1 Introduction

A bionic underwater robot, as the name suggests, is a new type of robot that imitates the propulsion mechanism and body structure of fish or other marine creatures living underwater using electromechanical components and intelligent materials (such as memory alloy materials, mixed materials, and rigid materials), which can adapt to different underwater environments and realize underwater propulsion (Chu et al. 2012). It has the characteristics of high efficiency, high mobility, and low noise (Chen et al. 2021a). For a long time, scholars have been committed to studying marine biological propulsion models and bionic underwater robots. Underwater vehicles can be classified into two groups based on their structural design: cabled underwater vehicles, commonly known as remotely operated vehicles (ROVs), and cableless underwater robots, traditionally known as autonomous underwater vehicles (AUVs) (Wynn et al. 2014). Moreover, they can be categorized by use into underwater investigation robots (observation, measurement, test material collection, etc.) and underwater operation robots (underwater welding, pipe twisting, underwater construction, underwater cutting, etc.) (Vu et al. 2018).

At present, most underwater robots are frame-based, similar to the rotating elongated body of a submarine. With the continuous development of bionic technology, the bionic fish shape and control modes of underwater robots will also evolve (** with high Froude number. Ocean Eng 268(1):113512" href="/article/10.1007/s44295-023-00010-3#ref-CR77" id="ref-link-section-d41130708e669">2023) and used a lightweight piezoelectric composite ceramic (PZT), a single crystal piezoelectric ceramic encased in glass/epoxy and carbon/epoxy resins, as the actuator material. The robot uses a crank, rack, and pinion structure (the size of the robot is increased by 400 mm due to the need for an additional device to achieve the movement). The robot has a maximum speed of 25.16 mm/s at an operating voltage of 300 vpp and an operating frequency of 0.9 Hz.

The miniature underwater mobile robot developed by Professor Toshio Fukuda of Nagoya University in Japan contains piezoelectric ceramics to drive the oscillation of two symmetrical legs to realize its movement. The two legs of the robot are equipped with a pair of symmetrical fins at a certain angle. The symmetrical structural design can offset the lateral force and strengthen the forward momentum. A 250-fold elastic hinge amplification mechanism is designed to amplify the PZT. The robot is 320 mm long and 190 mm wide, and the motion speed is 21.6–32.5 m/s. In 2009, the Marine Science Center of Northeastern University in the United States developed a robot fish with wave propulsion using PZT materials and chain rod structure (Zhou et al. 2008). Through lateral body fluctuations, the robotic eel drives itself through the water column and controls its floating depth. In 2008, DRAPER Lab launched VCUUV (Vorticity Control Unmanned Underwater Vehicle), a piezoelectric ceramic-driven robot fish designed after tuna (Suk and Hwan 2014). It is about 2.4 m long and weighs 300 lbs. Its maximum swing frequency is 1.5 Hz, with a maximum swimming speed of 1.25 m/s at 1 Hz. The goal of the laboratory is to develop an autonomous underwater vehicle using eddy current-controlled propulsion and show that PZT materials have good drag reduction characteristics, excellent maneuverability, depth-holding ability, and higher acceleration and deceleration ability through free swimming. Harbin Engineering University (Yue et al. 2015) developed a water microrobot with a PZT drive; the main feature of this drive is that it is a polymer material actuator only in water or wet environment work. The robot, which can be turned forward, left, or right, has a pair of driving wings driven by a pulse voltage to generate propulsion. PZT material can realize the continuous fluctuation deformation of the fluctuating fins of bionic fish, which makes it compact in structure, light in weight, and high in efficiency. This kind of robot has broad application prospects and value in microtubule detection and biomedicine.

1.4 Mixed materials

The underwater environment is complex, so the material requirements of the underwater bionic robot are very strict. Currently, polymer-metal composite materials are widely employed, combining the advantages of both the polymer and the metal. Polymers can withstand a certain degree of deformation in most environments. Both materials can make good adjustments to the impact of the external environment, with the polymer being lighter and the metal material being harder (Zheng et al. 2021) (Fig. 3c).

