Spray-Deposited Graphite Nanoplatelet Metasurfaces for Chipless Wireless Strain Sensing

Chipless passive wireless sensors (CLPWSs) are gaining traction for applications requiring battery-free, compact, and wireless monitoring capabilities. Among these, strain sensors leveraging metasurfaces offer a promising approach due to their tunable electromagnetic responses. This study presents the fabrication and characterization of a frequency-selective CLPWS using a spraydeposited nanocomposite based on graphite nanoplatelets (GNPs) on a polyetherimide (PEI) substrate. Characterized of electrical conductivity, strain sensitivity, and electromagnetic response in the X band (<tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$8-12 \text{GHz}$</tex>) of samples with different composition have been performed to evaluate the best case for the fabrication of the sensor array. A maximum electrical conductivity (<tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\sim 7.16 \times 10^{4} ~\mathrm{S} / \mathrm{m}$</tex>) was found for samples based on oriented GNPs at low frequencies (quasi-static condition). Strain sensitivity tests demonstrated a minimal resistance change under strain. X-band measurements and equivalent transmission line modeling were used to estimate material conductivity at high frequencies. The results highlight the feasibility of spraydeposited GNP-based metasurfaces for wireless strain sensing. Indeed, an array sensor of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$10 \times 10 ~\text{cm}^{2}$</tex> has been fabricated and preliminary tested in X-band.