Researchers Build Disposable Sensors With Organic Circuitry

The proof-of-concept semiconductor-and-battery combo aims to be biodegradable and nontoxic

3 min read

computer circuitry in the shape of a tree against a black background

Researchers at NTT Corp. and the University of Tokyo have developed the first combined battery-and-sensor circuitry from environmentally friendly materials that can transmit a communications signal. The proof-of-concept device is composed of carbon electrodes and wiring and other benign materials, including magnesium and cellulose, in place of metals such as lithium, copper, cobalt, and nickel that are potentially hazardous when carelessly discarded.

Kazuhiro Gomi, CEO of NTT Research in Sunnyvale, Calif., notes that the device is still in an early stage of development and there is much work to be done. “But now we have all the elements in place for an environmentally friendly battery and sensor, and we’ve proved the concept works.”

To prevent further environmental harm, Gomi says, such organic devices are going to be essential in the coming era of a trillion sensors and continued growth of the Internet of Things (IoT). He outlines a scenario where conventional sensors composed of hazardous materials will be employed in vast numbers globally, to monitor among other things soil conditions, crop growth, solar radiation, water and air quality, as well as changes in the weather. A problem arises when these sensors reach their end of life and are left uncollected or scattered around the environment in vast numbers to create e-waste.

“The growth of IoT sensors and batteries could absolutely add to the environmental burden of end-of-life technology,” says Geoff Walker, associate professor at the faculty of engineering at Queensland University of Technology, in Australia. “Especially given the potential of large volumes and short lifetimes of some applications.”

In 2018, NTT researchers developed the first iteration of a “return-to-earth battery,” explains Masaya Nohara, a research engineer involved in the project. The guiding concept is to use materials made of fertilizer ingredients and biodegradable materials. Eliminating toxic and rare metals in these devices creates an environmentally friendly battery that can be easily discarded.

While carbon seemed an obvious choice for the positive electrode, says Nohara, the powdered carbon typically used in its construction requires fluorine-based resins as binding agents. However, when used batteries are disposed of through incineration, the resins produce toxic gases, so they were unsuitable for the NTT project.

Instead, the researchers turned to biologically derived materials. After trial and error they came up with a suitable carbon material free from binding agents, and following the development of an electrostatic-spray-coating method of fabrication, they were able to construct porous carbonized electrodes without pollutants. For the negative electrode, magnesium is used, while the electrolyte is composed of magnesium acetate, and cellulose is used for the separator.

At present, the prototype battery has about one-tenth the capacity of a 6-cubic-centimeter commercial battery, enough to power a 0.1-watt LED lamp.

To test the effect of the fertilizer-based battery on plants, the researchers crushed several used batteries and mixed the parts with soil. Three kinds of soil were then used to grow mustard spinach: soil containing the crushed batteries, soil with crushed commercial batteries, and plain soil.

“Results confirmed that our return-to-earth battery had no adverse effect on the growth of the plant, unlike the soil containing the crushed commercial battery parts,” says Nohara.

But as Walker notes, “of course, further testing and thought is needed to understand the wider impact of these batteries—for example, as they find their way whole or decomposed into waterways and marine environments, or are ingested by animals or even humans.”

For the sensor circuitry, the researchers use organic matter including carbon, hydrogen, and sulfur for the semiconductors, polymide instead of silicon for the substrate, and carbon for the wiring, among other materials.

“From these we have developed carbon-electrode transistors, capacitors, and registers free of hazardous elements with Professor Junichi Takeya’s lab at the University of Tokyo,” says Gomi. “And we’ve used them to construct analog oscillation circuits and digital-modulation circuits using CMOS fabrication methods.”

For their proof-of-concept tests, the researchers chained 10 batteries in series to maintain a voltage of over 10 volts for more than 3 hours and produced a current of 1 milliampere. This was used to transmit a 140-hertz signal at 7 bits per second.

“This is too low right now to send a radio wave, so we transmitted the signal using conventional wiring to a speaker,” says Gomi. “Our next goal is to refine the circuitry to increase the frequency and speed.”

“The concept is a great idea,” says Walker. “But most battery-operated IoT sensors include some form of wireless interface, and for these, the proof-of-concept device has some way to go. Until these organic sensors are complex and fast enough to support a wireless interface, NTT should show they can build use cases which aren’t reliant on wireless interfaces.”

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