Wrinkled 'Skin' Created on Polymers

By controlling the direction and intensity of a focused ion beam, researchers have sculpted patterns on flat areas of a silicon-based organic polymer in much the same way that an airbrush moves over fabric. This new method for developing wrinkled hard skins on the surfaces of polymers could potentially be used to make biosensors, microfluidic devices and to help build custom-made cell templates for tissue engineering.

The work is a collaboration among applied scientists at Harvard University and Seoul National University (SNU) in Korea. The researchers sculpted the patterns on flat areas of polydimethylsiloxane, a silicon-based organic polymer more commonly known as the primary ingredient in Silly Putty.

The Harvard group consisted of John W. Hutchinson, the Abbott and James Lawrence Professor of Engineering; Myoung-Woon Moon, post-doctoral Fellow; and Ashkan Vaziri, lecturer on engineering and research associate in applied mechanics, all of Harvard Engineering and Applied Sciences. They have filed for a US patent.

"This technique is a one-step process for creating wrinkled skins," said Vaziri. "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," Vaziri said. "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."

The patterns can be used in the construction of microfluidic devices for particle separation and mixture and also have potential use in designing biological senors. The researchers have also started a close collaboration with scientists at the Harvard-MIT Division of Health Sciences and Technology aimed at exploring the behavior of living cells on these patterned substrates. Such research may lead to the development of an effective and robust method to build custom templates for engineering and growing tissues, the scientists said.

"We are approaching this field of research from various directions," said 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."

Vaziri, Moon and Hutchinson's co-authors are Sang Hoon Lee, Jeong-Yun Sun, and Kyu Hwan Oh, all from the School of Materials Science and Engineering at SNU. The research was recently published in the Proceedings of the National Academy of Sciences and was supported in part by the Korea Research Foundation, the Center for Advanced Materials Processing of the 21st Century Frontier R&D Program, the Office of Naval Research, and Harvard Engineering and Applied Sciences.

For more information, visit: www.deas.harvard.edu