The McKibben artificial muscle is a pneumatic soft actuator consisting of a rubber tube, with fibers covering it. In this study, we propose a new fabrication process to easily realize twisting artificial muscles by utilizing braiding technology and water-soluble fibers based on the McKibben artificial muscle. In the braiding process, half of the sleeve fibers of the artificial muscle are substituted with water-soluble fibers. However, the water-soluble fibers are removed by placing the muscle in warm water to realize a twisting motion. By changing the direction of the fibers, artificial muscles that twist clockwise or counterclockwise can be produced. We believe that the establishment of such a simple and efficient fabrication process will promote to the practical application of artificial muscles.

A McKibben artificial muscle is a fluid-driven soft actuator comprising sleeve fibres and rubber tube. However, as typical bulky and rigid displacement sensors are unsuitable as sensor elements in soft actuators, displacement sensing is challenging for the McKibben artificial muscle. Therefore, we propose an optical fibre-based smart artificial muscle (OSAM) to estimate self-displacement from the bending loss of the optical fibre used as the sleeve fibre. The optical fibre can be effortlessly integrated into the OSAM sleeve using a braiding machine, which is generally used for manufacturing strings, easing the mass production process. The radius of curvature of the optical fibre changed when the OSAM was driven. The displacement of the artificial muscle was estimated based on the sensor output. To demonstrate the usefulness of OSAM, displacement feedback control experiments were conducted using the optical fibre sensor integrated into OSAM. From the results, OSAM’s displacement showed a good response to the target displacement. Therefore, the developed artificial muscle can facilitate displacement feedback control without requiring external sensors, which in turn can improve the performance of rehabilitation and wearable devices.

A thin McKibben artificial muscle is a pneumatic actuator with an outer diameter of only 1.8 mm. We fabricated a string-shaped actuator called an “active string actuator,” which achieves a high contractile displacement by accumulating thin McKibben artificial muscles. To control the displacement, the length of the active string actuator should be estimated. However, this is difficult because bulky and rigid sensors are unsuitable for the sensor element of the active string actuator. Therefore, in this study, we propose a new sensing method for estimating the length of an active string actuator. The proposed sensing system is simple and comprises only three components: a step-index multimode optical fiber, a light emitter, and a light receiver. A step-index multimode optical fiber was combined with the active string actuator, and the length was estimated from the change in the amount of light propagating in the optical fiber when the active string actuator was driven. Fundamental experiments were conducted in this study, and the results demonstrated that the optical fiber sensor value changed with the actuator length. This suggests that it is possible to estimate the displacement of an active string actuator using an optical fiber sensor.

McKibben artificial muscle is a fluid-driven soft actuator comprising sleeve fibres and a rubber tube, which contracts by fluid pressure. A part of the side of the artificial muscle is reinforced with a reinforcement material to suppress contraction in that area, thereby achieving the bending motion. In this study, we propose a novel fabrication process to realise bending-type smart artificial muscles (B-SAMs) that sense their curvature by using optical fibres as the reinforcement material. The optical fibre also serves as a curvature sensor owing to the attachment of a light emitter and light receiver on each end of the fibre. A braiding machine is employed to fabricate B-SAM, allowing the optical fibre to be integrated with the sleeve fibres to cover the rubber tube, rendering mass production easy. The versatility of the proposed method is demonstrated through fabricating and evaluating a soft mechanism with two bending units using this fabrication technique. The proposed simple and efficient fabricating process can promote the practical application of artificial muscles.

We have fabricated a string-shaped actuator called “Active string” that has high contractile displacement/force by accumulating thin pneumatic artificial muscles using the string production process. However, displacement control of the active string is challenging because general bulky and rigid displacement sensors are not suitable for the sensor element of the active string. Therefore, in this report, a flexible optical fiber sensor is combined with the active string to enable sensing of its displacement. As the active string contracts, the radius of curvature of the optical fiber decreases, and light intensity propagating in the optical fiber decreases due to bending loss. The experimental results showed that the optical fiber sensor value changed with corresponding to the displacement of the active string. It shows the possibility that it is possible to make a displacement estimation of the displacement of the active string using an optical fiber sensor.