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Science & Technology

June 11, 2007
Volume 85, Number 24
pp. 30-34

Counterfeiting Countermeasures

Chemistry could play a major role in future deterrents against currency fraud

Sophie L. Rovner

SAY YOU'VE BOUGHT a soda with cash, and the store clerk hands you a $5 bill with your change. You probably stick it right in your wallet without a second glance. But what if it's a $100, or even a $20? Do you pay attention to the feel of the paper? Do you hold the banknote up to the light to check for an embedded plastic strip? Do you tilt the bill back and forth, looking for a shift in the color of ink used to print the denomination on the bottom right corner?

Robin Weiner/U.S. Newswire
Is it real? The plastic security strip in genuine U.S. banknotes glows under ultraviolet light.

These security features and many others challenge the skills of would-be counterfeiters. Nevertheless, out of $765 billion in circulation, some $65 million in fake U.S. currency was passed worldwide in fiscal 2006, according to Darrin Blackford, special agent and spokesman for the U.S. Secret Service, which was formed in 1865 to combat counterfeiting. While that amount is similar to the previous year, it represents a sharp rise over 2004, when the total value of circulated counterfeit bills stood at just $45 million.

An additional $44 million in counterfeit bills was seized before the notes could be circulated in 2004, compared with $53 million annually in 2005 and 2006.

"We do see counterfeiting as a big problem," Blackford says.

Indeed, the battle against counterfeiting is never really over, particularly as technology advances ease the production of fake money. "Historically, the value that made currency difficult to counterfeit was the quality of printing, primarily the detail," says Dennis J. Trevor, who recently helped prepare a National Research Council (NRC) report on counterfeiting. Trevor is the technical manager of the optical materials group at OFS Laboratories in Murray Hill, N.J., Lucent's former optical-fiber manufacturing unit.

Thirty years ago, a counterfeiter was typically a professional who could engrave a printing plate and then churn out a lot of fakes to make the laborious process worthwhile, explains Martin A. Crimp, a Michigan State University materials scientist who was also involved in production of the NRC report.

Brad Burris/H. W. Sands Corp.
Mood Money Banknotes could incorporate cholesteric liquid-crystal inks that would change color when warmed by a finger.

Today, modern digital desktop printing and graphic arts capabilities have broadened the pool of potential counterfeiters who can turn out decent fakes.

"The growing threat right now is someone running a little short at the end of the month, who whips out a couple of $20s on their ink-jet printer and passes them in a dark bar at night," Crimp says. "So the number of counterfeiting instances has exploded." The dollar value of each of those instances is quite low, but "the cumulative effect is a real problem."

To help stay a step ahead of the criminals, the Treasury Department's Bureau of Engraving & Printing, which produces U.S. banknotes, periodically asks NRC to conduct studies and prepare reports on counterfeiting. In February, the council released a draft of its latest report, "A Path to the Next Generation of U.S. Banknotes: Keeping Them Real." The bureau is currently evaluating the committee's findings.

The report describes counterfeiting threats and potential countermeasures, while providing a sense of the development work those measures would require. Rather than serving as an exhaustive list of potential security features, the report is intended to stimulate the generation of other ideas, says Robert E. Schafrik, general manager of materials and process engineering at GE Aviation and chair of the committee that prepared the report.

The committee included people with expertise in materials and optics, because "advanced currency is going to be a materials problem—and not so much a printing problem—for the future," Trevor says.

ADVANCED CURRENCY must include "features that are based on the molecular properties of the materials themselves," says Alan H. Goldstein, who was an Alfred University professor of biomaterials when he helped produce the report. "There's not a huge market for the home materials fabrication facility, the way there is a market for the home reprographic facility."

U.S. Secret Service
View Enlarged Image
Cross-check Multiple security features-including a watermark, color-shifting ink, microprinting of "USA 10" next to the torch, and a plastic strip that glows orange in ultraviolet light-can be used to authenticate the newest $10 bill.

Trevor believes that future generations of currency should also "interact more strongly with the person who's being asked to evaluate it." Interactive security features described in the report include chemical sensors and shape-changing wires.

U.S. currency is already protected by multiple security features, though many people may be aware of only a few. For example, banknotes worth $5 or more include a narrow plastic security strip that can be seen when the bill is held up to a light. But people might not realize that the strips in the different denominations are in different locations and fluoresce in different colors under UV light.

U.S. currency also features watermarks that echo the portrait printed on a bill, as well as intaglio printing, which deposits a raised ink pattern that can be felt on the surface of the banknote.

One of the key security characteristics is the "substrate," the material on which banknotes are printed. The paper substrate for U.S. banknotes is made from cotton and linen fibers rather than from wood pulp. The texture of the bills is unique so that "most counterfeits are identified by the feel of the substrate," Trevor says.

