Like anyone who designs computer chips for a living, James Myers is, at his core, a silicon guy. “Silicon is brilliant,” he says. Brilliant because it’s a natural semiconductor—able to both conduct electricity and act as an insulator, depending on the conditions—and because it can be engineered at small scale. Brilliant because it is the second-most-common element on Earth, probably clinging to the soles of your feet right now, and easily produced by heating sand. Those attributes have made it the bedrock of virtually every technology we use today. People like Myers, an engineer at the British semiconductor firm Arm, mostly spend their time thinking about how to pack more silicon into less space—an exponential march from thousands of transistors per chip in the 1970s to billions today. With Moore’s law, we are, as Myers puts it, “swimming in silicon.”
For the past few years, however, Myers has been looking beyond silicon to other materials, like plastic. That means starting again from the beginning. A few years ago, his team began designing plastic chips that contained dozens of transistors, then hundreds, and now, as reported in Nature on Wednesday, tens of thousands. The 32-bit microprocessor contains 18,000 logic gates—the electrical switches you get from combining transistors—and the basic lobes of a computer brain: processor, memory, controller, inputs and outputs, etc. As for what it can do? Think desktop from the early 1980s.
Why turn back the technological clock? Because modern silicon chips are brittle, inflexible wafers of electronics. Under stress, they crunch. And while silicon is cheap, and getting cheaper, there are some use cases where it may never be cheap enough. Consider a computer chip placed inside a milk carton, replacing a printed expiration date with a sensor that detects chemical signs of spoilage. Useful? Sorta! But it’s only worth adding to billions of cartons of milk if the cost is minimal. One application Arm is testing is a chest-mounted chip that monitors a patient for arrhythmia—an inconsistent, lilting heart beat—and is meant to be discarded after a few hours. For that, you want a computer that’s cheap but, even more importantly, one that bends. “It needs to move with you and not pop off,” Myers says.
A number of materials could theoretically meet those needs. Researchers have built transistors from organic materials and designed substrates—that’s the wafer the transistors go into—out of metal foils and even paper. The chip Myers’ team described Wednesday is composed of “thin-film transistors” made from metal oxides—a mix of indium, gallium, and zinc—that can be made thinner than their silicon counterparts. The substrate is polyimide, a kind of plastic, rather than a silicon wafer. It’s cheap, thin, and flexible—and also a bit of a pain to engineer. Plastic melts at a lower temperature than silicon, meaning some production techniques involving heat are no longer usable. And the thin transistors may contain imperfections, meaning energy doesn’t move around the circuitry in ways that chipmakers expect. Compared with modern chips, the design also uses a lot more power. These are the same issues that bedeviled chipmakers in the 1970s and ’80s, Myers points out. He can now sympathize with his older colleagues.
Compared with the billions found in modern 64-bit silicon processors, 18,000 gates doesn’t sound like much, but Myers speaks of them with pride. Sure, the microprocessor doesn’t do much; it just runs some test code he wrote five years ago that makes sure all the components are working. The chip can run the same sort of code as one of Arm’s common, silicon-based processors.
That consistency with silicon devices is key, explains Catherine Ramsdale, a coauthor of the research and senior vice president of technology at PragmatIC, which designs and produces the flexible chips with Arm. While the materials are new, the idea is to borrow as much as possible from the production process for silicon chips. That way, it’s easier to produce the chips en masse and hold down costs. Ramsdale says these chips might cost about one-tenth as much as comparable silicon chips, because of the cheap plastic and reduced equipment needs. It’s, yes, a “pragmatic” way of going about things, she says.