Figure 1 shows the architecture of

Figure 1 shows the architecture of the smart ring. It consists of a 40mAh Li-Ion battery with a solar energy harvester for power supply, a processing section with BLE connectivity (based on the TI CC2650 [33]) and two sensors, namely an oximeter (MAX30102) and an accelerometer (MIS2DH). As demonstrated in previous works, the accelerometer can be very useful to improve the quality of the measurement when the user is moving [31]. The accelerometer is not exploited in this work, but it will be used in future work. However, it is already present on the current version of the smart ring, and its power already counted in the energy evaluations. The pulse oximeter non-invasively measures the blood oxygenation by shining light at two different wavelengths into the finger and by analyzing the pulsatile component of the reflected intensities [26]. The oximeter sensor controls two LEDs and converts the analog signal from its photodiode into a digital representation for the connected microcontroller. The ring-form of the 3D-printed casing ensures conformal contact with the skin.