Formulating a new sustainable/soft material:  Using carbon fibre and PDMS (a silicone-based polymer) we built a scalable highly flexible and stretchable robust composite that behaves as a highly sensitive positive temperature coefficient thermistor.  Material choices followed  the objective of avoiding metal particles (for cost/sustainability) and adopting biocompatible materials.

A Microwave/RF (and not DC) readout: as with all positive temperature coefficient (PTC) thermistors, our material was highly sensitive, but only over a very limited temperature range; the DC resistance becomes unreadable beyond 50-80°C (>100 MOhm). The sheet resistance and observed bulk conductivity, on the other hand, remains moderately low at RF/microwave frequencies, due to the presence of the conductive particles (carbon fibres) inside the material. This meant, using the permittivity and RF conductivity, we were able to observe temperature/electrical changes beyond 200°C (maximum measurement temperatue was 230°C).

Creating a Seamless Wireless Interface: with the material properties being temperature-dependent, we designed planar antennas using the thermistor as a dielectric substrate. Thus,  controlling the amplitude of radiated signals using temperature. In a practical demonstration, we used an RFID tag to show that the signal strength (RSSI) of the backscattered signals from the battery-free tag could exhibit high temperature dependence. This means that we can have many distributed sensors with no on-board processing or power supplies. 

The findings and underlying data can be openly accessed at: https://doi.org/10.1038/s41467-024-44735-z