Researchers at Arm have designed a microprocessor built on flexible plastic, dubbed PlasticArm. The 32-bit microprocessor contains 18,000 logical gates and the primary lobes of a computer brain: processor, memory, controller, inputs and outputs. 

Modern silicone chips, although cheap, are brittle and inflexible and crush under stress. While silicon technology has embedded at least one microprocessor into every ‘smart’ device, its limitation lies in making everyday objects, such as milk bottles or food packages smarter. For instance, placing a computer chip inside a milk carton to replace the printed expiration date with a sensor that detects chemical signs of spoilage. According to Myers, an engineer at Arm, they spend a lot of time trying to pack more silicone chips in less space.

 “Arm Research, together with PragmatIC, began exploring the feasibility of an Arm-based flexible processor back in 2013”, said Charlotte Chrsitopherson, the company’s marketing manager, in a blog. In 2018, the team collaborated to create the PlasticArmPit project. The idea was to develop tiny smell sensors on plastic chips that can be embedded anywhere from food packaging to the armpit of a T-shirt.

The researchers worked on the bendable chip by implementing the same cell library, tool flow and process technology as the PlasticArmPit project. In one of the applications the company is testing, a chip is mounted in an arrhythmia patient’s chest to monitor for irregular heartbeats. 

The primary driver for the technology is not to make an increasingly complex circuitry. Myers cited a few more basic implementations of these plastic chips in smart patches for wound care and healthcare applications, identification and provenance tracking, safety or freshness indicators for fast-moving consumer goods, or waste management and packaging recycling. 

The chip is composed of thin-film transistors made from indium, gallium and zinc metal oxides. The researchers developed this using a flexible electronic fabrication technique with these oxides on polyamide substrates. Metal-oxide TFTs are low cost and can also be scaled down to the smaller geometries required for large-scale integration. 

According to Arm’s research paper on paper, the chip consists of the new FlexIC 0.8-μm design to reduce area and increase yield. The researchers have also changed the standard cell architecture to include MT1 (metal-tracking 1) pins to make it easier for the router to hook up the cells. To improve the overall quality of the logic synthesis, several complex AND-OR-INVERT and OR-AND-INVERT logic gates and some high-drive-strength simple logic gates were added to the library. 

A high-level overview of the PlasticArm test chip and its design (Image: Arm & PragmatIC)

TFT technology is not to replace silicon, researchers said. Silicon will likely maintain advantages in terms of performance, density and power efficiency. But, TFTs can enable electronic products with novel form factors at cost points unachievable with silicon. 

Since plastic melts at a lower temperature than silicon, production techniques involving heat can not be used. The design also uses more power in comparison to older chips. Eric Pop, an electrical engineer at Stanford University, explained that while the chip consumes 21 milliwatts of power, only 1 percent of that performs computations; the rest is wasted as the chip sits idle. This happens partly because the chip uses N-type transistors instead of P-type; since P-type is more challenging to engineer using the materials that Arm has chosen.

While the researchers have reported a fully functional PlasticARM FlexIC, the paper said “the current ROM implementation does not allow changing or updating the program code after fabrication.” Still, they are hopeful that this would be possible in future implementation.

For now, the microprocessor can run some test codes Myers wrote a few years ago. It can also run the same sort of code as one of Arm’s standard silicon-based processors. The researchers claimed the chip has up to 100,000 logic gates scaling capacity, which is the maximum energy efficiency it can attain. 

The plastic chip is still a proof of concept and will need more prototyping and testing before becoming a commercially viable project. That said, the prototype is so far the most complex flexible integrated circuit built with metal-oxide TFTs, being at least 12× higher than the best previous integrated circuit.