For the uninitiated, however, it’s hard to make out where the small “secret membrane” is located – for example on a door handle, a hinge, or a window pane. But that’s not all: Even the keys onto which the code is typed remain hidden. Only when these are touched in a predefined sequence is the door unlocked. The membrane is also coupled with a display to indicate whether the code was entered correctly, its developers at the Swiss Federal Laboratories for Materials Science and Technology (Empa) announced.

As the conductive ink for the keypad, the researchers used indium-tin-oxide nanoparticles, which have a brown color. When cured in an oven, the ink becomes invisible. Subsequently, the researchers submerged the flexible membrane in blue paint and exposed it to a high-intensity flash. The result: The blue layer incinerates, further increasing the transparency and conductivity of the printed sensor keys.

Versatile applications

The pattern of the printed keypad – for example, a square, a circle, or a heart – is determined during the initial inkjet printing process. The access code, i.e. the sequence in which the keys must be touched, is programmed at a later stage.

The invisible keypad could find applications in banks, hospitals, or even private homes.