Powered exoskeletons in fiction

Powered armor has appeared in a wide variety of fiction, beginning with E. E. Smith's Lensman series in 1937. Since then, it has featured in science fiction movies and literature, comic books, video games, and tabletop role-playing games. One of the most famous early versions was Robert A. Heinlein's 1959 novel Starship Troopers, which can be seen as spawning the entire sub-genre concept of military "powered armor."

In addition to heightened strength and protection provided by the exoskeleton, other popular features include internal life support for hostile environments, protection from environmental hazards such as radiation and vacuum, weapons targeting systems, firearms affixed directly to the suit itself, and transportation mechanisms that allow the wearer to fly, make giant leaps, or speed by on ground.

In some portrayals of powered armor, the suit is not much larger than a human. These depictions can be described as a battlesuit with mechanical and electronic mechanisms designed to augment the wearer's abilities. Other power armors are portrayed as being much larger, more like a bipedal vehicle the size of a tank or much larger. These latter are frequently termed Mecha, from the Japanese “メカ” (meka), an adaptation of the English “mechanical.” The line between mecha and power armor is necessarily vague. The usual distinction is that powered armor is form-fitting and worn; mecha have cockpits and are driven,^[1] or that powered exoskeletons augment the user's natural abilities, whilst mechas replace them entirely. However, the line between the two can be difficult to determine at times, especially considering that force feedback systems are often included for delicate maneuvers. Even in a larger mecha meant to be driven like a walking tank rather than worn, a realistic control system would have to be either cybernetic or form-fitting^{[citation needed]}: In the BattleTech universe, a cybernetic system is necessary to provide a sense of balance.

Another variation is Bio-Armour, which produces similar strength with organic technology (e.g. Peter F. Hamilton's novel Fallen Dragon, Jim Shooter's X-O Manowar comic book, and the Guyver Japanese animation series).

Most power armors carry an on-board, self-sufficient power source. Masamune Shirow's Landmates in Appleseed used simple internal combustion engines installed into the thigh assembly of the armor. The "hardsuits" of Bubblegum Crisis 2040 have a battery the size of an American football between their shoulderblades, though the functionality is never described. Still, more fantastic power sources have been introduced, for example, in Halo: Combat Evolved, Halo 2, and Halo 3, the Master Chief's MJOLNIR (pictured below) armor is powered by miniaturized fusion power reactors. In Privateer Press' Iron Kingdoms setting, a steam boiler powers an arcane conversion engine, which ultimately powers the suit. Similarly, in Final Fantasy: The Spirits Within, the suits are powered by single-celled organisms cultured in Ovo Packs. The Power Armor in the Fallout series, which is usually worn by the group the Brotherhood of Steel, a techno-religious group, is bulky and metallic. In these games the armor is described as being fueled by fusion power cells. However, they are made of titanium with a reflective coating against lasers. On some occasions, not even assault rifles breach them. The original armor have been bult by the "West Tek Research Division" and was studied by the Enclave (remnants of the US government), who designed and built Advanced Power Armors, which are bulkier, black and has a skull-like facemask.

