This paper presents an energy-autonomous bio-sensing system with the capability of proximity communication. The proposed biosensor includes a bio-fuel cell as a power source and sensing front-end associated with the integrated supply-sensing sensor. The sensor consists of a digital-based gate leakage timer, supply-insensitive time-domain temperature sensor, and current-driven inductive-coupling transmitter and achieves low-voltage operation. The timer converts the output supply-voltage from a bio-fuel cell to period output. The supply-insensitive temperature sensor provides PWM output without dependency of the supply voltage. The following inductive-coupling transmitter enables proximity communication. A test chip using 65-nm CMOS technology was operated with a supply of 0.19 V and consumed 53 μW to successfully demonstrate proximity communication with an asynchronous receiver. The measurement results show the possibility of energy-autonomous operation using bio-fuel cells.