Scientists from Duke University have recently created a technique that can print electronics onto organic surfaces such as the human skin. This innovation could lead to the creation of electronic tattoos and bandages that contain technology-equipped biosensors. This work was covered in two papers published on July 9 in Nanoscale and on October 3 in ACS Nano.
“When people hear the term ‘printed electronics,’ the expectation is that a person loads a substrate and the designs for an electronic circuit into a printer and, some reasonable time later, removes a fully functional electronic circuit,” said Aaron Franklin, the James L. and Elizabeth M. Vincent Associate Professor of Electrical and Computer Engineering at Duke. “Over the years there have been a slew of research papers promising these kinds of ‘fully printed electronics,’ but the reality is that the process actually involves taking the sample out multiple times to bake it, wash it or spin-coat materials onto it,” Franklin said. “Ours is the first where the reality matches the public perception.”
Printing Electronic Tattoos
Printing electronic devices onto human skin is a concept that was first developed in the late 2000s at the University of Illinois by John A. Rogers. These electronic tattoos are not injected into the skin as the ink tattoo is but rather printed on top of the skin as a thin, rubber patch with flexible electrical features. This film adheres to the skin like a temporary tattoo and was initially designed to contain sensors for neurological and cardiovascular measurement as well as muscle stimulation. Although these devices are encroaching on commercial development and implementation, there are still areas in which they must improve.
“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,” explained Franklin. “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 Initial Research
In their paper published in Nanoscale, Franklin’s lab collaborated with fellow Duke professor Benjamin Wiley’s lab to create a unique ink with silver nanowires. This novel substrate can be printed onto any substrate at low temperatures using an aerosol printer.
The product is a thin film that can conduct electricity as is, requiring no further modification after printing. The ink takes less than two minutes to dry and continues to be conductive even after a 50% bending strain over a thousand times. A video that came out with this first paper showed two leads being printed onto a researcher’s pinky finger, with an LED connected to the end of them. When a voltage is applied to the bottom of these leads, the light turns on and remains lit as the finger moves.
Furthering the Electronic Printing Technique
In the paper published in ACS Nano, Franklin and graduate student Shiheng Lu combined this conductive ink with two other printable components, forming functional transistors. In this approach, a semiconducting strip of carbon nanotubes is first put down and allowed to dry. Then, two silver nanowire leads are printed to extend several centimeters on each side. A non-conducting dielectric layer of hexagonal boron nitride is then printed over the top of the original semiconductor, proceeded by a silver nanowire gate electrode.
This procedure would typically require that the printed material be removed for a chemical bath, baking, or some type of additional processing. What is unique about Franklin’s approach is that the device can be printed in place, requiring none of these additional steps. In addition, this can be done at a processing temperature lower than any existing system.
“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,” said Franklin. “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.”
“Think about creating bespoke bandages that contain electronics like biosensors, where a nurse could just walk over to a work station and punch in what features were needed for a specific patient,” concluded Franklin. “This is the type of print-on-demand capability that could help drive that.”
— Exejobs (@exe_jobs) October 3, 2019