A team of scientists at the Wyss Institute for Biologically Inspired Engineering at Harvard University and the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) has evolved their microscale 3-D printing technology to the fourth dimension, time.

Inspired by natural structures like plants, which respond and change their form in response to environmental stimuli, the team has unveiled 4D-printed hydrogel composite structures that change shape when immersed in water.

Mimicking nature in 4D

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The method was inspired by the way plants change shape over time in response to environmental stimuli. This orchid-shaped structure is printed with a hydrogel composite ink containing aligned cellulose fibrils, which enable anisotropic swelling. A proprietary mathematical model developed by the team precisely predicts how the fibrils will swell in water. After printing, the 4D orchid is immersed in water to activate its shape transformation. Credit: A.S. Gladman, E. Matsumoto, L.K. Sanders, and J.A. Lewis / Wyss Institute at Harvard University

“This work represents an elegant advance in programmable materials assembly, made possible by a multidisciplinary approach,” said Jennifer Lewis, senior author on the new study. “We have now gone beyond integrating form and function to create transformable architectures.”

Lewis is a core faculty member at the Wyss Institute and the Hansjörg Wyss Professor of Biologically Inspired Engineering at SEAS. Another Wyss core faculty member, L. Mahadevan, the Lola England de Valpine Professor of Applied Mathematics, professor of organismic and evolutionary biology, and professor of physics at Harvard University and SEAS, is a co-author on the study. Their team also includes co-author Ralph Nuzzo, the G.L. Clark Professor of Chemistry at the University of Illinois at Urbana-Champaign.

In nature, the tissue composition and microstructures of flowers and plants have 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 or temperature, the 4D-printed hydrogel composites developed by Lewis and her team are programmed to contain precise, localized swelling. Importantly, the hydrogel composites contain cellulose fibrils that are derived from wood and resemble the microstructures that enable shape changes in plants.

And now, the hopping robot

Reported today in Nature Materials, the 4D printing advance combined materials science and mathematics through the involvement of the study’s co-lead authors: A. Sydney Gladman, a graduate research assistant advised by Lewis and specializing in the printing of polymers and composites at the Wyss Institute and SEAS, and Elisabetta Matsumoto, a postdoctoral fellow at the Wyss and SEAS advised by Mahadevan and specializing in condensed matter and material physics.

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. Just like wood, which splits more easily along the grain than across it, when it is immersed in water the hydrogel-cellulose fibril ink undergoes differential swelling along and orthogonal to the printing path. Combined with a proprietary mathematical model developed by the team that determines how a 4D object must be printed to achieve prescribed transformable shapes, the new method opens up new potential applications for 4D printing technology, 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,” said Gladman. “What’s more, we can interchange different materials to tune for properties such as conductivity or biocompatibility.”

The composite ink 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-response behavior, while the cellulose fibrils can be replaced with other anisotropic fillers, including conductive fillers.

“Our mathematical model prescribes the printing pathways required to achieve the desired shape-transforming response,” said Matsumoto. “We can control the curvature both discretely and continuously using our entirely tunable and programmable method.”

The mathematical modeling also solves the “inverse problem,” the challenge of predicting what the printing toolpath must be to encode swelling behaviors toward a desired shape.

“It is wonderful to be able to design and realize, in an engineered structure, some of nature’s solutions,” said Mahadevan, who has studied phenomena such as how botanical tendrils coil, how flowers bloom, and how pine cones open and close. “By solving the inverse problem, we are now able to reverse-engineer the problem and determine how to vary local inhomogeneity, i.e., the spacing between the printed ink filaments, and the anisotropy, i.e., the direction of these filaments, to control the spatiotemporal response of these shape-shifting sheets.”

“What’s remarkable about this 4D printing advance made by Jennifer and her team is that it enables the design of almost any arbitrary, transformable shape from a wide range of available materials with different properties and potential applications, truly establishing a new platform for printing self-assembling, dynamic microscale structures that could be applied to a broad range of industrial and medical applications,” said Wyss Institute Director Donald Ingber, the Judah Folkman Professor of Vascular Biology at Harvard Medical School and the vascular biology program at Boston Children’s Hospital and professor of bioengineering at SEAS.

This work was supported by funding from the Army Research Office and the National Science Foundation’s Materials Research Science and Engineering Center.