Air-Stable Ultra-Flexible Organic Photonic System for Cardiovascular Monitoring

Optical bioimaging techniques, such as imaging using fluorescent probes, photoacoustic imaging, and near-infrared spectroscopy, are noninvasive methods that is widely used in medical devices for measuring biological information from outside. In recent years, with the development of semiconductor technology, miniaturization of these imaging devices has become possible. In particular, organic optical devices have such features as high efficiency, flexibility, and low weight; hence, they are being increasingly applied to healthcare through integration into wearable devices. For example, a flexible device that integrates an organic light-emitting diode (OLED) and an organic photodetector (OPD) can be attached to a finger, and the pulse wave can be calculated by measuring the light reflected from the body. Measuring the blood oxygen ratio using two-color light sources, such as red and green, or red and near-infrared light-emitting diodes, is also possible. Other biometric data, such as veins and fingerprint images, can also be obtained using high-resolution imaging sensors. Improving the conformability of the device to the body is important to perform such <i>in vivo </i>imaging with high accuracy.<br/>Soft optical devices should be developed to improve the conformability of devices to the skin. Previously, extremely soft organic optical devices have been realized by ensuring the elasticity of the substrate and required materials. Owing to the development of new materials and processes, soft organic optical devices can achieve the same electrical properties as devices fabricated on glass or thick plastic film substrates. Furthermore, the thickness of the film substrate can be reduced to realize ultrathin organic optical devices with improved skin conformability. An OLED that is formed on a 1.5 μm-thick plastic substrate and can be attached to the skin has been reported.<br/>However, the integration of multiple types of organic optical device, such as OLEDs and OPDs, on the same thin substrate of less than 50 µm without using the lamination process has not been reported. This is because organic optical devices fabricated on a flexible substrate are not stable and require an encapsulating layer with high barrier properties, and making the film thinner while maintaining the barrier properties is difficult. Furthermore, to integrate different optical devices, it is necessary to fabricate the devices using a vacuum process, which results in insufficient characteristics of the organic photodiodes or organic photovoltaics. Therefore, it is necessary to form OPDs using a solution process and improve the stability of the organic optical devices to integrate high-performance organic optical devices on the same ultrathin substrates.<br/>In this study, we developed an ultra-flexible optical sensor system wherein OLEDs and organic photodiodes are integrated on the same substrate by improving the air stability of ultra-flexible OLEDs and photodiodes. By applying an inverted structure, the ultra-flexible OLEDs were operated for more than 10 days, even without a high-barrier passivation layer. The ultra-flexible OLEDs exhibited high air stability, wherein the current density was maintained at more than 50% of the initial state. The organic photodiodes also demonstrated high air stability, with less than 1% change in photocurrent and less than 10 nA/cm² dark current after 10 days. The sensor system integrating these air-stable optical devices is extremely thin, with a total thickness of 5 µm. Therefore, it can be directly attached to the skin, and the pulse wave can be measured easily. Furthermore, we succeeded in measuring the pulse wave propagation time by combining an 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. This value was measured using a conventionally used cuff-type blood pressure-monitoring system.