Security features exist to protect bank notes, documents and branded products against counterfeiting. Losses caused by product forgery and counterfeiting may be huge. According to the German Engineering Association, the damaged caused in 2016 by counterfeiting amounted to 7.3 billion euros. Researchers from the Karlsruhe Institute of Technology (KIT) and the ZEISS Company have proposed the use of 3D-printed microstructures instead of 2D structures, like holograms, to improve counterfeit protection.
"Today, optical security features, such as holograms, are frequently based on two-dimensional microstructures," said Professor Martin Wegener, expert for 3D printing of microstructures at the Institute of Nanotechnology of KIT. "By using 3D-printed fluorescent microstructures, counterfeit protection can be increased."
The new security features have a side length of about 100 micrometers and are barely visible with the eye or a conventional microscope. For their production and application, Wegener and his team have developed an innovative method that covers all processes from microstructure fabrication to the readout of information.
The microstructures of a 3D cross-grid scaffold and dots fluoresce in different colors and can be arranged variably in three dimensions within the grid. To produce and print these microstructures, the experts use a rapid and precise laser lithography device developed and commercialized by the Nanoscribe Company, a spinoff of KIT. It enables the highly precise manufacture of voluminous structures of a few millimeters in edge length or of microstructure surface of several square centimeters in dimension.
The special 3D printer produces the structures layer by layer from non-fluorescent and two fluorescent photoresists. A laser beam precisely passes certain points of the liquid photoresist. The material is exposed and hardened at the focus point of the laser beam. The resulting filigree structure is embedded in a transparent polymer in order to protect it against damage.
"Security features produced in this way are not only of individual character but also very complex in manufacture. This makes life difficult to forgers," said Frederik Mayer of the Institute of Nanotechnology of KIT.
The new process can be extended easily and is extremely versatile. The 3D features may be applied as an embedded foil in security tags to protect pharmaceuticals, spare car parts and mobile accumulators against counterfeiting. They may be integrated into the transparent windows of bank notes. For later authentication checks at the supermarket checkout or in the production hall, special readout instruments are required to detect the fluorescent 3D structures.
The results of this study were published in Advanced Materials Technologies.