Highly Stretchable and Positive-Piezoconductive Silicone Composite Using Nickel Powder and Ionic Liquids

In this study, a new conductive rubber with positive piezoconductivity, whose conductivity increases 10 million times with tensile strain, and which can be elongated by up to 600%, is proposed. The proposed material consists of silicone (Ecoflex00-30) as a matrix, nickel powder as a filler, and ionic liquid as an additive. In previous studies, maximum extensibility of only 100% was achieved because the matrix elasticity was limited by the large amount of conductive material required for the positive piezoconductivity. In this study, Positive piezoelectric conductivity due to the inclusion of a large amount of filler and high stretchability were both achieved by the gelation of silicone by the addition of ionic liquid, which caused an increase in the matrix stretchability.<br/><br/> Conductive rubber is a composite of stretchable rubber and conductive filler, like carbon nanotube, graphene, and metal particles. This material has elasticity due to the rubber and conductivity due to the filler. Therefore, the material is used for wearable devices and soft robotics.<br/><br/> In general, conductive rubbers decrease in conductivity with elongation. This is caused by the breakdown of electrical contact between conductive particles inside the conductive rubber as it elongates. This property is called negative piezoconductivity. Conductive rubbers with negative piezoelectric conductivity can be used as strain sensors by measuring their resistance, and are utilized for feedback control in wearable devices and soft robots.<br/><br/> On the contrary, conductive rubbers with positive piezoconductivity, whose conductivity increases with elongation, have also been studied. Some of them show a rapid increase in conductivity under tensile strain. These positive piezoconductive rubbers are used as strain sensors as well as those with negative piezoconductivity, or as switches in stretchable circuits that take advantage of the rapid change in conductivity.<br/><br/> However, it is necessary to mix a large amount of conductive filler to induce a sufficient resistance change as a circuit element for a positive piezoconductive rubber. On the other hand, an increase in the amount of conductive filler mixed with the rubber material causes a decrease in the elasticity of the overall composite. Therefore, both sufficient conductivity enhancement by strain and high elasticity are required for positive piezoconductive materials for applications with high deformability, such as stretchable devices and soft robots.<br/><br/> The conductive rubber developed in this studyas a maximum elongation of 600%, comparable to that of raw silicon rubber, in spite of holding three times as much Ni powder as rubber by mass. This occurs due to the ionic liquid being held in the silicone rubber in a liquid state.<br/><br/> The conductivity of stretched composite was increased 10 million times higher than the non-stretched condition. The increase in conductivity can be explained by the decrease in composite cross-sectional area due to tensile strain, which caused the aggregation of Ni powder. In addition, the elongation ratio at which the resistance drop begins can be controlled according to the mass of Ni powder contained inside.<br/><br/> The composite was demonstrated for application to soft robots and wearable devices. The composite was incorporated into an electrical circuit to read the deformation of the soft robot and elbow bending in a single series circuit. Unlike existing conductive rubbers with negative piezoelectric conductivity, which require an external evaluation circuit to switch by strain, the conductive rubbers developed in this study realized a switch by elongation without an external circuit.