New substrate material for flexible electronics could help fight e-waste

A new type of flexible substrate material developed at MIT, the University of Utah, and Meta has the potential to enable not only the recycling of materials and components at the end of a device’s useful life, but also the scalable production of more complex, multilayer circuits than currently available substrates.

The development of this new material is described in the journal RSC: Applied Polymersin a paper by MIT professor Thomas J. Wallin, University of Utah professor Chen Wang, and seven others.

“We recognize that electronic waste is an ongoing global crisis that will only get worse as we build more devices for the Internet of Things and as the rest of the world evolves,” says Wallin, an assistant professor in MIT’s Department of Materials Science and Engineering. Much of the research on this front so far has been aimed at developing alternatives to conventional substrates for flexible electronics, which primarily use a polymer called Kapton, a trade name for polyimide.

Most of these studies have focused on completely different polymer materials, but “that really ignores the commercial side of why people chose these materials in the first place,” Wallin says. Kapton has a number of advantages, including excellent thermal and insulation properties and easy access to the raw materials.

The polyimide market is expected to be worth $4 billion by 2030. “It’s everywhere, in basically every electronic device,” including parts like the flexible cables that connect different components inside a cellphone or laptop, Wang explains. It’s also widely used in aerospace applications because of its high heat tolerance. “It’s a classic material, but it hasn’t been updated in three or four decades,” he says.

However, it is virtually impossible to melt or dissolve Kapton, so it cannot be recycled. These same properties also make it difficult to manufacture circuits in advanced architectures, such as multilayer electronics. The traditional way to make Kapton involves heating the material to temperatures of 200 to 300 degrees Celsius. “It’s a pretty slow process. It takes hours,” Wang says.

The alternative material the team developed, which is itself a form of polyimide and therefore should be easily compatible with existing manufacturing infrastructure, is a light-cured polymer, similar to those currently used by dentists to create strong, permanent fillings that harden in seconds with ultraviolet light. This method of curing the material is not only relatively fast, but it can also work at room temperature.

The new material could serve as a substrate for multilayer circuits, offering a way to significantly increase the number of components that can be packed into a small form factor. Previously, because the Kapton substrate did not melt easily, the layers had to be glued together, which made the process longer and more expensive. The fact that the new material can be processed at low temperatures while also curing very quickly on demand could open up possibilities for new multilayer devices, Wang says.

In terms of recyclability, the team introduced subunits into the polymer backbone that could be quickly dissolved by a solution of alcohol and a catalyst. Then, the precious metals used in the circuits, as well as entire microchips, could be recovered from the solution and reused in new devices.

“We designed a polymer with ester groups in the backbone,” unlike traditional Kapton, Wang explains. These ester groups can be easily broken down with a fairly mild solution that removes the substrate while leaving the rest of the device intact.

“We break the polymer down into its original small molecules. Then we can collect the expensive electronic components and reuse them,” Wallin adds. “We all know about the shortage in the supply chain for chips and certain materials. The rare earth minerals that are in those components are very valuable. So we think there’s a huge economic incentive now, as well as an environmental incentive, to create these processes to recover those components.”