A nonmechanical, unidirectional valve, the Nematode Electrotactic-Response Valve (NERV), has been developed to control the motion of Caenorhabditis elegant (C. elegans) in microfluidic devices. By forming a region of lateral electric field, which can be switched or reversed, the NERV prevents worms from advancing through it if the field lines are angled against the movement of the worms. Upon arriving at the boundary of the NERV, the worms penetrate the field area briefly and then retreat and...