Materials scientists at the North Carolina State University have connected copper particles with gallium indium alloy bridges to form a conductive printing ink with gel-like properties that are well suited for 3D and 4D printing. Their results appear in the journal Matter.
Xing et al. report printable metallic gels (pendular suspensions) consisting of an aqueous suspension of copper particles connected by bridges of liquid eutectic gallium indium alloy (EGaIn). Pendular suspensions rely on capillary forces to form networks between solid particles with a composition-dependent rheology, but prior studies have focused on insulating suspensions. Here, the rheology of a conductive solid-liquid-liquid suspension is tuned for 3D printing by varying the composition and the pH; the latter promotes metallic wetting. The dry printed parts have metallic electrical conductivity without requiring a sintering step. Drying at elevated temperatures can accelerate the removal of water while creating stress that drives shape change (i.e., 4D printing). As a demonstration, the authors print a conductive spider that lifts and assembles its own body from an initially flat shape. Such conductive inks are promising for printing metallic structures under ambient conditions. Image credit: Xing et al., doi: 10.1016/j.matt.2023.06.015.
“3D printing has revolutionized manufacturing, but we’re not aware of previous technologies that allowed you to print 3D metal objects at room temperature in a single step,” said North Carolina State University’s Professor Michael Dickey, co-corresponding author of the paper.
“This opens the door to manufacturing a wide range of electronic components and devices.”
To create the metallic gel, Professor Dickey and his colleagues start with a solution of micron-scale copper particles suspended in water.
They then add a small amount of an indium-gallium alloy that is liquid metal at room temperature. The resulting mixture is then stirred together.
As the mixture is stirred, the liquid metal and copper particles essentially stick to each other, forming a metallic gel network within the aqueous solution.
“This gel-like consistency is important, because it means you have a fairly uniform distribution of copper particles throughout the material,” Professor Dickey said.
“This does two things. First, it means the network of particles connect to form electrical pathways. And second, it means that the copper particles aren’t settling out of solution and clogging the printer.”
The resulting gel can be printed using a conventional 3D printing nozzle and retains its shape when printed.
And, when allowed to dry at room temperature, the resulting 3D object becomes even more solid while retaining its shape.
However, if users decide to apply heat to the printed object while it is drying, some interesting things can happen.
The researchers found that the alignment of the particles influences how the material dries. For example, if you printed a cylindrical object, the sides would contract more than the top and bottom as it dries.
If something is drying at room temperature, the process is sufficiently slow that it doesn’t create structural change in the object.
However, if you apply heat — for example, put it under a heat lamp at 80 degrees Celsius — the rapid drying can cause structural deformation.
Because this deformation is predictable, that means you can make a printed object change shape after it is printed by controlling the pattern of the printed object and the amount of heat the object is exposed to while drying.
“Ultimately, this sort of 4D printing — the traditional three dimensions, plus time — is one more tool that can be used to create structures with the desired dimensions,” Professor Dickey said.
“But what we find most exciting about this material is its conductivity.”
“Because the printed objects end up being as much as 97.5% metal, they are highly conductive.”
“It’s obviously not as conductive as conventional copper wire, but it’s impossible to 3D print copper wire at room temperature.”
“And what we’ve developed is far more conductive than anything else that can be printed. We’re pretty excited about the applications here.”
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Ruizhe Xing et al. 2023. Metallic gels for conductive 3D and 4D printing. Matter 6 (7): 2248-2262; doi: 10.1016/j.matt.2023.06.015
