A nodeshell for such a cool word? Egad. Well. (/me rolls up sleeves) a surfactant is a substance that, when dissolved into or mixed with a liquid (typically water) has the effect of lessening the surface tension of the target fluid. The most common use of such a substance is soap, which is a surfactant and used to not only dissolve greasy liquids but to increase the 'wetness' of water.

If you're a car buff, there are products designed to do this in your radiator coolant. The point is to reduce the size of bubbles formed in the coolant (because the bubbles interfere with heat transfer, the radiator's job) and to ensure the coolant contacts all of the heat exchange surface. One popular brand is named, aptly enough, Water Wetter.

According to Princeton's online dictionary, the name was produced as follows:

surface plus active plus agent

Mature lungs produce surfactant naturally. Its function is to prevent the collapse of the tiny air sacks called alveoli by reducing the surface tension of the fluid within the lungs where oxygen and carbon dioxide are exchanged at a cellular level.

When a premature baby is born they may not be mature enough to produce surfactant yet. Production begins in the early weeks of gestation but may not be adequate for independent breathing until the 35th week of gestation. Infants without adequate surfactant may develop Respiratory Distress Syndrome (RDS) which may later develop into Bronchopulmonary Dysplasia (BPD).

Treatment is available. Oxygen, respirators and other medical treatments have long been the mainstay of treatment for the premature infant’s RDS. More recently artificial surfactant has become available and has made tremendous positive differences in many premature infant outcomes.

In 1963 the third child of Jackie and President Kennedy died from prematurity that was not really extreme in today’s terms. His gestational age was about 32/33 weeks. RDS and a lack of surfactant probably played a large part in his death. Today, the earliest age of viability of premature infants is approaching 23 weeks gestation. Granted this is the extreme end and there are many ethical as well as medical questions about these micro premies. More pertinent here, prematures of Patrick Bouvier Kennedy’s gestational age would rarely die today, in part because of artificial surfactant.

interesting reading on the wider subject of ethics in the treatment of premature infants: http://www.mnmed.org/Protected/99MNMED/9912/Lussky.html
“A Century of Neonatal Medicine”
Also used in photographic darkrooms (remember those?) to help avoid streaks on film as it dried.

In other words, as the liquid that was used in the film developing process (most likely the last liquid in that process being water) either dried or slipped off the surface of the developed film, the surfactant made sure that the liquid slipped off rapidly and evenly.

Therefore, voila!, no streaks.

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