Although membrane channel proteins are important to drug discovery and hold great promise as engineered nanopore sensing elements, their widespread application to these areas has been limited by difficulties in fabricating planar lipid-bilayer membranes. We present a method for forming these sub-5-nm-thick free-standing structures based on a self-assembly process driven by solvent extraction in a microfluidic channel. This facile automatable process forms high-quality membranes able to host channel proteins measurable at single-molecule conductance resolution.