Engineering news
A team of scientists at Harvard University has developed 4D-printed hydrogel composite structures that change shape when immersed in water, opening up uses in smart textiles, soft electronics and biomedical devices.
The team at the Wyss Institute for Biologically Inspired Engineering at Harvard University and the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) developed the technique that is inspired by natural structures like plants.
Jennifer Lewis, senior author on the study, said: "This work represents an elegant advance in programmable materials assembly, made possible by a multidisciplinary approach. 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 shapes that change according to their environments. Mimicking the variety of shape changes undergone by plant organs such as tendrils, leaves, and flowers in response to environmental stimuli like humidity or temperature, the 4D-printed hydrogel composites developed by Lewis and her team are programmed to contain precise, localised swelling behaviours.
Importantly, the hydrogel composites contain cellulose fibrils that are derived from wood and are similar to the microstructures that enable shape changes in plants.
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 specialising in the printing of polymers and composites at the Wyss Institute and SEAS, and Elisabetta Matsumoto, a postdoctoral fellow at the Wyss and SEAS specialising 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 (where it responds unequally to an external stimulus in different parts of the plant)
creates varied movements that can be predicted and controlled), just like wood, which can be split easier along the grain rather than across it. Similarly, when immersed in water, the hydrogel-cellulose fibril ink undergoes differential swelling behaviour along and at right angles to the printing path.
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 new method opens up 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 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.
"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."
Donald Ingber, Wyss Institute founding director, said: "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."