A fully print-in-place technique for electronics could enable technologies such as high-adhesion, embedded electronic tattoos and bandages with patient-specific biosensors.
Conventional electronic tattoos are thin, flexible patches of rubber that contain equally flexible electrical components. The thin film sticks to skin much like a temporary tattoo and early versions of the flexible electronics were made to contain heart and brain activity monitors and muscle stimulators. There are some arenas in which they are not well suited such as when direct modification of a surface by adding custom electronics is needed.
Researchers developed a novel ink containing silver nanowires that can be printed onto any substrate at low temperatures with an aerosol printer. It yields a thin film that maintains its conductivity without any further processing. After being printed, the ink is dry in less than two minutes and retains its high electrical performance even after enduring a 50 percent bending strain more than 1,000 times.
In a demonstration, two electronically active leads were printed along the underside of a pinky finger. Toward the end of the finger, the leads are connected to a small LED light. A voltage is then applied to the bottom of the two printed leads, causing the LED to stay lit even as the finger bends and moves.
The conductive ink can be combined with two other printable components to create functional transistors. The printer first puts down a semiconducting strip of carbon nanotubes. Once it dries and without removing the plastic or paper substrate from the printer, two silver nanowire leads that extend several centimeters from either side are printed. A non-conducting dielectric layer of a two-dimensional material (hexagonal boron nitride) is then printed on top of the original semiconductor strip, followed by a final silver nanowire gate electrode.
With today’s technologies, at least 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 don’t mix, or an extended bake to remove traces of organic material that can interfere with electric fields. The print-in-place technique requires none of these steps and despite the need for each layer to dry completely to avoid mixing materials, can be completed at the lowest overall processing temperature reported to date.
The printing method does not replace large-scale manufacturing processes for wearable electronics but has value for applications such as rapid prototyping or for bandages that contain biosensors.