Pile-o'-polymers breaks up on command.

Many children like to stack wooden blocks into towers. Most of those kids also like busting those edifices apart.

Likewise, materials scientists have been playing with ways to stack one-molecule-thick layers of electrically charged polymers. By controlling which polymers are in what layers, the experimenters have improved the properties of prototype, plastic-based electronic components and other devices. Now, Steve Granick of the University of Illinois at Urbana-Champaign and one of his colleagues have shown that they can trigger such stacks of polymers to break apart on cue.

The propensity to disintegrate under predictable conditions could make such structures useful for releasing drugs, pesticides, or other active compounds where and when they're needed, argue Granick and Svetlana A. Sukhishvili of Stevens Institute of Technology in Hoboken, N.J. The materials could also prove valuable whenever "you need to temporarily protect a surface from an unfavorable environment," such as in certain stages of manufacturing drugs, Sukhishvili says.

To make stacks with a built-in breakdown feature, the researchers used polymers for which the bonds between layers break in the presence of ions. In the Oct. 4 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, the scientists report that their piled polymers crumbled when the acidity of the surrounding liquid decreased a specified amount. An electric field can also destroy the stacks, they showed. With the help of shocking-pink dye, the experimenters demonstrated that compounds interspersed among the layers are released during breakdown.

--P.W.