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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…