This paper highlights the new advances in the field of energy-autonomous battery-free wireless sensors for IoT applications. The design and implementation challenges as well as optimized solutions are presented in a unified approach. At device level, the most relevant hardware architectures are presented with a focus of state-of-art experimental results obtained for battery-free LoRaWAN (Long Range Wide Area Network) and BLE (Bluetooth Low Energy) wireless sensors targeting IoT (Internet of Things) and SHM (Structure Health Monitoring) applications. At system level we are focusing on networks topologies and new security and identification mechanisms implemented on hardware/electromagnetic level by exploiting the wireless power transfer link capabilities in the context of the simultaneously wireless information and power transfer.