The gears above are just 380 microns across, or about four times thicker than a human hair. They're being turned by bacteria that are bumping into the spokes. Scientists at Argonne National Laboratory developed the bacteria-powered gears in an effort to develop "hybrid biomechanical systems." The speed at which the gears turn can be controlled by changing the amount of oxygen in the solution. From an Argonne National Lab press release:
The microgears with slanted spokes, produced in collaboration with Northwestern University, are placed in the solution along with common aerobic bacteria, Bacillus subtilis. Andrey Sokolov of Princeton University and Igor Aronson from Argonne, along with Bartosz A. Grzybowski and Mario M. Apodaca from Northwestern University, discovered that the bacteria appear to swim around the solution randomly, but occasionally the organisms will collide with the spokes of the gear and begin turning it in a definite direction."Argonne Scientists Use Bacteria to Power Simple Machines"
A few hundred bacteria are working together in order to turn the gear. When multiple gears are placed in the solution with the spokes connected like in a clock, the bacteria will begin turning both gears in opposite directions and it will cause the gears to rotate in synchrony for a long time.