A new method of creating microscale wrinkled surfaces with precise sizes and patterns can be compared to the way raisin skins dry, becoming stiff and wrinkly as the pulp inside dries out. This basic method could be harnessed for a variety of useful structures, including microfluidic systems for biological research, sensing and diagnostics; new photonic devices that can control lightwaves; controllable adhesive surfaces; antireflective coatings; and antifouling surfaces that prevent microbial buildup. The process developed at MIT uses two layers of material. The bottom layer, or substrate, is a silicon-based polymer that can be stretched, like canvas mounted on a stretcher frame. Then, a second layer of polymeric material is deposited through an initiated chemical vapor deposition (iCVD) process in which the material is heated in a vacuum so that it vaporizes, then lands on the stretched surface and bonds tightly to it. Lastly, and most importantly, the stretching is released first in one direction and then in the other, rather than all at once. Images taken with a 3-D microscope show wrinkled surfaces produced using a method developed at MIT. The size, spacing and angles of the wrinkles vary depending on how much the original underlying surface was stretched, and how the stretching was released. (Image: Jose Luis Yagüe and Felice Frankel) If the tension were released all at once, a jumbled, chaotic pattern of wrinkles would result — much like the surface of a raisin. But in MIT’s controlled, stepwise release system, a perfectly orderly herringbone pattern emerges. The size and spacing of the herringbone ribs are determined by exactly how much the underlying material was originally stretched in each direction, the coating’s thickness and in which order the two directions are released. For the first time, the team demonstrated the ability to control the exact size, periodic spacing and angles in both directions. Fundamentally, "it's the same process that gives you your fingerprints," said Karen Gleason, the Alexander and I. Michael Kasser Professor of Chemical Engineering. But in this case, precise control over the resulting patterns requires the iCVD process, which Gleason and her colleagues have been developing for years. This gives a high degree of control over the thickness of layers deposited, and also enables control of the surface chemistry of the coatings. The iCVD method also provides the high degree of adhesion that is needed to form buckled patterns. Without sufficient adhesion, the surface layer would separate from the substrate. "One distinguishing feature of what we're showing is the ability to create deterministic two-dimensional patterns of wrinkles," such as a zigzag herringbone pattern, said Mary Boyce, the Ford Professor of Engineering and head of MIT's department of mechanical engineering. "The deterministic nature of these patterns is very powerful and yields principles for designing desired surface topologies." The method could be used to measure ultrathin-film material’s stiffness and thickness by analyzing its pattern, said postdoc Jose Luis Yagüe. Another possible application, the researchers say, is microfluidic devices such as those used to test for molecules in a biological sample, where tiny channels of precise dimensions must be produced on a surface. These could be used as sensors for contaminants, or as medical diagnostic devices. The patterns could also be used to make surfaces adhere to one another — and in this case, the degree of adhesion can be controlled. "You can dynamically tune the patterning — direct stretching or other actuation can be used to tune the pattern and corresponding properties actively during use," Boyce said, even letting surfaces return to perfectly flat. This could, for example, be used to provide secure bonding with quick-release capability or to actively alter reflectivity or wettability. The new method is very simple, consisting of just two or three steps, and it can be used to make larger-size patterned surfaces, the team said. No external templates are needed to create the pattern, said Jie Yin, the lead author. The study, funded by the King Fahd University of Petroleum and Minerals in Saudi Arabia, appeared in Advanced Materials. For more information, visit: www.mit.edu