Think about the last time you held actual money in your hands – a crisp $20 fresh from the ATM, or a handful of coins jingling merrily in your pocket. Maybe you were driving on a cash-only toll road or had to pay back a friend. Although bills and coins are increasingly being replaced by cards or smartphone apps, our currency is still under attack by counterfeiters. Luckily, U.S. scientists have been continually inventing advanced anti-counterfeiting technologies as far back as Benjamin Franklin’s “nature prints,” which used actual leaves to create unique patterns on bills.

Until everyone owned a laser printer, counterfeiting money was really hard. You had to acquire the right paper, a printing press, and the engraved printing plate to get the details right. After ink-jet printers became a common household staple, the number of counterfeiters using advanced graphics programs to print low denomination bills, like $20s, increased sharply. If cashiers or other sellers can’t check for security measures – either because real bills don’t have them or they need special equipment – fake bills can be easily accepted. (Disclaimer: DO NOT try this at home.)

Security features are currently incorporated into money in several ways. First, there is the substrate – what the bills are made of. In the U.S., a blend of 75% cotton and 25% linen is used, which is just as durable as plastic substrates recently in use by other countries. The fabric can also have a watermark incorporated into it – easy to see when held up to the light. The ink and pattern printed onto the bills can also be a security feature. The more detailed the pattern, the harder to duplicate. For example, the raised printing on U.S. bills gives them a distinct texture. The ink itself can also be unique – “color shift” ink changes color as the bill is tilted. Currently, the U.S. uses all of these features.

Recently, exciting new features have been added to bills. These are things like the security strips or threads on current $5-$100 bills, which glow different colors under a black light, making it easy to confirm legitimate bills. Or the “3D ribbon” on the $100 which is embedded in the fibers of the bill and changes pattern when tilted.

This is where science has a lot to offer. Security features must be cheap, easy to mass produce, and highly durable, but also very difficult to duplicate – requirements that are somewhat at odds. But, being professional problem solvers, scientists have tons of ideas about the next big anti-counterfeiting technology.

The most promising features are also very strongly interactive. In order to validate authenticity, your sales clerk or bartender interacts with the bill in a simple way – with a black light, the touch of their finger, or the CO2 exhaled in their breath – and the bill must have an obvious response.

For example, a material called liquid crystals can be used in an ink that changes color with the heat of your finger – just like a mood ring. Using a flexible electronic circuit made of conducting polymers printed on the bill, a finger placed in the right spot can make a complete circuit, eliciting a twinkle from Benjamin Franklin’s eyes. Or, by adding to the circuit a piezoelectric material that moves in response to a voltage, detectable bumps can appear with a finger’s touch. A really exciting idea incorporates optical fibers into the bill itself so that when light is directed onto a specific location, bright spots appear on the bill’s surface in a unique pattern, like a barcode.

With estimates in the last ten years indicating $220 million of U.S. currency in circulation is fake, and 60% of that is ink-jet printed, counterfeiters aren’t going away any time soon. Counterfeiting is still a big problem that can devalue currency – no matter if that currency is in your hand or your bank account. So today, National Dollar Day, we say #ThanksScience for continuing to keep our money safe!

By Amanda Grennell

Posted by Science Buffs

A CU Boulder STEM Blog

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