Research ArticleAPPLIED PHYSICS

Flexible graphene photodetectors for wearable fitness monitoring

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Science Advances  13 Sep 2019:
Vol. 5, no. 9, eaaw7846
DOI: 10.1126/sciadv.aaw7846
  • Fig. 1 Health-tracking prototypes based on GQD PDs.

    (A) Photograph of the flexible and transparent GQD PD integrated in HR monitoring bracelet. (B) Zoomed-in photograph of the flexible PD containing a 1-mm2 graphene channel on the PEN substrate that is completely covered by a thin layer of PbS QDs (30 nm in thickness). The detector is visibly transparent and mechanically flexible. (C) Schematic illustration of the assembly of graphene and QDs on a flexible substrate. (D) Schematic of photoplethysmogram (PPG) in reflectance mode. Volumetric changes in the microvessels modulate the backscattered light reaching the GQD PD. (E) HR monitoring bracelet based on reflection mode PPG to extract vital signs from wrist. (F) Schematic of transmission mode PPG. Transmissive ambient light is modulated by the cardiac cycle and reaches the PD. (G and H) Photograph of the health patch on the mobile phone screen that uses transmission mode PPG to extract HR from finger. (I) Normalized PPG readings for transmission and reflectance modes of operation. High sensitivity and mechanical flexibility of GQD PDs allow health patches to operate accurately for long periods in both modes. Photo credit: Alina Hirschmann, ICFO–Institut de Ciencies Fotoniques.

  • Fig. 2 Electro-optical and mechanical characterization of the GQD PDs on flexible polymer substrates.

    (A) Photograph of macroscale PD on the PET substrate. Scale bar, 5 mm. (B) Photo-induced resistance change (ΔR/R) with respect to irradiance at 633 nm. Horizontal dashed line represents the noise floor of the device, which is measured to be 4.4 × 10−7 and corresponds to an NEI value of 3.7 × 10−11 W cm−2. Inset shows an individual GQD PD on the PEN substrate. Scale bar, 500 μm. (C) Mechanical stability of the flexible PD on the PEN substrate. Change in photoresponse due to applied uniaxial strain is minimal over 2000 cycles. (D) Photograph of the gated PDs on polyimide. Series of PDs were implemented on a gate structure containing a 50-nm Al covered by a 100-nm Al2O3 dielectric layer. Contact electrodes are extended along the substrate for interconnects. Inset shows a zoomed-in image of the GQD channel on the gate structure. Scale bar, 50 μm. (E) Responsivity versus applied gate voltage. Gate provides control on the speed, responsivity, and the transferred charge type. (F) Dynamic response of the detector for fixed gate voltages. Flexible PDs sustain high-frequency operation with a typical cutoff frequency on the order of 104 Hz. The inset shows the temporal response at 633 nm from which the response time is extracted to be 50 μs. Photo credit: Emre O. Polat, ICFO–Institut de Ciencies Fotoniques.

  • Fig. 3 HR and RR measurements by GQD health patches.

    (A) Flexible and transparent health patch on the mobile phone screen. Software is in an idle state. (B) Blood pulse monitoring from the user’s finger. Two-terminal health patch connects to a mobile readout unit, which processes and sends the data to the mobile phone via Bluetooth. The developed software allows monitoring of the HR in real time and displays the PPG trace. (C) PPG trace for visible and near-infrared wavelengths. Multiwavelength absorption of the GQD channel presents compatibility for optical measurements of SpO2. (D) Correlation plot of measurements from the health patch (HRGQD) and a state-of-the-art PPG sensor used in the clinical setting (HRSoA) (Nellcor OxiMax SpO2 module, Medtronic Capnostream 20p). The data from the two devices yield a concordance correlation coefficient of 0.988, representing a high correlation in between two simultaneous measurements. (E) Bland-Altman plot analysis of the health patch. The green line represents the mean difference (ΔHMean) of simultaneous measurements taken from both devices with a value of −0.078. The top and bottom blue lines represent the SD of both measurements (±1.96σ) that set the limit of agreement (LoA) of the measurements with the values of (−1.48, 1.18). The variations in between the limit of agreement (±LoA) state the high probability that the methods do not disagree. (F) Fourier transform of the recorded PPG. High-intensity peaks at 0.29 and 1.20 Hz represent the dominant RR and HR of the individual over 5 min of recording, which correspond to 17 breathe per minute and 72 bpm respectively. Harmonics at 0.25 and 0.33 Hz correspond to RRs of 15 and 20 bpm, and HR harmonics at 1.16 and 1.22 Hz correspond to 69 and 73 bpm. (G) Bland-Altman plot analysis for the extracted RR proving the good agreement with the values of ΔHMean = 0.39 and LoA = (−1.79, 2.53) for the GQD health patch and state-of-the-art capnograph in the clinical setting (Medtronic Capnostream 20p). Inset shows the correlation plot showing the linear agreement with a concordance correlation coefficient of 0.8421. Photo credit: Stijn Goossens, ICFO—Institut de Ciencies Fotoniques.

