A crown ether is a molecule containing hydrogen, carbon and oxygen atoms. Each oxygen atom is bound between two of the carbon atoms and arranged in a ring (hence "crown").
The original crown ether discovered by Charles Pedersen has six oxygen atoms exposed along the inside wall of the ring. When atoms of certain metalic elements such as sodium or potassium pass through the center of the ring, they attach themselves to the exposed oxygen atoms and fit like a key in a lock.
The crown compound then acts as "host," taking its "guest" to a place where it would not otherwise go - for instance, through the membrane that forms the wall of a cell. The high degree of selectivity enables the crown compound to "identify" the guest atom in a solution and wrap around it.
In accepting specific atoms lock and key fashion, crown ethers mimic in a relatively uncomplicated way the very complicated functions of biological materials such as enzymes. It is this mimicry that has scientists so excited. The study of crown compounds may indicate new approaches for developing pharmaceutical systems or a way to cross the blood-brain barrier. They may help explain how the body moves sodium and potassium, essential elements for life, into cells.
Crown ethers also have potential to be used as "scavengers" to remove certain elements like radioactive strontium from the environment or to regulate concentrations of sodium in the blood. Some people have even suggested that crown compounds could be used one day to extract uranium or gold from seawater.