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Novel 4D printing method blossoms from botanical inspiration

26 January 2016

The 4D-printed shape-shifting composite structures produced by Harvard researchers could lead to smart textiles, soft electronics and new biomedical devices.

Printing under way for the hydrogel composite structures (courtesy of the Wyss Institute at Harvard University)

A team of scientists at the Harvard School of Engineering and Applied Sciences (SEAS) and the Wyss Institute for Biologically Inspired Engineering at Harvard University has extended its microscale 3D printing technology to the fourth dimension: time.

Inspired by natural structures like plants, which change their form over time according to environmental stimuli, the team has unveiled 4D-printed hydrogel composite structures that change shape upon immersion in water.

“This work represents an elegant advance in programmable materials assembly, made possible by a multidisciplinary approach,” says SEAS' Professor Jennifer Lewis, the senior author of a paper describing th work in the journal, Nature Materials. “We have now gone beyond integrating form and function to create transformable architectures.”

In nature, flowers and plants have tissue composition and microstructures that result in dynamic morphologies that change according to their environments. Mimicking the variety of shape changes that plant organs such as tendrils, leaves, and flowers undergo in response to environmental stimuli such as humidity and/or temperature, the 4D-printed hydrogel composites developed by Lewis and her team are programmed to contain precise, localised swelling behaviours. The hydrogel composites contain cellulose fibrils that are derived from wood and are similar to the microstructures that enable shape changes in plants.

These images, courtesy of the Wyss Institute at Harvard University, shows the transformation of a 4D-printed hydrogel composite structure after its submersion in water

By aligning cellulose fibrils during printing, the hydrogel composite ink is encoded with anisotropic swelling and stiffness, which can be patterned to produce intricate shape changes. The anisotropic nature of the cellulose fibrils gives rise to varied directional properties that can be predicted and controlled.

Like wood, which can be split more easily along the grain than across it, the hydrogel-cellulose fibril ink undergoes differential swelling behaviour along and orthogonal to the printing path when immersed in water. Combined with a proprietary mathematical model developed by the team that predicts how a 4D object must be printed to achieve prescribed transformable shapes, the method opens up a potential range of applications for 4D printing, including smart textiles, soft electronics, biomedical devices, and tissue engineering.

“Using one composite ink printed in a single step, we can achieve shape-changing hydrogel geometries containing more complexity than any other technique, and we can do so simply by modifying the print path,” says Sydney Gladman, a graduate research assistant and co-author of the Nature Materials paper. “What’s more, we can interchange different materials to tune for properties such as conductivity or biocompatibility.”

The composite ink that the team uses flows like liquid through the printhead, yet rapidly solidifies once printed. A variety of hydrogel materials can be used interchangeably resulting in different stimuli-responsive behaviour, while the cellulose fibrils can be replaced with other anisotropic fillers of choice, including conductive fillers.

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