Scientists are tackling the growing problem of electronic waste by designing wearable devices made entirely from sustainable materials.        

Electronic waste, also known as e-waste, is a persistent health and environmental threat, with public concern centered around the billions of pounds of plastic that end up in oceans every year. Heavy metals and other toxic substances from e-waste also leach from landfills into the environment, amplifying the problem.

According to the team of researchers at North Carolina State University, devices that are simultaneously recyclable and biodegradable are rare. Although transient or edible electronics designed to disintegrate, dissolve, or even be eaten at the end of their service life partially solve this issue, their functional components — for example, their circuit boards — are usually neither biodegradable nor recyclable.

A sustainable wearable patch

The North Carolina team confronted this issue by designing a wearable electronic patch that can be simultaneously composted and recycled. The soft part of the device, called the substrate, is made from a renewable biomaterial, while the electrical circuit is composed of silver nanowires that can be recycled. This strategy avoids the loss of scarce, valuable metals commonly found in electronic devices.

“The electronic patch can, after further development, be used for a wide range of applications, such as human health monitoring, electronic textiles, sports performance monitoring, soft robotics, prosthetics, and human–machine interfaces,” Yong Zhu, a professor and one of the device’s developers, stated. Zhu’s research at NC State focuses on nanomechanics and nanoengineering.

Orlin Velev, a professor in the Department of Chemical and Biomolecular Engineering at NC State, described their approach. “We started with fabricating substrates in the form of gel films from agarose—a biopolymer derived from red seaweed—modified with glycerol, followed by printing conductive silver nanowire patterns on the agarose/glycerol gel film using screen printing,” he said.  

Velev, Zhu, and their team tested the sustainable electronic patch as an electrophysiological sensor. When worn on the wrist or forearm, the silver nanowire electrodes detected electrocardiogram and electromyography signals just as well as a commercial gel electrode, even when the skin was stretched, compressed, and twisted. They attributed the performance of the device to its excellent conformability and stretchability. 

Glycerol, a harmless byproduct of biofuel synthesis, acts as a plasticizer, enhancing the flexibility of agarose while maintaining its biodegradability. The researchers reported that glycerol increased the stretchability of the film from 24.8% to 68.1%—this degree of flexibility is essential for fabricating a device intended to be worn on the skin.

On the molecular level, the network of hydrogen bonds that form between agarose and glycerol are responsible for the improved stretchability of the device. These interactions between hydrogen and oxygen prevent the propagation of cracks during stretching. The researchers found that an agarose/glycerol ratio of around 1:5 produced a highly stretchable film without compromising other important mechanical properties like strain recovery and Young’s modulus.

Device disposal and drawbacks

The researchers claim that the electronic patch can be safely disposed of by the consumer. The agarose/glycerol gel films would biodegrade upon composting, and the silver nanowires do not pose a major threat to the environment. However, although the device can simply be thrown away, the nanowires would ideally be recovered and reused.

“After introducing options for device collection and recycling, the collected devices can be immersed in a microbial suspension formulated by a commercial compost activator,”  Zhu told us. The device will completely degrade after 65 days when treated this way.

“The silver nanowires could be collected and reused after biodegradation of the agarose/glycerol substrates,” he added.  

The current protype has a few disadvantages, including the leaching of glycerol from the film when it comes into prolonged contact with water. The researchers noted that a plasticizer with a higher molecular weight than could potentially solve this leaching problem.

“Basically the larger the molecular weight, the smaller the tendency to diffuse through the [agarose] matrix and leach,” Velev explained.

Another limitation has to do with the amount of strain the circuits can tolerate. Velev elaborated on this: “While the plasticized films are very soft and extensible, the printed circuits on top of them have a certain limit to [their] extension. Luckily, the printed nanowire circuits can tolerate larger extensions than the ones encountered on the skin on most parts of the body.”

To help protect the conductive pattern on the film, an external barrier could also be added.  

“The electromechanical stability of the printed silver nanowire electrodes can also be improved by adding an encapsulation layer made of biopolymer gel on the device,” Zhu told us.

In the paper, the researchers explained that they omitted this feature for the purpose of physiological monitoring, where electrodes have to be in direct contact with the skin. But in the future, this modification could make the patch more practical for other applications.  

“We’re not aiming for concrete functional wearables at this stage,” Velev said, “but aim to prove the principle that such soft devices could be made from materials that are completely sustainable.”

Reference: Orlin D. Velev, Yong Zhu, et al.,Sustainable Soft Electronics Combining Recyclable Metal Nanowire Circuits and Biodegradable Gel Film Substrates, Advanced Electronic Materials (2024). DOI: 10.1002/aelm.202300792

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