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Printed Origami Offers New Technique for Small, Complex Structures

Although it looks small and unassuming, a tiny origami crane, roughly the size of a penny and made of titanium, heralds a new method for creating complex three-dimensional structures for biocompatible devices, microscaffolding and other microsystems.

Unlike traditional paper origami cranes, this graceful bird began as a printed sheet of titanium hydride ink. The method, the result of a collaboration between Northwestern University and University of Illinois at Urbana-Champaign researchers, is published online by the journal Advanced Materials.

Small, intricate shapes made of metals, ceramics or polymers have a variety of applications, from biomedical devices to electronics to rapid prototyping, but they are difficult to create.

One method of fabricating such structures is by direct-write assembly in which a large printer deposits inks containing metallic, ceramic or plastic particles to assemble a structure layer by layer. Then, the structure is annealed at a high temperature to evaporate the liquid in the ink and bond the particles, leaving a solid object. The trouble is that as more layers are added the lower layers tend to sag or collapse.

Having encountered this problem while trying to manufacture titanium scaffolds for tissue engineering, the U. of I. researchers decided to try a different approach: Print a flat sheet, then roll it up into a spiral -- or even fold it into an assortment of shapes. But folding the printed sheets, which can become stiff and crack, was not easy.

The challenge, then, was to find a solution that would render the printed sheets pliable enough to manipulate yet firm enough to retain their shape after folding and during annealing. The method mimics wet-folding origami, in which paper is partially wet to enhance its foldability.

The Illinois researchers, led by Jennifer Lewis, the Thurnauer Professor of Materials Science and Engineering, used a mixture of fast-drying and slow-drying solvents in the ink so the sheet dries partway but stays flexible enough to fold through multiple steps -- 15, in the case of the crane.

In their first demonstration of this method, they used inks containing titanium hydride particles, a material suggested by Northwestern's David Dunand, the James and Margie Krebs Professor of Materials Science at the McCormick School of Engineering and Applied Science.

"I knew how to transform titanium hydride into metallic titanium without contamination from the ink, based on prior research in my lab," said Dunand who annealed the soft, titanium hydride origami structures into strong, metallic titanium objects.

The marriage of printing and origami techniques allows for greater structural complexity -- such as the crane's overhanging wings, a feature not producible by direct printing methods alone. In addition, the researchers can print sheets with a variety of patterns, adding yet another level of architectural detail.

"By combining these methods, you can rapidly assemble very complex structures that simply cannot be made by conventional fabrication methods," Lewis said.

Next, the team hopes to expand its origami repertoire to include much larger and much smaller structures, with an expanding array of inks. For example, the method can be extended to a variety of other ceramics and metals ranging from steels to nickel- and cobalt-based alloys to refractory and noble metals, according to Dunand.

The power of the new method is just starting to unfold, with the researchers planning to explore possible applications including lightweight structures, biomedical devices, sensors and more.

The Advanced Materials paper is titled "Printed Origami Structures." In addition to Dunand and Lewis, authors of the paper are Bok Yeop Ahn and Christopher J. Hansen, of the University of Illinois at Urbana-Champaign; Daisuke Shoji, of Hoya Corporation, Tokyo; and Eunji Hong, of Kookmin University, Seoul, who was at Northwestern as a visiting student during the study.

- Megan Fellman

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