3D printer active mixing [image credit: Thomas Ober, Harvard SEAS/Wyss Institute]

Multi-material printheads 'the next frontier' of 3D printed products

Harvard researchers have developed printheads that will allow for greater complexity and flexibility in products produced via 3D printing.

Traditionally, the process is only capable of using one material to produce a chosen design which creates limitations on what can be created.

The new multimaterial printheads are capable of mixing and printing concentrated viscoelastic inks that allow for the simultaneous control of composition and geometry during printing.

The researchers are touting the technology as “the next frontier in 3D printing” due to its ability to create electronic circuits with varying resistances in addition to flexible products.

Using active mixing and fast-switching nozzles, the material composition can be altered on the fly and could pave the way for entirely 3D-printed wearable devices, soft robots, and electronics.

The active mixer efficiently mixes a wide range of complex fluids by using a rotating impeller inside a microscale nozzle.

The research was conducted by Jennifer A. Lewis, Engineering Professor at Harvard’s School of Engineering and Applied Sciences, Thomas Ober, postdoctoral research scholar at the Wyss Institute and Daniele Foresti, the Society in Science Branco Weiss postdoctoral fellow.

"Passive mixtures don't guarantee perfectly mixed materials, especially highly viscous inks," said Ober, the paper's first author. "We developed a rational framework -- and verified it experimentally -- for designing active microfluidic mixers that can mix a wide variety of materials."

The research team demonstrated several uses of their active mixing technology. They showed that silicone elastomers can be seamlessly printed into gradient architectures composed of soft and rigid regions.

They also printed reactive materials, such as two-part epoxies, which typically harden quickly when the two parts are combined. Finally, they showed that conductive and resistive inks could be mixed on demand to embed electrical circuitry inside 3D printed objects.

Christopher Spadaccini, Director of the Center for Engineered Materials, Manufacturing and Optimization at Lawrence Livermore National Lab, said that the research represented a “significant advancement” in the field of additive manufacturing.

"By allowing for the mixing of two highly viscous materials on the fly, the promise of mixed material systems with disparate mechanical and functional properties becomes much more realistic,” he said.

“Before, this was really only a concept. This work will be foundational for applications which required integrated electrical and structural materials."

"This printhead design eliminates the need to align multiple nozzles as well as start and stop ink flow on demand," said James Hardin, first author of a recently published paper in Advanced Materials. This paper was co-authored by Ober, research fellow Alexander Valentine, and Lewis.

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