New 3D printed structures fold itself

New 3D printed structures can fold itself

7:18 AM, 20th September 2017
A new method produces a printable structure that begins to fold itself up as soon as it’s peeled off the printing platform.
A new method produces a printable structure that begins to fold itself up as soon as it’s peeled off the printing platform.

CAMBRIDGE, US: A team of researchers from MIT’s computer science and Artificial Intelligence Laboratory (CSAIL) and colleagues have reported something new: a printable structure that begins to fold itself up as soon as it’s peeled off the printing platform.

The study appears in the journal Applied Materials and Interfaces.

One of the big advantages of devices that self-fold without any outside stimulus, the researchers said, is that they can involve a wider range of materials and more delicate structures.

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.

“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. 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,” said Subramanian Sundaram, an MIT graduate student in electrical engineering and computer science and first author on the paper.

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 on 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.

While trying to develop an ink that yielded more flexible printed components, the CSAIL researchers inadvertently hit upon one that expanded slightly after it hardened. They immediately recognized the potential utility of expanding polymers and began experimenting with modifications of the mixture, until they arrived at a recipe that let them build joints that would expand enough to fold a printed device in half.

The ink that produces the most forceful expansion includes several long molecular chains and one much shorter chain, made up of the monomer isooctyl acrylate. When a layer of the ink is exposed to ultraviolet light — or “cured,” a process commonly used in 3-D printing to harden materials deposited as liquids — the long chains connect to each other, producing a rigid thicket of tangled molecules.

When another layer of the material is deposited on top of the first, the small chains of isooctyl acrylate in the top, liquid layer sink down into the lower, more rigid layer. There, they interact with the longer chains to exert an expansive force, which the adhesion to the printing platform temporarily resists.

“This work is exciting because it provides a way to create functional electronics on 3-D objects. 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," said Michael Dickey, a professor of chemical engineering at North Carolina State University.

Coauthors on the paper are Wojciech Matusik, an associate professor of electrical engineering and computer science (EECS) at MIT; Marc Baldo, a professor of EECS, who specializes in organic electronics; David Kim, a technical assistant in Matusik’s Computational Fabrication Group; and Ryan Hayward, a professor of polymer science and engineering at the University of Massachusetts at Amherst.

© MIT

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