Introducing a Wax-Based Temperature-Time-Dependent Threshold Sensor

Temperature monitoring is an important tool for a broad variety of sensitive goods to maintain their quality and integrity during supply chain operations. To balance cost and benefit, many applications use sensors that only monitor a specific reference temperature limit. These sensor elements should be lightweight, low cost, and ready for wireless readout. In this work, an innovative sensor concept is presented that can meet these requirements. It irreversibly changes its electrical resistance from low impedance to high impedance if a certain temperature value is violated. The sensor element uses wax as a temperature-dependent element. If its melting temperature is exceeded, the liquid wax starts to flow into a textile reservoir. The descending liquid level of the outflowing wax disrupts an inkjet-printed nanoparticle layer that electrically connects the sensor's input and output contact. A fluidic resistance between wax and textile reservoir regulates the speed of wax flow. Thus, it is possible to set a time delay for triggering the sensor after the threshold temperature is reached. This article demonstrates the feasibility of the proposed sensor concept experimentally. Furthermore, the sensor element is connected to a custom-made radio frequency identification (RFID) tag, which allows wireless readout of the sensor state. From the authors' point of view, the simple functional principle combined with the good scalability of the sensor concept offers a high potential for smart packaging applications.