Hollow fiber sorbents for gas separation benefit from the use of a heat transfer fluid transported through their bores for thermal moderation and heat recovery. Additionally, for thermal swing processes, the heat transfer fluid is necessary to actuate the temperature swing. An impermeable yet thermally conductive barrier layer is needed on the inner channel wall to prevent the heat transfer fluid from the flue gas diffusing through the pore structure of the fiber. This paper discusses the challenges and different available techniques to establish a barrier layer on the lumen wall of a hollow fiber sorbent. In particular, we discuss the deposition of a polyacrylonitrile (PAN) barrier onto a Torlon-based fiber sorbent after the fiber sorbent has been spun. Of the various barrier formation materials and methods explored, this method was found to provide the most effective barrier, with helium permeance reduced from 25 000 GPU to less than 0.4 GPU, even at a pressure differential of 120 psi across the barrier. The heat transfer fluid typically used is water, and this barrier was also found to resist the transport of liquid water from the lumen to the shell side, thereby proving its efficacy in containing the heat transfer fluid to the fiber bore.