Plastic banknotes: new fingerprint technique means criminals can’t avoid capture

“The UK has just introduced plastic banknotes, almost 30 years after they were used for the first time in Australia. The polymer notes are designed to last longer and be harder to forge. But the new notes, which will replace the old cotton paper ones entirely by 2020, come with a challenge for police detectives and forensic scientists.

The existing techniques for obtaining fingerprints from paper notes won’t necessarily work for the new plastic money. However, our team at the chemistry department of Loughborough University has developed a potential solution.

The use of fingerprints in forensic science may date back to the 19th century, but in the UK alone it still plays a key role in bringing charges in some 27,000 crimes a year, according to Home Office data we obtained. But new materials can pose significant challenges for fingerprinting. We’re forever trying to make things biodegradable, or handling devices that simply didn’t exist a decade or two ago.

The issue is that the new notes have been fashioned from “biaxially oriented” polypropylene, a type of plastic that has been strengthened by stretching it in two directions. They are also, as with all notes, deliberately fiddly in design. Illustrations and security features such as foil and transparent sections make it harder to develop a perfect print.

The key is to try to find a method that will make the design of the note invisible and just highlight the print. Conventional techniques, such as exposing the fingerprint to cyanoacrylate (“superglue”) fumes that stick to the moisture in the ridges of the print and turn them white, can struggle in such circumstances. The developed print simply appears white and so is harder to see against the background, and it leaves an indelible mark or stain that means the note can’t be returned to circulation.”

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