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Page 185 Yu et al. Intell Robot 2022;2:180-99 https://dx.doi.org/10.20517/ir.2022.10

Figure 4. Rigid-soft coupled robot fishes in BCF propulsion mode: (A) a robot fish with two caudal fins [22] ; and (B) wires actuators of the
robot fish [23] . BCF: Body and/or caudal fin.

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experiments. Tunabot could swim 9.1 km if it swam at 0.4 m·s or 4.2 km if it swam at 1 m·s while powered
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by a 10 Wh battery pack. This highlighted the capabilities of high-frequency swimming. This provided new
ideas to improve the swimming performance of robot fishes. The variable stiffness design of the robot fish is
[25]
also an imitation of fish. TenFiBot, a robot fish with variable stiffness, was designed by Chen and Jiang .
The whole structure of TenFiBot was a tandem structure with multiple variable-stiffness tensegrity joints
(VSTJs). The preload of the springs on the VSTJs could be adjusted to change the stiffness distribution on
the TenFiBot’s body. Experiments demonstrated that the change of stiffness distribution directly affected
the swimming performance (such as swimming speed) of the robot fish. By changing the stiffness
distribution of the robot fish, its swimming performance could be greatly improved.

2.2. Robot fishes in MPF propulsion mode
2.2.1. Soft robot fishes
This robot fish tends to be designed with smart materials and is smaller in size. As MPF propulsion mode is
adopted, it has greater maneuverability. Therefore, it is ideal for applications in special environments, such
as fine pipes, deep sea, etc. Inspired by the hadal snail-fish, which lives at 8000 m water depth, Li et al.
[26]
designed an untethered soft robot fish that could withstand extreme hydrostatic pressure . The robot fish
was driven by DEAs. The electronic components of the robot fish were decentralized on several smaller
printed circuit boards, which could effectively reduce the shear stress between components. This ensured
that the robot fish could withstand extreme water pressure. The robot fish successfully swam at a depth of
10,900 m in the Mariana Trench, showing great potential for application in deep-sea exploration.

2.2.2. Rigid-soft coupled robot fishes
A key condition to achieving high swimming performance is to adjust the distribution of soft and hard
structures in robot fishes. As shown in Figure 5A, a robot fish with cartilages and soft tissues was designed
by Yurugi et al. . Experiments revealed that adding cartilages to the fins of the robot fish could improve
[27]
swimming efficiency. The researchers also investigated the fish’s swimming behavior. As shown in
Figure 5B, Ma et al. designed a robot fish driven by the oscillating and twisting of the pectoral fins after
[28]
studying the pectoral fin movement of the cownose ray . The pectoral fins simultaneously realized
oscillating motion and chordwise twisting motion. The maximum swimming speed of the robot fish was
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0.94 BL·s , and the turning radius was nearly zero. This reflected the excellent turning performance and
high swimming speed of the robot fish. These authors should conduct additional research into the effect of
pectoral fin flexibility on swimming performance.