The precise art of origami has been folded down to a microscopic scale.
Scientists have manipulated flat sheets of silicon nitride into cubes, pyramids, half soccer-ball-shaped bowls and long triangular structures which resemble Toblerone chocolate bars.
The geometric objects are almost too tiny to see with the naked eye.
“While making 3-D 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,” said Antoine Legrain, a graduate student at the MESA+ Institute for Nanotechnology at the University of Twente in the Netherlands.
To help solve the challenge of building in miniature, researchers turned to self-assembly in which natural forces such as magnetism or surface tension trigger a shape change.
In the 1990s self-assembly became a way to help cram even more computing power into shrinking electronic devices. So-called solder assembly used the surface tension of melting solder to fold silicon, the electronic industry’s standard semiconductor material, into 3-D shapes.
The University of Twente team also created silicon-based shapes, but they used water to activate and control the folding.
They describe their water-based folding system in a paper in the Journal of Applied Physics.
To create their menagerie of 3-D shapes, the researchers used a custom software program to first design the flat starting pattern.
They then printed the design onto silicon wafers. To create hinges, the researchers etched away material just before depositing a thinner layer.
To fold the designs, the researchers pumped a small amount of water through a channel they had left in the silicon wafer. The capillary forces created by water molecules sticking to each other and to the silicon pulled the flat surfaces together to form fully three-dimensional creations.
The team also discovered that the final structures, which are about 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. 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.
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