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.