A cryogen tube is a life sustaining device used in Life Stations aboard Galactica.
In addition to monitoring and suspending a patient's life functions, cryogen tubes are also used to quarantine infected patients, thereby preventing pathogens from infecting life sciences practitioners or med techs during treatment.
Presumably, the cryogen tube works through some form of freezing, given that ice crystals form atop a patients' epidermis inside the tube.
Those infected by the virus brought back from the Otarsis Quadrant asteroid are placed in these chambers (TOS: "Lost Planet of the Gods, Part I").
These tubes are also utilized to ferry the "Earthlings" to and from their shuttle, Lunar Avion, after its interception by Apollo, so as to prevent any potential cross-contamination (TOS: "Greetings from Earth").