  • Fig. 4 Wireless and battery-free UV monitoring patch.

    (A) Photograph of the GQD-UV patch. GQD assembly was heterogeneously integrated onto a commercially available NFC patch (TIDM-RF430-TEMPSENSE, Texas Instruments), and the electrical connection between PD and the chip is obtained by deposited metal lines. Scale bar, 10 mm. (B) Block diagram of the wireless and battery-free UV monitoring system. NFC provides two-way communication by inductively powering the patch and wirelessly sending the data to smartphone. (C) Mobile UV index monitoring via UV patch placed on the arm. The patch uses a flexible short-pass filter on the front side that blocks the wavelengths greater than 400 nm, allowing an accurate monitoring of the environmental UV index. The developed software displays the actual UV index and informs the user about recommended remaining exposure time. (D) Modulation of output NFC signal with respect to irradiance at 285 nm. High sensitivity of 1 mW/m2 allow accurate and wireless UV index measurement. The color scale shows the severity of the UV exposure according to the Diffey weighted average. Photo credit: Alina Hirschmann, ICFO-Institut de Ciencies Fotoniques.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/9/eaaw7846/DC1

    Fig. S1. PPG readings of GQD Health Patch under various light conditions for the same applied bias.

    Fig. S2. Flexible and transparent GQD assembly on PEN and the dynamic range of flexible GQD PDs.

    Fig. S3. Noise characteristics of flexible GQD PDs.

    Fig. S4. Gate modulation of flexible gated GQD PDs.

    Fig. S5. Photosensitivity for various operation frequencies.

    Fig. S6. Temporal response of the flexible GQD PD at 633 nm.

    Fig. S7. Photoresponse of flexible GQD PDs at near-infrared wavelength.

    Fig. S8. Continuous PPG reading by GQD health patch.

    Fig. S9. Simultaneous measurements of GQD health patch and state-of-the-art capnograph.

    Fig. S10. Large-area Raman spectroscopy mapping of graphene on flexible polymer substrates.

    Table S1. Demonstrated device types and their specifications.

    Movie S1. GQD bracelet (reflection mode PPG).

    Movie S2. GQD health patch on the mobile phone screen (transmission mode PPG).

  • Supplementary Materials

    The PDF file includes:

    • Fig. S1. PPG readings of GQD Health Patch under various light conditions for the same applied bias.
    • Fig. S2. Flexible and transparent GQD assembly on PEN and the dynamic range of flexible GQD PDs.
    • Fig. S3. Noise characteristics of flexible GQD PDs.
    • Fig. S4. Gate modulation of flexible gated GQD PDs.
    • Fig. S5. Photosensitivity for various operation frequencies.
    • Fig. S6. Temporal response of the flexible GQD PD at 633 nm.
    • Fig. S7. Photoresponse of flexible GQD PDs at near-infrared wavelength.
    • Fig. S8. Continuous PPG reading by GQD health patch.
    • Fig. S9. Simultaneous measurements of GQD health patch and state-of-the-art capnograph.
    • Fig. S10. Large-area Raman spectroscopy mapping of graphene on flexible polymer substrates.
    • Table S1. Demonstrated device types and their specifications.
    • Legends for movies S1 and S2

    Download PDF

    Other Supplementary Material for this manuscript includes the following:

    • Movie S1 (.mp4 format). GQD bracelet (reflection mode PPG).
    • Movie S2 (.mp4 format). GQD health patch on the mobile phone screen (transmission mode PPG).

    Files in this Data Supplement:

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