Microsensors are already used in many applications, like detection of poisonous gases. They are integrated into miniaturized transmitter/receiver systems like RFID chips. But the sensors often contain precious metals that are harmful to both the environment and human health so they are not suitable for medical applications with direct contact with the human body or for inclusion in food products. There is a high level of interest, both in research and industry, in developing microsensors made from non-toxic materials that are biodegradable.
A team of researchers, led by Giovanni Salvatore, post-doc in the ETH Zurich Electronics Laboratory, has been working with scientists from other ETH institutes on the development of biodegradable microsensors for temperature measurement. The 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, and silicon dioxide and nitride are biocompatible and dissolvable in water. The polymer is produced from corn and potato starch and its composition compile with EU and US foodstuff legislation.
Salvatore is convinced that these biodegradable microsensors have a bright future. He says, “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 food with no threat to consumer health. The sensors need to be small, robust and flexible enough to survive in containers full of fish or other food products.
The sensor is 16 micrometers thick, thinner than a human hair, and weighs a fraction of a milligram. In the current form, the sensor dissolves completely in one-percent saline solution over the course of 67 days. Currently, the sensor continues to function for one day when completely submerged 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. The current sensor is so thin that it continues to function even if it is completely crumpled or folded. Even when it is stretched to about 10% 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 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.
Producing biocompatible microsensors is currently a time-consuming and expensive process. But Salvatore is confident that it will be possible to produce these sensors for the mass market soon, especially as the methods of printing electronics circuits are becoming increasingly sophisticated.
"Once the price of biosensors falls enough, they could be used virtually anywhere," Salvatore says.
The sensors would provide the link between 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 everyday life within the next 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 microsensors. A lot more research is still required before these components can be used without concerns for human health for the environment. The team is currently searching for a biocompatible energy source to power its sensor.
A paper on these sensors and the research was published in Advanced Functional Materials.