A mask-only artficial gill unit from the Traveller RPG universe. Art copyright Mike Jackson.

Dissolved Oxygen Artificial Gill
Tech Level: 12
Oxygen-Cracking Artificial Gill
Tech Level: 13

The aqualung and SCUBA gear created a revolution in undersea exploration, opening up vast areas of the planet for human exploitation that had been difficult to get to previously. The technology has gone through a tremendous amount of refinement and advancement since it was first introduced decades ago, and it seems poised to undergo another major change with the advent of artificial machine gills.

Basic diving systems depends on air tank capacity for its endurance; more advanced rebreather units recycle used air through a CO2 scrubber and other filters. For both systems, the upper limit usually hovers around four to six hours of life support under the surface. Artificial gills try to bypass these limits by taking oxygen directly from the water. Ideally, it would eliminate the need for heavy compressed air tanks, since a fresh supply of breathable air is always available.

Water of course already contains a great deal of oxygen. Not only is it the primary component of water molecules, but most seawater contains a small amount, about 1.5% by volume, of dissolved air. Simple in concept, but difficult to engineer, an artificial gill would extract the oxygen from the water by one method or another, and convert it into a breathable air mixture for the diver.

Both types of artificial gill described here would only be able to work at standard diving depths, down to a maximum of fifty meters at most if used with common gas mixtures. Beyond that, pressure issues make the gas mixtures provided by the basic gill units potentially dangerous for the diver to breathe.

Though this article generally refers to use of this technology by divers, both these systems described here can be scaled up and used to provide breathable air to both sub-surface watercraft and habitats.

Tech Level: 12

Currently being developed by independent Israeli inventor Alon Bodner under the project name of LikeAFish, this type or gill uses a centrifuge system to lower the pressure of seawater to release the free oxygen dissolved in it. The process is somewhat similar in general concept to opening a soda can; the act of popping the top causes the pressure within to lower, letting the dissolved carbon dioxide bubble up out of the soda.

Water is drawn in through one or more intakes, sent through the centrifuge system to bubble the dissolved air out of it, then expelled. The liberated oxygen in the gill is diverted to an airbag for use by the diver. The exhaled air is also expelled into the water unless part of a rebreather system.

The main advantage of this system is that it potentially can never run out of air as long as power is available. Current portable battery power could probably only provide about an hour or so of energy, but the diver could be connected to a surface boat or to a submerged generator by a long flexible umbilical, that could last as long as needed. However, this could greatly restrict the freedom of movement that modern divers enjoy.

The main disadvantage of this system is that 200 liters of water have to be processed per minute to provide enough oxygen for a diver under normal usage condition. For peak physical activity, the unit may be equipped with a heart monitor and other biosensors, which would increase the flow of water through the system in order to keep up with the demands of the diver’s body. This would mean an increase in water flow of three times or more, which in turn would require faster spin speeds and drain the battery quicker.

Another potential problem would be bulk. With current and near-future technology, a centrifuge system that can successfully process hundreds of liters of water per minute will neither be lightweight nor compact, especially if it need to carry along a mobile power source as well. It might prove somewhat lighter to carry in a backpack unit than modern rebreather units, but the system’s weight is not the only consideration in the unit’s comfort.

Because the heart of the unit is a rapidly spinning centrifuge, on a lone diver progressional instability issues will arise. Progression is the phenomenon of spinning objects to be tugged off center in the plane of the object’s spin. Its what keep bicycle wheels from wobbling while in motion. But on the back of a diver, the centrifuge would constantly be tugging gently at the diver and sliding him off form where he would want to be. Even though the force at any one moment would be very slight, it can easily build up over time if the diver is inattentive. One solution to this would be to add a second centrifuge, counter-rotating to he first, with both handling equal loads.

Another issue raised by experts is the existence of dead or depleted zones in the water, that may have reduced dissolved air content. The unit may have sensors on board that will pick up oxygen production if it encounters such, as well as having a small back-up tank of compressed air for such an emergency.

Because of its very large power supply requirements, this type of artificial gill may be best deployed on a large scale in underwater habitats as opposed to personal breathing rigs.

Tech Level: 13

A tremendous amount of oxygen already actually exists in water. The problem is, its usually bound up with hydrogen atoms in the water molecules themselves. An oxygen-cracking gill is a much more complex device than a dissolved-oxygen gill, as it involves ‘cracking’ water molecules apart with electrical current to get at the oxygen within. This type of gill is seen occasionally in science fiction, usually as small, compact face-mask units that may or may not be connected to small supplementary tanks and a power source.

These devices make judicious use of advanced semi-permeable membranes, which allow molecules of one type to pass through, but not others. They are for all intents and purposes nanoscale sieves.

These units would use a portable power supply to separate the hydrogen and oxygen atoms from water molecules via electric current. A semi-permeable membrane would allow the hydrogen to bubble out of the system but retain the oxygen. The freed oxygen is then fed into a holding tank and combined with a dilutable gas (typically air or nitrox, from a carried tank) which the diver uses for breathing. Another type of semi-permeable membrane allows exhaled carbon dioxide to escape, while retaining the other gasses in the system. The gasses are both fed back into the system through scrubbers and reused, much like in modern rebreather systems.

The oxygen-cracking apparatus itself is used mostly to supplement the recycled oxygen, and thus eliminates the need for heavy oxygen tanks with hours worth of the gas. Instead, the gill-fed oxygen bottle would contain only enough reserve for an emergency, typically fifteen minutes worth of breathing or so. It would allow rebreather systems to be constructed as much lighter and more compact. However, bubble-free systems, such as special-ops military rigs, will still need waste gas holding tanks and more extensive scrubbers, so the savings in bulk and mass may be more negligible.

Mask-only systems would be used for short-term casual dives, most likely for tourism or recreational purposes, but perhaps for emergency uses as well. Without supplemental air or nitrox tanks, the gill would produce only pure oxygen for breathing, meaning it could only be used safely to a maximum depth of nine meters, beyond which the pressure makes pure oxygen toxic to breathe.

An oxygen-cracking artificial gill will continue to work as long its battery holds out and its intakes remain in contact with water.


Article added 10 October 2008