Toward all-day wearable health monitoring: An ultralow-power, reflective organic pulse oximetry sensing patch

Abstract

Pulse oximetry sensors have been playing a key role as devices to monitor elemental yet critical human health states. Conventional pulse oximetry sensors, however, have relatively large power consumption, impeding their use as stand-alone, continuous monitoring systems that can easily be integrated with everyday life. Here, we exploit the design freedom offered by organic technologies to realize a reflective patch-type pulse oximetry sensor with ultralow power consumption. On the basis of flexible organic light-emitting diodes and organic photodiodes designed via an optical simulation of color-sensitive light propagation within human skin, the proposed monolithically integrated organic pulse oximetry sensor heads exhibit successful operation at electrical power as low as 24 μW on average. We thereby demonstrate that organic devices not only have form factor advantages for such applications but also hold great promise as enablers for all-day wearable health monitoring systems.

Fig. 1. Overview of the proposed OPO sensors.

(A) Schematic of the proposed OPO sensor with enlarged cross-sectional view to depict device arrangement and light receiving process through the skin medium. The picture of an OPO sensor in operation is also shown. Note that the 8-shaped OPD wraps around red and green OLEDs in operation. A microscope image of a red OLED and a part of the neighboring OPD are presented to show device-to-device alignment and electrode arrangement. Photo Credit: Hyeonwoo Lee, KAIST. (B) The device structure of the green OLED, the OPD, and the red OLED used for the proposed OPO sensor. (C) EQE versus luminance characteristics of green and red OLEDs. The detailed current density (J)–voltage (V)–luminance (L) characteristics are represented in section S1. (D) Spectral responsivity of the proposed OPD at 0 V and spectral emission plot of the proposed OLEDs. The responsivity values at 520 and 610 nm, which are the peak wavelengths of the green and red OLEDs, are found to be 0.29 and 0.21 A/W, respectively. The detailed J-V curves of the OLEDs and the OPD are shown in section S1. a.u., arbitrary units.

Fig. 2. Design considerations for low-power OPO sensors.

(A) Schematic of the proposed optical skin model used in this study. (B) Measured and simulated normalized power measured at a distance measured from the edge of an OLED. Box and line charts represent experimental and simulation results, respectively. (C) Top view of the simulated optical power distribution at the surface of a skin. The light from each OLED is optically coupled to the skin and scatters from various organelle back to the top surface. The radius of each OLED (r₀) is set at 0.4 mm. (D) Calculated normalized signal-to-noise ratio (SNR) versus the width (W) of the concentric, ring-type OPD for light originating from a circular OLED with r₀ = 0.4 mm. The inner radius (r₁) of the ring-type OPD is set at r₀ + 0.2 mm (= 0.6 mm) in consideration of fabrication margin. The SNR values obtained here are with f_H(r=r₁) and l₀ of 0.01 W/m² and 2.3 mm for red and 0.01 W/m² and 1.8 mm for green OLEDs. For details on estimation of SNR, refer to section S3.

Fig. 3. OPO sensor measurement at various body parts and power consumption comparison.

(A) The PPG signal, heart rate, and SpO₂ value obtained with R and G OLEDs from various body parts. H.R, heart rate; B.P.M., beats per minute. (B) Comparison of the power consumption of R and G light sources of oximetry sensors compared: †, discrete optical elements integrated on a PCB substrate with the edge-to-edge distance between elements being 2 mm (= PO1); ‡, a commercially available reflective pulse oximetry head (SFH7050, Osram Sylvania Inc.) (= PO2); ††, OPO sensor wherein rectangular OLEDs and OPDs are arranged side by side with the edge-to-edge distance of 2 mm (= PO3); ‡‡, the OPO sensor proposed in this work (= PO0). The layer configurations of OLEDs and OPD in the OPO sensor used in †† are identical to those used in this work.

Fig. 4. Significance of the optical coupling between the OPO sensor and the skin.

(A) Optical coupling monitored via reduction in substrate-confined light in the OLED. Spatial distribution of light seen across the edge of the substrate in the presence or absence of a finger contact on the OLED-emitting surface. Insets are side-view snapshots of the OLED glass/air and OLED glass/finger, respectively. Photo Credit: Hyeonwoo Lee, KAIST. (B) Extraction of substrate-confined mode from an OLED versus outcoupling methods shown in the inset diagrams: half-ball lens, MLA, and finger contact. (C and D) Cross-sectional view of ray diagrams: illustration (left) and optical simulation results (right) for the case where the substrate (PET) is optically coupled to the skin (C) and for the case where the substrate is not optically coupled to the skin due to the presence of air gap between the substrate and the skin (D).