Xenomorph blood is a potent molecular acid that Xenomorph XX121 contains within its body. A highly corrosive substance, it is capable of burning through virtually any natural or man-made material in short order. While the use of potent acid for blood provides the creature with an obvious defensive mechanism, the fluid's primary purpose is in fact in providing the Xenomorph with its energy. Acid blood is one of the Xenomorph's most recognizable physical attributes, and all stages of the creature's life cycle have been found to contain the fluid.
The exact composition of Xenomorph blood is a vigorously debated topic. Its extreme corrosiveness precludes study to a large degree. It has been suggested that it may consist of either sulfuric acid (H2SO4) or hydrofluoric acid (HF(H2O)), although this is largely theoretical and the exact chemical composition remains unknown. However, it is known that it is some form of molecular acid and that, while it is incredibly potent, it quickly oxidizes in air and is subsequently rendered neutral. It is typically a dull yellow in color, occasionally with hints of green. While generally accepted as the creature's blood (and typically described as such), some have suggested that, at least at the Facehugger stage, the acid is not blood but rather a defensive fluid maintained under pressure between a double layer of skin.
The specific composition of the acidic blood remains a mystery, with its incredibly corrosive properties no doubt limiting the degree to which it may be studied. However, it has been theorized that the blood could be some type of "hydrosulfuric" or hydrochloric acid composition due to its corrosiveness and its conspicuously toxic effects on living human tissue. It has also been proposed that the Xenomorphs are immune to their own acidic blood due to an endobiological build-up, similar to the human stomach's ability to protect itself from its own digestive fluids. It has also been theorized that the blood is fluorine-based, and that the Xenomorph's protection system against its own toxic acid is essentially a bio-organically produced Teflon insulation within its body, since polytetrafluoroethylene (PTFE, or Teflon), being a fluorine-based compound, does not react with hydrofluoric acid. It is known that Xenomorph chitin is resistant to the acid even after the creature has died or its chitin has been removed.
Other theories include the possibility of the blood being a fluorocarbon or chlorofluorocarbon compound; this would serve to explain why Xenomorphs that have been set alight will sometimes explode violently, as fluorocarbons are incredibly reactive with fire.
While Xenomorph acidic blood is primarily thought of as a passive defense mechanism used to deter attackers, studies show that it in fact plays a far more integral role in the creature's biology. Research by Lasalle Bionational indicates that the acid is primarily a component of a biological "battery", generating a powerful bio-electric charge by means of chemical reaction that provides the Xenomorph with its energy, replacing the need for traditional respiration and digestion of food altogether. This would help to explain how the creature is apparently able to survive in the vacuum of space for significant periods, and also helps to explain how the Ovomorph stage is seemingly able to remain dormant but alive for vastly extended periods of time. However, the means by which the creatures may "recharge" this battery remains a mystery. It is quite possible it simply cannot be renewed, and that Xenomorphs will eventually die naturally as a result of this energy source depleting.
Despite the blood's apparent purpose in "powering" the Xenomorph, it remains true that the highly corrosive liquid is a significant defensive device. While it does not pose a threat directly, killing or harming an Alien with any attack that breaks the skin will cause the acid to spill, potentially harming the attacker or, as would be the case on a starship, breaching a vessel's hull. This makes safely killing even a single Xenomorph a difficult proposition if only conventional projectile weapons are available, especially when encountered on board a starship.
Despite its typically passive nature, certain castes of Xenomorph have evolved to use their acid blood in a more offensive manner. Drones, for example, have developed a highly pressurized bloodstream that can cause them to physically explode if sufficiently damaged, showering the surrounding area in acid. Boilers intentionally use a similar process as a means of attack, rushing targets before suicidally rupturing their bodies and fatally drenching their victims in blood. The Drone and Runner castes have been shown to possess the ability to spit their blood at targets, although the quantity is small and consequently this means of attack is normally used to wound and disable targets rather than kill them. The spitting ability is used more aggressively by Praetorians and Spitters, which are capable of spitting lethal quantities of their blood significant distances, giving them a dangerous ranged attack other Xenomorphs do not possess.
Xenomorphs also use their blood in more tactical ways. Facehuggers have the ability to secrete small quantities of their blood in order to burn through obstructions and gain access to hosts. Similarly, fully grown Xenomorphs are shown to have an understanding of their blood's properties beyond simply spitting it at victims, and have been known to deliberately mutilate or even kill each other in order to use these properties to overcome obstacles. Eggs have been known to forcefully spray acid from any incisions made in their outer skin by way of defense, in order to deter, disable or kill whatever may be responsible.
Defense Against Xenomorph Blood
While Xenomorph acid is notoriously corrosive, there are some methods and materials that can resist it. For instance, on the Cold Forge, the station's outer hull and other critical containment surfaces were constructed of super-hydrophobic materials that repelled the acid and prevented it burning through. During the Hadley's Hope infestation, meanwhile, Facehugger specimens were suspended in alkaloid nahcolite water baths that rendered them unable to use their acidic blood to escape the containment tanks in which they were held.
Perhaps the most notable cases of acid-resistant technology reside with the Yautja, much of whose equipment and weaponry is constructed of metals that are immune to the effects of Xenomorph blood. Yautja blood itself is also known to be able to partially neutralize the acidity of Alien blood. However, human technology in this regard remains in its infancy. While basic systems have been prototyped (including the Apesuit acid-proof body armor) mankind has yet to develop truly effective methods for negating the corrosive effects of Alien blood.
Behind the Scenes
The idea of giving the Xenomorph acid blood came from conceptual artist Ron Cobb, in response to screenwriter Dan O'Bannon's insistence that the creature be a mortal organism, but have some characteristic that prevented the crew from killing it. In Alien and Alien3, shots of the acid blood dissolving through flooring were achieved using a combination of styrofoam for the solid surface and a mixture of chemicals for the blood. In Alien, the blood mixture was mainly composed of chloroform, acetone, cyclohexylamine and acetic acid, with other compounds in minor quantities. "We produced a mixture of chemicals that actually was pretty corrosive – you wouldn't want to get it on your skin, you know," explained special effects supervisor Brian Johnson. "That stuff just ate right through the styrofoam, but it left enough color behind so it really looked like metal."
- The effects of acidic blood are portrayed with wildly varying consistency in the films. In Alien, the blood (from the Facehugger attached to Kane) is seen to be capable of melting through two decks of the Nostromo in mere seconds, yet when Ripley shoots the fully-grown Alien with a grappling gun at the climax of the film, the spilled blood fails to melt the tether and free the creature. In Aliens, Corporal Hicks' armor is destroyed by acid blood, yet at many other points the substance does not damage the walls and floors around dying creatures like it should. In Alien Resurrection, Ripley 8 has acid blood capable of melting through steel and toughened glass, yet it has no effect on the surgical implements used to operate on her at the start of the movie.
- Beyond the acidic effects of the blood itself, the Xenomorph circulatory system is never examined in any detail in the film series. However, the novelization of Alien Resurrection states that Xenomorphs have a considerably faster pulse than that of a human.
- The clone Ripley 8 also possessed acidic blood thanks to the inclusion of Xenomorph DNA in her system, although it was nowhere near as potent as that of Xenomorphs.
- Despite their immunity to their own acid, Xenomorphs are vulnerable to hydrofluric acid, which was effectively used against them on LV-178.
- One species known to exhibit natural immunity to the Xenomorph's acidic blood are the predatory creatures that cohabit with and prey on them on Xenomorph Prime.
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