Microscopic barrels, bullets and worms of plastic

Michael J. Lander

Molded, stretched or extruded into shape, polymeric materials make up items from microscope knobs to culture plates. Increasingly, shaped polymers are playing a role at the micro- and nanoscale as well. But controlling the appearance of structures of this size can prove daunting, as few methods exist to finely manipulate plastic substances into predictable and reproducible forms.

Extensively stretching the film along the X- and Y-axes prior to liquefying the plastic pellets caused biconvex lens-shaped disks to emerge.

At the University of California, Santa Barbara, researchers led by Samir Mitragotri have synthesized nonspherical polymer particles using two related methods. Particles formed possess multiple shapes and distinct three-dimensional definition, unlike the varied but comparatively flat forms possible with photolithographic techniques.

Particles with a flat margin and bulged center — structures the researchers dubbed UFOs — arose when the film surrounding chemically liquefied polymer was stretched moderately in the X and Y directions simultaneously. Images have been colorized and are courtesy of Julie Champion, California Institute of Technology.

To begin both processes, the scientists started with sheets of water-soluble thin film containing spherical, water-insoluble polymer particles. One way of altering the spheres’ shape, they discovered, was to liquefy the polymer using heat or a solvent, to stretch the sheets, to allow the polymer to harden and, subsequently, to dissolve the film. The other method involved stretching the film prior to liquefying the particles, which created voids around them that they filled once melted or dissolved. With an FEI Co. scanning electron microscope, the researchers found that the products of each procedure were unique.

When the sheet of film was stretched and released before the polymer liquefied, it relaxed and took on a wrinkled appearance, which subsequently was transferred to the particles.

Aside from the sequential order of film pulling and melting, the main differences in the particles’ final shape resulted from polymer viscosity and degree of stretching. Solvents, for example, converted the polymer into thin syrup that readily filled stretching-induced voids, resulting in particles with pointier ends or thinner edges than those produced through heating. Variations of the schemes yielded particles with wrinkles or holes. A summary of the work and micrographs of many of the particles synthesized appear in the July 17 issue of the Proceedings of the National Academy of Sciences.

Pulling the film around mildly heated polymer beads along one axis resulted in the formation of long rectangular disks. Squared ends are the result of the material’s high viscosity at low temperatures.

Having begun the project to study the effect of shape on the role of polymer particles for drug delivery, the researchers plan to design nanoparticles whose shape makes them less prone to engulfment by macrophages and, thus, more likely to reach their intended destination. The scientists suggest that plastic pellets shaped like bacteria and functionalized with protein and other molecules could act as models in biological and environmental experiments.