Artificial gills are devices that exist in science fiction, and being developed in reality, to extract oxygen dissolved in water, thus allowing humans to survive underwater. Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the Water is a common Chemical substance that is essential for the survival of all known forms of Life. Human beings, humans or man (Origin 1590–1600 L homō man OL hemō the earthly one (see Humus Underwater is a term describing the realm below the surface of Water where the water exists in a natural feature (called a body of water) such as an Ocean
It is generally thought that they would be unwieldy and bulky, because of the massive amount of water that would have to be processed to extract enough oxygen to supply an active diver, as an alternative to a scuba set. A scuba set is an independent breathing set that provides a scuba diver with the Breathing gas necessary to breathe underwater during Scuba diving. However, Like-A-Fish is an ongoing attempt to develop such a system in the real world.
As sea water contains 7 ppm oxygen, 1,000,000 kg (1,000 tonnes) of sea water holds 7 kg (1,000 short tons holds 14 lb) of O2, the equivalent of 5,350 litres (1,400 U. The litre or liter (see spelling differences) is a unit of Volume. S. gallons) of oxygen gas at atmospheric pressure. A gallon is a measure of Volume. It is in current use in the United States and still has limited use in many other English-speaking countries
An average diver with a fully closed-circuit rebreather needs 1 litre (roughly 1 quart) of oxygen per minute. A rebreather is a type of Breathing set that provides a Breathing gas containing Oxygen and recycled exhaled gas As a result, at least 192 litres (51 gallons) of sea water per minute, or 3. 2 liters (3. 5 quarts) each second, would have to be passed through the system, and this system would not work in anoxic water. For other uses of the term "hypoxia" see Hypoxia. Hypoxia or oxygen depletion is a phenomenon that occurs in aquatic environments
Natural gills work because nearly all animals with gills are cold-blooded and so need much less oxygen than a warm-blooded animal the same size. A gill is an anatomical structure found in many aquatic organisms Cold-blooded organisms (called poikilotherms - "of varying temperature" maintain their body temperatures in ways different from Mammals and Birds In Biology, a warm-blooded Animal species is one whose members maintain thermal Homeostasis; that is they keep their body temperature at a roughly constant
Like-A-Fish Technologies is an Israeli business, founded by Alan Bodner in 2001, that is developing a human artificial gill system; they have developed a prototype.  Like-A-Fish's technology uses a centrifuge causing lower pressure at the center, where dissolved air comes out of the water. A centrifuge is a piece of equipment generally driven by a motor that puts an object in Rotation around a fixed axis, applying a force perpendicular to the axis 
The key issue remaining is battery life. In electronics a battery is a combination of two or more Electrochemical cells which store chemical Energy which can be converted into electrical energy Currently a one kilo battery would only last for one hour, whereas a regular scuba tank can last longer (depending on depth). Scuba diving is swimming underwater, or taking part in another activity while using a Scuba set. The biggest possibilities lie in underwater habitats, which have access to electricity, but need constant refilling of air tanks. Underwater habitats are Underwater structures in which people can live for extended periods and carry out most of the basic human functions of a 24-hour day, such Additional possible uses include systems for scuba divers and submarines, among others. Scuba diving is swimming underwater, or taking part in another activity while using a Scuba set. A submarine is a Watercraft that can operate independently below water as distinct from a Submersible that has only limited underwater capability