Electrochemically Driven One-Body Torsional Artificial Muscles from Polymer-Coated Carbon Nanotube Yarn

Torsional artificial muscles are being developed to replace electric motors in the field which needs miniaturization. Among the various types of torsional muscles, electrochemical torsional muscles have many advantages including fast reaction, easy control, high energy efficiency, and a natural latching state. For the electrochemical artificial muscle, carbon nanotubes (CNTs) are one of the best materials due to their remarkable mechanical strength and high capacitance. Previously reported electrochemical torsional CNT muscle used a CNT yarn made by twist insertion into the sheets. These CNT muscles showed great rotating performance. However, these muscles could only use a half of the available voltage range for actuation, which means that only one of the positive and negative voltage ranges could be used, so their rotation performance is limited. In addition, counter and reference electrodes made it difficult to make a compact muscle system. To solve these problems, we demonstrate a torsional muscles which rotate in one direction throughout the whole available voltage range. By coating the CNT yarn with an ion-exchange polymer, poly(sodium 4-styrenesulfonate) (PSS), the potential of zero charge of CNT yarn shifts to the outside to the electrochemical stability window of aqueous electrolytes. It means that only cations participate to a muscle rotation for the whole voltage range, resulting the rotation in the one direction from -1 to +1 V. The rotation degree and speed of the PSS-coated CNT muscles are higher than the neat CNT muscles, since the voltage range is doubled. Moreover, the PSS-coated CNT muscles are easy to make a compact one-body structure which consists of upper and lower muscles. The upper muscle and the lower muscle of the pedal rotates in the opposite direction when a voltage is applied to the one-body muscle system. One muscle works as electrochemical and mechanical counter for the other muscle, so the rotation of the pedal is maximized and its rotation also has high reversibility. This high-performance and one-body torsional muscle is useful for various applications, such as microbotics and microfluidics.