Thermal-Free, Wireless Patch-Type Tissue Oximeter with Radiative Cooling Structure

Abstract

For emerging skin-integrated electronics/optoelectronics, thermal management should be achieved for reliable signal acquisition as well as thermal comfort. However, the current layout of skin-integrated devices has limited their use to indoor settings, because of solar energy gain outdoors, particularly strongly in oximeters. Various approaches have been reported to release accumulated heat in the skin-integrated device, such as a thin metallic heat sink to dissipate the concentrated heat and a copper/paraffin bi-layer acting as a heat sink based on phase variation. However, metallic layers hinder wireless communication between the receiver and transmitter, and phase shift materials function irreversibly at high temperatures of ~ 50 °C. In particular, in conventional wearable oximeter reported in the literature, since black encapsulation layer is adopted to block ambient light, using the conventional layout in outdoor environment considerably exacerbates device heating owing to solar energy absorption. Previous works could not provide an obvious solution for these challenges. Here, we introduce a new platform of radiatively cooled wearable optoelectronics that is free from heat and light concerns, which enables assessing athletic performance in outdoor environments under direct sunlight. To maximize the radiative cooling performance, a nano-/micro-voids polymer (NMVP), a nonmetallic and flexible radiative cooler, is realized by applying bi-layer layout of two perforated polymers (i.e., polymethylmetacrylate (PMMA) and styrene-ethylene-butylene-styrene (SEBS)), which are optically and mechanically optimized for wearable optoelectronics. The NMVP boasts near-unity reflectivity and emissivity in the solar spectrum and atmospheric window (i.e., 8-13 μm-wavelength) with flexible feature. As a result, 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. Additionally, the integration of the optimum NMVP with the device enables maintaining temperature of ~ 34 °C on the skin under sunlight, whereas the normally used, black encapsulated device shows over 40 °C owing to solar absorption. The heated device exhibits an inaccurate tissue oxygen saturation (StO2) value of ~ 60 % compared with StO2 in a normal state (i.e., ~ 80 %). However, our thermally protected device presents reliable StO2 of ~ 80%. This successful demonstration provides a potential of thermal management in wearable devices for outdoor applications.