Scientists have developed a 3D-printed 'smart cap' that uses embedded sensors to wirelessly monitor the freshness of milk in cartons. Researchers from the University of California's Berkeley
Sensor and Actuator Center in collaboration with colleagues at Taiwan's National Chiao Tung University first used polymers and wax to enable the technology.
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Polymers are attractive materials in the world of 3D printing because their flexibility allows them to be formed into a variety of shapes. They then removed the wax, leaving hollow tubes into which liquid metal - in their experiments they used silver - was injected and then cured.
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The shape and design of the metal determined the function of different electrical components. For instance, thin wires acted as resistors, and flat plates were made into capacitors. The researchers then integrated the electronic components into a plastic milk carton cap to monitor signs of spoilage. The "smart cap" was fitted with a capacitor and an inductor to form a resonant circuit.
A quick flip of the carton allowed a bit of milk to get trapped in the cap's capacitor gap, and the entire carton was then left unopened at room temperature (about 22 degrees Celsius) for 36 hours. The circuit could detect the changes in electrical signals that accompany increased levels of bacteria. The researchers periodically monitored the changes with a wireless radio-frequency probe at the start of the experiment and every 12 hours thereafter, up to 36 hours.
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The property of milk changes gradually as it degrades, leading to variations in its electrical characteristics. Those changes were detected wirelessly using the 'smart cap', which
found that the peak vibration frequency of the room-temperature milk dropped by 4.3 per cent after 36 hours. In comparison, a carton of milk kept in refrigeration at 4 degrees Celsius saw a relatively minor 0.12 per cent shift in frequency over the same time period.
"This 3D-printing technology could eventually make electronic circuits cheap enough to be added to packaging to provide food safety alerts for consumers," said researcher Liwei Lin, a professor of mechanical engineering and co-director of the Berkeley Sensor and Actuator Center.
"You could imagine a scenario where you can use your cellphone to check the freshness of food while it's still on the store shelves," Lin said. The study was published in the journal Microsystems & Nanoengineering.