Fig. 3: Light-adaptive, power-efficient sensor for HR and SpO₂ by PPG measurements.

a Exploded view schematic illustration that highlights the energy harvesting system, a light-adaptive PPG sensing module, a PV cell, a BLE communication module, a pair of LiPo batteries, all attached via an fPCB platform and housed in a silicone encapsulation structure. b The PPG signal (black) and calculated HR value (red filled circle) measured from the response of the red PD activated with light from the PL component of the encapsulation structure. Measurements involve a period of 2.5 minutes in dark conditions after exposing the PL component to light from the sun. The red unfilled circles in the graph correspond to HR values measured from a reference system (Dash-3000, GE) on the finger. c Power consumption for measurements of HR and SpO₂ for four different ambient light conditions by controlling room light and an IR lamp, at 3 V operation. d Dependence of V_PV on operation time for different ambient light conditions. e Dependence of V_PV on time for sensors with and without the aPM module in a dark or sunlight environment. f Results of diffusion theory simulation of light propagation in the near-surface region of the skin due to illumination from PL. g Results of diffusion theory for red and IR light from the LEDs, and from sunlight that passes through windows in the top of the encapsulation structure. h Bland-Altman (BA) plots of HR and SpO₂ measured using the sensor and reference system under four different ambient light conditions from three healthy adult subjects, using a breath hold method to modulate SpO₂. i The power consumption of the App with in-sensor computing (60 Hz sampling rate) or without in-sensor computing (60 Hz sampling rate) for measurements of HR and SpO₂(n = 3, using different experimental units). The top and bottom lines represent the minimum and maximum values, and the box plot represents the interquartile range (IQR), which spans from the 25th to the 75th percentiles. Whiskers extend to 1.5 times the IQR. The circle shape within each box indicates the median of the dataset. j BA plots of HR and SpO₂ values determined using in-sensor computing and using the reference sensor for three healthy adult participants in air with controlled oxygen concentration(n = 3, using different experimental subjects). k Measurements of SpO₂ in a mouse model, determined using a commercial PPG sensor (blue) and using the sensor with post-processing (black) and in-sensor computing (red) under room lighting during cycles of normal (100% oxygen) and hypoxia states.