Wearable Multisensor Ring-Shaped Probe for Assessing Stress and Blood Oxygenation: Design and Preliminary Measurements

Figure 1. (a) Block diagram of the acquisition system. Both PPG and GSR electrodes were placed inside the elastic band (sensor ring) and transferred the data to the microcontroller unit (MCU). (b) Front and side views of the design (top) and implementation (bottom) of the proposed ring-shaped probe, including its dimensions.

Figure 2. (a) Absorbance curve of oxygenated and deoxygenated hemoglobin (readapted from [43]). (b) Frontal view of SpO2 Pulse Oxygen in Blood Sensor PRO for MySignals (eHealth Medical Development Platform) manufactured by Libelium and used for SpO2 calibration [44].

Figure 3. Scatter plot of the 25 data pairs of R to SpO2 used for calibration (blue dots) and the computed SpO2 regression line (red line).

Figure 4. (a) 25-s excerpt of the PPG signals acquired on a subject during the experimental protocol. Signals are represented as raw ADC values. Please also note that for visualization purposes the signals were inverted during post-processing to reflect a typical PPG waveform. The red- and blue-colored curves indicate the PPG signals acquired using red and infrared light, respectively. Black dots represent the minimum value of each PPG waveform, detected for each cardiac cycle to measure the PPI intervals. The vertical red line denotes the transition between the R1 and SF phase. (b) The inset shows a zoom of an exemplary PPG waveform acquired with our device. In the inset: systolic foot (black dots), systolic peak (red dot), diastolic notch (yellow dot), and diastolic peak (green dot).

Figure 5. Beat-to-beat heart rate during the analyzed 25-s window extracted from PPG signal. The vertical red line marks the transition between the R1 and the SF phase.

Figure 6. 25-s window of the GSR signal acquired during the experimental protocol and extraction of its components through LEDALAB toolbox. (a) GSR signal (blue area and black line) and SCL component (grey area); (b) SCR component in terms of phasic driver. The vertical red line denotes the transition between the R1 phase and the SF phase.

Figure 7. Instantaneous values of SpO2 during a 160-s window. The vertical red lines delimitate the 40-s BH phase.