A gas separation unit 102, 200, 300 for permeating a gas out from a pressurized feed mixture includes an input manifold 104, 204, an exhaust manifold, 106, 206 and a permeate assembly 108, 208, 303. The permeate assembly supports one or more permselective foils 130, 132, 218, 232, 318 over a hollow cavity 134, 272, 306 supported by a microscreen element 142, 144, 228, 230, 326. The microscreen element includes non-porous perimeter walls 190, 192, 278 supported on a frame surface and a porous central area 194, 280 supported over the hollow cavity. A porous spacer 138, 140, 174, 234 disposed inside the hollow cavity structurally supports the entire microscreen surface spanning the hollow cavity while also providing a void volume for receiving fluid passing through the porous central area and for conveying the fluid through the hollow cavity.