Fig. 3
figure 3

Mixed-material robot. a Composite robotic fish structure (Yan et al. 2021); b Prototype jellyfish-like biomimetic underwater microrobot (Shi et al. 2010); c Mechanism composition of the soft robotic fish (Zhang et al. 2021); d Dolphin robot (Shen et al. 2013); e Composite robotic fish (** patterns by using a biomimetic robot fish. IEEE Robot Automat Lett 5(1):64–71" href="/article/10.1007/s44295-023-00010-3#ref-CR135" id="ref-link-section-d41130708e752">2020); f Illustration of the robotic fish (Marras and Porfiri 2012)

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DE, which is widely used in robot drives, has good softness, and its outstanding advantages are that the relative adjustment rate after shape change is fast, the response is quite rapid, the energy consumption is less, and the mechanical and electrical conversion efficiencies are high. The dielectric elastic material-driven robot developed by Kagawa University is a jet propulsion robot simulating a pike (Bal et al. 2019). The driver is composed of SMA, ICPF, and rubber materials. The length of the motion direction is 46.1 mm, the diameter of the section is 36.3 mm, and the maximum speed of the robot is 6 mm/s. The dolphin robot developed by Beihang University consists of three parts: (1) a rigid plastic shell that acts as a body, (2) IPMC stripes that act as muscles, and (3) a plastic sheet that mimics a tail fin. The shell is designed based on the proportions of the dolphin’s streamlined body, made of nylon plastic, using a 3D printer, and covered with a black matte resin varnish, leading to a smooth surface. The IPMC is attached to the body by two small rectangular conductive copper plates, which act as clamps, with a flexible fin attached to the end of the IPMC, which is designed based on the shape of a natural dolphin fin. The robot can jump and swim freely like a dolphin (Shen et al. 2013) (Fig. 3d). The bionic robotic fish developed by the Chinese University of Hong Kong includes a rigid head, a wired-driven active body, and a flexible tail. A pair of SMA spring plates with the same stiffness pass through an active body comprising multiple connecting rods, which are like the backbone of a real fish, and then distribute a pair of wires along the spring plate to drive the moving body. The robotic fish tail is a flexible tail made of silicone and carbon fiber reinforced material that allows the robotic fish to swim in multiple modes, such as cruising, turning, rising, and descending (** patterns by using a biomimetic robot fish. IEEE Robot Automat Lett 5(1):64–71" href="/article/10.1007/s44295-023-00010-3#ref-CR135" id="ref-link-section-d41130708e782">2020) (Fig. 3e). The robotic fish designed by the New York University consists of a rigid acrylonitrile butadiene styrene (ABS) plastic body shell and a tail consisting of rigid ABS elements and flexible polyester tail fins. The robot fish uses a waterproof servo motor to control the tail, and a flexible tail fin allows the tail to bend and undulate to mimic the swimming of a live fish. The tail beat frequency and amplitude of the robot are controlled by an external microcontroller. The signals driven by the servo motor generate the periodic sinusoidal movement of the flexible polyester tail fin to mimic the movement of fish (Marras and Porfiri 2012) (Fig. 3f).

2 Underwater robot control system classification

At present, the commonly used motion control methods of underwater biomimetic robots are model-based control methods, sine controllers, and central mode generator (CPG)-based methods. As the structural components of marine biomimetic robots usually include power modules, sensors, chips, and driving components, the behavior of the bionic robot is controlled by the predefined program or the command controller (the power supply of the controller is mainly provided by traditional lithium batteries) (Chen et al. 2017). The robot adjusts its height through a buoyancy module, and a motor in the tail provides power and adjusts its direction. The flexible part of the robotic fish, designed by the State Key Laboratory of Complex Systems Management and Control at the Institute of Automation, Chinese Academy of Sciences, consists of three joints connected by an aluminum exoskeleton. Each joint is connected to an R/C servo motor that controls the rotation angle of the joint. The rubber caudal fin is connected to the third segment by the peduncle and is crescent-shaped with good coordination (Yu et al. 2016) (Fig. 6c). The robot fish designed by the University of the Chinese Academy of Sciences employs a magnetic actuator as a motor. The propulsion system is characterized by remote control using Bluetooth low power and easy operation through smart devices. By the electromagnetic induction law, the robot fish can swim quickly and turn flexibly. This miniature robot fish could be employed for animal behavior research and special underwater tasks (Chen et al. 2017) (Fig. 6d).

