Highly Reliable Wireless Patch-type Bio-integrated Sensor

Abstract

Various wearable sensors for monitoring bio-signals have been developed and researched with increasing people's interest in health. However, in order to enhance user comfort, miniaturization and device heating issue must be improved. Especially in optoelectronic devices such as photoplethysmography (PPG) sensors (ie., tissue oximeter, pulse oximeter), heat problems can hinder accurate bio-signal measurements as well as cause skin burns. Therefore, we present miniaturized patch-type tissue oximeter (PTO) with radiative cooling structures, which can measure tissue oxygenation and temperature under sunlight in reliable manner. The device was designed to operate wirelessly without a battery based on the NFC system, with flexible and compact feature. The bi-layer coil structure of PTO enable high power supply with compact size. The reliability of PTO is demonstrated by comparison with a commercial sensor. To maximize the radiative cooling performance, we introduce a nano-/micro-voids polymer (NMVP) by combining two perforated polymers to both reduce sunlight absorption and maximize thermal radiation. The optimized NMVP exhibits sub-ambient cooling of 6 °C in daytime under various conditions such as scattered/overcast clouds, high humidity, and clear weather. The NMVP-integrated PTO enables maintaining temperature within ~ 1 °C on the skin under sunlight relative to indoor measurement, whereas the normally used, black encapsulated PTO exhibits over 40 °C owing to solar absorption. The heated PTO shows an inaccurate tissue oxygen saturation (StO2) value of ~ 70 % compared with StO2 in a normal state (i.e., ~ 80 %). However, our thermally protected PTO presents reliable StO2 of ~ 80%.