3D printed smart cap detects sour milk

A smart cap tells whether the contents of a package (such as milk) is still suitable for consumption. What’s more, this "Smart Cap" is also made by 3D printing, which offers numerous advantages.

Researchers at UC Berkeley have teamed up with colleagues at the National Chiao Tung University of Taiwan and developed a protocol for making 3D printed microelectronics. To demonstrate the applicability of 3D printed microelectronics of this sort, they developed a prototype: the Smart Cap. This is a 3D printed cap that can passively measure and respond to the quality of liquid foodstuffs by wireless.


To make the smart cap, a polymer structure was first 3D printed in which microchannels and cavities were formed by using wax as a support material that could then be dissolved and removed after printing. A liquid metal paste (silver) was injected into these voids to create electrically conductive structures. Finally, the metal paste was hardened.

The working principle

The prototype of the smart cap, which fits on a milk carton, contains a built-in LC circuit that can serve as a passive, wireless sensor. By briefly inverting the package, a small amount of liquid comes between the two electrodes of the capacitor, where it acts as a dielectric material. The resonance frequency of the LC circuit is determined by the dielectric constant of the food product, which in turn is influenced by the degradation of the product. The value of the resonance frequency, which is read out via a radio frequency reader, provides information about the freshness of the liquid food product.

A practical test

The fact that the 'Smart Cap' also works effectively was shown when the researchers monitored the resonance frequency as a function of the storage time of milk at room temperature (22°C) and chilled milk (4°C). The resonance frequency remained virtually constant in the sample that was kept cool, but fell by 4.3 percent after 36 hours storage at room temperature.

Threshold-lowering technology

The flexibility of the 3D printing means that a wide range of microelectronic components (such as resistors, capacitors and inductors) can be produced.

What’s more, the technology makes developments such as the smart cap cheaper and therefore more practical for large-scale use in the food industry. In addition to food packaging applications, the combination of 3D printing and injecting a liquid metal paste has potential for other applications, such as developing smart implants that can transmit information from inside the body via electric circuits. In time, consumers would also be able to create and print their own electronic circuits or wireless sensors themselves at home.

Sirris also has in its SMALL-Lab the know-how needed for making small products with internal channels and equipped with microelectronics. Companies interested in developments like this can find what they’re looking for here!

(Photo: Sung-Yueh Wu)