Silk is a marvelously versatile material that is useful in an incredibly wide variety of settings besides luxurious underwear. You can find it in surgical sutures, flexible electronics, and other biologically friendly applications. And soon you might be finding it in your food. Scientists at Tufts University have now engineered the multitalented material into fully chewable food sensors. Pasted onto eggs, stamped onto fruit or floating in milk, they can warn you when your fruit is ripe, or when your milk has gone sour.
"We see a huge market for food," Hu "Tiger" Tao, a postdoc at Tufts University told Co.Exist. "People are always looking forward to some kind of sensor that’s easy to use and gives you information about spoilage."
The flexible sensors are made of gold antennae embedded in a purified silk film support (that Tao and his collaborators first prototyped in 2010). The gold bits are as thin as gold leaf found on some extra-fancy desserts. The silk substrate—made of pure protein—is easily digestible. The whole sensor is flexible, and can curve according to the shape of the fruit it’s being stuck on.
The silk film doubles up as the sensor’s glue, turning sticky when exposed to water. The sensor is then pasted directly onto the food that needs tracking, eliminating the need for an additional glue to keep it clinging on. Tao and his collaborators, led by Fiorenzo Omenetto, tested the sensors on bananas, eggs, apples, cheese, and milk, and published the results in Advanced Materials.
When a fruit ripens or rots, chemical changes churn around inside it. Those changes and differences in the stiffness of the fruit translate to what’s called their dielectric properties. Tao’s gold sensors pick up on that change, and emit a different electromagnetic signal when monitored with a reader. "We can tailor our sensor to be extremely sensitive to the change of the dielectric property," Tao says. Sensors for bananas, for example, are slightly bigger than sensors for milk. The working principles behind the sensors are based on existing RFID tech—the difference here is that the sensors aren’t hard electronics, they’re flexible, edible stickers.
In August last year, Tao was part of a collaboration, headed up by John Rogers at the University of Illinois, that published a paper in Science showing how flexible electronics in the form of an "electronic skin" could stick to the skin and wirelessly track vital health signs. Those weren’t made of silk, but a silk-based approach like those employed on food sensors could also work, says Tao.