This is a brief blog to share my thoughts on artificial gills for humans. Every few years, I stumble upon an article online that announces a dramatic “new breakthrough” in the development of artificial gills for human use. Usually, the detail provided goes no further than a concept and not without reason because there are some enormous hurdles to be overcome:
Gas Breathing Gills.
It is possible to extract oxygen from seawater. In fact, it is remarkably simple to allow oxygen and other gases to pass across a semi-permeable membrane to reach equilibrium with the gas on the other side. Take a container- with one or more surfaces made from a semi-permeable membrane- and immerse it in seawater. After a while (the time depending on the membrane efficiency and partial pressure differentials), the proportions of gases in the container would equate to those in seawater and would therefore form a breathable mixture akin to air. (Assuming that this experiment is performed in shallow water without unusual conditions or pollution). However, the quantities required for breathing at any sort of depth would be enormous: To supply sufficient oxygen for a slow swimmer at about 15 m would need all of the oxygen to be extracted from about 200L of seawater every minute. The gill membrane would have to be very large and very efficient to perform such a task. In any case, Nitrogen (or another inert gas such as Helium) must also be transferred into the breathing gas at a ratio of approximately 4: 1 (4 parts Nitrogen: 1 part Oxygen) in order to obviate the risk of oxygen toxicity. The membrane must therefore be able to differentially permit gas transfer in the correct proportions. Theoretically, such a system might be possible but, for now, the technology is not available to pack such a gas exchanger into a unit small enough to wear.
Liquid Breathing Gills.
The US Navy experimented with filling divers’ lungs with an oxygenated fluorocarbon liquid through which gas diffusion could occur. Similar experiments have been performed on mice and dogs and it is proven that liquid breathing mixtures can sustain life. Such systems might help to avoid injuries such as lung-expansion embolisms, narcosis and decompression sickness but would still require a gas exchange mechanism (or oxygen supply) to oxygenate the breathing liquid. Furthermore, the viscosity of the liquid makes breathing a tiring and painful process and also inhibits carbon-dioxide flushing from the lungs. Liquid breathing systems might one day be used in place of saturation diving systems but the need for a gas-exchange apparatus currently precludes their use with gill systems until a workable gill is available.
One Last Thought:
All of those animals that breath through gills are poikilothermic and therefore have a lower baseline metabolic oxygen demand. Perhaps, as homeotherms, we are being too ambitious in trying to supply our oxygen requirement through gills.