Fig. 6
figure 6

Single motor-driven bionic robot. a Soft robotic fish and diver interface module (Katzschmann et al. 2018); b Details of a soft-bodied robotic fish (Marchese et al. 2014); c Prototype of the robotic fish applied to the underwater robot competition (Yu et al. 2009) (Fig. 10d). To produce greater thrust, the choice of motor parameters becomes very significant.

Research on bionic flutter drive systems has never stopped. Still, due to the complexity of the drive mechanism and its unique motion characteristics and the different research methods, the forms of flutter wing propulsion are also different (Zhu 2018). So far, it has been impossible to conduct a theoretical study for various bionic drive mechanisms because many crucial technical and theoretical problems remain in the research stage, and the technical design of various bionic propulsion systems is still very backward and far from practical application.

3.3 Bionic wave oscillation rigid drive

At present, the main biomimetic fish propulsion systems are BCF models, such as dolphins, which are propelled by the caudal fin, and MPF models, such as manta rays, which are propelled by the pectoral fin.

The propulsion model has high thrust, stability, and maneuverability (Jung et al. 2002). It has an excellent performance in fast swimming under hydrostatic conditions and better start and stop functions but poor maneuverability in low-speed turns and turbulent environments. Peking University developed a robot fish consisting of a rigid head, a flexible body, and a tail fin. The hard head houses a control unit, a wireless communication module, and a set of batteries. The battery is placed at the bottom of the head to ensure the vertical stability of the robot while swimming. A pair of pectoral fins are fixed on both sides of the head to ensure the stability of the fins in water. The flexible body comprises three joints, each connected to a servo motor to adjust the deflection angle of the joint. The rubber tail fin is fixed on the third joint and acts with the water flow to move forward in waves (Li et al. 2020); d Underwater robot with elastic skin (Ma et al. 2015)

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California Institute of Technology established a piston jet model by studying the propulsion mechanism of a squid water jet (Wu et al. 2020) (Fig. 12c). The bionic pectoral fin of the manta ray robot developed by Beihang University can produce an effective angle of attack, and the thrust generated by the interaction with the current can effectively propel the robot fish. The experimental results exhibit that the maximum forward speed of the robot fish can reach 0.43 m/s (0.94 times body length/second) when it is swimming in the tank, and it has good small radius turning maneuverability (Ma et al. 2015) (Fig. 12d).

Due to different conditions, various bionic water jet propulsion systems cannot realize the same movement as real organisms, nor do they have extremely sensitive responses and fast movement ability. However, research on biomimetic water jet propulsion systems is still in its nascent stage: there is no relatively mature biomimetic propulsion system, the types of technologies are relatively small, there are several difficulties to be overcome, and there is a long way to go.

4 Applications

Oceans are vital to life on Earth; they are key to regulating climate and balancing various ecosystems (Park and Kim 2016). They are also home to countless creatures and diverse environments. In addition, the oceans are important channels for global transportation. They are indispensable sources of energy. Despite their vital significance, oceans remain underexplored due to their harsh conditions, making exploration impossible with traditional methods. Using underwater vehicles for ocean exploration is becoming increasingly popular as they allow people to conduct safe exploration in extreme environments for long periods. At present, underwater bionic robots are used in many fields, from oil and gas and fisheries to archaeology, search, rescue, and defense (Li et al. 2014c). In addition, underwater robots are of use in scientific missions, such as map** water composition and environmental parameters over time and space, exploring the characteristics of the seafloor in terms of depth, morphology, and composition, investigating glacial areas and icebergs, observing biological species in the environment, collecting biological and geological samples, searching for life in the deep ocean, and hel** protect the environment from pollution.

