1. Introduction

Engineering and technology are increasingly required to be utilized in “fields closer to humans” alongside changes in social conditions, such as a declining birth rate and aging population. A machine equipped with essential safety features is required to meet these needs, and a driving element (soft actuator) with features such as “flexibility” and “adaptable shape” are important. A McKibben pneumatic artificial muscle is a soft actuator with a structure where fibers are woven around a rubber tube, and it is the representative artificial muscle at the level of practical application currently. Using our technology, we have achieved a smaller diameter (minimum diameter of approximately 2 mm), longer length, and mass production of the McKibben pneumatic artificial muscle. Generally, the McKibben artificial muscle experiences decreased driving force and a considerable drop in durability as the diameter is reduced. In our technology, we investigated and prototyped the fiber and rubber type as well as the method of weaving the fibers to achieve a small-diameter McKibben artificial muscle with high operating performance and durability. The award winners have established a university-launched venture company based on the results of the industry-academia collaborative research at the university and are in the process of spreading and deploying this technology in society.

2. Content of technology

2.1 Smaller diameter / increased length
The McKibben artificial muscle is a pneumatic soft actuator composed of rubber tube and fibers woven around it. When air pressure is applied inside the rubber tube, the rubber tube deforms and the woven angle of the fibers changes, which simultaneously results in contraction in the axial direction and expansion in the radial direction. This artificial muscle was developed in Germany around the 1950s, and various applications focusing on welfare equipment have subsequently been attempted in the past 70 years. Even today, several companies still sell artificial muscles, but many of these are on the order of centimeters, and their lengths are approximately a few dozen centimeters.

The award winners continuously produced small-diameter artificial muscles with a diameter of 2–5 mm and succeeded in mass production. Various new drive mechanisms that were not previously possible have been developed using this small-diameter artificial muscle. The operating principle and structure are the same as conventional ones, but the smaller diameter and longer length have enabled various flexible drive mechanisms that were not previously possible.

Various phenomena occurring in artificial muscles with a normal size can also occur as the diameter decreases. One is the decreased contraction amount and contraction force. The effects of friction between the rubber tube and fiber as well as between the fibers themselves become considerably larger as the diameter decreases, and the operating performance deteriorates. Furthermore, even slight variations in rubber tube thickness or rubber hardness can result in heterogeneous rubber tube expansion or meandering braids, and the artificial muscle will cease to function. The increased contact stress and friction between the fiber and rubber tube surface also damages the rubber surface and dramatically increases the frequency of damage, thereby also affecting durability.

The award winners solved these problems by improving the rubber material, improving the fiber material / composition, optimally designing the woven structure, and thoroughly controlling quality. Thus, they achieved contraction characteristics of more than 20% and a drive durability of more than 1 million times despite the artificial muscle’s small diameter; these characteristics are superior to those of normal McKibben artificial muscles. Continuous production at the several-hundred m scale was also made possible. Currently, several types of artificial muscles are being sold, including a long roll-shaped small-diameter artificial muscle (left left) and small-diameter artificial muscles with a pneumatic one-touch connector (bottom right).

2.2 Application
The decreased diameter and longer length have enabled flexible mechanisms of various shapes by weaving and bundling the artificial muscles. In particular, various applied researches, such as comfortable power support suits (bottom figure, right) and rehabilitation equipment (bottom figure, left), are currently in progress at domestic and international research institutions and private companies.

Suzumori Koichi*1 (s-muscle Co., Ltd,/ Tokyo Institute of Technology)

Shuichi Wakimoto*2 (s-muscle Co., Ltd,/ Okayama University)

Shukushi Seita*3 (s-muscle Co., Ltd.)

Kazutoshi Kohno*3 (s-muscle Co., Ltd.)

*1 Fellow，Tokyo Institute of technology (12-1, Ookayama 2, Meguro-ku,, Tokyo 152-8550), s-muscle Co., Ltd.
*2 Member, Okayama University (1-1-1, Tsushima-naka, Kita-ku, Okayama, 700-8530), s-muscle Co., Ltd.
*3 s-muscle Co., Ltd.e (4-24, Karakoto 2, Kurashiki 711-0905)