Tiny and efficient metal muscles made of nanoscale-size platinum particles have been created by German researchers, and could one day used to develop robotic limbs.

Dr Jörg Weissmüller from the Institut für Nanotechnologie in Duisburg and colleagues report in the latest issue of the journal Science that they have been able to use a tiny electric charge to flex a piece of 'nanoporous' platinum - an artificial sandwich of platinum atoms riddled with tiny holes.

The platinum belongs to a class of alloys which change shape when heated, but then return to their original shape when cooled. Metal muscles made from such 'shape-memory' alloys hit the headlines several years ago when researchers developed special nickel-titanium alloy strips, which moved in response to an electrical stimulation.

However, such 'nano-muscles' have so far had limited efficiency. To flex, they require a large electrical current - up to 100 volts - which is then converted to heat energy. Methods to dissipate this heat generated then have to be included in the design, making the muscles bulkier.

The German team were able to achieve the same degree of movement as previous nanomuscles but without generating large volumes of heat. This is because their platinum nanoparticles have a much larger surface area that is electro-chemically accessible; this enables the alloy to store a large electric charge and yet only require a few volts to flex.

Nanomuscles weigh just one gram but can lift 140 grams, and are preferred to electric motors as they are far cheaper to produce: 50 cents each compared to US$300. They also make less noise and operate more smoothly. They could one day replace most small electric motors in toys, cameras and other devices, and one medical company is investigating nanomuscles in penile implants.

Efficient nanomuscles are in great demand because they can generate an enormous amount of energy for their weight and size. This becomes vital for the development of walking robots, because robots beyond a certain size need motors and batteries, which are heavier than they can carry.

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