: If the ionic gel actuator consisting three layers (cathode, ionic gel layer, anode) is considered as a parallel resister-capacitor circuit, current can be represented as I (1) I= I exp( t) RC where I is initial injected current, R is resistance, C is capacitor. The time dependence of the displacement of the actuator is similar to the above equation. Therefore, the displacement of the actuator is related to the charge stored in the ionic gel.
5.3 Dependence of Displacement on Input Voltage The dependence of displacement on voltage was examined. In this experiment, the displacement of the actuator was measured at 150 seconds later after input voltage was applied. Figure 6 shows the dependence of displacement on input voltage. This result indicates that the displacement of the actuator is proportional to the input voltage below 1. 1V. In the region of input voltage over 1.1 V, the displacement isn't proportional to the input voltage. Although the reason of this phenomenon is unclear, the change of the capacitance and resistance of the ionic gel may be one of the reasons. Owing to the linear response at the appropriate input voltage, the precise position control of the actuator can be realized.
1 0.05 E
Figure 7 The dependence of displacement on Yi (voltage and frequency are 1.5V and 0.025Hz).
5.4 Real-time Position Control Using Integration Valve of Injected Current We introduce the integration value of injected current (Zi) into the real-time position control of the actuator. Yi is the equivalent value for quantity of charge stored at electric-double-layer. To evaluate the availability as a control parameter of the displacement of the actuator, the dependence of the displacement on Yi was measured. In experiments, Yi and the displacement were measured in real-time, when rectangular-wave voltage was applied during 3 cycles. Figure 7 illustrates the relationship between the displacement and Yi. As shown in Fig. 7, in appropriate input voltage and input frequency, the displacement of the actuator was almost proportional to Zi without remarkable hysteresis. This unique feature offers the precise real-time position control of the ionic gel microactuators without external position sensors. Therefore, the position control using Zi is suitable both for miniaturization and for integration of microactuator systems. 6. CONCLUSION We developed a photopatternable ionic gel as a promising material for soft actuators that can be driven at low voltage
under atmosphere. A prototype of cantilever-type actuator using the photopatternable ionic gel was made and driven by applying voltage of below ±1.5V under atmosphere. The driving performance of the actuator was examined experimentally. We found that the integration value of injected current was useful for real-time precise position control. Since the photopatternable ionic gel can be easily patterned by UV light, mass production of soft microactuators can be realized by photolithographic approach. In addition, the use of the photopatternable ionic gel in 3-D optical microfabrication such as microstereolithography techniques [9. 10] can provide 3-D micromachines such as sophisticated micromanipulators. Acknowledgment. This research was supported in part by research grant from the Japan Society for the Promotion of Science [Scientific Research in Priority Areas: Science of Ionic liquids]. This research was also supported by a research grant from PRESTO, the Japan Science and Technology Agency.
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