Frequently, when we hear about self-folding 3D printable structures, they’re based on origami. But researchers from the MIT Computer Science and Artificial Intelligence Laboratory (CSAIL) have built a self-folding printable device prototype that’s actually a variation on their 3D printable multimaterial electronic device that was inspired by the golden tortoise beetle. While other researchers have been investigating printable structures that fold themselves into 3D shapes once they’ve been immersed in water or exposed to heat, the CSAIL researchers have developed a printable structure that starts to fold itself up the instant it’s removed from the print bed, without needing to be exposed to any outside stimulus.

“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. These materials are often very, very sensitive to moisture and temperature,” explained Subramanian Sundaram, an MIT graduate student in electrical engineering and computer science (EECS) who also worked on the golden tortoise beetle project. “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.”

The team built a prototype that includes electrical leads, as well as an expanding polymer pixel that reacts when a voltage is applied to it by changing from transparent to opaque; the device looks sort of like an ‘H,’ until each of its four legs fold themselves into opposite directions, reminiscent of a table. To illustrate that they are able to control the angle of joint folding, the team also built different versions with the same hinge design and tested them by attaching the hinges to a weight to straighten them by force. Once the weight was removed, the hinges reverted back to their original folds.

What makes the design possible is an unusual new printer-ink material the researchers discovered that expands after it solidifies; typically, printer-ink materials contract as they solidify, which limits what designers can do. The CSAIL researchers deposited the expanding material at specific locations in the top and bottom layers of their prototypes – because the bottom layer adheres to the print platform, it can keep the device flat while the rest of the layers are 3D printed. But once the device is removed, the joints made from the material expand and bend the device.

The team published a paper on their research, titled “3D-Printed Self-Folding Electronics,” in the American Chemical Society’s journal Applied Materials and Interfaces; Sundaram was the first author, and co-authors include David Kim, a technical assistant in the Computational Fabrication Group (CFG); associate professor of EECS Marc Baldo; Ryan Hayward, a professor of polymer science and engineering at the University of Massachusetts Amherst; and Sundaram’s advisor Wojciech Matusik, who’s an MIT associate professor of EECS and runs the CFG.

As is so often the case when it comes to research breakthroughs, the team discovered the material by accident. The CFG mostly uses printer materials that are combinations of polymers and monomers – long molecules that are made up of chain-like repetitions of single molecular components. Matusik and his team often mix these components to make printer inks that possess certain physical properties, and while the CSAIL researchers were working to develop an ink that could make more flexible printed components, they created one that would slightly expand once it had hardened. They realized the importance of expanding polymers, and started to modify the mixture until they developed a specific recipe that allows them to create expanding joints that could fold a 3D printed device.

Example of an accelerated fold. [Image: Tom Buehler/CSAIL]

Hayward helped the team explain exactly how the material was able to expand. The ink which produces the expansion includes some long molecular chains, as well as a shorter one that’s made up of the isooctyl acrylate monomer. When an link layer is UV cured, the chains connect to form a “rigid thicket of tangled molecules.”

Schematic representation of spontaneously folding electronics achieved by multimaterial 3D printing. [Image: MIT CSAIL research paper]

As the layers are deposited on top of each other in the 3D printing process, these short chains in the top layer then sink into the lower layers and interact with the long molecular chains to apply expansion force; the printing platform adhesion temporarily resists this expansion.

The researchers hope that by continuing to study the material, and why it expands, they will be able to design material for specific applications, such as materials that resist the small contraction percentage that happens with many printed polymers after the curing process.

“This work is exciting because it provides a way to create functional electronics on 3-D objects. Typically, electronic processing is done in a planar, 2-D fashion and thus needs a flat surface,” said Michael Dickey, a professor of chemical engineering at North Carolina State University who was not involved with the CSAIL team’s research. “The work here provides a route to create electronics using more conventional planar techniques on a 2-D surface and then transform them into a 3-D shape, while retaining the function of the electronics. The transformation happens by a clever trick to build stress into the materials during printing.”

Possible applications for the team’s technique could allow for custom manufacturing of displays, sensors, and antennas which have functionalities that depend on their 3D shape; long-term goals include the possibility of printable robots.

[Source/Images: MIT]

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