*Article* **A Batteryless, Wireless Strain Sensor Using Resonant Frequency Modulation**

**Kyeong Jae Lee 1, Namsun Chou <sup>2</sup> and Sohee Kim 1,\***


Received: 20 September 2018; Accepted: 9 November 2018; Published: 15 November 2018

**Abstract:** In this study, we demonstrated the feasibility of a wireless strain sensor using resonant frequency modulation through tensile impedance test and wireless sensing test. To achieve a high stretchability, the sensor was fabricated by embedding a copper wire with high conductivity in a silicone rubber with high stretchability, in which the resonant frequency can be modulated according to changes in strain. The characteristics of the sensor and the behavior of wireless sensing were calculated based on equations and simulated using finite element method. As the strain of the sensor increased, the inductance increased, resulting in the modulation of resonant frequency. In experimental measurement, as the strain of the sensor increased from 0% to 110%, its inductance was increased from 192 nH to 220 nH, changed by 14.5%, and the resonant frequency was shifted from 13.56 MHz to 12.72 MHz, decreased by 6.2%. It was demonstrated that using the proposed sensor, strains up to 110% could be detected wirelessly up to a few centimeters.

**Keywords:** batteryless strain sensor; wireless strain sensor; resonant frequency modulation; Ecoflex
