Three-dimensional (3D) printing, or additive manufacturing, has gone beyond the simply possible to become a widely used technology for the flexible manufacturing of countless commercially important objects from industrial components to personalized medical implants. The technology is also being explored for the production of biological materials (tissues) for drug discovery and development and ultimately organs for transplant. Researchers are now working on the next iteration—four-dimensional (4D) printing—in which printed objects change their shape over time in response to various stimuli. For instance, scientists led by Skylar Tibbits at Massachusetts Institute of Technology are investigating self-assembly of printed inks containing mixtures of rigid and expandable (upon exposure to water) materials. Very recently, researchers at the Wyss Institute for Biologically Inspired Engineering at Harvard University and the Harvard John A. Paulson School of Engineering and Applied Sciences developed a 4D printing process inspired by the behavior of natural plant structures.
The new inks are based on biomimetic hydrogel composites containing cellulose fibrils that are derived from wood and similar to the microstructures in plant cell walls that enable plant organs, such as tendrils, leaves, and flowers, to exhibit dynamic morphologies that change in response to environmental variations (humidity and/or temperature). Using a mathematical model, the researchers can predict the necessary 3D printed figuration with precise, localized swelling behaviors to achieve specific shape changes once the hydrogel composite structures are immersed in water. The key to the behavior of the inks is the incorporation of the cellulose fibrils, which align during printing, giving rise to directional properties that can be predicted and controlled and undergo differential swelling behaviors along and orthogonal to the printing path. Specific patterns can be created with anisotropic swelling and stiffness that result in intricate shape changes, including those that mimic the changes seen in different types of flowers.
The viscoelastic, aqueous ink was designed to be very tunable to different types of monomers and fillers, according to graduate student A. Sydney Gladman. “Hydrogels are a very versatile group of materials with properties that are easily tunable and have the capability to reversibly respond to environmental stimuli.” The original acrylamide matrix (N,N-dimethylacrylamide or N-isopropylacrylamide for thermally reversible systems) is biocompatible and undergoes a large degree of swelling in water. The ink also contains a photoinitiator, nanoclay, glucose oxidase, glucose, and the nanofibrillated cellulose (NFC). The nano clay serves both as the rheological modifier (enabling high viscosity, shear thinning properties) and as a multifunctional crosslinker for the monomer. “Using this clay, it is possible to maintain the correct rheological properties even with different monomers. Thus, we do not need to redesign the ink for each chemical composition.” The incorporation of glucose oxidase (a naturally occurring oxygen-scavenging enzyme) and glucose prevents oxygen inhibition, which is a typical problem associated with thin films generated via free-radical polymerization. The formulated ink is readily processable, printable, and curable under ambient conditions. The cellulose fibrils were chosen due to their high stiffness and high aspect ratio, and were derived directly from wood, making them sustainable and cost efficient.
The ink is printed in bilayer architectures, photopolymerized, and then physically crosslinked by the nanoclay particles, producing a biocompatible hydrogel matrix that swells readily in water. The fibrils undergo shear-induced alignment as the ink flows through the deposition nozzle, leading to a fourfold difference in the swelling strains in the longitudinal (along the filament length as defined by the printing path) and transverse directions. The extent of the fibril alignment (and therefore the magnitude of the anisotropic swelling) changes as a function of the nozzle diameter and the printing speed. In addition, the two layers of the bilayer structure swell differently, inducing positive or negative curvature depending on the 3D shape. “Through collaboration with colleagues with a great deal of experience in modeling the concepts behind plant-inspired shape change and predictive theoretical frameworks, we were able to develop a theoretical model to predict the three-dimensional structure produced by a prescribed print path when the two layers are printed anticlockwise to one another to a certain degree,” Gladman explains.
To demonstrate the effectiveness of the model, a series of functional, folding-flower architectures were printed and exposed to water. It is worth noting that water uptakes occur quickly, leading to shape changes within minutes, according to Gladman. The researchers also printed a complex structure to mimic the complex Dendrobium orchid helix using four distinct types of incorporated shape change using discrete bilayer orientations in each petal. Additionally, the group used the model to translate the surface of the calla lily flower (using the local curvatures, swelling ratio, elastic constants, height, and size of the structure) into a two-layered print path that would mimic its complex curvature, achieving good agreement between the final calculated and experimentally printed 3D shapes.
Currently, the researchers are developing a suite of inks capable of reversible, responsive shape change when subjected to a variety of external stimuli (i.e., pH, heat, and light) that may find use in transformable tissue engineering scaffolds or biomedical devices. The work involves extending the range of possible matrices beyond hydrogels to include, for example, liquid-crystal elastomers. The use of functional fillers to improve properties such as electrical conductivity or biocompatibility is also being investigated. Anisotropic fillers (e.g., metallic nanorods) that when combined with flow-induced anisotropy allow the creation of dynamically reconfigurable materials with tunable functionality are also being explored.
One advantage of the 4D printing technology, according to Gladman, is the ability to use one composite ink printed in a single step to achieve many different shape-changing hydrogel geometries containing more complexity than any other technique simply by changing the print path. The ability to interchange different materials to tune for properties such as conductivity or biocompatibility is a further advantage. “We think this approach has broad implications for manufacturing of complex shape-changing forms, and the tunability of the ink design could enable a broad variety of uses,” Gladman asserts. A number of commercial applications are being considered, particularly, due to the biocompatibility of the inks, shape-shifting architectures with applications in tissue engineering and biomedical devices. Additionally, soft robotics, smart textiles, and soft electronics are being considered.
Gladman’s coauthors on the new study published in Nature Materials include her adviser Jennifer Lewis, a core faculty member at the Wyss Institute for Biologically Inspired Engineering at Harvard University and the Hansjörg Wyss Professor of Biologically Inspired Engineering at the Harvard John A. Paulson School of Engineering and Applied Science (SEAS); Elisabetta Matsumoto, a postdoctoral fellow at the Wyss and SEAS who is advised by L. Mahadevan, a Wyss core faculty member as well as the Lola England de Valpine Professor of Applied Mathematics, Professor of Organismic and Evolutionary Biology, and Professor of Physics at Harvard University and Harvard SEAS; and Ralph Nuzzo, the G.L. Clark Professor of Chemistry at the University of Illinois at Urbana-Champaign.
For more information, see Gladman, A.S., Matsumoto, E.A., Nuzzo, R.G., Mahadevan, L., and Lewis, J.A., “Biomimetic 4D Printing,” Nature Materials (2016).