In a step toward more sustainable electronics, a team at North Carolina State University demonstrated a low-cost way to retrieve and reuse silver nanowires. The researchers say that this recycling method could potentially be extended to other materials beyond electronics, including nanoparticles and carbon nanotubes.

The researchers' paper, “Recycling of Nanowire Percolation Network for Sustainable Soft Electronics ,”co-authored by Yong Zhu, Andrew A. Adams Distinguished Professor of Mechanical and Aerospace Engineering at NC State, and Ph.D. students Yuxuan Liu and Hongyu Wang, is published in the journal Advanced Electronic Materials.", is published in the journal Advanced Electronic Materials.

The method described in the study differs from conventional recycling ideas.

Glass, for example, is melted down before it is reused. The N.C. State approach separates the silver nanowire network from the rest of the materials in a device.

In a proof-of-concept demonstration, the researchers created — and then recycled — a wearable health sensor patch that tracks a patient’s temperature and hydration. The sensor consisted of silver nanowire networks embedded in a polymer material.

The polymer material is soluble in solvents, but the nanowires are not.

Using a working solvent with low surface tension (isopropanol) and an optimal ultrasonication time (20 seconds), the silver-nanowire network film can be recycled multiple times — four times, in fact, for the health-sensor demo.

A Breakdown of the Process

Once a device has reached the limits of its use, the polymer matrix is dissolved, leaving behind the nanowires. When the nanowire mesh is hit with just the right amount of ultrasound, the nanowires dissemble from the network and are ready to be reused.

"It usually takes less than one hour to recycle one batch of nanowires," Zhu told Tech Briefs in a short Q&A below.

For Zhu's demonstration, the researchers dissolved the polymer in water, removed the nanowire network, broke the collection down into individual nanowires, and then used those nanowires to create a brand-new wearable sensor.

While there was minor degradation in the properties of the nanowire network after each “life cycle,” the team found that the nanowires could be recycled four times without harming the sensor’s performance.

“Using our approach, you get far more use from the nanowires,” said Prof. Zhu in an earlier press release . “And even after the nanowires have broken down many times, to the point where they can’t be reused, we can still use them as feedstock for conventional recycling. It’s a tremendous reduction in waste.”

In an email interview with Tech Briefs below, Zhu, in coordination with his colleagues, explains more about the promise of sustainable electronics.

Tech Briefs: There has been a growing interest in recycling of electronics. Why has electronics recycling been so challenging to accomplish, do you think, and how does your demonstration address that challenge?

Prof. Zhu: The recycling process of electronics waste with continuous films typically deposited in clean rooms requires specialized metallurgical processes, often involving high-temperature melting, which are complex, expensive, and tedious.

Nanomaterials have emerged as functional materials in electronics, especially soft electronics, due to their unique mechanical and electrical properties. Recycling of nanomaterials can be different from conventional recycling (of deposited films).

In our approach, a nanowire network is separated from the rest of the device, without destroying the nanowires. The nanowire network is then disassembled into separate nanowires in solution. These nanowires can be used in a new ink that will printed in the form of the nanowire network again, which is incorporated into a new sensor or other devices. This simple and low-cost method to recycle and reuse nanowires shows great promise in sustainable electronics. This recycling concept can be explored for other nanomaterials such as nanotubes and 2D materials.

Tech Briefs: How long does it take to recycle the nanowires? Is it a difficult process, given all the steps? I imagine that the ultrasound step could be difficult. How do you ensure that you apply the correct amount?

Prof. Zhu: It usually takes less than one hour to recycle one batch of nanowires. Given the large amount of materials that can be processed in one run, the yield can be large. As mentioned earlier, the process is simple and low-cost, with all the instruments widely used in industry and research labs. Ultrasonication is a critical step in this process, which can influence the mechanical and electrical properties of the recycled nanowires significantly. In our work, we used ultrasonication equipment with a fixed power, while studying the effect of ultrasonication time on recycled nanowires. The processing time needs to be carefully tuned to avoid both agglomeration and shortening of the nanowires.

Tech Briefs: Why is electronics recycling so important? In what applications is this kind of achievement most useful?

Prof. Zhu: More and more electronics are used in different facets of human life, yet the lifetime of these devices is showing a generally shortening trend (take smartphones, for example).

As a result, two important challenges have emerged: 1) a huge amount of waste from used electronics, posing a hazardous threat to the environment, and 2) a rapid consumption of scarce elements such as noble and rare-earth metals to make these electronic devices.

Recycling of these used electronics is important for reducing electronic waste and maximizing the use we get out of rare or costly materials. Our strategy in this work focuses on the recycling of functional materials in soft electronics, which cover a number of aspects of next-generation electronics, such as wearable healthcare devices, flexible displays, soft robotics, and so on. But the recycling concept presented can be applied to other electronic devices too, as long as nanomaterials in the form of a network are used.

Tech Briefs: What’s next for your research?

Prof. Zhu: For future work, we will investigate the fundamental aspects related to colloid stability in the recycling process to improve the recycling efficiency. This is a critically important step as currently there is still considerable degradation in the nanowire structure and properties after each recycling process. In addition, we will study the interactions between the nanowires, the substrate, and the dissolving solvent, such that more types of recyclable substrates can be used to achieve recycling of the entire electronic devices.

Furthermore, the recycling concept can be extended to other nanomaterials such as nanoparticles, carbon nanotubes, other types of NWs, and 2D materials, in the form of a percolation network. We will explore the recycling of different nanomaterials.

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