Investigation of RFID passive strain sensors based on carbon nanotubes using inkjet printing technology

Because of their remarkable properties, carbon nanotubes (CNTs) have been used in many applications [1-5]. Stain sensors based on CNTs can overcome several limitations of the existing conventional sensors, including limited monitoring locations and separation from the structure that is being monitored. Many deposition methods were applied to deposit CNTs on substrate; inkjet technology among others will possibly turn out as a fast and flexible manufacturing method for CNT-based sensors. We report about a wireless passive strain sensor made by inkjet-printing of CNT dispersions on flexible substrates. According to the radio frequency identification (RFID) principles, a square planar coil was patterned using inkjet orienting technology on a flexible substrate (Polyethylene terephthalate - PET) in order to measure strain wirelessly [6, 7]. Several settings of printing were investigated and optimized. Coils of different design parameters including variation of the number of windings, inner diameter, outer diameter and number of printed layers were manufactured using inkjet printing. The investigation of the readout mechanism of the sensor was done by applying the electrical impedance spectroscopy (EIS) analysis. By measuring the change in the complex impedance of the coupled coil, the changes in the resonance frequencies of the sensors were detected wirelessly. The change in the bandwidth is considered to be the main effect of the sensor under strain.