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A new method produces a printable structure that begins to fold itself up as soon as it’s peeled off the printing platform. (Credit: Courtesy of the researchers)

MIT researchers have developed a way to print flat electronics that can fold themselves into a desired shape. The researchers say the development could have applications in robotics and human-machine interfaces.

Up to now, folding printed devices into their desired shapes has required additional processing steps or specific conditions — such as light exposure or dunking the pieces into liquids — which is not always a good option for electronic products. To address these limitations, MIT researchers wanted to come up with a more practical approach.

The researchers formulated a new ink containing acrylate monomers and oligomers that can be cured with ultraviolet light. Energy is stored in specific regions of the printed part in the form of residual stress during the printing process. After the flat device is printed and removed from the printer platform, swelling forces cause it to fold itself into a predetermined shape without additional stimulus.

One of the big advantages of devices that self-fold without any outside stimulus, the researchers say, is that they can involve a wider range of materials and more delicate structures. “If you want to add printed electronics, you’re generally going to be using some organic materials, because a majority of printed electronics rely on them,” says Subramanian Sundaram, an MIT graduate student in electrical engineering and computer science. “These materials are often very, very sensitive to moisture and temperature. So if you have these electronics and parts, and you want to initiate folds in them, you wouldn’t want to dunk them in water or heat them, because then your electronics are going to degrade.”

To illustrate this idea, the researchers built a prototype self-folding printable device that includes electrical leads and a polymer “pixel” that changes from transparent to opaque when a voltage is applied to it. The device starts out looking something like the letter “H.” But each of the legs of the H folds itself in two different directions, producing a tabletop shape.

The researchers also built several different versions of the same basic hinge design, which show that they can control the precise angle at which a joint folds. In tests, they forcibly straightened the hinges by attaching them to a weight, but when the weight was removed, the hinges resumed their original folds. In the short term, the technique could enable the custom manufacture of sensors, displays, or antennas whose functionality depends on their three-dimensional shape. Longer term, the researchers envision the possibility of printable robots.

The key to the researchers’ design is a new printer-ink material that expands after it solidifies, which is unusual. Most printer-ink materials contract slightly as they solidify, a technical limitation that designers frequently have to work around.

Printed devices are built up in layers, and in their prototypes the MIT researchers deposit their expanding material at precise locations in either the top or bottom few layers. The bottom layer adheres slightly to the printer platform, and that adhesion is enough to hold the device flat as the layers are built up. But as soon as the finished device is peeled off the platform, the joints made from the new material begin to expand, bending the device in the opposite direction.

The researchers hope that a better theoretical understanding of the reason for the material’s expansion will enable them to design material tailored to specific applications. “This work is exciting because it provides a way to create functional electronics on 3D objects,” says Michael Dickey, a professor of chemical engineering at North Carolina State University. “Typically, electronic processing is done in a planar, 2D fashion and thus needs a flat surface. The work here provides a route to create electronics using more conventional planar techniques on a 2D surface and then transform them into a 3D shape, while retaining the function of the electronics. The transformation happens by a clever trick to build stress into the materials during printing.”

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