Outdoor-Useable, Wireless/Battery-Free Patch-Type Tissue Oximeter with Radiative Cooling

Abstract

For wearable electronics/optoelectronics, thermal management should be provided for accurate signal acquisition as well as thermal comfort. However, outdoor solar energy gain has restricted the efficiency of some wearable devices like oximeters. Herein, wireless/battery-free and thermally regulated patch-type tissue oximeter (PTO) with radiative cooling structures are presented, which can measure tissue oxygenation under sunlight in reliable manner and will benefit athlete training. To maximize the radiative cooling performance, a nano/microvoids polymer (NMVP) is introduced 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 shows over 40 °C owing to solar absorption. The heated PTO exhibits an inaccurate tissue oxygen saturation (StO₂) value of ≈67% compared with StO₂ in a normal state (i.e., ≈80%). However, the thermally protected PTO presents reliable StO₂ of ≈80%. This successful demonstration provides a feasible strategy of thermal management in wearable devices for outdoor applications.

Keywords: daytime radiative cooling; nonmetallic/flexible radiative cooler; outdoor useable oximeter; thermal management; wearable optoelectronics.

Figure 1

Wireless/battery‐free patch‐type tissue oximeter with radiative cooler. A) Exploded view of schematic illustration of the constituent layers. B) Photograph of the device mounted on forearm. C) Photograph and D) thermography of the samples such as black elastomer (BE), white elastomer (WE), and NMVP on human bodies. E) Reflectivity and emissivity spectra from the visible to far‐infrared wavelength range of NMVP. Optical images of F) unencapsulated device and G) encapsulated device. The wavelengths of LED1 and LED2 are 850 and 750 nm, respectively. Quarter coin highlights the miniaturization of the device. Photographs of the device during wireless operation with smartphone in H) the flat state and I) the bent state. J,K) Wirelessly obtained light and temperature data by photodetector and thermistor, simultaneously.

Figure 2

Structural, optical, and thermal characterizations of NMVP. A) Photograph of NMVP in bent state. B) Schematic illustration for improving solar reflection and LWIR emission owing to Mie scattering enhancement and antireflection effect of gradual refractive index. C) False‐colored scanning electron microscope (SEM) image of NVMP. Magnified SEM images of D) p‐PMMA and E) p‐SEBS. Pore size distributions of F) p‐PMMA and G) p‐SEBS. H) Electric field distributions for computational models of p‐PMMA and p‐SEBS at the wavelengths of (top) 0.5, (middle) 2.1, and (bottom) 9 µm. I) Measured reflectivity spectra of NMVP at the surfaces of p‐PMMA (green line) and p‐SEBS (red line). J) Reflectivity and K) emissivity of p‐SEBS, p‐PMMA, and NMVP. L) 38 h continuous temperature measurement of p‐SEBS, NMVP, and BE. Yellow line indicates the solar intensity, I _solar. M) Measurement reliability of NMVP cooling ability by monitoring for several days. ‘SC’, ‘HM’, ‘OC’, and ‘CL’ refer to scattered, humid, overcast, and clear.

Figure 3

Device performance evaluation for flexibility and reliability. A) Photograph of NMVP‐integrated PTO on finger connected with a smartphone. B,C) Cyclic bending test result for 200 bending cycles in 20 increments for PTO and NMVP. B) Resonance frequency and Q‐factor of PTO with bending cycle. C) Average solar reflectance and LWIR emissivity of NMVP with bending cycle. D,E) Result of vein occlusion test with NMVP integrated PTO at forearm. D) Measured raw data and E) variation in hemoglobin concentration calculated by data processing. Photograph of subjects for isometric exercising at each side of forearm using dumbbell with F) NMVP‐integrated PTO and G) commercial device. H) StO₂ measured by NMVP‐integrated PTO (blue line) and commercial device (gray dash line) during isometric exercise. I,J) Isotonic exercise with attached NMVP‐integrated PTO and the commercial device to each thigh. K) StO₂ measured by NMVP‐integrated PTO (blue line) and commercial device (gray dash line) during isotonic exercise.

Figure 4

Effect of heat on tissue oximeter and thermal protection by NMVP. A) Illustration image of cyclist with NMVP‐integrated and black elastomer‐integrated tissue oximeters on each thigh. The insets represent thermal effects: i) light absorption of black elastomer. The red gradation presents the heat transfer from black elastomer to device and skin. The inset image shows black‐elastomer‐covered skin with postexposure to direct sunlight, which turned reddish. ii) Thermal effect‐free NMVP. The sky‐blue arrows represent thermal radiation from NMVP. Investigation of the thermal interference with measuring B) hemoglobin concentration and C) StO₂. The red and gray areas indicate heating and exercising periods, respectively. D) Measured temperatures of NMVP, BE, and WE on skin under direct sunlight for 7 min. The dashed line expresses the indoor measured result (≈32.8 °C). E) The difference between indoor and outdoor skin temperatures. The measurement was performed on November 15, 2020. Comparison of measurements pre‐exposure and postexposure with F) BE‐integrated device, G) WE‐integrated device, and H) NMVP‐integrated device attached to forearm. Sky‐blue regions represent time period (4 min) of direct sunlight exposure. The measurements were performed on November 15, 2020.