Two populations of fluorescent neurons are connected through micro-tunnels, the geometry of which can be controlled to yield unidirectional connectivity. The microfluidic channels are wide enough for an axon to go through but the soma itself does not enter. Growth cones are seen at the front of the axons as they propagate forward through the micro channels. Only a fraction of neurons are fluorescent here.
Three populations device (two inputs and one output). Two and three populations devices constitute the building blocks of more elaborate devices.