Issue Date: September 18, 2017 | Web Date: September 14, 2017
Electronics take shape
After being peeled from the platform of a three-dimensional printer, a flat electronic component begins to fold its four legs. In just a few minutes, it has folded into its desired form and can stand. Researchers can now print self-folding electronic devices like this one thanks to a new polymer ink that builds up mechanical stress during the 3-D-printing process (ACS Appl. Mater. Interfaces 2017, DOI: 10.1021/acsami.7b10443).
Using 3-D printing to make electronic devices with unusual shapes is quite challenging, says Subramanian Sundaram of Massachusetts Institute of Technology, who developed the new ink in collaboration with Ryan C. Hayward, a polymer scientist at the University of Massachusetts, Amherst.
Semiconductor inks, which are runny because of the solvents used to make them, must be printed on a flat surface or else they will dribble out of bounds. Researchers typically print electronic devices flat, then reshape them into a 3-D form. This reshaping involves heating and bending, or using printable materials that expand in the presence of water or another solvent. But exposure to liquids and heat isn’t good for electronics, Sundaram says.
The ink he and his colleagues developed is made up of long-chain and short-chain acrylic polymers. As the 3-D printer deposits layers of the ink, the short polymer chains from the new layer migrate into the underlying, partially cured layer deposited earlier. The ink layers swell, causing mechanical stress to build up as the polymer becomes rigid. That residual stress cannot be released until the printed piece is pried off the printing platform.
By alternating layers of the ink with rigid polyacrylate and controlling different parameters, the researchers can program particular folds into a printed object and control the folding angles within a few degrees. As a proof of principle, the group made a self-folding, four-legged electrochromic device, which changes color in response to an applied voltage.
“The idea of using mechanically active substrate materials to transform planar electronics into 3-D, curvilinear shapes is interesting,” says John A. Rogers, a materials scientist at Northwestern University. With further development, he says, researchers could use this method to make bioinspired devices and medical devices.
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