Wearable, water-resistant light detector targets health monitoring

RIKEN physicists have developed an ultrathin, organic light detector that, when attached to an individual’s finger, can record the optical signature of their pulse even when submerged in water¹. It shows promise for wearable and implantable applications in biophotonics such as monitoring of vital signs for health purposes.

Researchers hope that thin, flexible, organic optical electronic devices will one day lead to the realization of comfortable and imperceptible health-monitoring devices that can be directly worn on the skin.

“Optical monitoring in healthcare is gaining prominence owing to its versatility and minimal invasiveness,” says Baocai Du of the RIKEN Center for Emergent Matter Science (CEMS). “In particular, light detectors are playing a crucial role.”

However, to be practical such thin and flexible devices need to be both mechanically robust and water resistant, which is an ongoing challenge.

Now, a team led by Kenjiro Fukuda, also of CEMS, has developed a light detector that satisfies those two criteria (Fig. 1).

“Our organic light detector has an excellent stability and is resistant to water immersion and mechanical stress,” says Du. “It can provide reliable vital signs under challenging conditions such as body motion and water immersion.”

The detector was able to detect visible and near-infrared light with a high optical responsivity. Even after being immersed in water for five hours, the detector showed a stable response to light, with the responsivity dropping by just 6%—far less than a control device, which was degraded by 25%.

The secret to realizing water resistance was including an elastomer called SEBS in the photoactive layer, which converts light into electrons and lies at the heart of the device. This addition rendered the active layer water resistant. The adhesive SEBS also realized excellent adhesion between the layers, which makes it harder for water to seep in.

In addition to being water resistant, the device is easy to wear. “Our organic light detector is ultrathin, just 3.2 micrometers thick,” says Du. “It is also highly flexible, so that it can conform closely to the body and is comfortable to use.”

It was the device’s thinness that made it hard to make the device water resistant. “Making the device simultaneously ultrathin and water resistant was the biggest challenge,” says Du. “Firstly, organic semiconductors are intrinsically sensitive to water. Furthermore, the ultrathin substrate and encapsulation layers cannot protect the device enough, so water can easily penetrate inside the device and cause device failure.”

Given the success of the detector design, the team is now considering making other ultrathin, organic electronic and photonic devices that are also water resistant.