Patterned Tapes Fold Into Devices

Current microfabrication techniques are ideal for making linear or two-dimensional planar objects, but three-dimensional objects are a challenge. Most fabrication techniques lead only to specific types of 3-D structures.

A method that harnesses the tools of planar fabrication to make 3-D objects with arbitrary shapes now has been described by chemistry professor George M. Whitesides, graduate student Derek A. Bruzewicz, and coworkers at Harvard University (J. Am. Chem. Soc., published online June 29, dx.doi.org/10.1021/ja062973q).

"Investment in planar microfabrication has been too large, and the technology has grown too sophisticated and effective to abandon for an entirely new technology that might only marginally improve 3-D fabrication," the researchers write.

The researchers were inspired by the folding of biological molecules such as proteins, which have linear structures that dictate how they will fold. "Proteins fold by having a 'tape' with the information for folding coded in shapes—amino acids—and 'sticky' regions-hydrogen bonds and hydrophobic regions," Whitesides says. "The underlying concept is the same, although the tape that we work with is very much simplified. We have shapes and sticky regions."

In Whitesides' method, the shapes are provided by metal patterning on a flexible planar surface, and the sticky regions are made of solder. The researchers start with two strips of Mylar tape. One is patterned with copper and crimped into zigzag shapes. A second flat piece of tape is attached to the first and the metal pattern is dip-coated with solder. The structure is suspended and agitated in water hotter than the melting point of the solder. Analogous to protein folding, the fusion of solder on neighboring faces of the crimped tape minimizes the free energy of the interface between the solder and the water, driving the tape to fold into a 3-D structure. By attaching light-detecting diodes to the uncrimped tape, Whitesides and coworkers construct a device that can detect light from any direction.

"Making complicated 3-D microstructures is a very challenging and expensive task based on the existing fabrication techniques," says L. James Lee, a chemical engineer at Ohio State University. "A proper integration of 2-D patterning and the self-folding concept is a very attractive approach to solving this problem because the 2-D patterning techniques in both micro- and nanoscale are well-developed and widely available."

Jennifer A. Lewis, a materials scientist at the University of Illinois, Urbana-Champaign, says Whitesides and coworkers "describe a highly creative method for achieving arbitrary 3-D shapes, which is inspired by biomacromolecular folding. This process must be scaled down for it to be applicable in commercial device fabrication. Nonetheless, their work is an important first step toward creating functional devices by guided self-assembly."