Small tires, big dreams
Whisk a pinch of s alt along with the correct polymer in a glass of solvent and add water. The molecules will rearrange themselves into a strange structure - tiny clusters of concentric, hollow rings. The structures, which are less than 1/1000 mm in size, could one day be used to dispense medication into the body in graduated doses or to build miniature electronic devices. Under the right conditions, polymers (long chains of molecules) naturally turn into complicated structures, like balls or rods. This is caused by electrostatic pushing and pulling forces. The final shape depends on the outcome of the struggle between the polymers' resistance to bending and the need to avoid sharp corners and ends in their shape. Adi Eisenberg, a chemist at McGill University, supports this thesis. For example, under certain circumstances, a "sheet" of polymers curls up to form a hollow, edgeless sphere.
Eisenberg and his colleagues painstakingly created new shapes from a polystyrene polymer linked to polyacrylic acid by dissolving the polymer in water and adding s alt. According to Eisenberg, the s alt ions weaken the electrostatic repulsion between the polymer molecules, which reduces the flexural resistance of the polymers. Completely unexpected for the researchers, however, was the formation of tire-like shapes. "For months we didn't know what we had created," Eisenberg recalls. However, by taking multiple electron micrographs from different angles, they discovered the circular structure of the particles (Physical Review Letters, December 22, 1997 issue). Now, Eisenberg says the hoop structure, some of which is less than 100 nanometers in diameter, is a logical shape because hoops, like spheres, have no edges. First, the polymer clumps together, then the hoops form.
Robert Langer, a chemical engineer at the Massachusetts Institute of Technology who pioneered timed drug delivery, says the tires made of polymers are "certainly interesting". In principle, they could be filled with medicines, which would then be slowly released again as the stomach acid dissolves one annular layer after the other. But for this the tires would probably have to be made of a different material. In Langer's opinion, it would be a big step "if you could demonstrate that it also works with other polymers."