Modifying Commercial RFID Tags using Polydimethylsiloxane based Polymers for Sensing Purposes

In the last decade, RFID sensing has grown rapidly. Passive RFID tags are attractive due to their low cost, theoretically infinite lifespans, small form factor and the ability to be read without a line of sight required. This has resulted in an enormous increase in research and commercial interest, with RFID sensing growing at an ever expanding rate. Previous RFID sensing has relied upon exploiting the reader side of RFID systems, or incorporating bespoke sensors into RFID systems. Newer classes of sensing tags now allow for useable sensing information to be provided by the tag antenna in the form of a sensor code that relates to a stimuli affecting the tag.

A popular research avenue for RFID has been utilising stimuli responsive materials. Using stimuli responsive polymers and other materials has previously relied upon exploiting the analogue changes that result when the stimuli responsive material is affected by an analyte of interest. The primary advantage of using stimuli responsive materials over bespoke sensing components is that the simpler components can be utilised. Nowadays, commercial sensing tags can be purchased for as little as £2.75, that can provide digital information about environmental conditions. Whilst dedicated sensors for other analytes would be expected to have a higher cost, incorporating polymeric materials into lower cost sensors and repurposing them to sense a range of analytes presents an attractive alternative. A commercially available RFMicron RFM-2100 AER moisture sensing tag was modified with polydimethylsiloxane (PDMS) to allow for sensing of aqueous electrolytes of variable concentration. Coating the tag with the hydrophobic layer also allowed for the system to return to be reusable. The system also demonstrated the ability measure the relative amount of water (or alcohol) in water/alcohol mixes. A two part reactive silicone cross-linked polymer was also investigated for the purpose of repurposing the tag to be used as a pH sensor, but was found not to produce enough (if any) changes in response with variable pH. Following this work, a preliminary investigation showed the potential for a PDMS/PANI (polyaniline) composite was tested as a pH responsive coating for the RFMicron RFM2100 AER. The system was capable of measuring 5 distinct ranges of pH, but only on the first use of the system, as after this the system became incapable of measuring pH changes aside from those associated with the large change in dielectric properties of extremely low pH solutions.