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Scientists Built a Robotic Fish That Can Swim Like a Real Fish Using ‘Smart Muscles'

Nature has spent millions of years improving the way animals move. Among the best examples are fish, which can swim through water with incredible speed, control, and efficiency. Fish do not simply move their bodies from side to side. They carefully control the flexibility and stiffness of their bodies and fins to create powerful and energy-saving movements.

Now, scientists are trying to bring this natural ability into the world of robotics. A research team led by Hu and colleagues has developed a new technology called Programmable Online Stiffness Modulation (POSM) that allows underwater robots to change their stiffness while swimming. This could help future robotic swimmers move more naturally, use less energy, and perform better underwater.

Learning From the Swimming Secrets of Fish

Fish are among the most efficient swimmers on Earth. Their bodies are designed to work with water instead of fighting against it. One important reason behind their success is their ability to control body stiffness.

When a fish swims, different parts of its body do not remain equally flexible all the time. Muscles and tendons help the fish adjust its body strength and flexibility depending on the situation.

For example, when a fish needs to move forward quickly, it can make parts of its body stiffer to create stronger pushes against the water. When it needs to turn or change direction, it can become more flexible for better control.

This ability to change stiffness during movement gives fish an advantage. However, most robotic swimmers today do not have this feature. Their joints usually have fixed stiffness, which limits their ability to adapt.

Researchers wanted to create a robot that could adjust itself in a similar way to living creatures.

A Robot That Can Change Its Stiffness

The team developed the POSM system to give underwater robots a new ability: changing their joint stiffness while they are moving.

The technology is based on a multi-link robotic arm structure. This means the robot is made of multiple connected sections, similar to the body parts of a fish.

Each joint in the robot can have its own stiffness level. This is important because different parts of a fish’s body contribute differently during swimming. The tail, body, and fins all perform unique roles.

With POSM, the robot can control each joint separately. One part of the robot can become stiff while another part remains flexible. The robot can also change these settings over time depending on the type of swimming movement it is performing.

This makes the robotic swimmer much more adaptable compared with traditional designs.

Understanding Movement Through Advanced Simulation

Creating a swimming robot is not easy because moving through water is complicated. Water creates resistance, pressure, and many other forces that affect how a robot moves.

To solve this challenge, the researchers created a detailed computer model of the robotic swimmer. They used a method called discrete variational mechanics to study how the robot moves while interacting with water.

This method helped them accurately calculate the movement of different robot parts and the effects of underwater forces.

The simulation allowed researchers to test different swimming strategies without needing to build a new robot for every experiment. They could study how changing stiffness at different times affected the robot’s performance.

This helped them discover the best ways to control stiffness for efficient swimming.

Finding the Best Swimming Strategy

After creating the simulation, the researchers used it to optimize the robot’s movements.

Instead of manually deciding when each joint should become stiff or flexible, they used the system to find the most effective stiffness patterns automatically.

They tested the robot using two different swimming styles.

Drag-Based Swimming

In drag-based swimming, the robot uses its fins to push against the water. This is similar to how many fish use their side fins, called pectoral fins, to move, balance, and control direction.

The researchers found that adding dynamic stiffness control greatly improved performance. The robot produced up to 89% more thrust compared with robots that used fixed stiffness.

This means the robot could push through water much more effectively.

Body-Caudal Fin Swimming

The second method is called body-caudal fin (BCF) propulsion. This is one of the most common swimming methods in fish, where the body and tail create waves that push water backward and move the animal forward.

Using POSM technology, the robotic swimmer achieved up to 19% improvement in thrust compared with traditional robots without stiffness control.

Although the improvement was smaller than drag-based swimming, it still showed that changing stiffness can make robotic movement more efficient.

Building and Testing a Real Robotic Fish

The researchers also created a real fish-inspired robot to test whether the technology would work outside computer simulations.

The untethered robotic fish was able to swim freely and perform both types of movement: body-and-tail swimming and pectoral-fin swimming.

The experiments showed that the robot could successfully adjust its stiffness while moving, just like the simulations predicted.

This is a major step because real-world underwater conditions are much more difficult than computer models. A robot must deal with unpredictable water movement, changing pressure, and other environmental factors.

The successful tests suggest that adaptive stiffness technology could become useful for future underwater robots.

Why This Breakthrough Is Important

The oceans cover most of our planet, but many underwater areas remain unexplored. Scientists and engineers use underwater robots to study marine life, monitor ecosystems, inspect underwater structures, and explore deep-sea environments.

However, many existing underwater robots have limitations. They may require a lot of energy, move slowly, or struggle in complex environments.

Robotic swimmers inspired by fish could solve many of these problems. By changing their stiffness and adapting their movement, they could travel more efficiently and operate for longer periods.

Possible future uses include:

  • Exploring deep oceans

  • Studying marine animals

  • Monitoring environmental changes

  • Searching underwater after disasters

  • Inspecting pipelines and underwater equipment

A Step Toward More Natural Robots

The development of POSM shows a new direction in robotics. Instead of only copying the appearance of animals, scientists are now copying the biological systems that make animals successful.

Fish are not efficient swimmers just because of their shape. Their real advantage comes from their ability to constantly adapt their bodies while moving.

By giving robots similar abilities, researchers are creating machines that are more flexible, intelligent, and efficient.

The work by Hu and the team demonstrates that dynamic stiffness control can greatly improve robotic swimming. In the future, underwater robots may become more like living creatures, allowing humans to explore the oceans in safer, smarter, and more effective ways.

Reference: Hu, J., Lee, J.H., Wu, T. et al. Programmable online stiffness modulation for optimized aquatic locomotion. npj Robot (2026). https://doi.org/10.1038/s44182-026-00099-8

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