rebreathers using artificial gills for O2 resupply?

Once you are enthusiastic about diving, it is likely that you want to spend more and more time underwater, but as soon as you cross the border from swimming and snorkelling to scuba diving, you promptly come across the technical and physiological limitations of the available diving equipment, which usually limit the duration of your underwater experience.

For the sake of brevity, I will focus on the technical aspects and leave out the physiological, although they are also being addressed to some extent by all of the alternative ways of "underwater breathing apparatus" listed below. So what if you were able to "do it like the fishies" and truly breathe underwater? So far, it seemed technically out of reach and unefficient, so people had to resort to a number of compromises instead.

1) Conventional "open system" scuba, relying on a tank of pressured air (or blended gas like nitrox or heliox etc.), work only as long as it takes you to deplete the tank's charge, which is commonly around 200- to 300x compressed air, or bar (sometimes also measured in psi, or pound/square inch, factor 14.5). So once your tank approaches the minimum pressure needed by your breathing regulator, your air supply breaks down. Not a fun thing to experience while being underwater...

2) Surface-supplied systems rely on large gas tanks or compressors constantly connected to you and your personal breathing regulator to supply you with air, usually also with communication and sometimes even hot water which circulates through your suit, to prevent you from getting too cold. It is regularly used by commercial divers, p.e. working at underwater construction sites or out of saturation diving bells, where the limited range and mobility imposed by moving on an "umbilical" is not a big problem. However, if something (or someone) fails to work properly with your surface supply, you can be in trouble, so there is both the radius/mobility issue as well as the dependency on the smooth operation of your supply system which is completely out of your hands.

2a) One funny spin-off of this is Snuba, a combination of "snorkel" and "scuba", and the pitch is offering the advantages of snorkelling, i.e. free and easy roaming of shallow inshore waters, with scuba, which basically allows a person to stay underwater longer than anyone can hold his or her breath. However, proper SCUBA diving has some inherent risks that require adequate training, while snorkelling really just requires kicking around with or even without fins. Get rid of the tank, which follows you on the surface mounted on a float, and you can feel as free and easy as a snorkeller while taking advantage of the air supply of a compressed air tank so you do not even have to hold your breath any more. Again, once the tank is close to empty, the fun is over.

3) Rebreathers offer another extension of your stay underwater, while still relying on compressed gas. Although this gas is usually Nitrox, a "blend" with an increased oxygen percentage, they are much more efficient in exploiting that gas reservoir by recycling or "rebreathing" a large part of the inhaled and exhaled gas volume.

But since you are forced to deal with both supply of oxygen into and transport of carbon dioxide (CO2) out of your respiratory system, you also need a "scrubber" or absorber that removes the exhaled carbon dioxide from the recycling or "rebreathing" process. So even though the improved gas efficiency of a rebreather has drastically prolonged your time underwater, you still have the same old problem: sooner or later, your supply of oxygen will run out.

But what if you could refill your tank on-the-go? Israeli inventor Alon Bodner thinks he came up with a technical solution, a battery-powered artificial gill. Basically, it pumps a large amount of water through a system where dissolved oxygen "bubbles out" of the water and becomes breathable gas - just like CO2 bubbles out of a soda bottle (or beer can ;-) once you pop the cap. This could be it, the answer to all problems: unlimited gas supply! (or at least as long as the system's power supply runs...)

But wait, just before we get into details and technicalities about how exactly it is possible to gain oxygen out of sea water just like the fish, there is that nasty detail: not only your gas tank, but also your CO2 scrubber is only good for a certain time (or rather, a certain amount of gas being "scrubbed") so even if you had unlimited supplies of oxygen, you still need to dump the CO2 somehow.

Using a rebreather system without adequate removal of CO2 is bound to slowly suffocate the user, and exchanging or even "recycling" the CO2-absorbing substance while being underwater is not really practical or even possible so far.

So although it seemed like there finally was a solution to the scuba diver's dream of unlimited underwater time, it is unlikely to become available as personal diving equipment until the CO2 scrubber problem also has been addressed.

Where it still might become a practical solution is in manned subsea installations (like Cousteau's famous "underwater habitats" Conshelf I, II and III and all it followers, or possibly in submarines, where access to an ample supply of energy and frequent removal of CO2 scrubbers are not a problem.

So we dream on about genetically manipulated fish-men with gill slits and webbed feet - while webbed fingers could prove to be rather impractical in the long run... ;-)