Formula 1 cars use(d?) flywheels, but that's neither "nano" nor easily applicable to cars. But i think I've also seen it used at a subway in Istanbul. Muscles

Formula 1 cars use(d?) flywheels, but that's neither "nano" nor easily applicable to cars. But i think I've also seen it used at a subway in Istanbul. Muscles are also excellent at storing energy in spring-like fibres that stretch/relax. But I can't think of any manufactured materials at that scale.