Tiny Heart Monitor Powered By the Sun

Japanese researchers have developed a solar-powered cardiac monitor that can be worn directly on the skin.

Scientists at Japanese research institution RIKEN and the University of Tokyo integrated what’s called an organic electrochemical transistor (OECT)—a device that can measure various biological functions—into a flexible, organic solar cell. In doing so, they have developed a sensor with new technology in solar-energy generation to develop a bio-compatible, self-powered heart monitor that people can wear directly on their skin.

“We have developed a human-friendly, ultra-flexible organic sensor powered by sunlight, which acts as a self-powered heart monitor,” said Kenjiro Fukuda, one of the researchers on the project from the RIKEN Center for Emergent Matter Science, in a RIKEN news release.

self-powered heart monitor
Researchers in Japan have developed a flexible heart monitor that can be worn directly on someone’s finger using new sensor and solar-energy-generating technology. (Image source: RIKEN)

No More Bulky Power Sources

Researchers have been working for some time to integrate energy-harvesting technology into wearables to eliminate the need for bulky power sources. Medical and fitness devices that track human vital signs and collect other potential diagnostic information are among some of the earliest types of this technology.

For next-generation wearables, researchers are eying the use of biocompatible materials—combined with reliable self-powering sources—so they can fit directly onto human skin. An advancement like the technology developed by RIKEN researchers is a step in this direction, eliminating not just the bulkiness, but biocompatibility and practicality issues associated with existing technology, researchers said in the news release.

Key to the technology developed by the RIKEN team, said Fukuda, is the use of a nano-grating surface on the light absorbers of the solar cell. This allows for high photo-conversion efficiency (PCE) and light-angle independency, providing unprecedented stability of the power source.

“Although the ultra-thin solar cells with high-power-conversion efficiency and mechanical/thermal stability have been achieved, the integration of ultra-thin solar cells with sensors has not yet been demonstrated because of their unstable output power under mechanical deformation and angular change,” Fukuda explained. “The benefit of using a nano-grating structure is very important for wearable power sources because they can be easily deformed onto complex three-dimensional biological tissues and skins.”

The nano-grating structure allowed researchers to reach a PCE of 10.5 percent and a high power-per-weight ratio of 11.46 watts per gram in the device, he said. This approaches what scientists consider the “magic number” of 15 percent that makes organic photovoltaic technology competitive with existing silicon-based technology. Moreover, under repetitive compression tests of 900 cycles, the device demonstrated a PCE decrease of only 25 percent from 9.82 percent to 7.33 percent—a higher PCE gain of 45 percent compared to non-grating devices under a 60-degree light angle, Fukuda said.

Researchers found that the device worked optimally at a lighting level of 10,000 lux—about the equivalent to the light seen in the shade on a clear sunny day. The device also experienced less noise than similar devices connected to a battery, presumably because of the lack of electric wires.

“The important feature is that the required voltage is very small—less than 1 volt—which is consistent with the integration with solar cells,” Fukuda explained. “Of course, there were a lot of tries and errors to achieve the integration of OECTs with solar cells.”

The technology already performs well enough to be used to monitor someone’s heartbeat as they exercise, Fukuda said. In the future, if accuracy is improved, it can be optimized to use as an imperceptible electrocardiogram system for medical use. Researchers plan to continue their work to improve the device to make the power supply even more stable, as well as to further integrate other types of sensors and technology to extend its capabilities, Fukuda added.

Researchers outline their path to developing the technology in a paper on their work published in the journal Nature.

Elizabeth Montalbano is a freelance writer who has written about technology and culture for 20 years. She has lived and worked as a professional journalist in Phoenix, San Francisco, and New York City. In her free time, she enjoys surfing, traveling, music, yoga, and cooking. She currently resides in a village on the southwest coast of Portugal.

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