Japanese scientists say they have developed a human-friendly, ultra-flexible organic sensor powered by sunlight, which acts as a self-powered heart monitor.
Researchers from the University of Tokyo and RIKEN in Japan integrated a sensory device, called an organic electrochemical transistor—that can be used to measure a variety of biological functions—into a flexible organic solar cell.
They were then able to measure the heartbeats of rats and humans under bright light conditions, according to the research published in the journal Nature.
"This is a nice step forward in the quest to make self-powered medical monitoring devices that can be placed on human tissue," said Kenjiro Fukuda of the RIKEN Center for Emergent Matter Science.
Self-powered devices that can be fitted directly on human skin or tissue have great potential for medical applications.
They could be used as physiological sensors for the real-time monitoring of heart or brain function in the human body, researchers said.
However, practical realisation has been impractical due to the bulkiness of batteries and insufficient power supply, or due to noise interference from the electrical supply, impeding conformability and long-term operation, they said.
The key requirement for such devices is a stable and adequate energy supply.
The study used a nano-grating surface on the light absorbers of the solar cell, allowing for high photo-conversion efficiency (PCE) and light angle independency.
The researchers were able to achieve a PCE of 10.5 per cent and a high power-per-weight ratio of 11.46 watts per gramme, approaching the "magic number" of 15 per cent that will make organic photovoltaics competitive with their silicon-based counterparts.
They demonstrated a PCE decrease of only 25 per cent (from 9.82 per cent to 7.33 per cent) under repetitive compression test (900 cycles) and a higher PCE gain of 45 per cent compared to non-grating devices under 60 degree light angle.
To demonstrate a practical application, organic electrochemical transistors were integrated with organic solar cells on an ultra-thin substrate, to allow the self-powered detection of heartbeats either on the skin or to record electrocardiographic (ECG) signals directly on the heart of a rat.
They found that the device worked well at a lighting level of 10,000 lux, which is equivalent to the light seen when one is in the shade on a clear sunny day, and experienced less noise than similar devices connected to a battery, presumably because of the lack of electric wires.
"Importantly, for the current experiments we worked on the analog part of our device, which powers the device and conducts the measurement," Fukuda said.
"There is also a digital silicon-based portion, for the transmission of data, and further work in that area will also help to make such devices practical," said Fukuda.