4.1 Application status of underwater robots

Since the second half of the twentieth century, underwater robots have begun to assist human exploration of the ocean, and with the continuous advancement of human reach and exploration depth, underwater robots performing various tasks have also been born. In 2017, Professor Yang Canjun of Zhejiang University designed an underwater robot that can automatically clean marine life 100 m below the surface of water. In its first sea test, the robot sent back a 'selfie' video underwater: firmly attached to the wall of the tube, spraying water filled with bubbles, and the accumulated shells were 'swept' away. The robot is specially designed to clean the marine organisms attached to the surface of a steel pipe of an oil drilling platform and has been successfully tested in the **hu oil and gas field in the East China Sea. In 2018, the underwater unmanned robot enterprise Yoken Robot launched a new product—BW Space Pro—which is the world’s first underwater UAV with intelligent functions, which is widely used in diving entertainment, underwater shooting, underwater survey, sea fishing, marine environmental protection, marine biological research, aquaculture, underwater archaeology, underwater search and rescue, and other fields. In 2019, Dr. Erik Engeberg of Florida Atlantic University in the United States developed a jellyfish robot that can autonomically shuttle between coral reefs and monitor jellyfish robots at close range. Besides assisting in research, the jellyfish robot can shoulder the task of defending the ocean and serve as a small spearhead in the front line of protecting the environment. In July 2020, the team of Professor Wen Li of Bei**g University of Aeronautics and Astronautics and Junzhi, a researcher from the Institute of Automation of the Chinese Academy of Sciences, designed and manufactured an underwater soft robot arm that can be applied to the natural environment of the near shallow sea, with the aim of establishing the kinematic model and rapid solution method of inverse kinematics to realize real-time kinematic control and finally realizing underwater grasp operations in the natural environment of the near shallow sea. With the upgrading and mature application of underwater robot technology, it can not only greatly reduce the risk of manual operation but also improve operation efficiency and reduce the corresponding expenditure cost. Meanwhile, driven by the integration of other innovative technologies, both the comprehensive performance and cost performance levels of underwater robots are continuing to improve, which can better complete the work and is conducive to promoting large-scale development of the industry.

4.2 Natural resource surveys

By duplicating the form of marine organisms, bionic robots can better adapt to harsh environments, such as high pressure, low temperature, and current, at the bottom of the sea. They are usually small in size and light in weight; thus, they can better collect various substances in their original conditions, which is of great significance for the study of natural resources at the bottom of the sea.

Underwater vehicles have been widely used in various marine geoscience research, initially focusing on seafloor map** but more recently expanding to water column and oceanographic surveys. The first underwater vehicle dedicated to marine was probably the IFREMER AUV, which was used in the early 1980s to map deep-sea manganese nodule fields. A Woods Hole Oceanographic Institution (WHOI) Sentry AUV was used to map the Deepwater Horizon oil spill in the Gulf of Mexico, which resulted in a hydrocarbon plume (Levshonkov et al. 2020), using robots carrying detectors to assess its impact on animals and habitats. Many underwater vehicles were deployed in 1995 and 1996 at the Juan de Fuca Ridge in the northwestern United States to detect and map new lava flows (Stenius et al. 2022). To use magnetometers to measure young lava flows at 2200 m east longitude, WHOI developed the mixed-material underwater vehicle Nereus for scientific exploration at 11000 m in the deepest part of the ocean. This was almost twice the depth range of the AUV at the time. In 2013, the French National Center for Marine Exploitation built Orca, an unmanned cable-free underwater vehicle with a maximum depth of 6000 m (Gao et al. 2013). In 2020, the French National Sea Bomb Development Center cooperated with a company to jointly develop the 'Eret' acoustic remote-control diving robot, which is used for underwater drilling rig inspection, submarine oil rig installation, oil pipeline auxiliary installation, anchor cable reinforcement, and other complex operations. In China, the underwater vehicle was first used in 2022 for subglacial surveys in the Arctic Ocean. Shortly after the scientific survey ship 'Ocean' began the third leg of the expedition, 'Ocean' conducted its first underwater robot operation in the East Pacific Sea for the first time with the underwater robot 'Sea Dragon 2', which was used to observe a rare giant chimney in the 'Bird’s nest' black chimney area and carried a robotic arm used to accurately capture about 7 kg of vulcanized black chimney ventilation samples. 'Hailong 2' relying on accurate dynamic positioning, accurately landed on the seabed in the black chimney area of the 'Bird’s nest' and performed camera observation and measurement of hydrothermal environmental parameters. The discovery marks China as one of the few countries worldwide that can use underwater robots to conduct hydrothermal surveys and sampling studies at mid-ocean ridges. The robot fish has the concealment of integrating into the fish, which can be used to collect information on the fish or guide the fish to schedule the distribution or cluster of the fish according to some algorithms (Marras and Porfiri 2012) (Fig. 13a). Thus, underwater bionic robots may effectively be used in marine environment observation, deep-sea resource exploration and development, and deep-sea and polar scientific investigation.

