Breathing in liquid, from the deep sea to space

380 million years ago, fish began to crawl onto land. After a long period of natural selection, a group of them evolved into humans in the intricate maze. Humans are undoubtedly the kings of the Earth's Golden Age. There are no islands on the Earth that we have not reached. However, our unique physiological structure determines that we are only suitable for survival in a gaseous environment. We have lost the ability to breathe in liquid like fish. Compared with settling in the deep sea, we now seem to prefer colonizing space. However, it seems a little unusual that the desire to breathe in liquid is not only about the ocean, it can also help us fly into space.

Pioneers

Human breathing is a very simple diffusion process. The oxygen in the air is absorbed into the blood by the lungs, and the carbon dioxide produced by metabolism is then transported from the lungs to the air. The whole breathing process is completed by satisfying this inhalation and exhalation. Compared with air, the dissolved oxygen in water is really pitiful. At the minimum, we need to inhale at least 45 liters of water per second to extract 0.05 grams of oxygen to maintain normal life. Human lungs are not high-pressure water pumps, so they naturally cannot do such "storm inhalation". But soon someone thought, if we use liquids with a high dissolved oxygen content, one breath can meet the oxygen supply demand. Does it mean that we can breathe in liquids?

To verify this, in 1962, scientists trapped mice in saline solution that dissolved a large amount of oxygen. Although it was much more difficult to expel liquid than gas, the mice successfully absorbed oxygen, but encountered difficulties in exhaling carbon dioxide: saline solution did not have a good property of dissolving carbon dioxide, and the carbon dioxide produced by breathing accumulated in the lungs. Poor Jerry only lived for a few minutes before dying of respiratory acidosis. Four years later, scientists discovered a perfluorocarbon organic liquid (hereinafter referred to as PFC). This liquid not only has an astonishing amount of dissolved oxygen, but also a much higher amount of dissolved carbon dioxide than saline. This time, little Jerry lived for 20 hours.

Little Jerry is breathing in the PFC liquid in the lower layer, while the fish in the upper layer of water have no idea what is happening. Image source: Researchgate

Although the conditions seemed immature, in 1969 the U.S. Navy conducted human experiments on liquid breathing. Francis J. Falejcyk, a diver with excellent lung capacity, became the first person in history to breathe oxygenated salt water and PFC at the same time. However, the experiment was not completely successful. The technology at the time could not drain all the water from his lungs, and he subsequently developed pneumonia due to residual fluid in his lungs.

Considering the possible tactical application prospects of liquid breathing underwater, in 1977, Duke University submitted a report entitled "The Feasibility of Human Liquid Breathing" to the U.S. Navy, which then launched a test exercise in 1980. The Navy SEALs tried liquid breathing in PFC for the first time. The whole process was quite strenuous, so much so that several divers "sucked" and broke their ribs.

To date, it is difficult to say that liquid breathing technology has made great progress. Although some liquid ventilation technologies have been successfully used in medical treatment and have improved the breathing conditions of some patients with abnormal lung function, we are still some distance away from real life in the sea. For now, we need liquids that are lighter and less viscous than PFC to make it easier to inhale and expel liquids. But if we stay on the surface of today's sea of ​​technology, we may not feel the undercurrents surging thousands of miles away due to short-sightedness. Now, let us go a step further, not limited to the technological level achieved by the pioneers, but use advanced thinking to imagine the future. We will find that just by soaking in liquids, the blueprint from the deep sea to space will slowly unfold to us.

Under the Waves

As the global climate warms, the glaciers in the Arctic and Antarctic are melting, the sea level is rising year by year, and coastal cities are facing the risk of being flooded and attacked by typhoons. Land reclamation is not a long-term solution. However, if we can turn retreat into progress and live directly in the sea, we can solve the problem of seawater flooding once and for all. This idea is by no means ironic. Living underwater is a logical solution to the problem of environmental collapse. If humans can live 50 meters below sea level, they can have an extra land area of ​​the entire Antarctica. Moreover, the underwater environment is not as dangerous as the myths circulate: there is no fire, the power of earthquakes is limited, and storms and tsunamis are difficult to affect underwater.

“History will repeat itself and humans will once again be forced into the ocean to make a living.”

