At present, single-fin propulsion modes, such as pectoral fins and caudal fins, cannot ensure the mobility and forward speed of bionic robotic fish, whereas a multi-fin and flexible body can better ensure a high propulsion efficiency – particularly with low-speed mobility and high-speed stability. Therefore, the cooperative propulsion mode of the pectoral fin and caudal fin has become a research hotspot in recent years. This paper systematically introduces various actuators of robotic fish, including traditional actuators and functional materials. The functional materials include shape-memory alloy wires, ionic polymer–metal composites and piezoelectric ceramics, and their performance characteristics and limitations are discussed. Functional materials can reduce the volume and weight of a robot and reduce noise. Underwater robots have the advantages of flexibility and the ability to perform complex motions. Therefore, flexible robotic fish that combine the flexible behaviour of real fish with robot technology are gradually emerging. Finally, future research directions for bionic robotic fish are presented.

To improve the performance of a 3.5 kW small H‐type vertical axis wind turbine (VAWT) with the airfoil of NACA0015, a design method for installing split flaps at the trailing edge of a blade is proposed. By solving the two‐dimensional URANS equations, the simulation of VAWTs is carried out. The influences of the length (10% c‐30% c), arrangement positions (90% c‐70% c), and deflection angles (10°‐30°) of the split flaps on the performance of VAWT are determined by orthogonal design when Reynolds number is 2 × 10⁵ and 1 < TSR < 3. In accordance with the influence degrees of these parameters and adopting the maximum wind energy utilization ratio C_Pmax and the maximum efficient operating range Δλ_max as research objectives, the optimum flap length is determined as 22% c, the optimum deflection angle is 10°, and the optimum arrangement position is 92% c. Further studies show that the C_Pmax is increased by 5.8% and its Δλ_max is increased by 25.9% after installing split flaps. Meanwhile, split flaps can change the Kutta‐Joukowsky trailing edge boundary condition, which increases the flow circulation around the airfoil. Also, split flaps can increase the positive pressure difference in the upwind area and reduce the negative pressure difference in the downwind area, which significantly improves the aerodynamic performance of VAWTs.

Hydrogel has been regarded as one significant biomaterial in biomedical and tissue engineering due to its high biocompatibility. This paper proposes a novel method to pattern calcium alginate hydrogel in a 3D way via electrodeposition process based on a piece of paper. Firstly, one insulating paper with patterned holes is placed on one indium tin oxide (ITO) glass surface, which is put below another ITO glass. Then, 1% sodium alginate solution with 0.25% CaCO3 nano particles is filled between these two glasses. In the bottom glass, patterns of electrodes followed patterns of holes on the insulating layer. Hydrogel forms on patterned electrodes when electrochemical potential is applied due to electrodeposition. The experiments demonstrate that the pattern of alginate hydrogels follows the pattern of electrodes exactly. In addition, the hydrogel’s height is controllable by applied potential and reaction time. An equivalent circuit model and a hydrogel growth model have been built to predict the electrodeposition current and hydrogel’s growth. This method for gel formation is easy and cheap since the main material is one piece of insulated paper, which provides an easy and controllable method for 3D hydrogel patterning.