Figure 15.

a) Basic working principle of PPG and the setups of the heart rate measurement using OPDs, along with the recorded pulse signals. Reproduced with permission.^{[ 72 ]} Copyright 2019, Wiley‐VCH. b) Schematic of the proposed OPO sensor with enlarged cross‐sectional view to depict device arrangement and light receiving process through the skin medium. Reproduced with permission.^{[ 131 ]} Copyright 2018, American Association for the Advancement of Science. c) The device structure and operation principle of the reflective pulse oximeter. Reproduced with permission.^{[ 132 ]} Copyright 2016, American Association for the Advancement of Science. d) The schematic configuration and photograph of the reflectance oximeter array. Reproduced with permission.^{[ 134 ]} Copyright 2018, The Authors. Published by PNAS.