We present the NERV, a nonmechanical, unidirectional valve, to control the locomotion of Caenorhabditis elegant (C. elegans) in microfluidic devices. This valve is created by establishing a region of lateral electric field which can be toggled between on and off states. We observed that C. elegans do not prefer to advance into this region when the field lines are facing their movement, so when they reach the boundary of the NERV, they partway enter the field, retreat, and switch direction. We...