Hydrostatic gel

Hydrostatic gel is a component of the MJOLNIR Powered Assault Armor. It is a blueish semi-liquid forming a layer inside the armor. It is likely one of the innermost layers of the armor, but it is not in contact with the operator's skin.

Overview
It was designed by Dr. Catherine Halsey, a civilian scientist in the employment of the UNSC Office of Naval Intelligence, Section Three, as a component of the MJOLNIR Powered Assault Armor. It is one of the main components of the MJOLNIR system. The MJOLNIR has an emergency exhaust port that releases hydrostatic gel when the suit becomes too hot, preventing the wearer from being boiled alive. Severe or sustained trauma to the suit can cause the gel layer to become viscous, rendering it partially or completely ineffective.

Purpose
The hydrostatic gel's main purpose is to conform to the wearer's shape and body temperature, keeping the wearer from getting too hot or cold. The gel also has the ability to be pressurized to various levels, allowing its wearer an amount of cushioning during hard impacts. This way, a wearer can survive falls from great heights or other similar conditions as long as the gel is pressurized.

Trivia

 * In the opening cutscene of Halo 3, John-117 is seen crashing down somewhere in the Kenyan jungle. It is later revealed that the hydrostatic gel layer absorbed most of this impact, as John-117 is relatively unharmed.
 * In Halo: First Strike, Frederic-104 ordered Red Team to over pressurize their hydrostatic gel layers just before impact. It is also stated that doing this was risking nitrogen embolisms, but this was the only way to make a "soft" landing.