In literature

E. E. Smith's Lensman series, (published from 1937 onwards), contains the earliest conceptualization of personal armour with both defensive and offensive capabilities for all environments.
Another early example were the Fat Man underwater suits (with mechanical pantograph arms and a propulsion system), which debuted in Tom Swift and His Jetmarine (1954).
The novel Starship Troopers (1959) details the tactics involved with powered armor. It was also the first work of fiction that widely popularized the concept. The first film adaptation was a Japanese OVA, Uchu no Senshi, produced by Sunrise with mechanical designs by Studio Nue. The later Hollywood version, however, did not use powered armor, opting instead for the Troopers to use simple (but clearly ineffective) body armor. In contrast, the 1999 television adaptation remained far more faithful to Heinlein's original vision.
Powered exoskeletons with arc weapons are used by inhabitants of Northworld in the Northworld Trilogy by David Drake
Standard issue battle armor in Joe Haldeman's book The Forever War is an exoskeleton using logarithmic force amplification.
Powered armor operated remotely by telepresence also feature in Haldeman's Forever Peace (which shares themes with the previous The Forever War but is not a sequel in terms of setting and characters).
The security troops of the interstellar company Zantiu-Braun are described as wearing biological "Skin" armour in Peter Hamilton's novel Fallen Dragon.
Very advanced suits based on nanotechnology and a very advanced computer assistance system are depicted in Legacy of the Aldenata by John Ringo. These suits are notable for being one of an extremely few designs that recognise traditional transparent 'eyes' in the helmet of the armour as being a weakness. These suits are often regarded as the spiritual descendants of Robert A Heinlein's Mobile Infantry Marauder, Command and Scout suits and in fact borrow similar terminology.
Armor by John Steakley, who admits that the work was inspired by Starship Troopers, features powered battle armor.
Dale Brown uses characters in many of his novels that wear a Tin man suit, which is a thin material that has an electric charge that flows though it to protect the user from all but very large explosions and very large caliber rounds. He also writes about CID units in his book, Act of War, which are Cybernetic Infantry Devices which are approximately 9 feet tall.
In Neal Stephenson's novel The Diamond Age, nanotech-derived Hoplite suits are used by modern infantry, and are full-body suits of powered armor.
In Iain M Banks's novel The Player of Games, a veteran wearing life-support powered body armor is forced to commit murder when the control systems of his suit are taken over from outside.
In David Weber's Honorverse and Mutineers moon universe, military units - in addition to unpowered 'skin' suits use powered armour hardsuits for combat operations. In addition in his books Path of the Fury and the later expansion In Fury Born, both Imperial Marines and the elite Drop Commandos of the Imperial Cadre utilise powered armour suits.
In the novel Ender's Game by Orson Scott Card the "armies" in battle school use a version of a powered exoskeleton for mock battles how ever these did not offer any advantages and simply froze soldiers who were "shot"

In comics and manga

The Marvel Comics superhero Iron Man, who wields a specialized suit of armor of his own design, is arguably the most popular American superhero who uses powered armor. Other characters in his long-running comic book series have also used such armor, including War Machine, Titanium Man, Crimson Dynamo, and most recently, Wondra. (Jubilee)^{[citation needed]}
Other notable powered armor users in Western comics include the supervillain Doctor Doom, Steel, Batman (sometimes, notably his Dark Knight Returns, Kingdom Come, Knightfall, Batman Beyond and The Batman permutations), X-O Manowar, and Guardian.
In Japanese manga Gantz, the characters wear a powered suit like exoskeleton, which gives them enormous strength and protection.
The Franco-Belgian comics heroine Yoko Tsuno wore one in Aventures électroniques (Electronic Adventures),the 4th graphic novel of her adventures.
The webcomic Schlock Mercenary includes a variety of powered armor, ranging from 'mini-tanks' down through various sorts of articulated hardsuits, to the 'low-profile' carbon fiber armor that is nearly indistinguishable from ordinary uniforms. All forms of power armor in the series to date has had some flight capacity, owing to the ubiquitous artificial gravity technology shown throughout the strip.
The Guyver is a biomechanical exo-suit from the long-running manga, anime and films of the same name in the franchise created by Yoshiki Takaya.
Yu Ominae in Spriggan uses the Armored Muscle Suit, which gives him 30x artificial strength and protection from bullets, fires, explosions and melee weapons.
During the Incredible Hulk's rampages, S.H.I.E.L.D is known to send HULKBUSTER units. "Hulkbuster" is also a famous Iron Man variant armor, actually an add-on to the famous Mark XI "Modular Armor."
In Alan Moore's Watchmen Graphic Novel, the second Nite Owl built an Owl Themed Exoskeleton. On its first test run it broke his arm and he never used it again.
In Greg Bear's Novel Moving Mars thick reactive nano armor was used to protect Martian leaders during an attack by earth from projectiles and made wearers dangerous to all escort personnel.
Superman's arch-enemy Lex Luthor is known to wear green and purple armor suits from time to time.
Tech Jacket's main character Zack Thompson has an exoskeleton that gives him his super powers.
In BIONICLE, the Toa Mata used Exo-Toa to battle the Bahrag.

