Chipless Wireless Strain Sensor Based on Stretchable Piezoresistive Materials

Piezoresistive (PZ) materials are frequently employed for the fabrication of wired strain sensors by exploiting the variation of the surface resistance observed when a mechanical stimulus is applied. However, this variation is measured by electrodes connected to the sheet. In this work, we propose to extract the strain information from the PZ material by using a completely wireless approach. The sensor is based on a resonant absorbing structure working at 7.39 GHz that exploits the interaction between an unstretchable metasurface and the PZ material used as ground plane. When the PZ sheet is stretched, a variation of the reflection coefficient at the resonance frequency is observed because of the sheet resistance variation. Differently from other chipless strain sensors based on stretchable substrates which are based on a deformation of the metallic pattern under applied strain, here, the strain is only applied to the PZ material. The proposed passive wireless sensor has been thoroughly modeled through a transmission line (TL) approach to clarify the physical mechanism that induces the scattering modulation. Finally, experimental results that verify the proposed strain sensing approach are presented. The experiments have been carried out several times to evaluate the measurement uncertainty. A sensor sensitivity of 0.5 dB/% has been estimated. The proposed approach considerably simplifies the realization of passive wireless strain sensors as it is not required to print resonators on stretchable substrates with ad hoc manufacturing processes, but it is sufficient to employ commercially available stretchable shielding materials.