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4D printing to create shapeshifting robots that could swim like jellyfish or jump like crickets

15 June 2020

New “reactive 4D printing” creates objects that can take on alternate forms in response to changes in temperature, electric current, or stress.

Rice engineer Rafael Verduzco and graduate student Morgan Barnes led the development of a method to 3D-print materials that morph from one shape to another through application of temperature, electric current or stress. (Image: Jeff Fitlow)

The method could bring soft robots and biomedical implants that reconfigure themselves upon demand closer to reality.

The researchers first reported their ability to make morphing structures in a mould in 2018. However, using the same chemistry for 3D printing limited structures to shapes that sat in the same plane. That meant no bumps or other complex curvatures could be programmed as the alternate shape.

The process starts with a catalyst bath for the material, then deswelling, then deformation into the programmed under UV curing shape.

“Overcoming that limitation to decouple the printing process from shaping is a significant step toward more useful materials,” says Rafael Verduzco, an Associate Professor of chemical and biomolecular engineering and of materials science and nanoengineering at Rice University.

“These materials, once fabricated, will change shape autonomously,” Verduzco says. “We needed a method to control and define this shape change. Our simple idea was to use multiple reactions in sequence to print the material and then dictate how it would change shape. Rather than trying to do this all in one step, our approach gives more flexibility in controlling the initial and final shapes and also allows us to print complex structures.”

The lab’s challenge was to create a liquid crystal polymer “ink” that incorporates mutually exclusive sets of chemical links between molecules. One establishes the original printed shape, and the other can be set by physically manipulating the printed-and-dried material. Curing the alternate form under ultraviolet light locks in those links.

Once the two programmed forms are set, the material can then morph back and forth when, for instance, it’s heated or cooled.

The researchers had to find a polymer mix that could be printed in a catalyst bath and still hold its original programmed shape.

“There were a lot of parameters we had to optimise – from the solvents and catalyst used to degree of swelling, and ink formula – to allow the ink to solidify rapidly enough to print while not inhibiting the desired final shape actuation,” Barnes says.

One remaining limitation of the reactive 4D printing process is the ability to print unsupported structures, like columns. To do so would require a solution that gels just enough to support itself during printing, she says. Gaining that ability will allow researchers to print far more complex combinations of shapes.

“Future work will further optimise the printing formula and use scaffold-assisted printing techniques to create actuators that transition between two different complex shapes,” Barnes says. “This opens the door to printing soft robotics that could swim like a jellyfish, jump like a cricket, or transport liquids like the heart.”

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