Scientists have developed a microscopic engine, the smallest in the world, that they say is the first one capable of driving nanobots, including medical robots that could travel through the body. The prototype device, known as an actuating nano-transducer or Ant, combines microscopic gold balls with a special polymer gel. It generates a propulsive force on a microscopic scale that is a hundred times greater per unit weight than any known motor or muscle. “People have been talking about making nanobots for many years but they do not exist yet,” said Professor Jeremy Baumberg, leader of the project at Cambridge university. “Why not? Because so far there has been no way of making them move through liquids — which is like swimming through treacle on the nano-scale because the molecular forces are so strong.” He says Ant engines, described for the first time in Proceedings of the National Academy of Sciences, would provide sufficient power. “Like real ants they provide large forces for their weight,” he said. “The challenge we now face is how to control the force for nano-machinery applications.” The Ant is powered by physical rather than chemical reactions. It contains gold nanoparticles, each about 0.06 microns, or a thousandth of the width of a human hair, in diameter in water with a gel-like polymer called pNIPAM. When the temperature is above the critical temperature of 32C, the gold particles are bound tightly together with the polymer through intermolecular attraction. When it falls below 32C, the polymer suddenly absorbs water and expands — and the gold particles are pushed rapidly apart like a spring. “It’s like an explosion,” said Tao Ding, another member of the team. “We have hundreds of gold balls flying apart in a millionth of a second when water molecules inflate the polymers around them.” The reaction is completely and rapidly reversible, experiments show. When the temperature rises again, the Ant stores large amount of elastic energy in a fraction of a second as the polymer coating expels water from the gel and contracts around the gold particles. “The whole process is like a nano-spring,” said Prof Baumberg. The prototype Ant uses laser light to control the system’s temperature but other mechanisms could be used instead. The transition point could also be adjusted, for example to set the energy release point close to 37C — the human body’s normal temperature. The Ant might drive a nanobot through a series of piston strokes, rather like a car engine but on a scale many billions of times smaller. “The concept can underpin a plethora of future designs,” Prof Baumberg said. The team is working with Cambridge Enterprise, the university’s commercialisation arm, to develop practical applications for the technology.