Friday, May 05, 2017

Self-Folding Structures Inspired by Origami

From the elegant crane to playful flowers, the intricate shapes created with origami are delightful and often astounding. They are also a source of inspiration for scientists. In areas ranging from microelectronics to biomedicine, there is a need for small, complicated three-dimensional objects. Last week in the journal Science Advances, a team of scientists from Georgia Institute of Technology and Peking University shared their work on an origami-inspired technique for creating such structures.

A new technique for producing self-folding three-dimensional origami
structures created these tiny samples, held in a hand for size comparison. 

Image Credit: Rob Felt, Georgia Tech.

The traditional art of origami involves folding a flat sheet of paper into an elaborate shape. It turns out that folding is not only an elegant way to transform paper into intricate shapes; it’s an elegant and versatile way to create three-dimensional forms from other materials too.

Imagine a doctor making a tiny incision and inserting a small, flat surgical tool that unfolds into a pair of forceps once inside the body. Imagine swallowing an ingestible robot that can patch a wound on the inside of the stomach. Imagine cheaper, faster, more efficient computers thanks to better-organized microelectronic structures. Scientists are currently working on these and other cutting-edge technologies using origami-inspired methods.

Research in these areas is progressing, but many techniques for creating folded structures require complicated setups or a multitude of tedious processing steps. In this new research, the team used a common LED projector, PowerPoint slides, and a polymer film to create self-folding, three-dimensional tables, capsules, birds, flowers, and other shapes, all about a half-inch in size.

The method is based on something called frontal photopolymerization. Although it sounds intimidating, the technique is fairly straightforward. You start with a layer of light-sensitive liquid resin. When you shine a light on the liquid, it starts to polymerize, or cure into a solid film. The area closest to the light cures first, and as the light penetrates deeper into the liquid, the rest of the liquid cures in succession.

The key to this origami-inspired technique is that as the liquid solidifies, the volume shrinks. Because the liquid solidifies nonuniformly, the volume shrinkage happens nonuniformly too. This creates stress in the material. In most applications where frontal photopolymerization is used, this stress causes unwanted distortions and weaknesses. However, the researchers realized, this stress can be harnessed.

The stress causes the film to want to bend, or fold, toward the side that most recently solidified. The amount of stress depends on several factors, including the brightness of the light and length of exposure. By designing a system that exposes the resin to carefully chosen patterns of light, the research noted, you can create a structure that will bend itself into a desired shape.

To make this a reality, the researchers turned to an LED projector and a PowerPoint slide. They created a grayscale pattern on the slide and projected it—not onto a screen, but onto a thin layer of liquid on top of a plate. The liquid was a mixture of a polymer, a substance that causes the polymer to cure when exposed to light, and photoabsorbers that attenuate light as it penetrates a material. The photoabsorbers make the stress even more nonuniform, but in a controllable way. After shining the projector on the liquid for a few seconds, the areas exposed to light cured into a solid film. The researchers removed the solid film from the plate and it folded itself right into the desired pattern.

According to the scientists, this process works well if you want to create a structure that only needs pieces to bend in one direction—like a table whose legs need to bend down—but if you want a structure that can fold in two directions (up and down), you need to illuminate the liquid from both directions. To accomplish this, the researchers used a similar process but injected the liquid into a flat mold. The liquid was exposed to light first from one side, and then the whole structure was turned around and the other side was illuminated.

In addition to building a practical setup, the researchers worked out a simplified mathematical model for the system. With it, they show how stress and flexibility are influenced by exposure time and light intensity. They also mapped out how the grayscale on their PowerPoint slides correlates to intensity. Other scientists can now use this information to design patterns and setups and build their own “origami” structures.

Although they are often considered to be opposite ends of the spectrum, art and science are not so far apart. Just as origami can transform a flat sheet of paper into beautiful, unexpected arrangements, science can transform a set of mathematical relationships into valuable, unexpected technologies. It’s pretty amazing what you can do with creative thinking and a few simple folds.

After a certain high level of technical skill is achieved, science and art tend to coalesce in esthetics, plasticity, and form. The greatest scientists are always artists as well.
—Albert Einstein

—Kendra Redmond

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