Nature has always been one of the greatest sources of inspiration for engineers. From birds inspiring airplanes to insects influencing tiny drones, scientists often look at animals to solve difficult engineering problems. Now, researchers have taken inspiration from fish to create a new kind of robotic swimmer that can work at many different sizes while using the same basic design.
This breakthrough could make underwater robots cheaper, more flexible, and useful for many important jobs, including ocean exploration, environmental monitoring, underwater inspections, and marine research.
Why Fish Are the Perfect Underwater Engineers
Fish have spent millions of years evolving to swim efficiently through water. Different species have different body shapes depending on where they live and how they survive.
Some fish are built for speed, while others are designed to move slowly through coral reefs. Their body size also affects how they swim, including the speed of their tail movement, the force they produce, and how much energy they use.
Because of this incredible efficiency, engineers have long tried to copy fish when designing underwater robots.
However, there has been one major challenge.
The Problem with Today's Robotic Fish
Most robotic fish are designed for only one specific size.
A robot that works well at 50 centimeters usually cannot simply be enlarged into a 2-meter version. The larger robot behaves differently in water because forces such as drag, flexibility, and weight do not increase in the same way.
As a result, engineers often have to redesign the entire robot whenever they want a different size. This process is expensive, time-consuming, and limits how widely robotic fish can be used.
Researchers wanted a solution that could work across many sizes without needing a completely new design each time.
A New Fish-Inspired Design
To solve this problem, researchers led by Obayashi developed a robotic fish inspired by subcarangiform swimmers.
Subcarangiform fish, such as trout and many other common fish, mainly move the back half of their body while swimming. Their front section remains relatively stiff, while the tail bends back and forth to generate thrust.
The researchers copied this natural movement.
Their robot has a rigid front body connected to a flexible tail. Instead of using many motors, the entire robot is powered by just one motor, making it much simpler than many existing robotic fish.
Despite this simple design, the robot swims smoothly and efficiently.
A Smart Flexible Tail
The secret behind the robot's performance lies in its specially designed tail.
The tail includes flexible fiberglass rods along with an innovative crossed tendon system that controls how the tail bends during swimming.
When the motor moves, the flexible tail naturally creates fish-like waves that push water backward and move the robot forward.
This flexible structure allows the robot to mimic the graceful swimming motion seen in real fish without requiring complex control systems.
The result is a lightweight, energy-efficient robot with fewer moving parts that are easier to maintain.
One Design, Many Sizes
Perhaps the most exciting achievement of the project is that the same design works across a huge range of sizes.
The researchers developed a mathematical scaling law based on the interaction between water forces and the flexible tail.
This scaling law tells engineers exactly how to adjust the tail's flexibility and structure as the robot becomes larger or smaller.
By following these rules, each robot—regardless of its size—produces nearly the same swimming motion.
This means engineers can build robots ranging from just 50 centimeters to more than 2.5 meters long while keeping almost the same overall design.
Instead of starting from scratch each time, they simply scale the design according to the mathematical model.
Three Working Prototypes
To prove their idea works, the research team built three different robotic fish.
The smallest measured about 50 centimeters, while the largest stretched over 2.5 meters.
Although their sizes were dramatically different, all three robots displayed remarkably similar swimming behavior.
Their tails moved in nearly identical patterns, producing efficient propulsion across every size.
This shows that the scaling approach successfully preserves the robot's performance as it grows larger.
Watching Water Move
The researchers also wanted to understand exactly how the robots pushed against the surrounding water.
To do this, they used a technique called particle image velocimetry.
In this method, tiny particles are added to the water and illuminated with lasers. High-speed cameras then track the movement of these particles.
This allows scientists to visualize water flow around the robot.
The experiments revealed that all three robotic fish produced organized swirling patterns known as vortex wakes.
These vortex patterns are similar to those created by real swimming fish and are essential for generating efficient forward motion.
The results showed that the robots were successfully reproducing one of nature's most effective swimming strategies.
Tested Beyond the Laboratory
The robotic fish were not only tested inside controlled laboratory tanks.
Researchers also deployed them in real aquatic environments to evaluate their durability and reliability.
The robots successfully operated under different conditions, demonstrating that the design is robust enough for practical use.
Whether operating in calm water or more challenging environments, the robots maintained stable swimming performance.
This versatility suggests that the design could eventually be adapted for many real-world underwater missions.
Why This Matters
Underwater robots play an increasingly important role around the world.
They help scientists study marine ecosystems, inspect underwater pipelines, monitor water quality, map the ocean floor, and even assist in search-and-rescue operations.
Different tasks often require different robot sizes.
Small robots can navigate narrow spaces, coral reefs, and underwater caves.
Larger robots can carry heavier scientific instruments, batteries, cameras, or sonar equipment for long-distance missions.
With this new scalable design, engineers may no longer need to develop entirely new robots for each application.
Instead, they can build robots of different sizes using the same proven blueprint.
This could significantly reduce development costs while improving reliability.
Simplicity Is the Key
Another major advantage of the new robotic fish is its simplicity.
Many existing robotic fish rely on multiple motors, numerous joints, and complicated electronic control systems.
These designs can become expensive and difficult to repair, especially during long underwater missions.
In contrast, this new robot uses only a single motor together with a flexible mechanical structure.
By allowing the body itself to naturally generate realistic swimming motions, the robot requires less complex control software while maintaining high efficiency.
Fewer parts also mean fewer potential failures.
Looking Toward the Future
Although this research represents an important step forward, scientists believe the design can be improved even further.
Future versions could include advanced sensors, underwater cameras, sonar systems, artificial intelligence, and autonomous navigation.
Large versions could explore deep oceans or inspect offshore wind farms, while smaller versions could monitor lakes, rivers, fish farms, and fragile coral reefs without disturbing marine life.
Because the same design can be scaled across multiple sizes, future development could become much faster than with traditional robotic fish.
A New Chapter for Underwater Robotics
This fish-inspired robotic platform demonstrates that nature still has valuable lessons to teach modern engineering.
By combining a rigid body, a flexible tail, and a carefully designed scaling law, researchers have created a robotic fish that maintains efficient swimming from 50 centimeters to more than 2.5 meters in length.
The study shows that simple, scalable designs can perform remarkably well across a wide range of sizes.
As underwater robots become increasingly important for science, industry, and environmental protection, this innovative robotic fish could provide the foundation for a new generation of adaptable, efficient, and affordable aquatic explorers.
Reference: Obayashi, N., Anastasiadis, A., Gumowski, J. et al. ScaFi: length-scalable, compliant, parametric robotic fish design for operation in multiple environmental niches. npj Robot (2026). https://doi.org/10.1038/s44182-026-00105-z
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