Researchers at the University of Twente in the Netherlands have successfully adapted the precise art of origami down to the microscopic scale. Using only a drop of water, the scientists managed to fold flat sheets of silicon nitride into cubes, pyramids, half soccer-ball-shaped bowls and long triangular structures that resemble Toblerone chocolate bars.
“While making 3D structures is natural in everyday life, it has always been extremely difficult to do so in microfabrication, especially if you want to build a large number of structures cheaply,” explained Antoine Legrain, a graduate student at the MESA+ Institute for Nanotechnology at the University of Twente.
To help solve the challenge of building in miniature, researchers adopted a self-assembly technique, in which natural forces such as magnetism or surface tension trigger a shape change. As Legrain notes, self-assembly became a popular method in the 1990s to help cram even more computing power into shrinking electronic devices. Indeed, so-called solder assembly used the surface tension of melting solder to fold silicon, the electronic industry’s standard semiconductor material, into 3D shapes that more efficiently filled a small space with electrical components.
The University of Twente team also created silicon-based shapes, albeit with a more ubiquitous liquid – water – to activate and control the folding.
“Water is everywhere, is biocompatible, cheap, and easy to apply,” said Legrain. “Using water instead of solder also speeds up the folding of each individual structure. If the water-based process is further developed to fold multiple structures at once, it could become cheaper than current self-folding approaches.”
To create their three shapes, the researchers employed a custom software program to first design the flat starting pattern. They then “printed” the design onto silicon wafers, while hinges were inserted by etching away material just before depositing a thinner layer.
“Possible shapes are in principle limitless, as long as they can first be made on a flat surface,” Legrain pointed out.
To fold the designs, the researchers pumped a small amount of water through a channel in the silicon wafer. The capillary forces created by water molecules sticking to each other and to the silicon puledl the flat surfaces together to form fully three-dimensional creations. The team also discovered that the final structures, which are approximately the size of a grain of sand, can be opened and closed up to 60 times without signs of wear, as long as they remain wet.
The ability to unfold and refold the structures could be useful in biomedical applications. For example, self-folding tools could deliver drugs exactly where they are needed in the body or grab a tiny amount of tissue for a micro-biopsy.
“Cleanroom fabrication at research level can be long, tricky and frustrating. It is a good feeling when we obtain such nice results out of it,” Legrain added.
For now, creating soccer ball and Toblerone shapes are fun ways for researchers to test their system and better understand its capabilities. In the future, the team hopes to design conductive hinges and create 3D sensors with their new technique.
The article “Controllable elastocapillary folding of three-dimensional micro-objects through-wafer filling” is authored by A. Legrain, T. G. Janson, J. W. Berenschot, L. Abelmann and N. R. Tas. It was published in the Journal of Applied Physics on June 3, 2014 (DOI: 10.1063/1.4878460) and can be read here.
