Optimizing a novel PPG sensor patch via optical simulations towards accurate heart rates

This study proposes the design and optimization of the flexible OLED–OPD photoplethysmography (PPG) sensor patch to estimate the long time continuous heart rate. Using optical simulation, the distance between OLED–OPD and the aperture area of the OLED–OPD has been optimized to enhance the AC/DC ratio of the receive PPG signal. The optical simulation incorporates an empirical optical skin model. All the patches incorporate green OLED@525 nm wavelength and red OLED@630 nm wavelength. Simulation results show that the optimized AC/DC ratio of the cross-type patch for the green and the red OLED is 2.16% and 6.25%, respectively. Similarly, the optimized AC/DC ratio of the square-type patch for the green, and the red OLED is 9.6% and 5.8%, respectively. Experiment results show that the received PPG signal AC/DC ratio for the square type and cross-type are 2%@green OLED and 4.5% @green OLED, respectively. Also, the AC/DC ratio of the received PPG signal from the square type and cross-type are 1.4%@red OLED and 1.1%@red OLED, respectively. The AC/DC ratio is reduced because the skin and blood itself act as a lossy medium so that the DC signal increased more; as a result, the overall AC/DC ratio decrease. The best design of the optical patch is the square-type OPD patch due to the wide area of the OPD. The OLED drive current ranges between 0.1 and 0.4 mA. The average OPD current is 800 nA. The flexibility of the design PPG sensor patch is 130°. The non-invasive square-type PPG sensor patch is applied to the wrist artery of 40 subjects for sensing the PPG pulsation of the blood vessel. The heart rate measurement accuracy is 95%, whereas the standard error rate is 0.37 ± 1.96 bpm, respectively.