Chemical drivers for tiny Brownian motors.

A microscopic particle suspended in a liquid is continually bombarded by molecules of the solvent. Because these random collisions aren't evenly distributed at all times, the particle gets shoved this way and that and moves about erratically.

Known as Brownian motion, this constant jiggling has attracted the attention of researchers interested in harnessing such movement to create tiny motors and pumps and to develop improved methods of separating different-size particles or transporting various molecules. Systems of this sort could potentially serve as models of cellular ion pumps and biomolecular motors (SN: 3/22/97, p. 173), which drive muscle contraction and other biological processes.

Now, biochemist R. Dean Astumian of the University of Chicago and his coworkers have proposed a scheme in which chemical energy deposited in appropriate locations can bias Brownian motion to push particles and large molecules in a chosen direction. He outlines the method in the May 9 Science.

In a conventional electric motor, a start-up coil provides the energy that sets the rotor spinning in a particular direction and inertia keeps it going. Continued energy input compensates for friction in the system.

A protein immersed in water faces considerably more resistance to motion. "It's like trying to swim in butter," says Marcelo O. Magnasco of Rockefeller University in New York.

The trick is to keep nudging a molecule or microscopic particle to maintain its motion in a certain direction. That can be done by using the microscopic equivalent of a notched ratchet wheel that can rotate in only one direction.

In recent years, researchers have proposed and investigated several methods of creating the required ratchet configuration and supplying energy to produce fluctuations that the mechanism can rectify into motion in a particular direction. For example, two microscopic, comblike electrodes with interleaved teeth, made of metal deposited on a glass slide, can generate an oscillating electric field that allows a particle immersed in a liquid to move only in a favored direction.

Because particles of different sizes experience different amounts of friction and Brownian motion, it's possible to set up conditions so that larger particles move in a direction opposite to that of smaller particles, researchers have suggested.

Astumian now proposes that chemical reactions, rather than an oscillating electric field, could provide the energy fluctuations needed to drive particle transport. In this case, changes in the electric charge of molecules or ions in a reaction could supply the impetus. For example, the biochemical molecule ATP typically has a negative charge in solution. When it binds to a protein, it changes the protein's net charge. In principle, two proteins that react with ATP at different rates would move differently, allowing them to be separated.

"Several technological hurdles remain, however, before a practical device can be constructed," Astumian cautions.