Electronic waste is piling up around the world , partly because to recover useful materials from discarded gadgets. When processed improperly, spent electronics to lead, mercury and other toxic chemicals. Without systemic changes, our global appetite for electronics could produce an annual .

This conundrum inspired a team at the ����Ӱ�Ӵ�ý to create an easily recyclable material that could one day replace many traditional circuit boards, the foundation of most electronics. The new material is flexible, self-healing and can be made conductive without additional components.

This suite of features could help produce a more sustainable generation of wearable electronics, soft robotics and more.

“We created a lot of functionality within one material,” said senior author , a UW assistant professor of mechanical engineering. “Our goal is to build a widely useful platform for flexible, reusable devices.”

in Advanced Functional Materials. 

Conventional circuit boards pass electrical signals through conductive metal traces, which are bonded to a rigid board commonly made of fiberglass and resin. In contrast, the new material is a soft and stretchable composite made from a recyclable polymer infused with microscopic droplets of a liquid metal alloy based on gallium. A circuit can be created on this composite by lightly scoring a pattern into its surface, which connects adjacent embedded droplets and allows electricity to flow. The rest of the material remains electrically insulating. 

has been experimenting with liquid metal-infused polymers since 2019 — the team uses . It’s proven to be a promising class of materials, but the rising cost of the liquid metal motivated the team to focus on reusability.

The new composite has a few tricks up its sleeve. The polymer holding the liquid metal droplets is still stretchy and strong, but it can be broken down through a simple chemical process, freeing the metal for reuse. In experiments, researchers recovered 94% of the metal from their samples.

The composite also has self-healing properties. Users can cut the material into pieces, rearrange them, and bond them back together using only heat and pressure. An electrical circuit chopped up in this manner will still function when reconnected in a new configuration.

Malakooti envisions a new wave of electronics built with composites like this one, but also a new paradigm for use and reuse. Instead of mass producing gadgets and then tossing them out, he argues, we could design devices and their components to be used, repaired, reconfigured and ultimately recycled. 

“We’re trying to make a difference now to shape the future of flexible and wearable electronics,” Malakooti said. “We can’t make all these devices and then go back and try to figure out how to recycle them. That’s how we ended up with the electronic waste problem we face today. I want to tackle this problem from the very start.”

Co-authors include , a UW doctoral student of mechanical engineering; , a UW undergraduate student of mechanical engineering; and , and at the Oak Ridge National Laboratory.

This research was funded by the National Science Foundation and the Department of Energy.

For more information, contact Malakooti at malakoot@uw.edu.

One of the drawbacks of fitness trackers and other wearable devices is that their batteries eventually run out of juice. But what if in the future, wearable technology could use body heat to power itself?

UW researchers have developed a flexible, durable electronic prototype that can harvest energy from body heat and turn it into electricity that can be used to power small electronics, such as batteries, sensors or LEDs. This device is also resilient — it still functions even after being pierced several times and then stretched 2,000 times.

The team published Aug. 30 in Advanced Materials.

“I had this vision a long time ago,” said senior author , UW assistant professor of mechanical engineering. “When you put this device on your skin, it uses your body heat to directly power an LED. As soon as you put the device on, the LED lights up. This wasn’t possible before.”

Traditionally, devices that use heat to generate electricity are rigid and brittle, but Malakooti and team so that it can conform to the shape of someone’s arm.

This device was designed from scratch. The researchers started with simulations to determine the best combination of materials and device structures and then created almost all the components in the lab.

It has three main layers. At the center are rigid thermoelectric semiconductors that do the work of converting heat to electricity. These semiconductors are surrounded by 3D-printed composites with low thermal conductivity, which enhances energy conversion and reduces the device’s weight. To provide stretchability, conductivity and electrical self-healing, the semiconductors are connected with printed liquid metal traces. Additionally, liquid metal droplets are embedded in the outer layers to improve heat transfer to the semiconductors and maintain flexibility because the metal remains liquid at room temperature. Everything except the semiconductors was designed and developed in .

In addition to wearables, these devices could be useful in other applications, Malakooti said. One idea involves using these devices with electronics that get hot.

“You can imagine sticking these onto warm electronics and using that excess heat to power small sensors,” Malakooti said. “This could be especially helpful in data centers, where servers and computing equipment consume substantial electricity and generate heat, requiring even more electricity to keep them cool. Our devices can capture that heat and repurpose it to power temperature and humidity sensors. This approach is more sustainable because it creates a standalone system that monitors conditions while reducing overall energy consumption. Plus, there’s no need to worry about maintenance, changing batteries or adding new wiring.”

These devices also work in reverse, in that adding electricity allows them to heat or cool surfaces, which opens up another avenue for applications.

“We’re hoping someday to add this technology to virtual reality systems and other wearable accessories to create hot and cold sensations on the skin or enhance overall comfort,” Malakooti said. “But we’re not there yet. For now, we’re starting with wearables that are efficient, durable and provide temperature feedback.”

Additional co-authors are , a UW doctoral student in mechanical engineering, and , who completed this research as a UW postdoctoral scholar in mechanical engineering and is now an assistant professor at Izmir Institute of Technology. Malakooti and Han are both members of the UW Institute for Nano-Engineered Systems. This research was funded by the National Science Foundation, Meta and The Boeing Company.

For more information, contact Malakooti at malakoot@uw.edu.