His latest work in progress is a submarine, complete with half-tonne propeller, that will take centre stage in a children's playground.

BBC: How to build a bigger, better corkscrew

Ordinary rotors do not work on such a small scale, so Friend's micro-robot project has a special flagellated propeller, mimicking the swimming behaviour of E.coli bacteria.

CNN: Nanobots get to the heart of the matter