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Even nanotechnologists are sceptical about futuristic plans to send miniature submarines (above) into our bloodstreams to diagnose and fight disease. But researchers in Japan are going further: they think microscopic robotic fish will do the job better. Shuxiang Guo in the intelligent mechanical systems lab at Kagawa University says the flapping motion of a fishtail is more efficient than a motor-driven screw, so it will run for longer. He has made a prototype swimming fish that uses electrically activated artificial muscles to flap its tail fin. But at 4.5 centimetres long, Guo’s prototype is way too big to swim about inside you. He’s now working on miniaturising the fish.