Non-faradaic junction sensing

A non-faradaic junction (NFJ) is a connection between an ionic conductor and an electronic conductor in which no electrochemical reaction takes place. The junction behaves like a capacitor and couples the ionic and electronic currents through chemistry, electricity and entropy. Its charge–voltage curve is sensitive to various environmental signals, allowing it to function as a sensor; because no reaction occurs, the sensing is non-destructive and long-lasting. NFJ sensors have high sensitivity, rapid response and small size, and they can be self-powered. These sensors are familiarly used in electrophysiology of the heart, brain and muscles, and applications are emerging in wearable and implantable devices and soft robotics, as well as in sensing pressure, sound, temperature and chemicals. In this Review, we discuss NFJ sensors, emphasizing the development of devices and materials for each side of the junction. The flexibility in choosing materials enables NFJ sensors to fulfil challenging requirements, such as softness, stretchability, transparency and degradability.