**3.2. Prototype construction for principle verification**

Experimental prototype was constructed an experimental model to verify the proposed principle, as shown in Fig.21. A commercially available toy water snake was used as a cylindrical balloon, which is made of a thermoplastic elastomer. Two types of magnetic particles (TEC-BALL, JFE Techno-Research Corporation)[18], one with specific gravity less than that of a liquid, and the other with specific gravity higher than that of a liquid were placed inside the cylindrical balloon, and the balloon was filled with water. These magnetic particles push the top and bottom parts of the cylindrical balloon simultaneously and generate a well-balanced rotation. The dimensions of the cylindrical balloon became 115 mm in length, 35 mm in height, and 42 mm in width. There is no balloon of dimensions other than this, the size of the abdominal wall model was united with that of the balloon. The black band, which is a material joint, in the center of the cylindrical balloon is used to verify the sliding movement.

**Figure 19.** Schematic diagrams of the proposed movement principle of the cylindrical soft robot. Initial state (1), Magnetic particles inside the water balloon are affected by the external magnetic field (2), push the inner side of a balloon, rotate the water balloon to the direction of a magnetic field (3)(4), Magnetic field disappears and rotational movement stops (5).

**Figure 20.** Parameters, *F*, *a*, *b* and *c* for analyzing the condition that the proposal principle enables the movement.

**Figure 21.** Experimental prototype for the principle verification, it consists of a commercially available toy water snake, two types of magnetic particles and water.
