Low-Cost Printable Electronics Fabrication
The need for low-cost and environmentally friendly processes for fabricating printable electronics and biosensor chips is rapidly growing. NASA has developed a unique approach for an atmospheric pressure plasma-based process for fabricating printable electronics and functional coatings. This system involves aerosol-assisted, room-temperature printing in which an aerosol carrying the desired material for deposition is introduced into a cold plasma jet operated at atmospheric pressure.
The deposition is the result of the interaction of the aerosol containing the precursor material with the atmospheric pressure plasma containing a primary gas. Aerosol-assisted plasma deposition is a high throughput and facile process for printing and patterning that is easily scalable for industrial production. Multiple jets can be used for depositing different materials, and the approach can be adapted to a variety of platforms.
Commercial applications for the system include biomedical technology, consumer electronics, e-paper, security, and communications.
“Stamping” Electronics Using Nanotubes
Imagine food packaging that displays a digital warning that the food is about to spoil, or a window in your house that displays the weather forecast based on measurements of the temperature and humidity levels outside.
Engineers at MIT invented a fast, precise printing process that could enable these electronic surfaces. The team developed a stamp made of carbon nanotubes that can print electronic inks onto rigid and flexible surfaces. The process should be able to print transistors small enough to control individual pixels in high-resolution displays and touchscreens. The process also may provide a relatively inexpensive, quick way to manufacture other electronic surfaces.
Because techniques such as inkjet printing are difficult to control at very small scales, they tend to produce “coffee ring” patterns where ink spills over the borders, or uneven prints that can lead to incomplete circuits. The new technique uses a nanoporous stamp that allows a solution of nanoparticles, or “ink,” to flow uniformly through the stamp and onto whatever surface is to be printed.
The carbon nanotubes are grown on a surface of silicon in various patterns, including honeycomb-like hexagons and flower-shaped designs. The nanotubes are coated with a thin polymer layer to ensure the ink would penetrate throughout the nanotubes, and the nanotubes would not shrink after the ink was stamped. The stamp is then infused with a small volume of electronic ink containing nanoparticles such as silver, zinc oxide, or semiconductor quantum dots.
The key to printing precise, highresolution patterns is in the amount of pressure applied to stamp the ink. A model was developed to predict the amount of force necessary to stamp an even layer of ink onto a substrate, and the concentration of nanoparticles in the ink. After stamping ink patterns of various designs, the team tested the printed patterns’ electrical conductivity. After heating the designs after stamping, the printed patterns were highly conductive, and could serve as high-performance transparent electrodes. Going forward, the team plans to pursue the possibility of fully printed electronics.
Printed Graphene Treated with Lasers Enables Paper Electronics
The graphene carbon honeycomb is just an atom thick, conducts electricity and heat, and is strong and stable. Recent projects that used inkjet printers to print multi-layer graphene circuits and electrodes have led to graphene's use for flexible, wearable, low-cost electronics. But once printed, the graphene has to be treated to improve electrical conductivity and device performance, which usually means high temperatures or chemicals that could degrade flexible or disposable printing surfaces such as plastic films or even paper.
Iowa State University researchers developed a method that uses lasers to treat the graphene. By treating inkjet-printed, multi-layer graphene electric circuits and electrodes with a pulsed-laser process, electrical conductivity was improved without damaging paper, polymers, or other fragile printing surfaces.
The inkjet-printed graphene is transformed into a conductive material capable of being used in new applications such as sensors with biological uses, energy storage systems, electrical conducting components, and paper-based electronics.
The engineers developed computer-controlled laser technology that selectively irradiates inkjet-printed graphene oxide. The treatment removes ink binders and reduces graphene oxide to graphene, physically stitching together millions of tiny graphene flakes. The process makes electrical conductivity more than a thousand times better. The localized laser processing also changes the shape and structure of the printed graphene from a flat surface to one with raised, 3D nanostructures that resemble tiny petals rising from the surface. The rough and ridged structure increases the electrochemical reactivity of the graphene, making it useful for chemical and biological sensors.
The work paves the way for the creation of low-cost and disposable graphene-based electrochemical electrodes for applications including sensors, biosensors, fuel cells, and medical devices.
Electronic Devices “Printed” with Magnetic Ink
University of California San Diego (UCSD) engineers developed a magnetic ink that can be used to make self-healing batteries, electrochemical sensors, and wearable, textile-based electrical circuits. The ink is made of microparticles oriented in a certain configuration by a magnetic field that enables particles on both sides of a tear to be magnetically attracted to one another, causing a device printed with the ink to heal itself. The devices repair tears as wide as 3 millimeters.
Existing self-healing materials require an external trigger to kickstart the healing process. They also take anywhere between a few minutes to several days to work. The new system requires no outside catalyst to work, and damage is repaired within about 0.05 second.
The ink was used to print batteries, electrochemical sensors, and wearable, textile-based electrical circuits. Then the devices were damaged by cutting them and pulling them apart to create increasingly wide gaps. The devices still healed themselves and recovered their function while losing a minimum amount of conductivity.
A self-healing circuit was printed on the sleeve of a T-shirt and connected with an LED light and a coin battery. The circuit and the fabric it was printed on were both cut. At that point, the LED turned off. Within a few seconds, the LED started turning back on as the two sides of the circuit came together again and healed themselves, restoring conductivity. In the future, engineers envision making different inks with different ingredients for a wide range of applications.