Robotics

The robotic fish makes a splash with a breakthrough motion

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The robotic fish is equipped with a twisted and coiled polymer (TCP) to propel it forward, a lightweight, low-cost device that relies on temperature changes to generate movement, which also limits its speed.

TCP works by contracting like a muscle when heated, converting energy into mechanical movement. The TCP used in this work is warmed by Joule heating – the flow of current through an electrical conductor generates heat energy and heats the conductor. By minimizing the distance between the TCP on one side of the robot fish and the spring on the other, it activates the fins on the back, allowing the robot fish to reach new speeds. The wavy flaps of their rear fins are measured at 2Hz, two waves per second. The frequency of the electric current is the same as the frequency of the tail flaps.

Findings published at the 6th IEEE-RAS International Conference on Soft Robots (RoboSoft 2023), provide a new route for increasing actuation – the act that causes a machine or device to operate – TCP frequency through thermomechanical design and demonstrate the possibility of using TCP at high frequencies in environments watery.

Main author Tsam Lung You from Bristol Department of Mathematical Engineering said: “Twisted and Coiled Polymer (TCP) actuators are promising new actuators, demonstrating the attractive properties of light weight, high energy density at low cost, and simple fabrication processes.

“They can be made from materials that are very easy to value like fishing line and they contract and provide linear actuation when heated. However, because of the time required for heat dissipation during the relaxation phase, this makes them slow.”

By optimizing the structural design of the TCP-spring antagonist muscle pair and bringing their anchor points closer together, it allows the posterior fin to swing at a greater angle for the same amount of TCP actuation.

antagonist muscle. Image credit: Tsam Lung You

Despite the greater force required, the TCP is a powerful actuator with a high work energy density, and is still able to move the fins.

Until recently, TCP was mostly used for applications such as wearables and robotic arms. This work opens up more application areas where TCP can be used, such as marine robotics for underwater exploration and monitoring.

Tsam Lung You added: “Our robotic fish swim at the fastest actuation frequency found in a real TCP application and also the highest motion speed of any TCP application so far.

“This is very exciting because it opens up more opportunities for TCP applications in various areas.”

The team now plans to scale-up and develop a TCP-driven, knifefish-inspired tapefin robot that can swim nimbly through water.


The University of Bristol is one of the most popular and successful universities in England.

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