Our team developed "Roboswimimer," a swimming fish-inspired soft robot, under the guidance of Dr. Suyi Li. As is the inspiration for many biomimetic designs, we wanted our design to follow something that already exists in nature. We naturally thought of a fish – capable of swimming fast, slow, in deep water as well as in shallow waters – exactly what we wanted to achieve with our design.

Drawing inspiration from carangiform swimmers like tuna and mackerel (Lauder & Tytell, 2006), our soft robot harnesses undulatory propulsion where movement is primarily concentrated in the posterior third of the body. Unlike traditional rigid underwater vehicles that rely on propellers or thrusters, our design utilizes a tendon-actuated flexible spine system that more closely replicates the natural swimming mechanics of fish.

Our team decided to adopt tendon actuation as the method to control our soft robot because it was a relatively intuitive technique and feasible to implement given our available time period and design requirements. This biomimetic approach offers several advantages for underwater locomotion, including improved energy efficiency, enhanced maneuverability, and quieter operation compared to conventional underwater robots (Katzschmann et al., 2018).

The design involves a fish head fabricated with 3D printing that acts as the housing for the electronics. The body is made of 3 structural rings as shown in Figure 2. A flexible polyurethane pipe goes through the middle of the rings, acting as the spine, while the outer holes are made for the tendons. The end of the body features a 3D-printed PLA caudal fin which serves as the key body part that causes the fish to propel forward in water. By replicating the body-caudal fin (BCF) propulsion method found in nature, our robot achieves forward thrust through the rhythmic lateral oscillation of its flexible tail (Sfakiotakis et al., 1999). The electronics used for this robot include an Arduino Nano, a waterproof servo motor, and LiPo batteries.

Our soft robotic fish combines rigid and flexible components to achieve efficient underwater locomotion while staying within a 6-inch size constraint. The design features a 3D-printed rigid head housing that contains all electronic components (Arduino Nano, 7.4V LiPo battery, and power/mode selection switch), providing waterproof protection while maintaining a hydrodynamic profile.

The propulsion system centers around a waterproof servo motor positioned at the junction between the head and flexible tail. This servo drives the tendon actuation system, where fishing line "tendons" run through guide channels in the structural rings. When the servo rotates, it pulls the tendons asymmetrically, causing the flexible polyurethane (PU) spine to bend in controlled, fish-like undulations.

Initial testing revealed challenges with our original design. With the tail and head in a 1:1 ratio and constrained by the 6-inch size limit, we couldn't extend the tail length to generate sufficient thrust. Our solution was to increase the caudal fin's height, enhancing the surface area for propulsion without violating size constraints. Additionally, we observed directional instability during swimming tests, which we resolved by adding a stabilizer fin to the underside of the robot. This design modification significantly improved straight-line swimming capability by preventing unwanted rolling and yawing motions.

The three structural rings along the spine serve as guide points for the tendons while preventing torsional twisting during the swimming motion. This combination of rigid support elements with the flexible PU spine creates an efficient undulatory motion that mimics natural fish swimming, allowing "Roboswimimer" to navigate effectively in various water conditions.