The release of synaptogenic factors by the nerve terminal plays a central role in the aggregation of neurotransmitter receptors at the postsynaptic membrane, a precisely timed and localized process that is essential for the correct formation and functioning of the synapse. This process has been difficult to re-capitulate in cell culture because present cell stimulation methods do not have sufficient spatiotemporal control of the delivery of soluble signals. We cultured myotubes atop nanofabricated planar apertures (2â 8 µm diameter) to focally stimulate the muscle cell membrane with neural agrin, a synaptogenic factor released by motor neurons during development. Focal agrin delivery through the apertures after myotube fusion results in local aggregation of acetylcholine receptors (AChRs) in the vicinity of the apertures, a process reminiscent of AChR clustering at innervation sites. Since the apertures are spatially organized in microarrays, multiple experiments can be run in parallel on one device. The technique has wide applicability in cellâ cell communication studies and cell-based bioassays.