Fig. 13
figure 13

Application of voice-activated soft machine fish. a Robot fish collect information about shoals of fish (Marras and Porfiri 2012); b Deep-sea exploration (Li et al. 2021); c Underwater positioning (Wang et al. 2020); d Underwater imaging (Katzschmann et al. 2018)

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4.3 Biodiversity research

Through the bionic robot’s similarity in appearance to marine life, marine life can be studied without disturbing its normal activities, enabling close observation of marine life and potentially becoming a new platform for studying and interacting with underwater species (Wang et al. 2020). Underwater bionic robots play an important role in marine ecological protection. First, they can be used to collect marine environmental data. Using underwater robots, scientists can obtain detailed geographic images of the ocean and the conditions at the bottom of the ocean. This data is crucial for understanding the health and pollution levels of marine ecosystems. Professor Li Tiefeng’s team at Zhejiang University began research on a bionic deep-sea soft robot based on lionfish. Based on the dispersion and fusion of lionfish head bones in soft tissues, the project team performed the mechanical design of the structure and material of electronic devices and soft matrix and optimized the stress state in the robot body under a high-pressure environment. By designing materials and structures that adjust the devices and software, the robot could withstand a deep-sea pressure of 10000 m without a pressure-resistant shell and successfully conducted exploration missions in the Mariana Trench (Li et al. 2021) (Fig. 13b). Underwater robots can also be used to monitor the population and activity areas of marine life. With cameras and sensors, scientists can observe and record the behavior of many aquatic organisms in real time, providing evidence for their conservation.

In addition to data collection and monitoring, ROV maps can help protect marine life. They can remove debris and harmful substances from the ocean. Many marine creatures often die by ingesting waste. The underwater vehicle can collect this waste through its robotic arm and bring it to a safe location for disposal. Some underwater robot maps can even perform deep seabed cleanup operations to help restore the health of the ocean (Wang et al. 2002) (Fig. 13c). The University of Icahnx developed a new kind of robotic fish for detecting pollution in river water and drawing 3D pollution maps of the river (Gomatam et al. 2012). Each robotic fish is about 50 cm long, 15 cm high, and 12 cm wide. Each is equipped with pollution detection sensors and Global Positioning System (GPS), can 'smell' harmful substances in the water, and can work together, even if there is no one to direct. When they 'sniff out' the harmful substances in the water, they communicate with each other through a Wi-Fi wireless connection. The GPS navigation system allows them to swim freely without human operation, and once they find pollutants, they will send an alert to the environmental protection department personnel (Skorohod et al. 2020).

Biosensors were first deployed on an underwater robot when an NERC autonomous submersible AUV was fitted with an in situ dissolved manganese analyzer (Skorohod et al. 2020). This deployment showed how an autonomous underwater robot carrying a biosensor could detect small-scale changes in species distribution that traditional sampling methods could not address. Since then, chemical sensors on underwater robots used for marine purposes have been used mainly to search the water column for active hydrothermal columns and to study species distributions, and a suite of sensors for detecting hazardous liquid spills have been deployed on underwater robots in the North Sea Sleipner project for frequent, high time scale studies of areas of potential spills to protect the ecological environment (Tran and Park 2020). By application of underwater robot map**, people can better protect the diversity of marine ecosystems and marine life. They help people understand and solve the problems of the marine environment and provide a guarantee for the rational use of marine resources.

