Sep 13, 2017 | By Benedict

MIT researchers have developed a technique for 3D printing expanding polymers that self-fold without heating or water. The technique could be used for printed electronics and, someday, to make 3D printed robots.

If you 3D printed an object and it immediately folded in on itself on the print bed, you’d probably be worried. Did I use the wrong filament? Is there crazy humidity in the air? Am i seeing things? But for a group of MIT researchers, self-folding 3D printed structures could be incredibly useful.

The researchers, who are developing a 3D printing technique for self-folding structures, say that devices that fold without outside stimulus—heating, water immersion etc.—can provide key advantages over typical printed objects. They can be used to make delicate structures, for example, and can be made with a wider variety of materials.

Self-folding structures can even be particularly useful for printed electronics. Why? Just try bending an electronic device with water immersion and see where that gets you.

“These materials are often very, very sensitive to moisture and temperature,” explains Subramanian Sundaram, an MIT graduate student and first author of the study. “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 solution—which, according to the MIT researchers, came about entirely by accident—is a 3D printed self-folding device, made with electrical leads and a polymer “pixel” that goes from transparent to opaque when a voltage is applied to it.

If that process sounds familiar, you might have read about it in a previous project by the MIT group involving 3D printed robot skin. In that project, 3D printed “goldbugs” were made which turn from gold to red when prodded.

But instead of changing color, these new 3D printed devices change shape, contorting their rigid "H" shape into a table-like form. And because the researchers can control the precise angle at which a joint of the device folds, they have been able to create a number of different versions which deform in different ways.

One of the most important aspects of the study (and the thing that was discovered accidentally) is the use of a new printer-ink material that displays the unusual trait of expanding after it solidifies. Most printer-ink materials contract slightly as they solidify—something that makers will know all about.

Another thing makers will know lots about is the phenomenon of prints sticking to the print bed. This can be really annoying when you have to get the scraper out, but in this particular study it proved incredibly useful.

That’s because the printed structure’s adherence to the print bed keeps the object exactly where it needs to be during printing. If there was no stickiness at the bottom, the structure would start folding before printing was complete, throwing the layers out of line.

As it stands, however, printing the ink works smoothy. Then, as soon as the structure is peeled off the bed, the expanding material in either the top or bottom few layers starts to take effect, folding up the structure.

The “folding” behavior of the 3D printed device results from chemical interactions within the ink. The 3D printable ink contains several long molecular chains and one much shorter chain, made up of the monomer isooctyl acrylate. When cured with UV light, the long chains connect to each other, producing what the MIT researchers describe as a “rigid thicket of tangled molecules.”

The small chains of isooctyl acrylate in the liquid upper layer sink down into the lower, more rigid layer when another layer of material is deposited on top, causing an expansive force.

But the researchers want to learn more about how this process works, so that they might better be able to control it and harness its power.

At present, the researchers hope to use the technique to make sensors, displays, or antennas “whose functionality depends on their three-dimensional shape.” The method could also someday be used to make 3D printed robots.

The researchers—Sundaram, advisor Wojciech Matusik, Marc Baldo, David Kim, and Ryan Hayward—have had their study published in the American Chemical Society’s journal Applied Materials and Interfaces. It can be read here.



Posted in 3D Printing Application



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