We've previously seen self-folding structures that are triggered by light, and it's a technique that works well for turning flat planes into simple 3D shapes like cubes and pyramids. In an effort to make these smart materials smarter, scientists have now developed a new method for making complex self-folding structures by varying the color of the light and the "hinges" so that each section of the material folds in a specific order.

The new study builds on a previous project from NC State University back in 2011, when researchers made 2D templates that could fold themselves into a given 3D shape when exposed to infrared light. The key was the bold black lines at the hinges, which would absorb more of the energy than the transparent material around them and contract, pulling that segment into the desired position. The team could control the speed and angle of the fold by adjusting the thickness of each line.

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One drawback of that scenario was that light exposure triggered all folds to begin at the same time. Now the researchers have developed a way to control the order that each fold is triggered by changing the color of both the light and the hinges.

"A longstanding challenge in the field has been finding a way to control the sequence in which a 2D sheet will fold itself into a 3D object," says Michael Dickey, co-corresponding author of the study. "And as anyone who has done origami – or folded their laundry – can tell you, the order in which you make the folds can be extremely important."

The new system works on the principle that different colors absorb different wavelengths of light at different rates. By printing hinges in certain colors of ink, the researchers can then expose the material to colored light that causes some, but not all, hinges to react. Say, for example, one hinge is yellow and another is blue. Shining blue light on the shape will make the yellow hinge fold but not the blue one, because the blue ink won't absorb the blue light. Red light, meanwhile, will trigger the blue hinge when it's needed.

This approach allows the team to build a 3D shape with a carefully choreographed light show of different colors, but a sequence of folds could also be controlled without changing the color of the light. Different ink colors will absorb certain wavelengths of light at different rates, meaning one hinge could react quickly while another takes its time.

Being able to give self-folding materials and structures a detailed set of instructions opens up a wide range of potential applications, from ingestible robots that unfold to perform a function in the body, to solar panel arrays that fold flat for blast-off and unfurl in space.

"This is a proof-of-concept paper, but it opens the door to a range of potential applications using a simple and inexpensive process," says Dickey. "Ultimately, people are interested in self-assembling structures for multiple reasons, from shipping things in a flat package and having them assemble on site to having devices self-assemble in 'clean' environments for medical or electronic applications."

The research was published in the journal Science Advances. The material can be seen folding itself in sequence in the video below.

Source: NC State University

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