"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 Centre.
Polymers are used in 3D printing because their flexibility allows them to be formed into a variety of shapes. However, they are poor conductors of electricity. To get around this, the researchers built a system using polymers and wax. They then removed the wax, leaving hollow tubes into which liquid metal 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 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 flip of the carton allowed a bit of milk to get trapped in the cap's capacitor gap, and the carton was then left unopened at room temperature for 36 hours.
The circuit detected the changes in electrical signals that accompany increased levels of bacteria. The researchers 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 smart cap found that the peak vibration frequency of the room-temperature milk dropped by 4.3% after 36 hours. In comparison, a carton of milk kept at 4°C saw a relatively minor 0.12% 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," Prof Lin said. "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 his lab is 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.
