Vol.9 No.3 2017
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Research paper : Development of human-friendly polymeric actuators based on nano-carbon electrodes (Kinji Asaka)−118−Synthesiology - English edition Vol.9 No.3 (2017) this technology for active catheters, tactile displays, aquatic micro-robots, and others.[3] Although this actuator was an excellent device, there were many problems that hampered practical use in devices. The main problems were as follows: 1) material costs as precious metals such as gold or platinum had to be used as electrodes; 2) manufacturing costs because the method of electroless deposition took time; 3) surface area was too small to achieve high electrode performance (i.e. capacitance or ability to store ion was small); and 4) since the uorine ion exchange resin must contain water to maintain ion conductivity, it was difcult to operate in air, and practical use was limited. To solve these problems, we started reviewing the development of a low-voltage-driven soft actuator around 2000 with the following characteristics: a) use of low cost materials; b) manufacturable by a simple process; c) use of electrode materials with large capacitance; and d) use of ion conductive polymers that can be operated in air.3 Research scenario for the polymeric actuators using nanocarbon and ionic liquidsIn 1999, a famous paper on carbon nanotube (CNT) actuators was published in Science.[4] This paper reports that when paper-like electrodes are made from single-layered CNTs and voltage is applied to the counter electrode in the aqueous electrolyte solution such as sodium chloride, the CNT electrodes expand and contract. The paper explains that when the voltage is applied, the ions with different charge adhere to the surface of CNTs that compose the electrodes, the electric double layer is created and the CNTs become charged, the state of the graphene bond that comprises the CNTs changes due to a quantum effect, and the CNTs expand and contract. The paper also predicts computationally that an actuator with extremely high energy and power densities can be created considering the large Young’s modulus Fig. 1 Schematic diagram of the structure of an ion conductive polymeric actuator and the bending response principleand electroconductivity of CNTs. Moreover, other papers found that CNTs have high electroconductivity,[5] strong mechanical strength,[6] and large specific surface area i.e. large capacitance,[7] and may be ideal as actuator electrodes. Therefore, we decided to use nanocarbons such as CNTs for our electrode material.For in-air operability that is another issue, we attempted coating the aqueous element surface or impregnating the ion exchange resin with a solvent with a high boiling point instead of water. However, we were unable to obtain any practical element due to various problems. On the other hand, the research of ionic liquids, which is salt that maintains a liquid form at room temperature (Fig. 2), became active around 2000, and this became available for use in device research. This is an organic substance as shown in Fig. 2, and has the characteristics of being liquid at room temperature, of refractory, high conductivity, and stability. That is, it became possible to use this substance like water at room temperature without concern about evaporation.Based on the above findings, we started reviewing the use of CNTs as the electrodes of ion conductive polymeric actuators, and the use of the ionic liquid system as the electrolytes. At the time, single-layered CNTs available were extremely expensive, but we expected that the price would decrease if they were mass produced in the future. Therefore, we decided the following: to use nanocarbon electrodes such as CNTs to solve problems 1) and 3) described above; to form using a process of printing dispersed CNT electrodes for problem 2); and to use an ionic liquid gel for problem 4).4 Polymeric actuator research using the bucky gelAt rst, we engaged in research starting from improvement of ion conductive polymeric actuators, pasted the aforementioned CNT paper electrodes to the uorine ion exchange membrane, :Ion exchange resin (anion):Solvent (water etc.) molecule:CationVoltageElectrodesIon conductive polymer (polymeric electrolyte gel)EMIBF4: R=C2H5, X=BF4EMITFSI: R=C2H5, X(CF3SO2)2NEMITFS: R=C2H5, X=CF3SO3BMIBF4: R=C4H9, X=BF4BMIPF6: R=C4H9, X=PF6BMITFSI: R=C4H9, X=(CF3SO2)2NNNRXH3CFig. 2 Structural formula of the ionic liquids used in polymeric gel actuators

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