In television and film

In the movie Aliens (1986), mechanized exosuits (actually Caterpillar Power Loaders J-5000^[2]) are used in loading cargo for spaceships, as well as Ripley's duel with the queen alien. A military version of this loader can be seen in the video game Aliens versus Predator 2.
The Hardman gear Raimi wears in the 1994 movie Death Machine.
In the 1994 TV series M.A.N.T.I.S. (Mechanically Augmented Neuro-Transmitter Interactive System) Dr. Miles Hawkins, a paralyzed scientist portrayed by Carl Lumbly, creates a super-powered exoskeleton in order to walk again, but ends up using it for crimefighting.
The Armored Personnel Units seen in Matrix Revolutions (2003), although these walk the line between powered armor and mecha.
Many of the Kamen Rider series use more complex, mechanical looking armours that seem to be powered by an unknown source.
The Muscle Gear, used by the Space Criminals Alienizer in the 2004 Super Sentai series Tokusou Sentai Dekaranger, is a powered exoskeleton that reflects all attacks and is loaded with weapons.
In the Animatrix segment The Second Renaissance part 2, human forces in the first Man-Machine War use powered armor which is a more sophisticated ancestor of the armor used in Matrix Revolutions.
In the 2005 film Sky High, Gwen Grayson/Royal Pain uses an armored battlesuit.
In Iron Man there are suits with built-in weapons, and allowing flying with vertical take-off and sub-sonic flight.
In the TV series Dark Angel Logan "Eyes Only" Cale uses a partial exoskeleton (only covering the lower half of his body) to overcome his disability, allowing him to walk again. In addition the exoskeleton grants increased physical performance to his lower body than a normal human.

In animation

File:Uchu no Senshi.gif

Powered armor from Starship Troopers (1988).

The Zentraedi male & female powered armors from Robotech (the English adaptation of the Japanese Macross T.V. anime series) feature energy & projectile weaponry with both atmospheric & space travel capabilities.
In the CGI cartoon ReBoot, Matrix and AndrAIa wear yellow exosuits similar to the ones from the movie Aliens.
In the American action cartoon series Centurions, the Centurions used a uniform like powered exoframes with several hard points, which enabled them to be with merged with multiple weapon systems.
Exoskeletal vehicles named "E-frames" were one of the central aspects of the American animated television series Exosquad.
In the anime series Bubblegum Crisis the heroines use a type of powered armor known as a "Hard Suit" which deviates from the normal look of rugged military issue armor and keeps a more feminine appearance.
Exoskeletal suits named "EMACS" (Energized Monster Armed Containment Suit) or simply power suits were a standard weapon in the animated cartoon series Monster Force.
In The Transformers: The Movie (1986), as well as its 3rd and 4th seasons and Japanese spin-offs (TF: Super God Masterforce in particular), "exo-suits" were created, allowing humanoid supporting characters to transform alongside their Autobot and Decepticon counterparts. This technology was further used to allow humans to combine with Transformers, resulting in the Headmasters, Targetmasters, and Powermasters.
Many Japanese animation featuring mechanical objects also have humans controlling gigantic exoskeletons, such as The Vision of Escaflowne, Full Metal Panic, Bubblegum Crisis, Tekkaman Blade, Gundam or Ghost in the Shell: Stand Alone Complex. Although many of these are not exactly exoskeletons because of their non-humanoid forms, the main principle is identical. (See below.) The term "mobile suit" in the Mobile Suit Gundam series in particular, is believed to derived directly from the Mobile Infantry powered suits of Starship Troopers.
The CGI television series Roughnecks: Starship Troopers Chronicles (based on the Heinlein novel) made extensive use of power armor, though somewhat different from the armor in the original book.
In Episode 11 of Batman Beyond ("Disappearing Inque"), Bruce Wayne shows Terry McGinnis the powered armor exoskeleton he used briefly before creating the more advanced Powered Batsuit. The exoskeleton is mostly silver, with a black trim on the gauntlets and helmet, and the familiar shape of the Batman logo tapering from the neck across the chest.

In video games

In Star Warrior, the player is a member of the Furies, a mercenary group that uses small groups of operatives in powered armor.
In Earthworm Jim, Earthworm Jim uses the Ultra-high-tech-indestructible-super-space-cyber-suit which responds to his psychic commands, allowing him to progress through the game.
In Terra Nova: Strike Force Centauri, a 1st person tactical game by Looking Glass Studios, powered armor is the centerpiece, featuring many types of powered armor and loadout combinations. Among those found in electronic games, Terra Nova's powered armor suits are also the ones most similar to the drop suits and powered armor found in Robert Heinlein's novel, Starship Troopers.
Gordon Freeman's Hazardous Environment (HEV) Suit from Half-Life and Half-Life 2 is an exoskeleton originally designed for working with hazardous materials, exploration and possibly combat due its projectile resistant active armour and weapons recognition systems.

