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Little Hoops, Big Dreams
8 January 1998 7:00 pm
Stir a dash of salt together with the right polymer in a glass of solvent and add water. Like magic, the molecules will assemble themselves into a strange new structure--tiny masses of concentric, hollow rings, like barrels completely filled with hoops--as a group of physicists reports in the 22 December 1997 Physical Review Letters. The structures, less than 1000th of a millimeter long, might one day be used to deliver drugs in time-released doses or to build miniature electronic devices.
Under the right conditions, polymers (long molecular chains) will "self-assemble" into elaborate structures such as balls and rods. The electrostatic push and pull between the molecules drives the self-assembly, and the final shape depends on a competition between the polymers' resistance to bending and their aversion to having a sharp edge or end, says Adi Eisenberg, a McGill University chemist who worked on the project. In certain circumstances, for instance, a sheet of polymers will curl to form a hollow sphere, which doesn't have any edges.
Eisenberg and colleagues set out to coax a polymer of polystyrene connected to polyacrylic acid into new shapes by dissolving it in water and adding salt. The salt ions weaken the electrostatic repulsion between the polymer molecules, which reduces their resistance to bending, says Eisenberg. The researchers didn't expect to form hoops, however. "For months, we didn't know what we had made," Eisenberg recalls, but by making a series of electron microscope images at different angles, they discovered the circular structure of the particles. Eisenberg now says that hoops, some of which are less than 100 nanometers in diameter, are a logical structure for the chainlike molecules to form, because like spheres, they have no edges. The polymer first sticks together to form clumps, then the hoops form inside.
The hoops "are certainly interesting," says Robert Langer, a chemical engineer at the Massachusetts Institute of Technology. Langer, who has done pioneering work on time-released drugs, says the hoops could in principle be filled with medicine that would be slowly released as stomach acids gradually dissolve one layer of hoops after another, like a Russian doll being opened. But for that to work, he says, the hoops would probably have to be made of a different material. "It would be great," Langer says, "if they could show it were possible with other polymers."