As reported in the 27 Oct 2001 issue of Science News, Japanese researchers have found a way to control movement of microtubules. They wanted to get the tubules to move around a circular track in one direction, but the tubules would move randomly. Their solution? Simply point the way.
More precisely, etch arrowheads into the circular track, and the tubules will move in that direction only. A good example showing that building micro-machines requires a different way of thinking compared to building macro-machines.