Membranes with ''chemical valves'' such as described here could be use
d for controlled drug release or chemical sensing. The membranes conta
in hexagonally packed and uniformly sized nanochannels that are partia
lly filled with poly(acrylic acid) (PAA), as shown in the Figure. A mo
de of operation is discussed for the large change in water permeabilit
y caused by variations in pH, ionic strength, and ion type.