Structures with field responsive permeation control

Devices are useful to capture or dispense target molecules and include an electric field-responsive valve unit. The valve unit includes an active control structure having at least one monomolecular layer and being up to about 10 nm thick. The active control structure is formed by a majority of molecular species with a dipolar moment greater than about 5 Debye, and operable in response to an electric field at a threshold value. A particularly preferred embodiment has the active control structure carried on a microporous membrane with pores in the nanometer range. Applications for inventive embodiments include drug delivery and target molecule capture during electrophoresis separations.