A realistic laser-operated molecular locomotive has been proposed by a Texas A&M researcher (Zhisong Wang, nargate@jewel.tamu.edu).

For those designing materials at the smallest workable scales, a major dream is to build nano-locomotives that would move through a molecular-scale track to perform various tasks, such as transporting building blocks for nanomachines. Earlier proposals have outlined some innovative designs for these nanoengines (for example, see http://www.aip.org/pnu/2000/split/pnu490-1.htm).

While nano-locomotives are still in the blueprint stage, a new model brings the idea closer to reality by incorporating the latest working knowledge of nanomaterials as well as the probabilistic, jiggly nature of the molecular world. In Wang's design, a nano-locomotive would have a main body consisting of cars each made up of a linear polymer chain.

Either end of the train would have a chemically tailored "head" group that could bind to or break from a track, which could be a cylinder-shaped microtubule found in biology. Either end of the locomotive could attach via covalent bonds to special molecular groups on the track. Laser pulses would move the train: one light pulse would break the bond from one of the train's ends and another laser pulse would cause each car of the train to change its molecular configuration, and expand its size to reach the next part of the track.

Thermal fluctuations of the motor itself and the environment play a vital role, for example as the train's head seeks the next binding site on the track. Wang has proposed a multistep "optomechanical work cycle" that precisely outlines the laser steps needed to move the train, and even reverse its direction.

The locomotive would work not only as a motor, but a powerful molecular engine that could generate a pulling force 10 times greater than of the natural biomotor kinesin. Such forces, of about 100 piconewton, could allow the nano-locomotive to break molecular bonds and help in constructing nanomaterials while delivering cargo.

(Wang, Physical Review E, 15 September 2004; also see http://focus.aps.org/story/v13/st27).