When manufacturing medicines, it is especially important to provide a pure product. This task is often complicated because many drug molecules are produced in so-called chiral pairs (nonsuperposable mirror images) of which only one form is beneficial; the other may be useless or even harmful. A new technique detailed today in the journal Science provides a novel approach to this problem. Scientists describe a "smart" membrane containing tiny silica nanotubes that is capable of separating two forms of a cancer-fighting drug molecule.

Receptor sites in the human body are incredibly selective, which is why different chiral forms of the same molecule can have such a variety of consequences. Sang Bok Lee of the University of Florida and his colleagues exploited the inherent choosiness of antibodies in designing their nano-filter. The researchers first engineered an antibody that would attach only to the desired drug and leave its chiral counterpart, or enantiomer, untouched. The team then attached the antibodies to the inside walls of nanotubes composed of silica and inserted the tubes into the 35-nanometer-wide pores of a membrane. When exposed to a mixture of molecules, the antibodies latched on to the desired drug molecule and moved it along the tube "like a bucket brigade," according to study co-author Charles R. Martin of the University of Florida. As a result, the flow of the preferred molecule across the film was five times as fast as that of its undesirable mirror image. The membrane, Martin explains, "knows what molecule it wants and goes and grabs it."

It is still too early to evaluate the membrane's commercial potential; the scientists propose that it will be at least five years before industrial applications become feasible. Still, "it's a fascinating nanotechnology approach to a very difficult separation problem," says Chad A. Mirkin, director of the Institute for Nanotechnology at Northwestern University. "If it could be scaled up, it could have enormous potential."