Advancing the Field of Flexible Electronics with 3D Printable Liquid Metal Alloy

Whenever I hear about liquid metal, I will always picture the T-1000 from Terminator 2 first. But in reality, 3D printed liquid metal can be used to develop innovations that are far better for humanity than a nearly indestructible robotic killing machine, such as custom cars and functional electronics.

A team of researchers from Oregon State University’s College of Engineering is now using a modified, 3D printable liquid metal to take the next step toward rapid manufacturing of stretchable electronic devices, like flexible computer screens and soft robots. The ultimate goal is to 3D print tall, complex structures using a conductive gallium alloy.

Inexpensive gallium alloys are already used to conduct electricity in flexible electronics, thanks to their low toxicity and self-healing capabilities. But its printability had previously been restricted to 2D only, until the OSU team, from the engineering college’s Collaborative Robotics and Intelligent Systems Institute (CoRIS), developed a modification that used sound energy, or sonication.

The researchers used this modification to mix nickel nanoparticles and oxidized gallium into galinstan, a liquid metal alloy previously used to make 3D printed wearable temperature sensors. The combination then thickened into a 3D printable paste.

“The runny alloy was impossible to layer into tall structures. With the paste-like texture, it can be layered while maintaining its capacity to flow, and to stretch inside of rubber tubes,” explained Yiğit Mengüç, Assistant Professor of Mechanical Engineering. “We demonstrated the potential of our discovery by 3D printing a very stretchy two-layered circuit whose layers weave in and out of each other without touching.”

The team recently published a paper on their study findings, titled “Rheological Modification of Liquid Metal for Additive Manufacturing of Stretchable Electronics,” in the journal Advanced Materials Technologies.

The Young Investigator Program of the Office of Naval Research supported the work, and co-authors of the study include Uranbileg Daalkhaijav, PhD candidate in chemical engineering; Osman Doğan Yirmibeşoğlu, a robotics PhD student; Stephanie Walker; and Mengüç.

The abstract reads, “One of the challenges to rapidly manufacturing flexible electronics is the complexity involved in printing circuitry from stretchable conductors. Eutectic gallium alloys are typically used as the conductive material because they have unique high conductivity, self-healing, and stretchable properties. However, limited 3D printing has been demonstrated by leveraging the structural stabilization provided by the thin gallium oxide film. Vertical structures are difficult to print with a liquid metal (LM) due to the low viscosity and high surface tension of the gallium alloy, which easily leads to coalescence. A method is presented to alter the physical structure of the liquid metal through the incorporation of a conductive nano- or micronickel fillers. The resulting rheological modification of the liquid metal to a paste drastically increases the fluidic elastic modulus and yield stress, rendering it 3D printable. Further, the modification retains the high electrical conductivity (3.9 × 10⁶ ± 9.5 × 10⁵ S m⁻¹) and stretchability (over 350% strain) of pure liquid metal. The ability to print 3D standing structures using this highly conductive metal paste opens up new opportunities to manufacture more complex stretchable electronics.”

The researchers designed a custom extruder head that could handle the gallium alloy paste, and printed it on a Stratasys 3D printer. Then, they tested their work by 3D printing structures up to 10 mm high and 20 mm wide. One of the things they learned during their research was that the liquid metal material evolves continuously, which means that it takes the paste over four hours to structurally stabilize once it’s been sonicated.

“Liquid metal printing is integral to the flexible electronics field. Additive manufacturing enables fast fabrication of intricate designs and circuitry,” said Yirmibeşoğlu.

“The future is very bright. It’s easy to imagine making soft robots that are ready for operation, that will just walk out of the printer.”

(a) Metal paste printer setup with the Ultimus high precision dispenser on the left. (b) 3D printed custom design extruder head with attached 10 cc barrel.

According to co-corresponding author Daalkhaijav, the gallium alloy paste demonstrates multiple features that are new to the field of flexible electronics.

“It can be made easily and quickly. The structural change is permanent, the electrical properties of the paste are comparable to pure liquid metal, and the paste retains self-healing characteristics,” said Daalkhaijav.

Liquid metal processing and printing setup. (a) Sample sonication setup and (b) a diagram of the Galinstan and nickel mixture in a cooling bath. Pictures of pure Galinstan without sonication (c), after 2 (d) and 4 (e) minutes of sonication. Pictures of Galinstan and nickel paste after 2 (f), 4 (g), and 6 (h) minutes of sonication exposed to air.

The flexible electronics field covers a wide range of products, such as antennae, biomedical sensors and sensors for torque, bendable displays, electrically conductive textiles, and wearable sensor suits, like the kind used by video game developers.

The OSU team will continue their research in the future by determining how the nickel particles are stabilized, exploring the paste’s structure, and investigating how the structure changes as the paste continues to age.

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