Fully recyclable printed electronics created

There are already over 20 billion interconnected electronic devices in the world and the number is growing rapidly. One of the side effects of the emergence of the Internet of Things is an ever-growing pile of electronics discarded at the end of their useful life or when replaced by a newer model.

According to a UN estimate, less than a quarter of the electronics thrown away every year is recycled. Part of the problem is that electronic devices are very difficult to recycle, containing many different materials, including non-recyclable silicon chips.

The Duke University researchers have offered an alternative to conventional electronics, demonstrating that a transistor can be created with three carbon-based inks. The study has been published in Nature Electronics.

“Silicon-based computer components are probably never going away and we don’t expect easily recyclable electronics like ours to replace the technology and devices that are already widely used,” said Professor Aaron Franklin, an electrical engineer at Duke University. “But we hope that by creating new, fully recyclable, easily printed electronics and showing what they can do, that they might become widely used in future applications.”

Franklin and his colleagues used carbon nanotube (for the semiconductors) and graphene (for the conductors) inks to create a recyclable transistor which can be printed on paper. While these materials have been used in printed electronics, the path to recyclability was only cleared with the development of a wood-derived insulating dielectric ink, nanocellulose.

“Nanocellulose is a biodegradable and has been used in applications like packaging for years,” said Franklin. “While people have long known about its potential applications as an insulator in electronics, nobody has figured out how to use it in a printable ink before. That’s one of the keys to making these fully recyclable devices functional.”

The researchers found a technique for suspending nanocellulose crystals extracted from wood fibres that – with a sprinkling of salt – yields an ink which performs well as an insulator in their printed transistors. Their transistors can be created from the three carbon-based inks using an aerosol jet printer and remain usable in a variety of applications even six months later.

They demonstrated the recyclability of their transistors by submerging them in a series of baths, vibrating them with sound waves and centrifuging the resulting solution; the carbon nanotubes and graphene are sequentially recovered with an average yield of nearly 100 per cent. The materials can then be reused in the same printing process while losing little performance viability. Meanwhile, the nanocellulose, being derived from wood, can be recycled along with the paper substrate.

By initially demonstrating this fully recyclable, multifunctional printed transistor, the researchers hope to make a first step toward their technique being used commercially to manufacture simple devices. For instance, Franklin says the technology could be used in a large building which requires thousands of simple environmental sensors to monitor energy use or in custom biosensing patches for medical monitoring.

“Recyclable electronics like this aren't going to go out and replace an entire half-trillion-dollar industry by any means and we're certainly nowhere near printing recyclable computer processors,” Franklin said, “but demonstrating these types of new materials and their functionality is hopefully a stepping stone in the right direction for a new type of electronics lifecycle.”