fig4

Figure 4. Improved fidelity of PPG physiological-signal acquisition enabled by SPCI. (A) Photograph of the PPG-sensing FPCB; (B) Exploded-view schematic of the encapsulated PPG device; (C) Photograph of the device under bending deformation; (D) Photograph of the device attached to a fingertip; (E) Device temperature evolution during continuous outdoor operation (control group: encapsulated with Ecoflex-30); (F) Corresponding infrared thermography images for (E) (Test during 3 November, 11:00-13:00, Shenzhen, China); (G) Schematic of thermal-exchange processes in the device; (H) Fingertip PPG signals corresponding to the conditions in (E) and (F). Photographed by the authors. SPCI: Stretchable passive cooling interface; PPG: photoplethysmography; FPCB: flexible printed circuit board; MCU: microcontroller unit; BLE: bluetooth low energy; LDO: low dropout regulator.