Artificial gills that mimics air traps of aquatic beetles could help people breathe underwater.

English researchers investigated super-water-repellent surfaces of the insects' air traps. These have infinitesimally tiny structures that rise like trees in a forest, on which water droplets rest. The structures trap air between the surface they jut up from and the water on top of them.

Aquatic insects - like the great diving beetle Dytiscus marginalis (foto) - have rigid hairs on their abdomen that repel water so much they create a silvery film of air that does not collapse. The silk nests of some terrestrial spiders and the eggshells of some insects also can develop these sheaths of air.

These air layers can essentially behave as gills, allowing oxygen in the water to flow in and carbon dioxide in the air to diffuse out.

Glen McHale, a physicist at Nottingham Trent University in England, and his team investigated whether synthetic materials could mimic this natural effect. They created a hollow cylinder made of super-water-repellent porous foam, inside which they sealed a device that consumed oxygen. When submerged in aerated water, sensors within the cylinder showed oxygen made its way inside.

This effect could be used to supply oxygen for fuel cells to enable mini machines to work underwater, without need for stored or external oxygen supplies, the scientists say.

They also note that a surface area of about 970 square feet could provide enough oxygen for a single person to survive. "The actual surface area required would depend on the gases in the water, water movement and the level of oxygen required," McHale told LiveScience.

The surface for a person could get folded up to some extent for greater portability. However, corrugating the surface like that would just deplete nearby water of usable oxygen more efficiently, researcher Neil Shirtcliffe explained. "As with fish and sharks, unless water is pumped past or the 'gill' is moving there is little advantage to structuring the surface," he said.

How long a machine or person could stay underwater "is indefinite, provided the water does not become deoxygenated," McHale said. However, "if other gases, such as methane, were in elevated levels in the water, such as in stagnant pools, these gases would be transferred into the breathing air as efficiently as oxygen and could potentially cause serious problems."

McHale also cautioned they did not measure carbon dioxide levels within the cylinder, "and it is a buildup of carbon dioxide that would kill people before lack of oxygen." Still, "the rate of removal of carbon dioxide is expected to be high as the gas is very soluble in water."

About the effectiveness of these artificial gills on human beings, we also have to take into account the fact that natural gills work because all - or most - animals with gills are cold blooded and so use much less oxygen than a warm blooded animal the same size. No aquatic mammal or bird developed gills. Because oxygen level - even in most oxygenated waters - is lower than in the air.