Researchers at North Carolina State University (NC State) published their findings in a new paper demonstrating a method for printing flexible circuits on curved surfaces.
Such surfaces include a latex glove and a contact lens. How did they do this? Read on for the answers.
Printing of curved circuits
Direct printing of circuitry, both 2D and 3D, is nothing new, but each process presents its own set of challenges, and printing complex, high-resolution patterns of metal nanowires onto curvilinear substrates has so far been challenging.
The NC State team has successfully developed a new method that does exactly that, and the process has been dubbed MIMIC, which stands for “MIcroMolding In Capillary.”
The process works by first using a photolithography (UV) step to pattern a stencil in a photoresist material that features a set of microscale grooves that represent the circuit to be printed.
A liquid silicone (PDMS) is then poured onto the photoresist and cured, before peeling it off to leave a microchannel in the shape of the photoresist in the cured silicone. Entry and exit holes are drilled in the PDMS mold.
The mold is placed on the curved (or flat) substrate on which the circuit is required, and a solution of ethanol and silver nanowires (the ink) is dropped into the gate.
Capillary action then pushes the solution through the microchannels where the mold is shaken and the ink is allowed to dry at room temperature.
After removing the mold, the desired geometry of the circuit track is left intact on the surface.
You can see the process in the following image.
“There are many existing techniques for creating printed electronic products using various materials, but there are limitations,” says Yong Zhu, professor of Mechanical and Aerospace Engineering at NC State and corresponding author of the paper.
“One challenge is that existing techniques require the use of polymer binding agents in the ‘ink’ used to print the circuitry. This impairs the conductivity of the circuit, so you must incorporate an extra step to remove those binding agents after printing. A second challenge is that these printing techniques generally require you to print on flat surfaces, but many applications require non-flat surfaces.”
The researchers performed various tests on the deposited circuits and found that the resistances and conductivities of the sheets appear to be uniform throughout the pattern. This was because the ink reached equilibrium after completely filling the microchannel.
“We have developed a technique that does not require binding agents and that allows us to print on a variety of curvilinear surfaces,” says Yuxuan Liu, a doctoral student and first author of the paper.
“It also allows us to print the circuits as grid structures with a uniform thickness.”
The researchers printed the nanowires on a variety of substrates including glass, polyethylene terephthalate (PET), PDMS, cellulose film, a latex glove, and a plastic petri dish.
Three working prototypes were also manufactured to demonstrate the process.
It involved a “smart” contact lens with built-in circuitry to measure fluid pressure in the eye, a flexible, transparent electrode with circuitry printed in a grid pattern, and a latex glove with circuitry that served as pressure sensors.
“We think this could be scaled up quite easily, in terms of manufacturing,” Zhu said.
“We are open to talking with industries that are interested in exploring the potential of this technique.”
And we know what you’re thinking…
Is this really 3D printing?
And our answer to that question is…
The ink was printed on a curved surface, in three dimensions.
So, from our perspective, it’s certainly a form of 3D printing, even if it’s not exactly additive manufacturing in the usual way that we write about on these pages.
Take a look at the research paper, titled “Curvilinear Soft Electronics Using Microcasting of Metal Nanowires in Capillaries” (at this link) and decide for yourself.