Sample collection tubes and methods of producing the same are provided. Contemplated collection tubes comprise a tube having a separator substance disposed therein. In some aspects, the separator substance preferably maintains a predetermined flowability during irradiation or heat sterilization, and can subsequently polymerize upon exposure to a UV light or other suitable source. In other aspects, the separator substance preferably includes a soft gel component (thixotropic gel), and a photocurable sealant component that is formulated to polymerize and form a solid barrier between fractions of a liquid.