Bacteria swim in water solution with a random movement resembling brownian motion. Imagine they while impacting randomly on a micro-gear. They will transfer some energy to the gear, but due to random movements the resultant will not provide any directed motion. However, you should remember from physics101 that a principle for the conservation of the force does NOT exist: imagine each tooth of the gear as a lever arm, and do design gears with asymmetric teeth. In this way, bacteria impacting on one tooth side, will transfer more energy than bacteria impacting on the other side. And here comes the magic: motion!
By conventional photolitography, Sokolov and colleagues from Argonne National Laboratory, report on PNAS the design of microscopic gears (6 ug mass) able to extract useful work from swimming Bacillus subtilis sworms. According to their calculations, few hundred bacteria work together in order to turn the gear at a velocity of 1-2 rpm, generating femtowatt power.
Here the concept is that brownian motion can generate directed motion (in non-equilibrium conditions). Since Middle Age, oxen, horses and other animals have been used to generate power to move mills and other machines (in non-equilibrium conditions). Bacteria, and perhaps other floating cells/molecules might provide new energy sources for new micro-machines. Applications are wide and may impact also molecular and synthetic biology. What about designing proteins with asymmetrical teeth? Merry Christmas.
Sokolov, A., Apodaca, M., Grzybowski, B., & Aranson, I. (2009). Swimming bacteria power microscopic gears Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.0913015107