Research ArticleAPPLIED SCIENCES AND ENGINEERING

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

See allHide authors and affiliations

Science Advances  09 Nov 2018:
Vol. 4, no. 11, eaas9530
DOI: 10.1126/sciadv.aas9530
  • 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 fH(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.

    Photo Credit: Hyeonwoo Lee, KAIST.
  • 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).

  • Table 1 Performance summary of OLEDs and OPD studied in this work.
    OLED*OPD*†
    GreenRedAt λ = 520 nmAt λ = 610 nm
    EQE (%)21.920.8R (A/W)0.290.21
    Power efficiency (lm W−1)59.829.3D*(ideal) (Jones)4.3 × 10123.1 × 1012
    Turn on voltage (V)2.82.1D*(typical) (Jones)3.2 × 10102.3 × 1010

    *The detailed device characteristics are presented in section S1. EQE and power efficiency values were measured at 1000 cd m−2.

    D* was estimated using the magnitude of dark current recorded at V = −60 mV and equivalent shunt resistance estimated from the dark J-V characteristics. For details on the estimation of D*, please refer to the description provided in section S1.

    Supplementary Materials

    • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/11/eaas9530/DC1

      Section S1. Device characteristics of OLEDs and OPDs

      Section S2. Detailed description on optical simulation

      Section S3. Estimation of SNR ratio in a concentric configuration where a circular OLED is surrounded by a ring-type OPD sharing the center

      Section S4. Summary of overall fabrication process and mask-in-mask concept used for precise alignment among different patterns during thermal evaporation

      Section S5. Face seal encapsulation and interconnection procedure and characteristics of encapsulated OLEDs and OPDs

      Section S6. Application of proposed OPOs onto various body parts

      Section S7. Driving and signal acquisition scheme

      Section S8. SpO2 calibration method for OPOs

      Fig. S1. Opto-electrical characteristics of OLEDs and OPDs used in this work.

      Fig. S2. Ray-tracing example of the proposed simulation model.

      Fig. S3. Estimation of photo current, total noise, and SNR (= IPH/Itn) of each OLED case.

      Fig. S4. Overview of fabrication process and pattern definition by jigsaw puzzle–type, mask-in-mask shadow masking method.

      Fig. S5. Encapsulation and interconnection process and characteristics of the encapsulated devices.

      Fig. S6. Pictures of the working OPO sensors taken on the various body parts.

      Fig. S7. Schematic diagram showing the circuit functional blocks for driving of OLEDs and signal acquisition from the OPD of the proposed OPO sensors.

      Fig. S8. Overview of the calibration method.

      Fig. S9. Example of SpO2 calibration done at a forefinger.

      Table S1. Parameters used to estimate D* of the OPD under study.

      Table S2. Optical parameter of the human skin layers applied to the simulation.

      References (38, 39)

    • Supplementary Materials

      This PDF file includes:

      • Section S1. Device characteristics of OLEDs and OPDs
      • Section S2. Detailed description on optical simulation
      • Section S3. Estimation of SNR ratio in a concentric configuration where a circular OLED is surrounded by a ring-type OPD sharing the center
      • Section S4. Summary of overall fabrication process and mask-in-mask concept used for precise alignment among different patterns during thermal evaporation
      • Section S5. Face seal encapsulation and interconnection procedure and characteristics of encapsulated OLEDs and OPDs
      • Section S6. Application of proposed OPOs onto various body parts
      • Section S7. Driving and signal acquisition scheme
      • Section S8. SpO2 calibration method for OPOs
      • Fig. S1. Opto-electrical characteristics of OLEDs and OPDs used in this work.
      • Fig. S2. Ray-tracing example of the proposed simulation model.
      • Fig. S3. Estimation of photo current, total noise, and SNR (= IPH/Itn) of each OLED case.
      • Fig. S4. Overview of fabrication process and pattern definition by jigsaw puzzle–type, mask-in-mask shadow masking method.
      • Fig. S5. Encapsulation and interconnection process and characteristics of the encapsulated devices.
      • Fig. S6. Pictures of the working OPO sensors taken on the various body parts.
      • Fig. S7. Schematic diagram showing the circuit functional blocks for driving of OLEDs and signal acquisition from the OPD of the proposed OPO sensors.
      • Fig. S8. Overview of the calibration method.
      • Fig. S9. Example of SpO2 calibration done at a forefinger.
      • Table S1. Parameters used to estimate D* of the OPD under study.
      • Table S2. Optical parameter of the human skin layers applied to the simulation.
      • References (38, 39)

      Download PDF

      Files in this Data Supplement:

    Navigate This Article