© 2019 IEEE. A new flexible trans-nail photoplethysmographic (PPG) sensor system is proposed and characterized from both aspects of mechanical and electrical properties in this study. The unique FHE (flexible hybrid electronics) structure is consisting of an elastomer as a flexible substrate in which Si LSI dielets having photodiode and LED driver circuits etc. are embedded based on a FOWLP concept. Stress buffer layers (SBLs) as a key material are inserted between interdielet wirings and the substrate to mitigate mechanical stress and enhance wire reliability. The impact of the Young's moduli of the SBLs on the repeated bendability of the FHE systems is described. In addition, we evaluate the electrical properties of the photodiode circuits between before and after bending for comparison.