Materials in nature are rarely straight. In fact, helical structures are ubiquitous in nature and impart unique mechanical properties and multi-functionality. In our bodies, proteins assemble into helical filaments which allow our muscles to contract. Plants change shape because cellulose fibers are arranged helically within their cell walls. Researchers from the Harvard John A. Paulson School of Engineering and Applied Sciences and the Wyss Institute for Biologically Inspired Engineering at Harvard University have now developed a rotational multimaterial 3D printing method – RM 3DP – for creating helical filaments. Using this new approach, the team designed and fabricated artificial muscles and springy lattices for use in soft robotics and structural applications.
The team’s research, led by Professor Jennifer Lewis and the Harvard Lewis Lab, known for decades of advanced research and breakthroughs in 3D printing/bioprinting technologies and materials, was published in Nature (and is available here).
Rotational multimaterial 3D printing – RM 3DP
In the RM 3DP process, the printhead consists of four ink cartridges, each of which can contain different materials. The inks are then fed through a complex nozzle that allows multiple materials to be printed simultaneously. As the nozzle rotates and translates, the extruded inks form a filament with embedded helical features. The technique builds upon previous work on rotational 3D printing for composites also conducted by John A. Paulson School of Engineering and Applied Sciences (SEAS) researchers and Jennifer Lewis’s team.
“Our additive manufacturing platform opens new avenues to generating multifunctional architected matter in bio-inspired motifs,” said Jennifer Lewis, the Hansjorg Wyss Professor of Biologically Inspired Engineering at SEAS and senior author of the study. Lewis is also a Core Faculty member of the Wyss Institute. The Lewis Lab was among the first in the world to develop methods for multimaterial bioprinting of capillary structures nearly a decade ago and has been working on soft robotics for the past several years.
“Rotational multimaterial printing allows us to generate functional helical filaments and structural lattices with precisely controlled architecture and, ultimately, performance,” said Natalie Larson, a postdoctoral fellow at SEAS and first author of the study.
As the scientists explain in the study’s abstract until today synthetic architectures that mimic these natural systems have been fabricated by winding, twisting and braiding of individual filaments, microfluidics, self-shaping, and [various] printing methods. However, those fabrication methods are unable to simultaneously create and pattern multimaterial, helically architected filaments with subvoxel control in arbitrary two-dimensional (2D) and three-dimensional (3D) motifs from a broad range of materials.
Towards this goal, both multimaterial and rotational 3D printing of architected filaments have recently been reported; however, the integration of these two capabilities has yet to be realized. The new rotational multimaterial 3D printing (RM-3DP) platform developed for helical printing in this study enables subvoxel control over the local orientation of azimuthally (with respect to the angle above a plane) heterogeneous architected filaments.
By continuously rotating a multimaterial nozzle with a controlled ratio of angular-to-translational velocity, the researchers have created helical filaments with programmable helix angle, layer thickness and interfacial area between several materials within a given cylindrical voxel. Using this integrated method, functional artifcial muscles were created, composed of helical dielectric elastomer actuators with high fidelity and individually addressable conductive helical channels embedded within a dielectric elastomer matrix. The Harvard researchers have also fabricated hierarchical lattices comprising architected helical struts containing stiff springs within a compliant matrix. This new additive-manufacturing platform is expected to open new avenues to generating multifunctional architected matter in bioinspired motifs.
Flexing artificial muscles
Among other practical applications of RM 3DP, in collaboration with David Clarke, the Extended Tarr Family Professor of Materials, the team printed artificial muscles in the form of helical dielectric elastomer actuator filaments that can contract under an applied voltage. The conductive electrodes form intertwining helices encapsulated in a soft elastomer matrix. By tuning how tightly those helical electrodes are coiled, one can program the contractile response of these actuators.
The team also designed structural lattices with varying stiffness by embedding stiff helical springs within a soft, compliant matrix—like metal springs in a soft mattress. The overall stiffness of the material can be tuned by tuning the tightness of the springs inside the matrix. These tunable helical structures could be used to make joints or hinges in soft robotic systems.
Next, the team aims to harness the capabilities of this novel 3D printing method to create even more complex structures. “By designing and building nozzles with more extreme internal features, the resolution, complexity, and performance of these hierarchical bioinspired structures could be further enhanced,” said Larson.
The research was co-authored by Jochen Mueller, Alex Chortos, and Zoey Davidson at SEAS. It was supported by the National Science Foundation under MRSEC (DMR- 2011754), NSF Designing Materials to Revolutionize and Engineer our Future (DMREF-15-33985), the Vannevar Bush Faculty Fellowship Program, sponsored by the Basic Research Office for the Assistant Secretary of Defense for Research and Engineering through the Office of Naval Research Grant N00014- 21-1-2958, and the GETTYLAB.
To get an even better understanding of what is going on during helical 3D printing, the company Particleworks Europe, which distributes the Particleworks fluid simulation software, created a video simulation (below). Particleworks is a CAE software (compatible with Ansys) for the simulation of liquid flows based on the Moving Particle Simulation method. Its mesh-less solver and intuitive interface simplify the simulation process for complex geometries with moving parts, like gears and shafts in a complete transmission or in an engine. Its Navier-Stokes solver is based on a deterministic Lagrangian method.