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Samus Aran and her Chozo made power suit

The Half-life expansion pack Opposing force featured Adrian Shephard of the HECU wearing a Powered Combat Vest (PCV), designed as a high tech bullet proof vest to absorb the damage of attacks. It is assumed that in the story context this is a new or newly mainstream technology funded by the government for the military.
The Half-Life mod known as Natural Selection features a full armor suit, an advanced technology that Marines can acquire, which absorbs much damage for the wearer, is powered so the wearer maintains agility almost equal to an unencumbered person, and can be repaired when damaged.
The Half-Life 2 mod known as Dystopia features power armor for its Medium class. Additionally, all classes can equip themselves with "leg boosters", which increase jump height and brace against high falls.
In Deus Ex, the Majestic 12 Commando units wear power armor (dubbed 'obsidian' armor by their creators) that is outfitted with two 7.62 mm machineguns and two rocket launchers.
In Deus Ex: Invisible War, the Templar Paladins wear Power Armour to combat their biomodified foes.
Samus Aran from the Metroid series of video games wears an exoskeleton, the Power Suit, designed by the Chozo and enhanced with an attached arm cannon, which grants her the ability to roll into a ball around one meter in diameter and/or perform very high spinning jumps. In addition, it allows her to survive almost indefinitely underwater, as well as in the vacuum of space. The suit is modular in design, allowing the incorporation of additional weapons, detection equipment, movement enhancements, and protective shielding. Samus' flexibility as well as agility seem to suggest she either has a light suit or is so used to it that she can move around freely. It also includes some biological components, as evidenced in Metroid Fusion. Throughout the series, similar, if more primitive, armor is worn by Federation soldiers, Space Pirates, and other warriors within the universe.
The Fallout computer role-playing game series is notable for its use of powered armors in retro-'50s style. It increases strength, such as the exoskeleton intended to do so in real life and has an advanced version - Advanced Power Armor. It is powered by a Micro-Fusion reactor. In addition to providing physical protection, Power Armor also serves as a radiation shield.

