Small isn't always beautiful in electronics, it turns out...and neither is newer always better: An innovation from Princeton University takes ideas from an invention of 1920s vintage and combines it with really large scale circuitry to turn huge areas of plastic into electronic devices, including—critically for our wireless world—radios.
When you think of a circuit you probably imagine either something printed in copper on a motherboard, a collection of wires on a breadboard or perhaps the microscopic tracks of semiconductor on a chip.
But recent innovations have seen various research teams placing circuits on much more exotic, perhaps even bendy materials...often as part of a move to improve sensor tech or display screens. Printing circuitry on plastic is part of this revolution, and it's been going on for a short while...but with limited success. That's because when you try to fashion a conducting circuit onto plastic you often need high temperatures—conductors tend to be metals, after all—and this can deform the plastic substrate. That's where some super clever thinking from Naveen Varma's team at Princeton comes in.
Essentially the Princeton research has improved on recent inventions in creating not just circuits but actual electronic components onto plastic substrates. These came from another Princeton team who realized they could create electronic structures, like thin-film transistors, using amorphous silicon instead of crystalline silicon. Crystalline silicon is the hard, hyper-precise gray material that you'll know as forming part of traditional silicon chips. Amorphous silicon is acutally a lot more randomly arranged, and the innovation in creating a thin film transistor out of it is due to the fact it can be produced at temperatures closer to 300ºC instead of around 1000ºC for crystalline silicon. That temperature is just about tolerable for plastics. But to make this work, the scientists had to sacrifice some of the electrical properties of the material. This made making plastic-backed transistors out of the stuff possible, but the devices were unsuitable for more complex electronics needs—for example, in creating a radio.
Varma's team took this research and combined it with some 1922-era research by the chap who invented FM radio, Edwin Armstrong. In his time the transistor was fantasy, and he and fellow engineers and scientists had to perfect circuit designs using valves—powerful, but very tricky analog circuits. The Princeton research has co-opted one of these designs, called the super rengenerative circuit, into their new plastic-printing technology. And lo and behold if it doesn't make amorphous silicon transistors work in a much more reliable way.
Why is this tech, which sounds like so much arbitrary stuff, exciting? It's actually huge. As in actually "huge." By allowing the creation of large-scale flexible radio circuits and sensors on a plastic backing, the team's innovation could create very cheap, very large sized pieces of connected electronics. Think of room sensors embedded in the wallpaper that scan for occupants, then wirelessly share that data with a security system or a home automation device (possibly requiring no independent power, if a thin-film solar cell is part of the plastic-backed circuit too). Or how about a totally invisible comms system that could send data across large buildings at will, without being troubled by the normal sorts of radio interference that affects Wi-Fi.
One application could be structural monitoring of large components, such as load-bearing plates or joints in bridges or buildings. Modern ways of assessing the integrity of these objects do work, but they're often imperfect and stunted in effectiveness by the sheer size of the object in question. Think of bridges that automatically report on cracks in huge joists, or aircraft wings that radio in when they detect a structural flaw.