It is often said that size matters. At the nano-scale level, where a lot of current research is being done, this adage also holds true, and several scientific teams have laid claim to creating the "world's smallest engine" built from particles of ever-shrinking dimensions. The latest, a nano-scale engine made from tiny charged particles of gold and developed by researchers at the University of Cambridge, is claimed to be the smallest of them all.

The latest nano-motor has been built from microscopic charged particles of gold that are held together in a gel by using temperature-sensitive polymers, whilst the whole contraption is suspended in water. When heated with a laser, the nano-engine almost immediately takes on and stores a large amount of energy and stores it as mechanical (elastic) energy by forcing the gold nano-particles together into tightly-bonded clusters.

When the machine is subsequently cooled, the polymer gel absorbs water from its surroundings and rapidly expands, forcefully pushing the gold nano-particles apart with the release of the stored mechanical energy.

"It's like an explosion," said Dr Tao Ding from Cambridge's Cavendish Laboratory. "We have hundreds of gold balls flying apart in a millionth of a second when water molecules inflate the polymers around them."

The team claims that the forces exerted in this way are orders of magnitude larger than those for any similar device. A similar charged-particle engine, the single-atom engine from JGU, is claimed to produce energy at an efficiency of around 33 percent, whereas the Cambridge unit is spoken of in force unit per weight, so it is difficult to compare. Nevertheless, the Cambridge researchers still claim that pound for pound (or nanogram for nanogram), their device produces a force nearly 100 times better than any motor or muscle.

Though engines in general, and nano-engines in particular, operate on similar principles, the effect is less like the Stirling engine process, and more like a stored-energy device such as a spring.

"The whole process is like a nano-spring," said Professor Jeremy Baumberg also from the Cavendish Laboratory. "The smart part here is we make use of Van de Waals attraction of heavy metal particles to set the springs (polymers) and water molecules to release them, which is very reversible and reproducible."

The Cambridge engines have been dubbed "ANTs" (Actuating Nano-Transducers) for the way that their operation, whilst engine-like, is more akin to the operations observed where energy is transformed from one form to another in a high-pressure pushing motion.

"Like real ants, they produce large forces for their weight," said Professor Baumberg. "The challenge we now face is how to control that force for nano-machinery applications."

In this vein, the creators believe one day their pint-sized power-plant could provide the motivation for a range of microscopic robots that may, for example, make their way around the human body to directly fight viruses and diseases at their own level.

Further work and commercialization of the invention is now in the offing for the Cambridge team, particularly around developing the technology for micro-fluidics bio-applications.

The researchers recently published the results of their work in the journal Proceedings of the National Academy of Sciences of the United States.