Emergent sensing technologies such as wearable electronic devices and skins, monitors for human-computer interaction, etc., are turning to versatile hydrogel chemistry to facilitate the detection of physical and chemical signals. These technologies require portable, non-invasive sensors able to work autonomously for extended periods of time in different environments. The ability of the sensor to self-generate the energy required for its operation is critical because these sensors often work isolated from a power source in highly mobile scenarios. We aim to inform researchers working in similar and allied areas that could benefit from the technology and help those already working in this field to form a comprehensive picture of the state of the art.