4.4 Underwater imaging

There is an increasing demand for exploration of the seabed environment, and the imaging requirements for marine resources and the underwater world are also getting higher and higher (Liang et al. 2010). Due to the uncertainty of the underwater environment, such as interference of the current and limited sensing ability, conventional underwater navigation equipment has limitations; thus, bionic robots designed for different underwater environments have great advantages.

The bionic underwater foot robot studied by the National Metrology Institute of Japan (Maeda et al. 2020) imitates the appearance and behavior of crabs and can walk and jump underwater. Compared to traditional AUV and ROV, it is better adapted to complex underwater terrains and has a higher affinity for underwater organisms. Due to their bionic appearance, the natural movements of underwater creatures can be well imaged. National Institute of Ocean Technology (Ramesh et al. 2017) used the bionic fish REMUS to map the habitat at 1–2 m water depth in the Juan Strait in the northern United States. It used underwater video data for ground truth measurements. Underwater robots have been used to map various seafloor morphological features, including under ice sheets inaccessible to research ships. For instance, State Marine Technical University (Siek and Sakovich 2019) used the underwater vehicle NERC Autosub3 to investigate the retreat of the Pine Island Glacier (PIG) in West Antarctica. The robot performed six missions in 94 h, collecting 510 km of orbital data under the PIG ice shelf 50 km above the ice surface.

Underwater vehicles are also being used to image sedimentary features in submarine canyons. The University of Kanagawa used an underwater vehicle carrying a high-resolution multibeam waveform acoustic system (0.7 m lateral resolution) and a submarine profiler (0.1 m vertical resolution) to conduct underwater imaging experiments, collecting data from La Jolla Canyon on the Southern California coast. To understand the processes that produce observational patterns on a scale comparable to the surface (Tsukioka et al. 2002), the Science and Technology on Underwater Vehicle Laboratory used underwater robotic fish diving to collect deep-sea data and provide vibration core samples for sediment dating (Liu et al. 2020) (Fig. 13d). In the article on acoustically controlled soft robotic fish studied by the University of Zagreb (Kapetanovic et al. 2020), it is possible to approach underwater organisms without disturbing their normal life and to image underwater organisms and underwater landscapes through shape features similar to those of fish (Katzschmann et al. 2018).

When conducting underwater imaging, conventional underwater vehicles have higher accuracy in the tangential direction of the seabed and lower accuracy in the vertical direction of the seabed. In comparison, underwater bionic robots have lower accuracy in the tangential direction of the seabed and higher accuracy in the vertical direction of the seabed (Wang et al. 2014a). Moreover, the underwater bionic robot has high stability and adaptability to the seabed environment, and the combination of the two can obtain higher-quality underwater imaging maps.

4.5 Underwater search and rescue

Underwater robots can be used to check whether explosives are installed on dams and bridge piers, remote-reconnaissance structural conditions or dangerous goods, and closely inspect underwater evidence. In 2010, underwater robots could walk at 3–6 km/h in the deepest underwater world of 6000 m (Brown and Clark 2010). The forward-looking and downward-looking radar gives it 'good eyesight'. The accompanying camera, video camera, and precise navigation system allow it to 'overlook'. The underwater robot WHOI provided in 2012 took just a few days to find the wreckage of an Air France flight in 4000 km2 of ocean after two years of fruitless searching by various ships and aircraft. Underwater robots have great potential and application value in rescue missions. When encountering dangerous situations, underwater bionic robots can play a greater role in on-site situation assessment and positioning, providing important information for the next step. Through the underwater high-definition camera group, sonar, and a variety of sensors carried by the underwater robot itself, rescue workers can grasp the water depth and temperature on the shore. They can determine the obstacles in the water and remove the danger of entering the water (Wang et al. 2019c). In salvage and other operations, the underwater robot can quickly locate the location of underwater objects. Armed with this information, commanders can better formulate a reasonable and efficient rescue plan. Another major advantage of underwater robots lies in search and rescue missions. In dangerous waters such as rapids and low temperatures, it can take the lead in entering underwater areas that rescuers cannot reach to detect the location and situation of trapped people. The robot operator can control the movement of the robot by manipulating the handle or wireless sensing device on the shore. Carrying tools such as robotic arms can also assist rescue workers in completing tasks such as clearing and salvaging. The water environment where the danger occurs is not always ideal, and low visibility is one of the most significant problems. Bionic fluorescent robot fish can provide rescuers with a light source, and rescuers can also determine the location of the target and search for risks by referring to the umbilical cable connected to the underwater robot (Asadnia et al. 2015). The emergence of underwater robots makes underwater rescue work safer and more efficient.

