The Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) has made a tremendous amount of progress in the field of materials science, and much of that work has to do with 3D printing. The school’s researchers have used a hybrid 3D printing technique to create stretchable wearable electronics, and have developed novel 3D printing materials as well as metamaterials. Now SEAS has used 3D printing in the development of structural composite materials optimized for strength, stiffness and damage tolerance.

The new 3D printing method offers unprecedented control over the arrangement of short fibers embedded in polymer matrices. The researchers used the method to program fiber orientation within epoxy composites in specified locations to create the structural materials. The method has been named rotational 3D printing, and it could lend itself to a wide variety of applications. Because of the modular nature of the ink designs, several different filler and matrix combinations can be used to tailor the 3D printed objects’ electrical, optical or thermal properties.

“Being able to locally control fiber orientation within engineered composites has been a grand challenge,” said the study’s senior author, Jennifer A. Lewis, Hansjorg Wyss Professor of Biologically Inspired Engineering at Harvard SEAS and a Core Faculty Member of the Wyss Institute for Biologically Inspired Engineering at Harvard.

“We can now pattern materials in a hierarchical manner, akin to the way that nature builds.”

How it works is that the researchers very precisely choreograph the speed and rotation of a 3D printer nozzle to program the arrangement of embedded fibers in polymer matrices. They do this by equipping a rotational printhead system with a stepper motor to guide the angular velocity of the rotating nozzle as the ink is extruded.

“Rotational 3D printing can be used to achieve optimal, or near optimal, fiber arrangements at every location in the printed part, resulting in higher strength and stiffness with less material,” said then-postdoctoral fellow Brett Compton, now Assistant Professor in Mechanical Engineering at the University of Tennessee, Knoxville. “Rather than using magnetic or electric fields to orient fibers, we control the flow of the viscous ink itself to impart the desired fiber orientation.”

According to Compton, the concept can be applied to any material extrusion printing method, including FDM/FFF, direct ink writing, and large-scale thermoplastic additive manufacturing, using any filler material from carbon and glass fiber to metallic or ceramic whiskers and platelets. The method allows for the 3D printing of engineered materials that can be spatially programmed to achieve specific performance goals. The orientation of the fibers, for example, can be locally optimized to increase the damage tolerance at locations that would be likely to undergo the greatest stress during loading, hardening potential failure points.

“One of the exciting things about this work is that it offers a new avenue to produce complex microstructures, and to controllably vary the microstructure from region to region. More control over structure means more control over the resulting properties, which vastly expands the design space that can be exploited to optimize properties further,” said then-postdoctoral fellow Jordan Raney, now Assistant Professor of Mechanical Engineering and Applied Mechanics at the University of Pennsylvania.

“Biological composite materials often have remarkable mechanical properties: high stiffness and strength per unit weight and high toughness. One of the outstanding challenges of designing engineering materials inspired by biological composites is control of fiber orientation at small length scales and at the local level,” said Lorna J. Gibson, Professor of Materials Science and Engineering at MIT, who was not involved in the research. “This remarkable paper from the Lewis group demonstrates a way of doing just that. This represents a huge leap forward in the design of bio-inspired composites.”

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[Source/Images: SEAS]


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