a polymer or other substrate optimized for growing cells is described , which takes the form of a micro - thin bag with gas permeable sides . sides of the bag can be held at a fixed distance from one another with a multitude of tiny micropillars or other spacers extending between them , keeping the bag at a predetermined thickness and preventing the bag from collapsing and the sides from sticking together . in other embodiments , the sides may be held apart by gas pressure alone . a 0 . 01 μm to 1000 μm parylene or other biocompatible coating over the bag outsides controls the permeability of the bag material and provides a bio - safe area for cell growth . an alternate configuration uses open - cell foam with skins coated with a biocompatible coating . tubes going into multiple bags can be connected to a manifold that delivers gaseous oxygen or removes carbon dioxide and other waste gases . multiple bags can be stacked together tightly , with o - ring spacers in between , and housed within a vessel to form a high - surface area , ultra - compact cell growing system . for cells growing on the bags , liquid nutrients can be fed by way of the tube spacers , and oxygen and waste gases permeated through the bag sides and transported within the bags .