Today, let's swim like fish. The University of Houston's College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

So why can't you and I breath indefinitely under water the way fish do. They need oxygen just as surely as we. And the waters of our lakes and oceans contain plenty of it. I don't mean the oxygen that's bound up with hydrogen to make H2O; I mean the free oxygen that's dissolved into water. 

If you want to kill a fish, just put him in water that's been boiled for some time, then cooled without exposing it to air. All the gases will've been driven out of solution and carried away by steam bubbles. Cool that water, put a fish in it, and (since fish are oxygen breathers) it'll suffocate.

So why is it that fish can breathe in water and we can't? And might that situation be remedied for us? Well, let's look at fish gills. They include a feathery structure of membranes that offer a large surface area to a great deal of water. As water passes through them, dissolved oxygen flows into the membranes just as it flows into the membranes of our lungs.

Now writer Michael LePage describes recent work in which experimenters have been building artificial gills. And it's easier than we might think. A watertight box made from an appropriate membrane is all we need. The gas inside the box will come into equilibrium with the gas content of the water outside.

Unfortunately, while the mixture in the box will be life-sustaining, it has more carbon dioxide and less oxygen than the air around us. It'll leave us woozy. Therefore, people working on artificial gills are developing techniques for processing the gas that crosses the membrane to make it more oxygen-rich. But, so far, their systems are much clumsier than scuba gear. 

A diver is still better off carrying an air tank. And another problem: Very little nitrogen dissolves in water. Without it, pure oxygen becomes toxic at pressures thirty feet below the surface. Even with an artificial gill, a diver would still need nitrogen.

So, is this a fool's game? Well, no, it isn't. Think about submarines. Gill membranes not only acquire new oxygen; they also scrub out CO2. In the closed environment of a submarine, nitrogen is preserved. Artificial gills, not for the passengers but for the submarine itself, might be the wave of the future -- providing oxygen, not only to breathe, but to supply power fuel cells as well. 

We humans have always looked at animals and wondered why we're denied what they have. We finally solved the problem of getting ourselves into the air a century ago. We did so, not by growing wings, but by finally despairing of growing wings. We do not fly like birds; rather we build machines that carry us beyond birds.

And we've always wanted to swim like fish. That goal has been more elusive. Perhaps we've just come one step closer, now that we're thinking of taking our needed oxygen from the water, just as fish themselves have long been able to do.

I'm John Lienhard, at the University of Houston, where we're interested in the way inventive minds work.

M. LePage, Oceans of air. New Scientist, January 7-13, 2006, pp. 28-31.

The gills of a prawn (see b).  (From A. Buckley, Life and Her Children. 1892.)