Air-Stable Ultra-Flexible Organic Photonic System for Health Care Application

Optical bio-imaging is a non-invasive method to measure biological information from outside of the body, such as fluorescent probes, photoacoustic imaging, and near-infrared spectroscopy have been widely used as medical devices. In recent years, along with the development of semiconductor technology, the miniaturization of these imaging devices has been progressing. In particular, organic optical devices are being actively applied to healthcare by integrating them into wearable devices due to the featured characteristics such as high efficiency, flexibility, lightweight. For example, a flexible device that integrates an OLEDs and an OPD can be attached to a finger and measure the pulse wave by measuring the light reflected in the body. It is also possible to measure the blood oxygen ratio by a two-color light source such as red and green, or red and near-infrared light. Other biometric data such as veins and fingerprints image can also be taken by using high-resolution imaging sensors. It is important to improve the adhesion between the device and the body in order to perform such in vivo imaging with high accuracy. In this study, we developed an ultra-flexible photonic system in which the organic light-emitting diode and organic photodiode are integrated on the same substrate by improving the air stability of ultra-flexible organic light-emitting diodes and photodiodes. By applying an inverted structure, the ultra-flexible organic light-emitting diode has been operated over 10 days even without a high-barrier passivation layer. The ultra-flexible OLED shows high air stability in which the current density is maintained at more than 50% of the initial state. The organic photodiodes also show high air stability, with less than 1% change in photocurrent and less than 10 nA/cm² dark current after 10 days. The sensor system that integrates these air-stable optical devices is extremely thin with a total thickness of 3 µm. Therefore, it can be directly attached to the skin, and the pulse wave can be measured simply. Furthermore, we succeeded in measuring the pulse wave propagation time by combining the optical system with an electrocardiogram. When the blood pressure value was calculated from the measured pulse wave propagation time, a high correlation coefficient of 0.89 was achieved with the value measured by a conventionally used cuff-type blood pressure monitor system.