Flat Foldable Kirigami for Chipless Wireless Sensing

This chapter explores foldable chipless and wireless mechanical sensors. The effects of varying geometric aspects of the foldable kirigami tag are evaluated. Possible designs of printing counterparts for sensors are tested based on the characterized effects. The measured response of the tags can vary in two ways, either the magnitude of the signal strength or the resonant frequency. This response is affected by the change in the Real and Imaginary components of the tag’s impedance, respectively. The sensor tags are produced using two methods: (1) kirigami cutting and folding on the sheet of copper tape and paper, and (2) printing with an inkjet printer on a Polyethylene sheet using a silver nanoparticle ink. A Vector Network Analyser is used to provide the transmission and reception of RF signals and is connected to a Near-field Magnetic field probe in a variable height setup constructed from non-conductive materials. Tags are secured to the setup, and their responses to RF signals from 0 to 900 MHz are measured. The resonant frequency of the tag is denoted by a dip in the reflected power, namely the S11 parameter. The magnitude and position of the S11 parameter are measured at the resonant frequency. The effects with varying resistance, capacitance, and inductance of the tags are characterized. A logarithmic trend was identified in the S11 magnitude of the tags with varying resistance. A curve of $$\frac{1}{\sqrt{C}}$$ is identified in the resonant frequency of the tag with varying capacitance. Correspondingly, the inductance of the tags is varied by changing the number of turns in the tag. The resonant frequency of the tags decreased with an increase in the number of turns. Five sensor designs, three compressions, and two extensions were tested. All the sensors produced the expected results of changing resonant frequency when deformed. Two of the compression sensors produced discrete results, while the three remaining sensors were measured along with a range of values, and each followed a linearly decreasing trend in resonant frequency. The changes in a foldable mechanical structure can be transmitted wirelessly through chipless sensors with folding or inkjet printing. The sensors can be produced through the inkjet method for consistency.