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.