Soft Actuator Based on Carbon-Nanotube-Composite Papers Using Kirigami Technique

We propose a unique soft actuator based on carbon nanotube (CNT)-composite papers. In recent years, there are demand for soft actuators that are lightweight, flexible, and compact actuators in medical and welfare applications. Generally, the soft actuators are made of polymer or rubber materials and respond to external stimuli such as electric field, light, temperature, and pH. And they generate a basic motion stretching and shrinking, bending, or twisting. CNT is nano-carbon material that has lightweight, high electrical conductive and large specific surface area. On the other hand, they generally exist at nanoscale and in powder form. Therefore, it is difficult to handle on its own and is used in composites with other materials. For this, we succeeded in developing CNT-composite papers that compose CNT with paper that is a familiar material. We here focus on our CNT-composite paper to develop our soft actuator.
The soft actuator in this study is an ion-conductive soft actuator responding to electric field. It is a three-layered device. It consists of two sheets of CNT-composite papers as electrode layers and one sheet of paper (electrolyte layer) containing an ionic liquid (only of cations and anions). When voltage is applied to electrodes, each ion moves to the electrode layers and is injected. The size of the cations and anions vary that causes the electrode stretches and the other electrode shrinks. And the soft actuator generates bending motion.
CNT-composite paper is made by following a papermaking method based the traditional Japanese washi papermaking method. First, CNT dispersion is prepared by ultrasonic dispersion and pulp dispersion is also prepared by stirring, and each is mixed. The mixed dispersion is poured into a fine mesh and dehydrated. After that, it is dried by a heat press and the CNT-composite paper completes. It is cut into two sheets of the same size for electrode layers. At the same time, a tissue paper is cut into one sheet of the same size and contain ion liquid as the electrolyte layer. The tissue paper with ionic liquid is sandwiched between two sheets of CNT-composite papers and stacked by pressing and soft actuator based on CNT-composite papers completes.
In this study, Kirigami technique was introduced to our CNT-composite papers to incorporate paper-derived mechanical properties into the soft actuator. In this case, we use a cutter knife for simple fabrication. First, a vertical incision was made in the center of the CNT-composite papers. Next, the soft actuator was constructed using them, and the displacement, which indicates the degree of bending motion, was measured. As a result, we measured displacement of 4 mm. It was higher displacement than previous samples. We thought that this was because the strength of the paper was lowered by making vertical incision in the CNT-composite papers.
Since improvement in displacement was observed by making the vertical incision, we then considered making horizontal incision. First, horizontal incisions were made in CNT-composite papers at equal intervals to give the paper flexibility. Next, the soft actuator was constructed using them, and the displacement was measured as described above. As a result, we measured displacement of 1 mm. This was a lower than we had expected. However, at this time, we confirmed that the soft actuator was not bending in a two-dimensional direction, but rather twisting in a three-dimensional direction. We thought that by horizontal incisions, this motion was induced by the transition from in-plane deformation to out-of-plane deformation. Until now, few soft actuators have both bending and twisting motions have been reported for polymer materials and responding to electric field. This soft actuator was found to be an excellent soft actuator that can be used for twisting motion as well as bending motion by selective incision in our CNT-composite paper.