Flicking the switch on a molecular gate

The use of external stimuli to manipulate the properties of well-defined materials may find applications in on-demand drug delivery, separation of molecules, sensing, smart coatings, or artificial tissues (1). Most of these applications rely on changes in the shape, stiffness, pore size, or other properties of soft materials in response to external pressure, temperature changes, or electric or magnetic fields. However, the lack of long-range order in polymers makes it difficult to control their porosity and, hence, permeability. On page 347 of this issue, Knebel et al. (2) instead manipulate metal-organic framework (MOF) membranes. They show that the membrane pore size can change upon exposure to an external electric field, enabling precise separation of different gas molecules.