The Bacterial Pump

It is a fairly well-known story how Friedrich Kekule solved the structure of the benzene ring after having a dream of a snake biting its own tale. Also there is the case of Elias Howe, who succeeded in building a workable sewing machine only after dreaming of being menaced by natives wielding spears with holes in the tips. My dreams have yet to provoke any brilliant revelations, but have once or twice included some modestly useful ideas.

Last night I dreamed of a pump to be powered by bacteria. The idea was to confine a large number of swimming bacteria to a box with water-permeable membranes at each end. If all the bacteria could persuaded to swim in the same direction, then the water in the box would be pushed in the opposite direction. In my dream this was to be accomplished by engineering the bacteria to swim oriented to a magnetic field or to seek light.

In the cold light of day this at first seemed like a totally infeasible idea, but now I'm not so sure. I see that some forward-thinking people have measured the force produced by a single bull sperm and found it to be 0.000025 dynes. Elsewhere I find that the average density of sperm (for humans) is 65,000,000 per milliliter. I'm not sure what the density is for bulls but I don't think it can be that much different. If I combine these numbers I get a total force of 1625 dynes per milliliter. And 62 milliliters of bull sperm yield a force of over 1 newton--in other words, enough to accelerate a liter of water at over one meter per second squared.

The flagellum, the so-called "whiplike tail" of the sperm and some types of bacteria, is not really whiplike but more like a rotating corkscrew-shaped propeller. We don't usually think of living things as manifesting wheel-and-axle type structures, but bacteria (and sperm) do. Another waking thought I had was that the base of the flagellum should provide an excellent model of a rotating motor for nanotechnology enthusiasts. Naturally I am not the first to think of this.