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Disposable sensor could report when food isn’t fresh

(Credit: Salvatore et al, Adv Functional Materials, 2017 via ETH Zurich)

Researchers have created a small, thin, biodegradable sensor that could monitor the temperature of food in transit.

Microsensors are already used in many different applications today, such as the detection of poisonous gases. They are also part of miniaturized transmitter/receiver systems, such as the ubiquitous RFID chips.

As the sensors often contain precious metals that are harmful to both the environment and human health, however, they are not suitable for medical applications involving direct contact with the human body or for inclusion in food products. There is therefore a high level of interest, both in research and industry, in developing microsensors made from non-toxic materials that are also biodegradable.

biodegradable food sensor
The biodegradable microsensor for food monitoring. (Credit: Salvatore et al., Adv Functional Materials, 2017 via ETH Zurich)

The new biocompatible microsensors are created by encapsulating a superfine, tightly wound electrical filament made of magnesium, silicon dioxide, and nitride in a compostable polymer. Magnesium is an important component of our diet, while silicon dioxide and nitride are biocompatible and dissolvable in water. The polymer in question is produced from corn and potato starch, and its composition complies with EU and US foodstuff legislation.

Giovanni Salvatore, a postdoctoral researcher in the Electronics Laboratory at ETH Zurich and leader of the research team, is convinced that these biodegradable microsensors have a bright future. As an example, he cites one potential application: “In preparation for transport to Europe, fish from Japan could be fitted with tiny temperature sensors, allowing them to be continuously monitored to ensure they are kept at a cool enough temperature.”

This requires sensors that are suitable for use in foodstuffs and are no threat to consumer health. The sensors also need to be small, robust, and flexible enough to survive in containers full of fish or other food products.

The sensor developed by researchers is only 16 micrometers thick, making it much thinner than a human hair (100 micrometers), and—being only a few millimeters in length—weighs no more than a fraction of a milligram.

sensor on fish skin
The sensor also adheres to fish skin and would help monitor the freshness of a cargo. (Credit: Salvatore et al., Adv. Func. Materials, 2017 via ETH Zurich)

In its current form, the sensor dissolves completely in a one-percent saline solution over the course of 67 days. At present, the sensor continues to function for one day when completely submersed in water. This time would be sufficient to monitor a shipment of fish from Japan to Europe. “But it’s relatively easy to extend the operating life by adjusting the thickness of the polymer,” Salvatore says.

A thicker sensor would be less flexible, however. The current sensor is so thin that it continues to function even if it is completely crumpled or folded. Even when stretched by around 10 percent of its original size, the sensor remains intact.

For the power supply, researchers have connected the sensor to an external micro battery using ultra-thin, biodegradable zinc cables. On the same (non-biodegradable) chip there is a microprocessor and a transmitter that sends the temperature data via Bluetooth to an external computer. This makes it possible to monitor the temperature of a product over a range of 10 to 20 meters.

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Producing biocompatible microsensors is currently a very time-consuming and expensive process. However, Salvatore is confident that it will soon be possible to produce such sensors for the mass market, especially as the methods of printing electronic circuits are becoming increasingly sophisticated.

“Once the price of biosensors falls enough, they could be used virtually anywhere,” Salvatore says.

Sensors would provide the link between the physical and digital world, bringing food products into the “Internet of Things”. Their use would not be limited to temperature measurement either: similar microsensors could be deployed to monitor pressure, gas build-up, and UV exposure.

Salvatore predicts that these biodegradable sensors will be part of our everyday lives within 5 to 10 years, depending on the level of interest shown by industry. By that time, the battery, processor, and transmitter would probably be integrated into the microsensor, Salvatore explains.

A lot more research is still required before these components can be used without concerns for human health or the environment. The team is therefore currently searching for a biocompatible energy source to power its sensor.

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The findings appear in Advanced Functional Materials.

Source: Samuel Schlaefli for ETH Zurich