A stillsuit is a full body suit worn in the open desert of the planet Arrakis that was designed to preserve the body's moisture. It consisted of various layers that would firstly absorb the body's moisture through sweating and urination, and then filter the impurities so that drinkable water would be circulated to catchpockets. The individual could then drink the reclaimed water from a tube attached to the neck.
The stillsuit typically covered the entire body, up to the neck and wrists. For the hands, gloves could be worn. However, when delicate work was performed, Fremen discarded the gloves and rubbed their hands with leaves from the creosote bush. This allowed them to work, while the residue from the creosote leaves prevented perspiration.
A facial mask could also be worn so that moisture from the mouth and face could also be reclaimed.
"It's basically a micro-sandwich — a high-efficiency filter and heat-exchange system. The skin-contact layer's porous. Perspiration passes through it, having cooled the body ... near-normal evaporation process. The next two layers . . . include heat exchange filaments and salt precipitators. Salt's reclaimed. Motions of the body, especially breathing and some osmotic action provide the pumping force. Reclaimed water circulates to catchpockets from which you draw it through this tube in the clip at your neck... Urine and feces are processed in the thigh pads. In the open desert, you wear this filter across your face, this tube in the nostrils with these plugs to ensure a tight fit. Breathe in through the mouth filter, out through the nose tube. With a Fremen suit in good working order, you won't lose more than a thimbleful of moisture a day..."
During the Butlerian Jihad but far from it, in Arrakis, the outlaws of Selim Wormrider had developed distilling suits with internal recycling systems and protection and survival during times when they must be abroad in the open desert.
During the time of the Faufreluches, stillsuits were manufactured by Fremen as well as private entrepreneurs. However, Fremen manufactured stillsuits were widely considered the best, as they were the only ones that could preserve the body's moisture in the deep desert. Only an average of a thimbleful of water a day was lost when a Fremen stillsuit was worn.
Fremen stillsuit discipline was very strict. Since an individual's body water belonged to the tribe, it was everybody's personal responsibility to preserve as much of their water as possible. Moreover, when crossing the Deep Desert, utter adherence to maximum stillsuit discipline was the only way one would have a chance of surviving. While in sietch, stillsuit discipline was more relaxed.
One of the measures of the progress of Arrakis' transformation from a desert planet to a lush world was when Fremen no longer required to wear stillsuits in the open air.