Inkjet-printed sub-zero temperature sensor for real-time monitoring of cold environments

Real-time monitoring of low temperatures (usually below 0 °C) or cold environments is a specific requirement that finds its high demand in the aerospace, pharmaceutical, food, and beverage industries to maintain the temperature at high altitudes or in refrigerators and cold storage. In general, this purpose is achieved by using a sub-zero temperature sensor coupled with a control system. However, the market available such temperature sensors are very expensive, and bulky, thus not being suitable for portable operation, and also they suffer from poor accuracy. Therefore, the development of high-performance, low-cost, lightweight, and portable sub-zero temperature sensors is highly desired. In our recent work, we developed such sensors and integrated them with auxiliary electronics to demonstrate their wireless operation for the continuous and real-time monitoring of cold environments. So, in order to obtain low-cost sensors a cost-effective inkjet printing technology was employed for the fabrication of devices. A lightweight polydimethylsiloxane (PDMS) was used as the substrate and an electrically conducting graphene nanocomposite was used as the temperature-sensing material. To obtain a functional graphene nanocomposite film with a thickness of 530 nm and a conductivity of ~189 S m^-1, the printed graphene nanocomposite was photonically sintered using a xenon flash lamp. This step was crucial for obtaining a sensor on the soft PDMS platform. The graphene nanocomposite film exhibited a positive temperature coefficient resistance value of approximately 0.119 %/°C, and its resistance values varied almost linearly (with an Adjusted R² value (model accuracy) of 0.99) with temperature within the operating range of -30 °C to 80 °C. The sensor was properly encapsulated for protection without significantly affecting its performance. The sensors demonstrated sufficient flexibility, with a bending radius of 20 mm, and sustained 500 continuous bending cycles. Finally, the real-time operation of the sensors was demonstrated by wirelessly transmitting and monitoring the temperature over a smartphone platform.