Researchers at Duke University have created a fully print-in-place technique for customized electronics. The technique is gentle enough to be applied directly onto the skin or a piece of paper. The development could allow new technologies such as high-adhesion, embedded electronic tattoos and bandages to be outfitted with patient-specific biosensors.
The idea of electronic tattoos has been around for nearly a decade. Unlike a traditional tattoo that is injected into the skin, electronic tattoos are thin, flexible patches of rubber that contain flexible electrical components.
Some types of printed electronics, such as those that contain heart and brain activity monitors and music stimulators, are on the path to commercialization and large-scale manufacturing. However, not all are suited for direct modification of a surface, such as when custom electronics are needed.
"For direct or additive printing to ever really be useful, you're going to need to be able to print the entirety of whatever you're printing in one step," said Aaron Franklin, associate professor of electrical and computer engineering at Duke.
"Some of the more exotic applications include intimately connected electronic tattoos that could be used for biological tagging or unique detection mechanisms, rapid prototyping for on-the-fly custom electronics, and paper-based diagnostics that could be integrated readily into customized bandages."
The first test of the printed electronics involved an ink containing silver nanowires that is printed onto a substrate at low temperatures with an aerosol printer. The resulting thin-film conductor dries in less than two minutes and retains its conductivity after bending more than 1,000 times.
As an example, researchers printed two electronically active leads along the underside of a finger. At the end of the finger is a small LED light that stays lit while bending or moving the finger.
The team was also printed a semiconducting strip of carbon nanotubes. Once dried, two silver nanowire leads are printed, extending several centimeters from either side, without removing the plastic or paper substrate from the printer. Another layer is then printed onto the semiconductor strip, which is a non-conducting dielectric layer of a 2D hexagonal born nitride. A final layer is then printed with a silver nanowire gate electrode.
In conventional printed electronics, one of these steps would require the substrate to be removed for additional processing, such as a chemical bath to rinse away unwanted material, a hardening process to ensure layers do not mix or an extended bake to remove traces of organic material that can interfere with electric fields. But the print-in-place method from Duke University does not require these steps, although it does need time for each layer to dry.
"Nobody thought the aerosolized ink, especially for boron nitride, would deliver the properties needed to make functional electronics without being baked for at least an hour and a half," Franklin said.
"But not only did we get it to work, we showed that baking it for two hours after printing doesn't improve its performance. It was as good as it could get just using our fully print-in-place process."
Researchers do not believe this printing method will replace mass-scale manufacturing for wearables, but it could be useful in rapid prototyping or customization.
The full research can be found in the journal ACS Publications.