Vast flows of goods circle the globe every day. They include particularly delicate shipments, such as certain vaccines, medicines and food products. To ensure that these products arrive safely at their destination, they must remain within a certain temperature and humidity range throughout the entire supply chain. But how do we ensure this? It is costly and unsustainable to equip every single shipment with silicon-based sensors and chips. And measurements at nodes in the supply chain tell you nothing about what has already happened to the delicate goods on their way thus far.

Researchers from Empa, EPFL and CSEM have taken up this challenge in a four-year project called Greenspack. Together, they have developed a smart sensor tag that measures the temperature and relative humidity, and can “remember” when a certain temperature threshold has been exceeded. The small sticker is not only silicon-free, but also completely biodegradable. The project was funded by the Swiss National Science Foundation (SNSF) and Innosuisse as part of the BRIDGE Discovery program. The researchers published their findings in the scientific journal Nature Communications.

Electrical circuits with a memory

To do its job, the smart tag needs neither a battery nor a transmitter. Instead, it works in a similar way to an RFID chip. It contains printed paths of conductive materials that form electrical circuits with resistive and capacitive elements. If these circuits are exposed to an electromagnetic field, for example from a tag reader, a resonance is created that can be decoded by the tag reader. The ingenious part: The conductivity and capacitance of the individual circuits changes depending on the ambient temperature or humidity, also changing their resonance. This alteration provides information about the current temperature and humidity – without any complicated measurement technology.

But that's not all – the researchers also wanted to give the label a kind of “memory”. If the temperature of 25° Celsius is exceeded, a tiny element in one of the circuits melts, irreparably interrupting the circuit. The next time the label is read, it shows: This shipment was once too warm. “Talking about vaccinations, for example, this could mean that the shipment can no longer be used or that the expiry date is invalid,” explains Gustav Nyström, head of Empa's Cellulose and Wood Materials laboratory, who led the research project.

This technology reduces the burden on the supply chain and lowers its carbon footprint: Potentially damaged goods are detected earlier and do not have to be shipped any further. If the goods have simply become less durable due to the temperature exposure, the shipment can be redirected to a closer location if necessary. “Depending on the materials we use, we can set different temperature thresholds,” adds Nyström. Labels for frozen goods, for example, would be conceivable.

Biodegradable and sustainable

Once the shipment has reached its destination, the tag is envisioned to be composted or included with the cardboard recycling, as it is completely biodegradable. The Empa researchers have developed a dedicated material for the substrate, which consists of a biopolymer and cellulose fibers. The researchers at Empa and EPFL then used a customized ink containing the bioabsorbable metal zinc to print the conductive sensing elements. Meanwhile, the CSEM researchers worked on the tag design and readout technology.

Working with biodegradable materials is always a challenge – they should only decompose once their work is done. In addition, the individual components of the sensor tag only had to respond very selectively to the environmental conditions: “We didn't want the temperature sensor to react to moisture and vice versa,” says Nyström. Together, the project partners succeeded in solving these problems. Two EPFL researchers are now working on commercializing the findings from Greenspack with a start-up called Circelec. The Empa researchers around Gustav Nyström want to delve even deeper into the field of green electronics and explore the potential of smart labels as sensors for agriculture and environmental monitoring.