Experimental Study of the Bending Effect on LC Wireless HumiditySensors Fabricated on Flexible PET Substrates

This letter has experimentally investigated the effect of the external bending on inductor-capacitor (
LC
) wireless humidity sensors fabricated on flexible polyethylene terephthalate (PET) substrates. The
LC
humidity sensor consisting of interdigitated electrodes for the capacitor was fabricated on the PET substrate by a screen-printing method, and graphene oxide was utilized as the humidity sensing material. The measured sensitivity of the
LC
humidity sensor is -136.1 kHz/%RH. The behavior of the
LC
sensor when subjected to circular bending implemented by a customized bending setup has been tested through changes in resonant frequency. The ratio of the projected area under bending constraints to the initial area of the sensor is proposed to characterize the curvature of the sensor. It shows that the bending causes the ratio to decrease, which results in the resonant frequency ascent with the fitting slope being approximately 10.8 MHz/ratio for the ratio from 1 to 0.6. The reduction in capacitance and inductance of the
LC
sensor is subjected to circular bending accounts for the resonant frequency ascent. The results presented here would be a valuable reference for the future applications of the
LC
sensor on the flexible substrates. [2018-0066].