Currently, package labels contain certain information – such as barcodes, serial numbers or buyers' addresses – that would be best left unseen by wrongdoers. Newly-developed rewritable labels could address that issue, as they're blank and transparent unless exposed to a certain type of light.

Developed by a team at Germany's Dresden University of Technology, the labels are made of plastic foil containing organic luminescent molecules. At a thickness of under 50 micrometers, each foil is thinner than a human hair.

Ordinarily, the molecules remain in a dark, inactive state. This is because oxygen in the plastic draws light energy from them. When select areas of the foil are exposed to ultraviolet light, however, a chemical reaction occurs which removes the oxygen from those areas. This allows the molecules to become active, and thus emit light.

By applying the UV light using either a finely-focused laser beam or a mask that only allows certain areas of the label to be exposed, it's possible to write text or other information with a resolution similar to that of a laser printer.

Although that light-written text is visible right after being "printed," it proceeds to fade as the luminescent molecules settle back down, leaving the label clear and unreadable. When subsequently exposed to a flash of UV light, though, the molecules are temporarily reactivated, allowing the information to be read. A specific ultraviolet wavelength has to be used, so not just anyone with a UV lamp is able to read the label.

Once the information is no longer needed, the label can be erased by exposing the foil to infrared light for about one minute. This raises the temperature of the plastic, increasing its oxygen permeability and thus reintroducing oxygen to the previously UV-written areas. The label can then be reused up to 40 times, getting new information written on it each time.

It's possible to manufacture the labels in any size, at a cost of under €2 (about US$2.30) per square meter.

"Invisible and re-writable tags can be used in a multitude of ways," says study leader Prof. Sebastian Reineke. "We can manufacture such tags thinner than conventional barcode stickers. These tags can become a versatile alternative to many frequently technology-laden solutions for information exchange in our daily life."

A paper on the research was recently published in the journal Science Advances.