The invention provides a membrane comprising tubes extending through a polymer, wherein substantially all of the tubes are parallel with each other. Also provided is a method for producing a membrane, the method comprising: placing tubes on a substrate, subjecting the tubes to a magnetic field for a time and at a magnetic field strength to cause the tubes to align parallel with each other while simultaneously causing depending ends of the tubes to embed within the substrate; applying polymer to the tubes and substrate in an amount to affix the tubes relative to each other and relative to the substrate, and applying an etchant that cleaves a specific type of the bonds within the polymer to unblock the upstream ends of the nanotubes.