Major step toward fully 3D-printed active electronics

During the Covid-19 pandemic, the lack of widespread semiconductor fabrication facilities was one cause of a worldwide electronics shortage, which drove up costs for consumers and had implications in everything from economic growth to national defense. The ability to 3D print an entire, active electronic device without the need for semiconductors could bring electronics fabrication to businesses, labs, and homes across the globe.

While this idea is still far off, MIT researchers have taken an important step in that direction by demonstrating fully 3D-printed resettable fuses, which are key components of active electronics that usually require semiconductors.

The researchers' semiconductor-free devices, which they produced using standard 3D printing hardware and an inexpensive, biodegradable material, can perform the same switching functions as the semiconductor-based transistors used for processing operations in active electronics.

Although still far from achieving the performance of semiconductor transistors, the 3D-printed devices could be used for basic control operations like regulating the speed of an electric motor.

"This technology has real legs. While we cannot compete with silicon as a semiconductor, our idea is not to necessarily replace what is existing, but to push 3D printing technology into uncharted territory. In a nutshell, this is really about democratizing technology. This could allow anyone to create smart hardware far from traditional manufacturing centers," says Luis Fernando Velásquez-García, a principal research scientist in MIT's Microsystems Technology Laboratories (MTL) and senior author of a paper describing the devices, which appears in Virtual and Physical Prototyping.

He is joined on the paper by lead author Jorge Cañada, an electrical engineering and computer science graduate student.

An unexpected project

Semiconductors, including silicon, are materials with electrical properties that can be tailored by adding certain impurities. A silicon device can have conductive and insulating regions, depending on how it is engineered. These properties make silicon ideal for producing transistors, which are a basic building block of modern electronics.

However, the researchers didn't set out to 3D-print semiconductor-free devices that could behave like silicon-based transistors.

This project grew out of another in which they were fabricating magnetic coils using extrusion printing, a process where the printer melts filament and squirts material through a nozzle, fabricating an object layer-by-layer.

They saw an interesting phenomenon in the material they were using, a polymer filament doped with copper nanoparticles.

If they passed a large amount of electric current into the material, it would exhibit a huge spike in resistance but would return to its original level shortly after the current flow stopped.

This property enables engineers to make transistors that can operate as switches, something that is typically only associated with silicon and other semiconductors. Transistors, which switch on and off to process binary data, are used to form logic gates which perform computation.

"We saw that this was something that could help take 3D printing hardware to the next level. It offers a clear way to provide some degree of 'smart' to an electronic device," Velásquez-García says.

The researchers tried to replicate the same phenomenon with other 3D printing filaments, testing polymers doped with carbon, carbon nanotubes, and graphene. In the end, they could not find another printable material that could function as a resettable fuse.

They hypothesize that the copper particles in the material spread out when it is heated by the electric current, which causes a spike in resistance that comes back down when the material cools and the copper particles move closer together. They also think the polymer base of the material changes from crystalline to amorphous when heated, then returns to crystalline when cooled down -- a phenomenon known as the polymeric positive temperature coefficient.

"For now, that is our best explanation, but that is not the full answer because that doesn't explain why it only happened in this combination of materials. We need to do more research, but there is no doubt that this phenomenon is real," he says.

3D-printing active electronics

The team leveraged the phenomenon to print switches in a single step that could be used to form semiconductor-free logic gates.

The devices are made from thin, 3D-printed traces of the copper-doped polymer. They contain intersecting conductive regions that enable the researchers to regulate the resistance by controlling the voltage fed into the switch.

While the devices did not perform as well as silicon-based transistors, they could be used for simpler control and processing functions, such as turning a motor on and off. Their experiments showed that, even after 4,000 cycles of switching, the devices showed no signs of deterioration.

But there are limits to how small the researchers can make the switches, based on the physics of extrusion printing and the properties of the material. They could print devices that were a few hundred microns, but transistors in state-of-the-art electronics are only few nanometers in diameter.

"The reality is that there are many engineering situations that don't require the best chips. At the end of the day, all you care about is whether your device can do the task. This technology is able to satisfy a constraint like that," he says.

However, unlike semiconductor fabrication, their technique uses a biodegradable material and the process uses less energy and produces less waste. The polymer filament could also be doped with other materials, like magnetic microparticles that could enable additional functionalities.

In the future, the researchers want to use this technology to print fully functional electronics. They are striving to fabricate a working magnetic motor using only extrusion 3D printing. They also want to finetune the process so they could build more complex circuits

This work is funded, in part, by Empiriko Corporation.