Evaluation of Gel Touch Sensor

Abstract

The polymer gel is composed of polymeric chains which are crosslinked in a three-dimensional (3D) network structure and liquid which is widely applied to tissue engineering, medical treatment, and solid state electrochemical devices. The gel contains a large amount of water inside the three-dimensional network structure. The human body is also mostly composed of water, hence, the percentage of water in human body is relatively high similar to hydrogel. Among various kind of gel, ion gel possessing ionic liquid is beneficial for applications since ionic liquids are liquid salts at room temperature and have useful properties such as non-volatility, non-inflammability, electrochemical stability, thermal stability and high ionic conductivity. In this study, we investigated the piezoelectric properties of ion gels to apply them for touch and pressure sensors in soft-matter robotics. In this study, the used ion gel was made utilizing thiol-ene reaction and was composed of thiol monomer. The thiol-ene reaction is hardly influenced by the surrounding environment, hence, we can easily make ion gel. In addition, it is possible to form any objects using a photopolymerization initiator and a 3D printer. The aim of this research is to develop a tactile sensor using ion gel having non-volatility, non-flammability, thermal stability, high conductivity and softness close to humans. For the preparation of ion gels, the monomers pentaery thritol tetralcis (3-mercaptobutylate) and dipentaerythritol hexakis (3-mercaptopropionate), crosslinker 1,6-Hexanediol diacrylate, initiator benzophenone, solvent ionic liquid 1 - buty - 3methylimidazolium bis (fluorosulfonyl) imide were used. For the experiment, a friction tester, LCR meter and temperature controller were used. Here we report the relationship between frequency of applied alternating current, electric resistance of ion gel, applied load, and temperature. The sizes of sample gel plates are 50 mm × 10 mm × 2 mm and 40 mm × 10 mm × 2 mm. Resistance of ion gel in 10 kHz is 3.4 kΩ and 4.8 kΩ taking for 200 g and without loads, respectively. It is speculated that the electric resistance decreased with the shape change due to load application. Resistance of ion gel in 10 kHz is 12.3 kΩ, 5.86 kΩ and 3.48 kΩ at temperature of 40 ℃, 60 ℃ and 80 ℃, respectively. We confirmed the temperature dependency of impedance of the gel sample. The experimental results indicate that impedance was decreased with increase in frequency, load and temperature. Currently, we are developing a human finger-modeled pressure and temperature sensor. Ion gel is shaped into human finger using a 3D printer. The LCR meter used for resistance measurement is made using Arduino. The technique shown in this study is expected to be applied to soft robot and robot hand.