File:Master Chief in Halo 3.png

Halo's Master Chief wearing the MJOLNIR battle armor

Powered armor is an integral gameplay element of the Tribes video game series, with all major characters wearing it most of the time. Additionally, all armor suits are outfitted with jet packs, adding a similarity to the original Mobile Infantry equipment in Heinlein's book.
The super-soldier Master Chief in the video game series Halo is clad in energy-shielded and strength-enhancing armored suit weighing 1000 pounds, called the MJOLNIR battle armor, that can allow him to turn over armored vehicles, quickly dispatch foes in melee combat, and house starship-grade AI in a layer of crystals, the same type used to create the AI's core, in between the armor and the inner body suit to allow him to override enemy electronic defenses. The character is so heavily associated with the suit that he is never depicted outside of the armor, though at both the end of Halo and the start of Halo 2, the Master Chief is helmetless, with his head offscreen. The reaction time improves by a factor of five and the strength is increased three folds, which is powerful enough to kill a normal human without the augmentations made to Master Chief.
Also, the Clone commandos in Star Wars: Republic Commando sport a version of powered armor, called Katarn-class powered armor. (though it is more armor than power).
Exoskeletons have surfaced in many other video games. Some examples would be certain marines from Unreal II: The Awakening and MAX Units from PlanetSide.
In Starcraft and its expansion pack, most of the Terran foot soldiers: the Marines, Firebats, Medics (in the expansion), and possibly ghosts, use powered armor. The basic Terran building unit, the SCV (Space Construction Vehicle) also appears to be a bulky powered exoskeleton. The Goliath also resembles powered armor, and has been described as such in some Starcraft books, but, like the Armored Personnel Unit of the Matrix, blurs the line between power armor and Mecha. Also the Protoss unit Dragoon is a severely wounded soldier on life support in a powered exo-skeleton, yet in Starcraft 2 the art for a Dragoon makes it resemble a mecha
After extensive research, troops in the computer game X-Com have access to power armor that allows damage resistance and flight.
In the FPS Bet on Soldier player, as well as enemies, can carry an "exo armor", which covers the whole body, and even get into a war bipod called "exoskeleton."
In the MMORPG Neocron 2 high level players have access to several types of power armor.
In the video game series Metal Gear Solid, the first game features the character Cyborg Ninja (Gray Fox), who possesses a powerful exoskeleton with stealth camouflage. In Metal Gear Solid 2 Olga Gurlukovich wears a similar exoskeleton. Also in MGS2 Solidus Snake wears another type of exoskeleton equipped with missile firing tentacles and the Arsenal Tengus who protect Arsenal Gear and the Metal Gear RAY's in MGS2 appear to be wearing a military prototype combat power suit. In Metal Gear Solid 4 Raiden uses an advanced Cyborg Ninja type exoskeleton and the Beauty and the Beast group featured in the game all wear a different type exoskeleton armor.
In Eugen System's RTS Act of War: Direct Action(Along with Act of War: High Treason), Task Force Talon's signature unit is the SHIELD Unit (Super High Infantry Electronic Defense Unit), which is a powered armor combat exoskeleton outfitted with a GAU-19 20 mm Vulcan Cannon and a Javelin Missile Launcher. These units have medium armor and used mostly as fast hit-and-run raiders that can take down helicopters in large groups, but do not fare too well against tanks and heavy artillery. They can be upgraded with a milimetric radar system to allow their pilots to detect stealth units.
Monolith Productions' first-person shooter F.E.A.R features an exoskeleton named the R.E.V.E. Power Armor. Arguably one of the game's most dangerous enemies, the R.E.V.E. is heavily armored and wields powerful weaponry such as rocket launchers and laser cannons, yet is almost as fast and maneuverable as a standard human soldier.
In Crytek's "Crysis", the protagonist, Nomad, wears a US Military prototype "Nano Muscle Suit" that allows the user to have extra protection against projectiles, superhuman strength, cloaking ability, underwater breathing, enhanced speed, and the ability to treat wounds out on the battlefield. These actions uses rechargeable energy reserves that power the suit.
In Red Faction Parkers Miner Suit (similar to Gordan Freeman's HEV suit) is a form of powered armour
In Time Crisis 4, some of the enemies wear hi-tech powered armour. Unlike normal troops, they take a lot of hits to kill, and have a machine gun instead of a pistol.
In Command & Conquer: Tiberian Sun there are several GDI units that can be purchased that are named the Wolverine Powered Combat Suit, the Titan Medium Mechanized Battle Walker, and the Juggernaut Heavy Artillery Platform which returns in the sequel. In Command & Conquer 3: Tiberium Wars Nod created its own bipedal mech warrior called the Nod Avatar Warmech. GDI also fields squads of Zone Troopers, infantry clad in swift, large powered armor that protects them from tiberium fields and being run over by vehicles and equipped with jump jets and powerful railguns that let them cut through anything but aircraft with ease. Nod Black Hand and Tiberium Troopers (Kane's Wrath) use powered armor, presumably to carry the great ammount of weight their liquid ammunition entails. It doesn't offer much in the way of actual protection, though.
In Command & Conquer: Red Alert 2 the Soviet Tesla Troopers are most likely wearing a powered exoskeleton. The Desolator unit (which is only available in skirmish/multilayer games and some campaign missions) also wears a type of powered light exoskeleton, but it provides the trooper protection from radiation.
In S.T.A.L.K.E.R the expert stalkers wear special exoskeleton suits which increases the carry weight limit from 50kg to 70kg and gives additional bullet protection, but makes them unable to sprint. The suits have minimal radiation protection and are very expensive/hard to find, which makes them somewhat impractical to stalkers venturing deep into the Zone.
In the computer game War Front, the Wehrmacht deploys troopers in exoskeletons, simply called "Exoskeleton." The design of these units is not influenced by science-fiction but rather resembles 20th century technology. Classified as "Light Tank", these Exoskeletons are armed with machine guns and a special rocket launcher ability that must be researched separately and can only be used every few minutes.
In the Sierra game Time Shift, the player wears the experimental "Beta Suit", which, apart from rechargeable shields, can cause time to slow, stop, or reverse, while leaving the player free to act normally. This suit seems to be a cross between power armor and a skin suit, but is similar to the suit used in Half Life 2.
Admiral Galak Fyyar uses a heavy full-body armour in Star Wars Jedi Knight II - Jedi Outcast. It has an inbuilt shield-generator which is characterised by reflecting Lightsabers.