—Alistair Hardy, marine biologist

A conception of an ocean city. Image source: AI generated

Not all animals that breathe with lungs are unaccustomed to water. Sea lions and seals can shrink their lungs into a ball to dive into the deep sea, but human lungs are not so elastic. Under the pressure of deep water, the lung cavity structure is compressed too much and will rupture on a large scale, causing suffocation. The ability to withstand pressure has limited human exploration of the ocean. The world record for the deepest free diving is held by New Zealander William Trubridge. In 2010, without the help of any auxiliary equipment such as scuba diving and fins, he dived to 116m under the Atlantic Ocean with only one breath. The water pressure at a depth of 116m will compress the nitrogen in the air in the lungs into the fat components and nerve tissue, producing a nitrogen narcosis effect like "anesthesia", and the compressed lungs will over-expand when floating up, causing air embolism.

To overcome the pressure of deep water, one approach is to fill the diving suit with breathable oxygen-rich liquid and immerse the person in the liquid to improve the ability to withstand pressure. To understand this approach, we only need to imagine a balloon immersed in the sea. If the balloon is inflated, it will be squeezed and burst by the water pressure in shallow waters; but if the balloon is filled with water, then even if it is placed in the Mariana Trench, the deepest part of the earth, the balloon will not change in shape. The reason is that water is incompressible, and the density of the water in the Mariana Trench is only 5% higher than that of surface water. Therefore, for a person completely immersed in liquid, in a deep water environment, the hydrostatic pressure evenly acts on the tissue structure and surface of the human body, causing the pressure inside and outside the body to increase synchronously, and the pressure difference between the inside and outside is balanced. The lungs will become difficult to compress, thereby withstanding a very high load. At this time, people have the super pressure resistance of deep-sea animals.

The mysterious fireworks jellyfish, whose body is made of transparent soft tissue, can withstand the water pressure at a depth of 1,000 meters. Image source: wiki

Above the Sky

The blue planet under our feet is our only mother in the solar system. However, even if all the matter in the solar system except the sun is added together, their total mass is less than 1% of the sun. There are at least 100 billion stars like the sun in the Milky Way, which is equal to the total number of humans who have lived on the earth. It only takes 0.133 seconds for light to circle the earth, but it takes 46 billion years to reach the edge of the known universe. The vastness and depth of unknown things, unsolved problems and unfinished business far exceeds our practical understanding. Futurists have long accepted the view that human survival will depend on the ability to survive in outer space.

"The future of humanity has two choices: either soar into space and become multi-planetary, or confine ourselves to our home planet Earth until mass extinction occurs." - Elon Musk, founder of SpaceX

Imagined multi-planetary colonization. Image source: AI generated

To conduct space exploration, especially to reach any distance quickly, we cannot avoid talking about high-G acceleration. Take the "Forward Four" propulsion of the star-class battleship in "The Three-Body Problem" as an example: under 120G acceleration, after 0.5 seconds your speed will far exceed any racing car on Earth; after 7 seconds you will be on par with the speed of the Dongfeng-41 intercontinental strategic nuclear missile; within 14 seconds you can reach the escape velocity to get rid of the gravitational pull of the sun.

But just as the human body's cavity structure cannot withstand deep water pressure, it cannot withstand high acceleration loads. Ordinary people can withstand 2-4G acceleration loads, and trained pilots can reach 8-9G. There have been experiments on the human body's extreme acceleration tolerance in history. In 1954, Air Force pilot John Stapp boarded a special rocket, accelerated to nearly the speed of sound within 5 seconds, and withstood an acceleration load of up to 46.2G. Although this acceleration load only lasted for a moment, John Stapp still broke many ribs, burst many blood vessels throughout the body, and detached his retina. He was sent directly to the hospital. Afterwards, when people asked him about his opinion on the experiment, this blind man said: His biggest gain from this experiment was a cane and a guide dog. Such calmness and unrestrainedness came from Stapp's fearlessness of death. You know, just before the experiment began, he refused a donut handed to him by someone else, on the grounds that food in the stomach would affect the anatomy.

Just like soaking in liquid to resist deep water pressure, it is a natural idea to use it to resist acceleration load. Liquid has good pressure transmission performance. For people completely immersed in liquid, the acceleration load is uniform inside and outside. There is no local force pressure difference in the human tissue structure. There is an extremely uniform pressure gradient throughout the body, which greatly reduces the impact of overload. The idea of ​​using liquid immersion to achieve high G acceleration is also mentioned in some science fiction works. The most widely known to Chinese readers is the deep sea state entered by the crew in Liu Cixin's "The Three-Body Problem":

"When at the highest propulsion power, the spacecraft will accelerate to 120G, and the resulting overweight is more than ten times the limit of the human body under normal conditions. At this time, it is necessary to enter the deep sea state, that is, fill the cabin with a liquid called "deep sea acceleration liquid". This liquid is rich in oxygen, and trained personnel can breathe directly in the liquid. During the breathing process, the liquid fills the lungs and then fills each organ in turn.

