London: World's smallest engine developed by scientists - just a few billionths of a metre in size. An engine is powered by light and may help develop nano-machines that can navigate in water, sense the environment around them, or even enter living cells to fight disease.

For the device, tiny charged particles of gold, have been used which bound together with temperature-responsive polymers in the form of a gel.

This device stores large amounts of elastic energy in a fraction of a second, as the polymer coatings expel all the water from the gel and collapse when the 'nano-engine' is heated to a certain temperature with a laser.

When the device is cooled, the polymers take on water and expand, and the gold nano-particles are strongly and quickly pushed apart, like a spring. "It's like an explosion.

We have hundreds of gold balls flying apart in a millionth of a second when water molecules inflate the polymers around them," said Tao Ding from University of Cambridge in the UK.

"The smart part here is we make use of Van der Waals attraction of heavy metal particles to set the springs (polymers) and water molecules to release them, which is very reversible and reproducible," said Baumberg.

It has been a dream of scientists to develop a nano-machines.