I. Introduction
A battery-less sensor, as discussed in this letter, is one whereby the entire operation of sensing and result indication is driven solely by the sensing element. This concept excludes the use of any additional power source or energy harvesters, as known in the literature [1], [2], other than the sensing element itself. Unassisted by any power-operated device or instrumentation, the sensing results need to be quantitatively perceivable to the users with indicative signal directly generated from the sensor’s transduction output.