A 3D-printed ‘smart cap’ that can tell whether milk has gone off has been created by researchers from the USA.
The polymer cap includes electrical components, a world first for 3D printing according to the engineers from the University of California, Berkeley, who collaborated with colleagues at Taiwan's National Chiao Tung University.
The group printed a wireless ‘smart cap’ for a milk carton that identifed signs of spoilage using embedded sensors that can detect the changes in electrical signals that accompany increased levels of bacteria.
"Our paper describes the first demonstration of 3D printing for working basic electrical components, as well as a working wireless sensor," said senior author Liwei Lin, a professor of mechanical engineering and co-director of the Berkeley Sensor and Actuator Center.
"One day, people may simply download 3D-printing files from the Internet with customised shapes and colours and print out useful devices at home."
A paper published yesterday in journal Microsystems & Nanoengineering describes how the researchers started off by building the frame of the cap using polymers and wax, before removing the wax to leave hollow tubes that could be filled with liquid metal – in this case silver – and then cured.
By altering the shape and design of the hollow tubes, the engineers were able to make electrical components with different functions – thin wires acted as resistors, and flat plates were made into capacitors.
To test the concept the "smart cap" was fitted with a capacitor and an inductor to form a resonant circuit and a quick flip of the carton allowed some milk to get trapped in the cap's capacitor gap.
The entire carton was then left unopened at room temperature for 36 hours and the researchers periodically monitored the changes in the electrical signals with a wireless radio-frequency probe at the start of the experiment and every 12 hours thereafter.
As milk degrades its properties change, leading to variations in its electrical characteristics and those changes were detected wirelessly using the probe.
The system 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 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 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."
Lin’s lab is now working on developing this technology for health applications, such as implantable devices with embedded transducers that can monitor blood pressure, muscle strain and drug concentrations.