Heat pipes are hollow, contain a small amount of liquid and are generally very thin walled.
They work by phase change, the liquid gets boiled at the hot area and turned into a vapor, this changeing from liquid to gas state change absorbs far more heat.
The vaporised liquid expands up the heat pipe cooling as it goes, as it cools it condenses releaseing the heat it absorbed.
The liquid is then drawn back down the heat pipe by cappilery action, by ether a wick inside the heat pipe or a rough texture on the inside of the walls of the heat pipe.
This phase change mechanic means a heat pipe conducts heat very effeciently within a given tempture range.
The liquid used and the pressure it is at effects the active heat range of a heat pipe.
Commonly used is distilled water or a Alcohol & distilled water mix, at a pressure below 1 atmosphere (this allows the boiling point to be reduced below 100c)
It also means they are hollow and self contained, so if they ruptured or pierced they are ruined.
Sorry for this slight topic side track.