While 3D printing is currently poised at the forefront of technology, 4D printing is nipping right at its heels. This next dimension in fabrication is really just 3D printing accentuated with the use of smart materials such as shape memory polymers (SMPs) that can be reformed into new shapes, and then counted on to revert back to their original shapes when an external stimulus, such as heat, is applied. (We’ve written a lot about 4D printing in the past, and you can find a quick, good overview here.)
One area that hasn’t seen much from SMPs is that of flexible electronics, due to inadequate processing techniques. A group of researchers from the Casali Center for Applied Chemistry at the Hebrew University of Jerusalem are in the process of integrating SMPs into the flexible electronics field, using 3D printing.
Matt Zarek, Michael Layani, Ido Cooperstein, Ela Sachyani, Daniel Cohn and Shlomo Magdassi have, for the first time, used an SLA 3D printer to make shape memory polymers. Until now, 4D printing has used low-level extrusion printers or high level multi-jet printers. The researchers at the Casali Center, by using an Asiga Pico Plus39 DLP SLA printer, were able to print SMPs at higher resolution than the more commonly used printers are capable of doing.
“Furthermore,” Layani told 3DPrint.com, “up until now 4D printed structures were based on responsive segments which were only a fraction of the total structure and limited the shape memory behavior to a specific actuated route, whereas our structures have total freedom to program the shape memory route.”
In a recent article submitted to the scientific journal Advanced Materials, the researchers explain how they set out to integrate SMPs into the field of flexible electronics. The current methods of 4D printing involve pre-programming specific parts of a structure to respond to a stimulus; for example, particular materials are designed to swell and expand when placed in water, while other materials rely on heat to stimulate movement.
For this study, described in the paper ‘3D Printing of Shape Memory Polymers for Flexible Electronic Devices,’ the researchers developed a process that would allow them to create a structure that could move its full form, rather than only particular parts. They created a customized heated resin bath, into which the print platform of the Asiga printer was lowered before the printing of each layer. By UV curing each layer at the bottom of the reservoir, they were able to avoid the effects of molecular oxygen inhibition. Several open source .stl files were downloaded and used to print basic models such as a bird, an Eiffel Tower, and a vascular stent.
The models, which were rigid and waxlike at room temperature, became pliant and re-shapable when they were raised above their melting temperature using a heat gun. Any alterations made to the shapes of the models were fixed by cooling them to below the melting temperature. When they were reheated, the shape memory was triggered and the models resumed their original shapes.
To apply the technique to flexible electronics, the SMPs were integrated into conductive materials to trigger mobility. A flat SMP circuit, printed with conductive silver nanoparticle ink, can be opened and closed by applying and withdrawing voltage to trigger its movement.
“Such responsive objects can be used in fabrication of soft robotics, minimal invasive medical devices, sensors, and wearable electronics,” said the researchers. “The use of 3D printing overcomes the poor processing characteristics of thermosets and enables complex geometries that are not easily accessible by other techniques.”
Below is a video of a dragon printed during the study, reverting to its original shape under heat stimulus at 70° C, at a 16x real speed: