Printable electronics — flexible circuitry that is deposited on some type of plastic substrate — has been a major area of research for decades. But the ability to print the substrate itself greatly increases the range of devices the technique can yield.
The choice of substrate limits the types of materials that can be deposited on top of it. Because a printed substrate could consist of many materials interlocked in intricate, but regular patterns, it broadens the range of functional materials that printable electronics can use. Printed substrates also open the possibility of devices that, although printed as flat sheets, can fold themselves up into more complex, three-dimensional shapes.
Researchers have designed and built a device that responds to mechanical stresses by changing the color of a spot on its surface. This technology demonstrates the feasibility of flexible, printable electronics that combine sensors and processing circuitry, and can act on their environments. The device was inspired by the golden tortoise beetle (or goldbug), an insect whose exterior usually appears golden, but turns to reddish-orange if the insect is poked or prodded; that is, mechanically stressed.
The new device is approximately T-shaped, but with a wide, squat base and an elongated crossbar (Figure 1). The crossbar is made from an elastic plastic, with a strip of silver running its length; electrodes were connected to the crossbar's ends. The base of the T is made from a more rigid plastic. It includes two printed transistors and what the researchers call a “pixel” — a circle of semiconducting polymer whose color changes when the crossbars stretch, modifying the electrical resistance of the silver strip (Figure 2).
The transistors and the pixel are made from the same material — the transistors also change color slightly when the crossbars stretch. The effect is more dramatic in the pixel because the transistors amplify the electrical signal from the crossbar. Demonstrating working transistors was essential because large, dense sensor arrays require some capacity for onboard signal processing.
The researchers used a custom 3D printer with two different print heads — one for emitting hot materials and one for cool — and an array of ultraviolet light-emitting diodes. Using ultraviolet radiation to “cure” fluids deposited by the print heads produces the device's substrate. A copper and ceramic heater was added to deposit the semiconducting plastic. The plastic is suspended in a fluid that's sprayed onto the device surface, and the heater evaporates the fluid, leaving behind a layer of plastic only 200 nanometers thick.
In a standard transistor, there is an insulator between the gate and the semiconductor to prevent the gate current from leaking into the semiconductor channel. The transistors in the new device instead separate the gate and the semiconductor with a layer of water containing a potassium salt. Charging the gate drives potassium ions into the semiconductor, changing its conductivity. The layer of saltwater lowers the device's operational voltage, so that it can be powered with an ordinary 1.5-Volt battery.