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The KIT/Zeiss technique envisions laser nanoprinted 3-D microstructures—consisting of fluorescent markers embedded in a rectangular scaffold less than 100 microns square and 30 microns thick—that could be attached to packages, products and currency to improve anti-piracy protection. [Image: Frederik Mayer/KIT]

We’re all familiar with the 2-D holograms and fluorescent codes used to guard against fakery in everything from credit cards to passports to currency. A team of engineers from Germany has put on the table a method that would literally take those optical security measures to another dimension.

Specifically, the research team—from the Karlsruher Institute Für Technologie (KIT) and Carl Zeiss AG—has demonstrated a technique that uses laser nanoprinting to create tiny, 3-D fluorescent codes (Adv. Mater. Tech., doi: 10.1002/admt.201700212). The nanoprinted codes, the team suggests, can then be popped onto a variety of products and packages to ratchet up security against counterfeiting and piracy. Less than a tenth of a square millimeter in area and 30 microns thick, the 3-D micro-barcodes are read out using confocal fluorescence microscopy.

The researchers argue that, by producing highly individual, complex, easily embedded structures that take advantage of additional degrees of freedom (such as fluorescence color), the system “leverages the level of counterfeit protection to an unprecedented level.” And, they add, “commercial applications appear to be in reach.”

From random to deterministic

It’s not difficult to see how a 3-D security barcode could significantly enhance security over the 2-D versions now in use—and, indeed, the KIT/Zeiss team’s effort isn’t the first time such an approach has been tried. In particular, some labs have demonstrated random or pseudo-random 3-D structures of dispersed nanoparticles or quantum dots that could provide individualized, hard-to-counterfeit microcodes. But validating those codes requires a query back to a database, which makes the system potentially vulnerable to software hacking.

A potentially more secure approach lies in creating deterministic 3-D microstructures that are readable without reference to an outside database. The ability to fashion such deterministic structures cost-effectively, however, didn’t come on the scene until fairly recently, with the advent of 3-D laser micro- and nanoprinting.

Scaffold and dots

The demo from the KIT/Zeiss team (led by OSA Fellow Martin Wegener) employed a commercially available 3-D nanolithography device from the company Nanoscribe—itself a KIT spinoff several years ago. The team used that device to build up, on a tiny square patch 97 μm on a side, alternate layers of a transparent, cross-gridded polymer scaffold and a polymer photoresist including embedded fluorescent quantum dots. The nanoprinting process drops fluorescent markers from the photoresist at precise, preprogrammed locations into a rectangular arrangement on the scaffold layer.

By using multiple fluorescent photoresists during the second step, the technique allows different, deterministically generated patterns and color combinations to be deposited on any given scaffold layer. The nanoprinter builds the structure up, layer by layer, creating a 3-D array of fluorescent nanoparticle markers within a nonfluorescent 3-D scaffold only 31 μm thick. The completed, tiny 3-D code is then embedded in a thin, transparent polymer film that can be attached to objects, for later readout at the point of use via confocal laser scanning fluorescence microscopy.

Arbitrarily increased complexity

In tests using two fluorescent-marker colors, the team created microstructures with simple embedded patterns (specifically numerals) that could be read out in 3-D. The team also showed that the patterns did not fall prey to photobleaching on readout—an advantage of using quantum dots rather than fluorescent dyes in the photresist layers.

A particular edge for the technique, according to the KIT/Zeiss team, is that the codes’ complexity—and, thus, their overall resistance to counterfeiting—can be arbitrarily expanded by adding photoresist layers containing fluorescent markers with different colors or different spectral characteristics. A potential hurdle to commercial rollout, however, is that the system would require new readout equipment to detect the microstructures at point-of-validation sites such as supermarket checkouts and factory floors.

Nonetheless, the team does not view that as an insurmountable obstacle, given the other pluses they see for these 3-D micro-barcodes. “Security features produced in this way are not only of individual character, but also very complex in manufacture,” the paper’s first author, KIT researcher Frederik Mayer, noted in a press release. “This makes life difficult to forgers.”