In the past two decades, synthesis of responsive membranes with pores that could be opened and closed by changing chemical or physical properties of their environment has been the subject of many publications (see Ulbricht's work for a recent review). [1][2][3][4][5][6][7][8] In most studies the variable pore permeability was attained by the surface modification of commercial microfiltration membranes using polymers that expand or contract in response to external stimuli. Radiation-and plasma-induced graft polymerizations were employed to immobilize a monolayer of surface-attached responsive polymer chains (brushlike layers) or crosslinked polymer networks (gels) on the membrane and/or pore surfaces. Stimuli-induced changes of the conformation of the grafted-polymer chains affected the permeability of nanometer-sized pores in the membranes. Responsive gels were used to fill the interior of larger submicrometer/micrometer pores and regulate the membrane permeability. Membranes sensitive to changes in temperature, pH, ionic strength, light intensity, reduction-oxidation state of functional groups, and concentration of various substances have thus been fabricated based on the above-mentioned principles. The application of such stimulus-responsive membranes or "chemical valves" (functional gates) includes flow control, size-selective filtration, chemical and bioseparation, controlled release of chemical substances and drugs, and chemical sensors.In the present study, we report on a novel method for the fabrication of flexible stimulus-responsive polymer gel (PG) membranes. These membranes are thin porous films made of a crosslinked polyelectrolyte. Past research has focused on porous thin-film membranes made of polyelectrolyte complexes. [9,10] In our approach the porous films are formed via phase separation of a polyelectrolyte and a volatile additive. This approach provides a broad possibility to regulate pore sizes and the membrane responsiveness. In our method, the PG membranes can be prepared on any flat substrate with a low surface roughness (e.g., Si wafer); afterward, the membrane can be transferred (and attached chemically, if necessary) onto various porous or nonporous supports (with flat, profiled, and even curved surfaces, e.g., membrane filter, fabrics, chemical sensor, or human skin). The fabrication of these membranes consists of three very simple steps, described in detail below.The PG membranes operate in a similar manner to those prepared by the grafting of polymers on the surface of porous substrates. That is, 3D swelling of the PG upon an external stimulus leads to shrinkage of the pores and, consequently, to the regulation of the membrane permeability. However, the PG membranes, unlike the membranes reported in literature, respond via swelling and shrinking of the entire body of the membrane. The shrinking allows for the stimuli-responsive mechanism of pore-size regulation in a very broad range, from completely closed pores (pore opening size = 0) to large open pores (pore opening size = 0.3 lm)....