Korean Researchers 3D Printing with Liquid Metal

In ‘High Resolution, reconfigurable printing of liquid metals with three-dimensional structures’ authors Young-Geun Park, Hyeon Seok An, Ju-Young Kim, and Jang-Ung Park explore a new technique outside the usual realm of metal 3D printing. With liquid metal 3D printing, the authors can create ‘stretchable’ 3D integrations formed into ‘diverse 3D structures.’ As an example in this study, they create a reconfigurable antenna.

High-resolution printing of liquid metals. (A) Schematic illustration of a printing system. (B) SEM image of 2D and 3D high-resolution EGaIn patterns. Scale bar,100 mm. Inset: Magnified SEM image of the 3D structures. Scale bar, 100 mm. (C) AFM image and cross-sectional profile of printed EGaIn line. Scale bar, 2 mm. (D) SEM image of 1.9-mm-wide EGaIn patterns. Scale bar, 10 mm. (E) SEM image of 3D patterns of EGaIn on a PET film and epoxy (SU-8). Scale bar, 10 mm. (F) Photograph of printed high resolution EGaIn patterns in (B). Scale bar, 1 cm. (G) Photograph of interconnect patterns of EGaIn. Inset: Top-view photograph. Scale bars, 5 mm. (H) Optical micrographs of printed EGaIn lines according to printing velocities. Scale bar, 40 mm. (I) The plot of line widths versus printing velocities. (J) The plot of line widths versus inner diameters of nozzles. Error bars in (I) and (J) indicate the SD. (Photo credit: Young-Geun Park, Yonsei University).

Deformity in devices is a focus here, centered around applications in ‘freeform electronics’ like:

• Stretchable electronics
• Wearable electronics
• Soft actuators
• Robotics

Previously, there have been challenges in finding suitable materials for such devices that require movable parts that are also a comfortable fit for the consumer, or easy to manipulate as functional objects. The authors point out that brittleness is often an issue, although conductive materials have been developed like wavy metals, metallic networks, and a variety of composites. While promising, such methods are not always scalable to 3D printing, and resolution may be an issue.

“While filament-based direct ink writing methods using inks of metal nanoparticles (e.g., Ag or Cu) have shown some feasibility for high resolution printing, they require additional thermal annealing or a drying process to form conductive pathways, which can cause damage to soft, tissue-like substrates. In addition, these printed and thermally annealed patterns of metals are relatively rigid and stiff; hence, repetitive device deformations can lead to cracking or failure in these metallic electrodes.”

The researchers discuss liquid metals like eutectic gallium-indium alloy (EGaIn) and gallium-indium-tin alloy (Galinstan), both stretchable materials that also exhibit low levels of toxicity and very little volatility. In comparison to solid metals, they also demonstrate excellent conductivity. While microfluidics or lithography can be used for patterning the liquid metals, their structures are limited to the 2D realm. Using a fine nozzle to print liquid metal under ambient conditions, the authors are able to create high-resolution structures. The use of narrow metallic filament allows for free-standing structures to be fabricated from liquid metal; in fact, they can even be lifted by the nozzle and moved.

Reconfiguration of liquid metals into 3D structures. (A) Schematic illustrations of each step of reconfiguration. (B) Schematic illustration of two adhesion forces during reconfiguration. (C) Photograph of lift-off (left) and cutoff (right) of EGaIn from the substrate. Scale bar, 100 mm. (D) The plot of the state of line versus the nozzle lift-off velocity. (E) Optical micrographs of reconfiguration. The printed horizontal line (left) is lifted off and reconfigured (right). Scale bars, 200 mm. (F) SEM images of reconfigured square coils. The end of the inner line in the square coil (left) is lifted and reconfigured (right). Scale bars, 200 mm. (G) SEM images of 3D bridges of EGaIn. Scale bar, 500 mm. Inset: Magnified SEM image of 3D bridge. Scale bar, 200 mm. (H) Plots of the applied biases and responding current densities in EGaIn. (Photo credit: Young-Geun Park, Yonsei University)

The high-resolution antennas were 3D printed as samples for the research, using a nozzle mounted to a syringe, and a substrate placed on the five-axis stage. The team also created free-standing structures of electrodes, allowing for minimization of interconnections—and ‘an aim toward higher integrations for miniaturized devices.’

“We believe that this high-resolution 3D reconfiguration method offers a promising strategy as an additive process that can be combined with conventional fabrication techniques for highly integrated and stretchable devices, indicating substantial promise for use in next-generation electronics,” stated the researchers.

While many industrial users are enjoying benefits such as the ability to build complex geometries that are strong yet lightweight, metal is being explored as the strongest medium for 3D printing, whether in creating porous metallic biomaterials, automated sheet metal production, or patented metals with high carbide content. What do you think of this news? Let us know your thoughts! Join the discussion of this and other 3D printing topics at 3DPrintBoard.com.

The electrical contact of direct-printed and reconfigured liquid metals. (A) Schematic illustrations of direct printing (left) and reconfiguration (right). (B) Dependence of total resistance on the length of the channel. Error bars represent the SD. (C) Current-voltage characteristics between Ag pads and direct-printed EGaIn. (D) Current-voltage characteristics between Ag pads and reconfigured EGaIn. (E and F) SEM images of EGaIn on an Ag pad after 7 hours of direct printing. (G and H) SEM images of EGaIn after 7 hours of reconfiguration. Scale bars, 200 mm.