In role-playing and board games

In the Battletech (Also known as MechWarrior) universe, genetically engineered Clan soldiers are bred for strength and other qualities to wear Elemental powered armor. Elemental armor also provides advanced medical technologies to keep the wearer alive in case of severe injury or trauma during combat. Following the Clan Invasion, many other governments began to deploy Battle Armoured suits, but often of lesser quality then the original Elemental Armour.
Powered armor is heavily used in science fiction role-playing games, such as Rifts, to allow weak and mundane humans to compete in combat with supernatural and super-powerful adversaries. Two common examples of Powered Armor (or just Power Armor) in that series are the 10 foot Glitter Boy, which is covered in a coat of mirror-like, laser-resistant alloys; and the jet flight capable Strategic Armor Military Assault Suit (commonly called the SAMAS or Sam).
Privateer Press' Iron Kingdoms, the setting for the tabletop game Warmachine, includes Warcasters and characters that don suits of steam powered armor. The suits require coal to keep the furnace lit as well as water in their boiler.
In Star Wars Knights of the Old Republic there are several different types of power armor available including Corellian Power Armor. Although the armor types in the game are of little importance, power armor is available for use.
Several examples can be found in the table-top strategy Warhammer 40,000. The most prominent examples are Power Armour, Terminator Armour and Dreadnought carapace used by the Space Marine and Chaos Space Marines, others include Tau Empire battle suits and Ork mech armour.

Skinsuits

An variant of the powered armor would be the skinsuit, a very thin (hence the name) and flexible powered armor variant. The skinsuit can be used as an environmental-protection suit, similar to spacesuit (for example, in the Honorverse universe), or may have some artificial muscle that increases strength, resistance and endurance, but in that case sacrifices environmental protection, sensory equipment, and built-in weaponry. The suits seen in the anime and manga versions of Spriggan and Gantz or skull suit from Metal Gear Solid 2: Sons of Liberty would be prime example of this form of armor. Another would be the Nano Suit in Crysis: http://en.wikipedia.org/wiki/Crysis The military uniforms in the webcomic Schlock Mercenary are almost universally skinsuits, utilizing buckminsterfullerene tubule-weave cloth and incorporating antigrav systems to allow a soldier flight capabilities.

Many Japanese tokusatsu television series and movies feature heroes (and more-rarely villains) in technological skin-tight armor. Popular examples include:
- the long-running sub-genre Super Sentai saga (which, each following year, is Americanized as the Power Rangers saga). One exception to this would be the heroes from 2005's Mahou Sentai MagiRanger (2006's Power Rangers Mystic Force) who use literal magic spells to create their suits; whereas all other Super Sentai series have some form of on-person (the hero carry a device with them) or projected (the suit is transmitted invisibly from a fixed location or another more-powerful sentient being) technology involved.
- Early versions of the darker Kamen Rider saga used skin-tight armor in addition to extended thick armor; though later generations of series have moved on in favor of more familiar metallic-looking powered armor suits that are also form-fitting.
- The Ultraman series of superheroes (like Kamen Riders) uses a combination of skin suits augmented with thicker armor. This is debatable, however, as some Ultraman heroes are completely alien and merely appear as humans on the outside (hence the definition of what is armor and what is their body is undefinable), they feature a human merged with an alien symbiote which provides their powers, or the powers are gifted to the human character through a merely-mechanical alien device.

As with powered exoskeletal armor, it has become apparent that even the skinsuit may become a possibility in the near future: The Massachusetts Institute of Technology has reopened research into the space activity suit, a type of spacesuit that equalizes pressure through mechanical pressure as the suit with the breathing gas and DARPA is researching using carbon nanofiber tubes as artificial muscles for powered uniforms as opposed to an exoskeleton rig.

In Dan Simmons' Hyperion books, one of the protagonists is aided by a woman from the very distant future; she uses advanced technology to sheathe the character in an energy 'skinsuit' that not only acts as a powered body armor, but tends to injuries, allows the wearer to focus on objects with perfect clarity from many kilometers away, and allows the wearer to phase-shift in incredible speeds during combat. A person 'skinsuited' appeared to the viewer to be covered in a micrometres-thin layer of mercury.