Switchable passive wireless vapour sensors from inkjet printed electronic components on poly(dimethylsiloxane)

A potential route to printed, inexpensive and disposable Radio Frequency Identification (RFID) sensor tags for chemical sensing such as the monitoring of food spoilage is described. The stimuli responsive material poly(dimethylsiloxane) (PDMS), is known to swell upon exposure to organic vapors. Colloidal silver ink solutions were printed and sintered onto surface modified PDMS to give conductive silver feed loops. These loops act as the active sensing component in antennae for passive (battery-free) (RFID) tags. When the tags were exposed to certain solvent vapors (e.g. ether, dichloromethane, acetaldehyde) the printed feed loop fractured. This was accompanied by a rapid increase in resistance and ultimately loss of conductivity. This led to a change in the transmitted power and read range of the wireless device. Remarkably upon removal from the vapor, the fractured feed loops reassemble and become conductive again, making them switchable and "multi-use". The selectivity for the response to the vapors could be directly correlated to a function of the Hansen solubility parameters and vapor pressures of the solvents giving rise to the vapours. Significant differences in the solubility parameters between PDMS and the organic volatile and/or low vapor pressures lead to no significant response (e.g. methanol, acetic acid, popan-1-ol). This work paves the way to a fully inkjet printed RFID substrate for vapor detection.