(Credit: Frederik Mayer/KIT)
The technology, developed by researchers at Karlsruhe Institute of Technology (KIT) and optical manufacturing specialist ZEISS, uses a 3D cross-grid scaffold and dots that fluoresce in different colours and can be arranged variably in three dimensions within this grid. With a side length of about 100 μm, the new security features are barely visible to the human eye or even a conventional microscope. The work is published in Advanced Materials Technologies.
"Today, optical security features, such as holograms, are frequently based on two-dimensional microstructures," said Martin Wegener, a Professor specialising in 3D printing of microstructures at the Institute of Nanotechnology of KIT. "By using 3D-printed fluorescent microstructures, counterfeit protection can be increased."
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To produce the microstructures, the team used a rapid and precise laser lithography device developed and commercialised by Nanoscribe, a spinoff of KIT. According to the researchers, it enables highly precise manufacture of voluminous structures of a few millimetres edge length or of microstructured surfaces of several cm² in dimension. The 3D printer produces the structures layer by layer from non-fluorescent and two fluorescent photoresists. A laser then very precisely passes certain points of the liquid photoresist, and the material is exposed and hardened at the point of the beam.
The resulting structure is then embedded in a transparent polymer to protect it against damage. As well as being suitable for the transparent windows of bank notes, the nanostructures could be used as an embedded foil in security tags to protect high-value goods such as pharmaceuticals or car components. For authentication, special readout instruments are required to detect the fluorescent 3D structures. These could located at a supermarket checkout for banknotes, or in a production hall for security tags.
"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.
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