Researchers are working on fluorescent 3D polymers that, when embedded into bank notes and product tags, help tackle forging and counterfeiting. Ines Nastali reports.
While bank notes, documents and branded products are being equipped with more and more security features such as watermarks or raised print, losses from forging and counterfeiting remain high. The German Mechanical Engineering Industry Association reported losses of around £7bln in 2016, due to pirated products being used in construction. In an effort to solve this problem, the Karlsruhe Institute of Technology (KIT) has developed a fluorescent 3D cross-grid scaffold that is barely visible to the naked eye and which can be printed and embedded within a product, making it superior to 2D security structures such as holograms.
The 3D features may be applied as a foil in security tags of products, car spare parts, or mobile accumulators against counterfeiting. They could also be integrated into the see-through windows of bank notes.
The researchers describe their counter-crime efforts in the Advanced Materials Technologies journal. Firstly, they developed a laser lithography device to print the 3D structure. ‘A special 3D printer produces the structures layer by layer from non-fluorescent and two fluorescent photoresists,’ the researchers stated in the paper. ‘A laser beam very 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 then embedded in a transparent polymer in order to protect it against damage.’
Frederik Mayer, from the Institute of Nanotechnology at KIT and lead author of the paper, titled 3D Fluorescence-based Security Features by 3D Laser Lithography, explains in more detail what material the photoresists are made of. He told Materials World, ‘Most importantly, they consist of acrylate monomers and a photo initiator. By exposing the liquid photoresists to light, a radical polymerisation is triggered by the photo initiator, and the acrylate monomers are cross-linked to form polyacrylate units. Therefore, the printed structures consist of a highly cross-linked polyacrylate.’
‘To be specific, we used the monomers pentaerythritol triacrylate and tricycle-[5.2.1.02,6]-decanedimethanol diacrylate. Also, Irgacure 819 was added as the photoinitiator. For the two fluorescent photoresists, we added fluorescent CdSeS/ZnS quantum dots with different emission wavelengths.’
Check the money
The result is an individual print that is complex to manufacture and therefore requires specialist equipment to duplicate, making life harder for forgers. In order to confirm the authenticity of a product, some kind of instrument is needed to check for the fluorescent structure. ‘In our paper, we used a confocal laser scanning microscope to read out the structures. Using this device, the precise structure of the security features can be imaged in 3D and therefore their genuity can be verified,’ Mayer said, adding, ‘For a commercial application, it would be necessary to develop readout devices that employ the same technique, but are small, as cheap as possible and easy to use.’
This would likely result in additional investment for the end user, but Mayer said ‘At the current stage of the project, it is not possible to give estimates about costs. Typically, we would expect the costs to scale advantageously when mass-producing the security features and the readout devices.’
The next step for the research is focused on further developing the security features and to improve the level of counterfeiting security that they offer. ‘For example, one can increase the number of fluorescent colours or scale down all structure dimensions to obtain security features that are even more difficult to counterfeit,’ said Mayer.
To be sure that their own invention is not being pirated, Mayer and his colleagues have a patent application running.
Find the paper online: bit.ly/2jSLGKs