Supplementary Note 5. Experiments on the robotic fin

The structure of the robotic fin is presented in Figure 4a and b, whose materials and fabrication method are detailed in Methods- 4.6. Materials and fabrication of the robots in applications. We fixed the robotic fin on the end of a manipulator to stabilize the actuator during actuation (Figure S8a). The propulsion force is measured through a force sensor connected between the robotic fin and the manipulator. The experimental procedures are as follows.
    i) Move the manipulator downward to immerse the robotic fin in the water. The top of the robotic          fin is about 5 mm below the water surface.
    ii) Clear the force value.
    iii) Follow the same experimental steps i-v as testing Double Bellows (Supplementary Note 4).               Notably, the real-time propulsion force is also recorded.
    iv) Move the manipulator upward and complete the experiments.
    v) Each specified working condition is measured seven times to reduce random error.
For the experiments on the robotic fin, the working pressure \(p_{tank}\)  with 50,60,70,80,90,100, and 75 kPa are covered, and  is \(\Delta p\)  set as  a constant value of 10 kPa.
Experimental results are shown in Figure 4c-f, the peak force is the maximum instantaneous propulsion force of the robotic fin, and the average force is a weighted average force in a flapping period of the robotic fin. According to the propulsion force formula \(F=\text{ }\rho u^2s_aC_t\text{ } \) , a larger average force \(F\)  can produce a larger average swimming velocity \(u\). [53]