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3D-printed 'smart cap' uses electronics to sense spoiled food

20 July 2015

Engineers at UC Berkeley have 3D-printed a wireless 'smart cap' for milk cartons that is able to detect signs of spoilage using embedded sensors.

UC Berkeley's 'smart cap', featuring embedded electronics to wirelessly monitor the freshness of milk (photo: Sung-Yueh Wu)

Engineers at the University of California, Berkeley, in collaboration with colleagues at Taiwan's National Chiao Tung University, are expanding their impressive portfolio of 3D printing technologies to include electrical components such as resistors, inductors, capacitors and integrated wireless electrical sensing systems. They have put the new technology to the test by printing a wireless 'smart cap' for a milk carton that detected signs of spoilage using embedded sensors.

"Our paper [published in the open-access journal, Microsystems & Nanoengineering] describes the first demonstration of 3D printing for working basic electrical components, as well as a working wireless sensor," says senior author, UC Berkeley's Professor Liwei Lin. "One day, people may simply download 3D-printing files from the Internet with customised shapes and colours and print out useful devices at home."

Polymers are attractive materials in the world of 3D printing because their flexibility allows them to be formed into a variety of shapes. However, such materials are poor conductors of electricity, and thus bad candidates for electronic devices. To get around this, the researchers started off by building a system using polymers and wax. They would then remove the wax, leaving hollow tubes into which liquid metal - in their experiments they used silver - was injected and then cured.

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 integrated electronic components manufactured in this way 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 for 36 hours.

The circuit was able to detect 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. 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 percent after 36 hours. In comparison, a carton of milk kept in refrigeration at around 4°C saw a relatively minor 0.12 percent 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," says Lin. "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."

As 3D printers become cheaper and better, the options for electronics will expand, though Lin does not think people will be printing out their own smartphones or computers any time soon.

Instead, his lab is working on developing the technology for health applications, such as implantable devices with embedded transducers that can monitor blood pressure, muscle strain and drug concentrations.

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