——"The Three-Body Problem 2: The Dark Forest"

To successfully achieve this deep-sea state, the acceleration fluid must not only meet the requirements of safe breathing but also have to have exactly the same average density as the human body: if the human body's density is less than that of the liquid, it will be pushed to the rear bulkhead during acceleration, and vice versa. The force provided by the bulkhead to the human body is not the hydrostatic pressure of uniform pressure, so once it sticks to the bulkhead, it is very likely to be pressed into a thin sheet on the bulkhead due to the local pressure difference, just like the crew members in "The Three-Body Problem" who accelerated directly without entering the deep-sea mode.

The acceleration when entering the deep sea state cannot be infinite, because too high an acceleration will make the "differential centrifugation" phenomenon obvious. Differential centrifugation means that under different intensities of centrifugal force, substances of different densities will separate according to the magnitude of the centrifugal force. There are huge density differences in human muscle, fat, blood and bones. Under too high an acceleration, when the connection between human tissues is not enough to constrain the separation of its constituent molecules, the bones with high density will sink to the bottom, while the fat with low density will float upward. The structure of the human body as a carbon-based organism limits the further increase of the acceleration load. To overcome this limitation, perhaps it is necessary to use a force field to fix all the molecules of the human body, as Michio Kaku imagined in "Space Wars", so that no matter how high the acceleration is, it will not lead to tissue stratification, and mankind's desire for extreme acceleration for thousands of years can finally be realized.

Differential centrifugation diagram, gradually increasing the centrifugal force to separate organelles of different sizes, source: wiki

Successors

It took a thousand years for humans to enter the steam age from the iron age, and only a hundred years from the steam age to the electrical age. In the dust of the galloping science, we see that the development speed of human civilization far exceeds the evolution speed of the human body. People today are no longer satisfied with being selected by the environment for a long time. They have learned to reclaim land, fix sand and cultivate land, and transform the environment to adapt to themselves. Perhaps in the near future, by breathing in liquid, humans can truly get rid of the limitations of the flesh and become "amphibians" that can catch turtles in the five oceans and reach the moon in the nine heavens. For this day to come, the successors still have a long way to go.

References:

[1]Shaffer, Thomas H., Marla R. Wolfson, and Leland C. Clark Jr. "Liquid ventilation." (1999).

[2]Clark Jr, Leland C., and Frank Gollan. "Survival of mammals breathing organic liquids equilibrated with oxygen at atmospheric pressure." Science 152.3730 (1966): 1755-1756.

[3] Chen Ruiyong, Jin Hai, Li Mengxing, Yang Feng, Gu Jinghua. Application prospects of liquid breathing technology in submarine rescue[J]. Journal of Naval Medicine, 2022, 43(12): 1320-1324

[4]Hirschl, Ronald B., et al. "Prospective, randomized, controlled pilot study of partial liquid ventilation in adult acute respiratory distress syndrome." American journal of respiratory and critical care medicine 165.6 (2002): 781-787.

[5] Xu Mingyi, Du Wanming. Challenges and ideas for developing marine cities[J]. Frontiers of Marine Science, 2020, 7(3): 59-67

[6]Rozwadowski H M. “Bringing Humanity Full Circle Back into the Sea” Homo aquaticus, Evolution, and the Ocean[J]. Environmental Humanities, 2022, 14(1): 1-28.

[7]https://www.todayifoundout.com/index.php/2021/08/can-humans-breathe-liquid-like-in-the-abyss/

[8]https://www.bbc.com/future/article/20130930-can-we-build-underwater-cities

Author: Li Wenjie Member of the Science Popularization Volunteer Association of Changchun Institute of Optics and Fine Mechanics, University of Chinese Academy of Sciences

Reviewer: Sun Yifei, Director of the Medical Education History Research Office, Hebei Medical University

The article is produced by Science Popularization China-Creation Cultivation Program. Please indicate the source when reprinting.