Engineers in the US say they have invented a hydrogen-powered robot that moves through water like a jellyfish.

Development of the robot, nicknamed Robojelly, is in the early stages but researchers hope it could eventually be used in underwater rescue operations.

Writing inSmart Materials and Structures, Yonas Tadesse said the jellyfish's simple swimming action made it an ideal model for a vehicle.

The fuel that the robot uses makes it unique, its inventors say.

Lead author Dr Tadesse, from the University of Texas at Dallas, said: "To our knowledge, this is the first successful powering of an underwater robot using external hydrogen as a fuel source."

Artificial muscles

A jellyfish moves using circular muscles in the inside of its umbrella-like bell.

As they contract, the bell closes in on itself and ejects water to propel itself forward. When the muscles relax, the bell regains its original shape.

To replicate this, the vehicle uses shape memory alloys - materials that "remember" their original shape.

These are wrapped in carbon nanotubes - tiny "straws" of pure carbon that are renowned for their electrical properties - and coated with a platinum black powder.

The robot is powered by heat-producing chemical reactions between the oxygen and hydrogen in water and the platinum on its surface.

The heat from the reactions is transferred to the artificial muscles of the robot, and reshapes them.

Currently, the jellyfish flexes its eight bell segments at the same time but there are plans to look at how to control each segment individually.

This would mean the robot could be more closely controlled and move in different directions.

The study was sponsored by the US Office of Naval Research, which invests in projects to benefit the US Navy and Marine Corps.