Wrinkled 'Skin' on Polymers

By controlling the direction and intensity of the ion beam, the researchers literally sculpted patterns on flat areas of polydimethylsiloxane, a silicon-based organic polymer. The technique has potential use for biological sensors and microfluidic devices and may offer new ways to build custom-made cell templates for tissue engineering.

"This technique is a one-step process for creating wrinkled skins," explains Ashkan Vaziri, Lecturer on Engineering and Research Associate in Applied Mechanics of Harvard Engineering and Applied Sciences. "The method is more robust compared with traditional techniques. The patterns can be generated along desired paths by simply controlling the relative movement of the ion beam and polymeric substrate. It's almost like using an airbrush on fabric. At a smaller scale the desired morphology of wrinkles can be achieved by controlling the ion beam intensity."

Because only the areas exposed to the beam are affected, the method enabled the scientists to create a variety of patterns - from simple one-dimensional wrinkles to peculiar and complex hierarchical nested wrinkles - along desired paths. Specific examples to date include "S" shapes, circular patterns, and long horizontal channels akin to the repeating tines of a closed zipper.

"Irradiation by the ion beam alters the chemical composition of the polymer close to its surface and forms a thin stiff skin which wants to expand," explains Vaziri. "The consequent mismatch between the mechanical strain of the generated stiff skin and the underlying polymeric substrate, almost like a tug-of-war, buckles the skin and forms the wrinkle patterns."

"We are approaching this field of research from various directions," says Vaziri. "At the moment we are looking at the effect of ion beam energy and have been able to reduce the wavelength of the wrinkles to 50 nanometers. Manipulation at such a small scale makes this method even more attractive. We are also building multifunctional microfluidic devices for the mixing of flow at very small scales and stretching of proteins and DNA. These new efforts, while at early stages of development, are very promising."

COMPAMED.de; Source: Harvard University