Polymer Wrinkled Skin

An American-Korean team developed a new method for making wrinkled hard skins on the surface of polymers employing a focused ion beam.

By controlling the direction and intensity of the ion beam, they literally carved patterns on flat areas of polydimethylsiloxane, a silicon-based organic polymer (the main ingredient in Silly Putty), by changing direction and intensity of an ion beam.

The find could have applications for biological sensors, microfluidic devices and 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, all 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."

The method can make a variety of patterns, from simple dimensional wrinkles to peculiar and complex hierarchical wrinkles with desired paths: "S" shapes, circular patterns, and long horizontal channels like the repeating pattern 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."

These patterns are suited for making microfluidic devices for particle separation and mixture or designing biosensors. The scientists also want to see how living cells behave on these patterned substrates, which would assess the potential for making templates for engineering and growing tissues. "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."

Photo credit: Moon et al