Duke University researchers have devised a fully print-in-place technique for electronics that can work on delicate surfaces such as paper and human skin, paving the way for the development of high-adhesion, embedded electronic tattoos and bandages with patient-specific biosensors. The techniques are described several papers published online July 9 in the journal Nanoscale and on October 3 in the journal ACS Nano.
“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,” said Aaron Franklin, Associate Professor of Electrical and Computer Engineering at Duke, in an article on the university’s website. “Ours is the first where the reality matches the public perception.”
The concept of electronic tattoos was first developed in the late 2000s at the University of Illinois by John A. Rogers, now Professor of Materials Science and Engineering at Northwestern University. Rogers’s electronic tattoos are not permanently embedded into the skin but are thin, flexible patches of rubber containing flexible electrical components.
“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 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.”
In the July paper, Franklin’s lab and the laboratory of Benjamin Wiley, professor of chemistry at Duke, developed a novel ink containing silver nanowires that can be printed onto any substrate at low temperatures with an aerosol printer. It produces a highly conductive thin film that, once printed, dries in less than two minutes and retains its high electrical performance even after undergoing a 50 percent bending strain more than a thousand times.
In the second paper, Franklin and graduate student Shiheng Lu combine the conductive ink with other printable components to create functional transistors. The printer first deposits a semiconducting strip of carbon nanotubes. Once dried, two silver nanowire leads extending several centimeters from either side are printed. A non-conducting dielectric layer is then printed on top of the original semiconductor strip, followed by a final silver nanowire gate electrode.
According to the researchers, current technologies would require the substrate to be removed for additional processing, such as chemical baths, hardening, or extended baking.
Franklin’s print-in-place technique would eliminate these steps, and the drying of each layer to avoid mixing of materials can be done at lower overall processing temperatures. Franklin sees his printing method being adapted to rapid prototyping or custom prototyping situations where one size doesn’t fit all.
“Think about creating bespoke bandages that contain electronics like biosensors, where a nurse could just walk over to a workstation and punch in what features were needed for a specific patient,” said Franklin. “This is the type of print-on-demand capability that could help drive that.”