Novel printheads actively mix complex fluids without stopping

Researchers from Harvard have designed multimaterial printheads that mix and print concentrated viscoelastic inks that allow for simultaneous control of composition and geometry during printing. Using active mixing and fast-switching nozzles, the printheads change material composition on the fly and could pave the way for entirely 3D-printed wearable devices, soft robots, and electronics.

Mixing complex fluids is fundamental for printing a broad range of materials. But most mixing approaches are passive, wherein two streams of fluids converge into a single channel where they undergo diffusive mixing. This method works well with low-viscosity fluids, but is ineffective with high-viscosity fluids, like gels, especially in small volumes over short timescales.

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

"Passive mixtures don't guarantee perfectly mixed materials, especially highly viscous inks," said Thomas Ober, the paper's first author. "We developed a rational framework, and verified it experimentally, for designing active microfluidic mixers that can mix a 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. These structures may find potential application in flexible electronics, wearable devices, and soft robotics. 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.

The team also designed another printhead that can rapidly switch between multiple inks within a single nozzle, eliminating the structural defects that often occur during the start-and-stop process of switching materials.

"Together, these active mixing and switching printheads provide an important advance for multimaterial 3D printing," said Jennifer Lewis, Professor of Biologically Inspired Engineering at the Harvard John A. Paulson School of Engineering and Applied Sciences. "They allow one to programmably control both materials composition and structure at the microscale, opening new avenues for creating materials by design."