As a prisoner, the Patriots used Raiden as a test subject for experiments in exoskeletal enhancement surgery, during which he had his head and spine excised from his body from his lower jaw down and subsequently grafted into an enhanced synthetic body.

Is Raiden a Cyborg Ninja?

Raiden is the only Cyborg Ninja in the series to survive the events of the game in which he is featured. He is also the only Ninja to have never fought Solid Snake in a main game, either during gameplay (Gray Fox) or off-screen (Olga Gurlukovich).

Why was Raiden called snake?

The real reason why Raiden was called “Snake” is because he represents the player having taken the role of Solid Snake in previous games/VR missions/Tanker Chapter. Minor nitpick, but a solidus isn't a gas, it represents a point between solid and liquid states.