This paper presents flexible electronics with a bio-compatible material interface for bio-signal monitoring applications. Organic thin-film transistors (TFT) are integrated with the bio-compatible gel interface in a 1.2- mu m-thick film substrate. An electrocardiogram (ECG) of a rat is amplified by an organic TFT amplifier circuit that is mounted directly on the surface of the heart. In addition to the flexible electronics implantation, an implantable wireless sensor is introduced in this report. Integration between the flexible organic electronics and the silicon devices produces a promising technology for next-generation internet-of-things (IoT) sensors.