5 Summary and outlook

From the above summarized research results, it can be observed that research on bionic underwater robots has grown considerably. Rapid turning, path tracking, autonomous operation, and other actions have been achieved on some prototypes, and there is a great improvement in speed and mobility, but there is still a very obvious gap with real fish. Underwater bionic robot development is high-end manufacturing industry supported by the Chinese government and plays the role of a 'strategic commanding height'. In China and abroad, a series of work has been conducted on the mechanical structure design, materials, and control methods of underwater bionic robots, and the related research has grown considerably. Due to the complexity of underwater, the mechanical structure design and control technology of underwater robots still require further optimization and improvement to truly achieve a life-like system that integrates the structure and biological characteristics. By enhancing the characteristics of underwater robots, such as self-control and self-perception, and through the coordinated control of robot systems, underwater robots can better integrate into the underwater environment to complete the work, pursue sustainability on the road to development, and make this technology more mature.

Research on bionic underwater robots has become more in-depth and expanded, and some prototypes have realized multimodal motion, fast turning, path tracking, autonomous operation, and other actions, which have greatly improved in speed and maneuverability. However, there is still a very obvious gap with real fish. In the future, bionic underwater robots should be developed into autonomous, intelligent, and collaborative tools. To further improve the performance of the bionic underwater robot system, further work should be conducted in the following main research directions: (1) Mechanism design and optimization. Most bionic underwater robots are driven by motors. Research can be conducted in terms of streamlined low-resistance shapes, intelligent driving materials, and rigid and flexible coupling efficient transmission mechanisms to improve the motion performance of bionic underwater robots. (2) Underwater environment perception and modeling are significant for bionic underwater robots to perform underwater tasks. Information fusion technology of various sensors can be examined and combined with the technology to conduct underwater environment modeling and improve the autonomy intelligence of bionic underwater robots. (3) Intelligent control methods, such as artificial intelligence, are a hot field right now. Some artificial intelligence technologies, such as reinforcement learning and transfer learning, can be applied to the intelligent control of bionic underwater robots so that they can learn various motor skills independently. (4) Multibionic underwater robot cooperation. In nature, fish is often in the form of clusters for foraging, defense, and cruising. The use of multiple bionic underwater vehicles to form a cooperative system is helpful in improving operational efficiency. Due to the complexity of underwater, the particularity of the propulsion mechanism, and the bottleneck of underwater communication, sensing, positioning, and other technologies, the collaboration of multibionic underwater robots will be a very challenging direction.

Due to the complexity of the marine environment, underwater bionic robots will face problems such as the drastic change in water velocity, the difference in pressure under different water depths, and their waterproofing, which poses a great challenge to the structural design of robots. To address these issues, the structure of the future underwater bionic robot needs to be more detailed and more lightweight, and the application of materials should also meet the requirements of the underwater environment. Miniaturization is the current trend of robot development because small structures are easier to adapt to the environment, reduce the contact area, and thus reduce the impact of underwater pressure on the machine structure to a greater extent. The most prominent point is that miniaturized robots are closer to the physiological structure of marine organisms and fundamentally realize the bionic effect rather than just the imitation of appearance. Marine space is generally unsuitable for human survival, and large-scale development and utilization of marine resources have a great dependence on robotics technology. Replacing humans with robots to promote and realize unmanned marine equipment has far-reaching strategic significance. Thus, future bionic underwater robots should be further developed, mainly in the direction of autonomy, intelligence, and synergy, to enhance the performance of bionic underwater robotic systems.