Several countries, including Australia, print their currency on a flexible plastic substrate that feels much like a new paper banknote. The substrate, which sometimes includes a clear window, is often made of biaxially oriented polypropylene. Australian officials say use of the polymer substrate has reduced counterfeiting, and the material is more durable than traditional currency paper. However, the committee that prepared the NRC report notes that it might not be cost-effective to switch over to the plastic substrate in the U.S. given the exceptional durability of the current U.S. substrate.

Nevertheless, the committee believes that utilizing a plastic substrate for low-denomination notes could offer an advantage. Counterfeiters sometimes bleach low-value U.S. banknotes and then print a higher denomination on the bleached substrate—one of the techniques used to "raise notes." Using a different substrate for $1 and $5 bills than for higher denominations would curtail this practice, Crimp says.

Some countries include holograms on their currency, but holograms aren't very durable, says Crimp, who studies metals and materials durability. Counterfeiters can simulate a worn-out hologram by attaching a piece of aluminum foil to a fake note that was crumpled to make it look old.

Mirkin Group/Northwestern University
Small print Dip-pen nanolithography uses an atomic force microscope tip to write with molecular "inks." The technique could be used to inscribe banknotes with print that has dimensions in the nanometer range.

Surprisingly, piling too many security features onto a note can backfire. Some banknotes, such as the euro, are so loaded with complex features that they confuse consumers, who can't remember what to check for, Goldstein says. Japan has opted instead for a pared-down approach that points consumers to one dominant anticounterfeiting feature: In the center of each banknote is a large, empty oval. It virtually begs the user to hold the bill up to the light to verify that a watermark portrait appears.

Looking ahead, Crimp notes that the next generation of U.S. banknotes must satisfy "a lot of competing priorities." First off, the new notes will have to be aesthetically pleasing. Long-term durability is also important. "You have to have deterrents that will survive in these notes," Crimp says. "You've got to be able to put them through your washing machine and dryer. You've got to be able to fold them a lot of times."

In addition, any new security feature has to be "really cheap. You've got to be able to incorporate it into the note for pennies," Crimp says. At the same time, "it can't be something that's easy to do." The electronics industry offers an instructive model: Microchips require sophisticated and expensive manufacturing equipment and facilities but are cheap to produce on a per-unit basis.

In fact, the electronics business could serve as more than a model. Flexible organic circuits could be embedded in banknotes and used to verify a bill's authenticity quite readily, Goldstein notes. Among other options, the electronics could be powered by a photocell printed on the currency.

The downside? Embedded electronics could enable the government and others to track money and transactions, removing the sometimes desirable anonymity that distinguishes paying with cash.

So the committee proposes that any new security feature should engage in dynamic behavior only when directly stimulated by a user. For instance, a cashier could squeeze a bill or hook it up to a battery to induce a response that verifies the bill's authenticity.

One such response could rely on piezoelectric materials. Voltage provided by a battery can reversibly change the shape of these materials, which are typically based on quartz or lead zirconate titanate. Polymer-based piezoelectric elements are also under development. If these materials were embedded in a section of a banknote, their shape change could, for example, raise bumps that alter the surface texture from smooth to rough and back.

ALTERNATIVELY, a consumer could squeeze a banknote containing a piezoelectric material hooked up to an organic light-emitting diode. The pressure would generate voltage that could cause the eyes in a portrait on the note to twinkle. In addition to being hard to counterfeit, Goldstein says, the feature "would have a certain cool factor."

Banknotes could conceivably incorporate superelastic and shape-memory materials, which return to their original shape after being deformed. Often based on NiTi (a nickel titanium alloy), these materials are already used in products such as eyeglass frames. A banknote containing a superelastic wire or thin-foil pattern would spring back to its original shape after being folded. A note containing a temperature-sensitive shape-memory feature could be induced to change shape by the heat from a finger.

Banknotes could also be printed with temperature-sensitive inks made of compounds such as thermotropic liquid crystals, which are used in mood rings. For instance, warming a bill with a finger could change the color of a portrait of George Washington or cause it to disappear.

The clear plastic window used in some currencies could be adapted to hold a liquid crystal or a self-assembling structure that would change color or transparency when someone rubbed or put pressure on it, Trevor says.

He sees a lot of potential in nanotechnology, including nanocrystals that can be used to make pigments that aren't easy to duplicate and have spectral properties that can be easily verified by machine. "Unlike regular organic pigment molecules," he explains, nanocrystal pigments "have very narrow absorption windows." Nanocrystals made of semiconductor materials such as cadmium telluride and zinc sulfide or of metals such as gold and silver are already available. Color can be tuned by changing the size of the particles.

The committee also thinks lithography on a minute scale shows promise as a deterrent, Trevor says. Techniques including dip-pen lithography—in which the stylus of an atomic force microscope is used as a pen—could be used to print currency with text, images, or other patterns with dimensions in the nanometer to micrometer range. Such features could be viewed with a scanning optical microscope.

A banknote could also incorporate a chemical sensor that could, say, briefly change color when someone touched it or breathed on it, Goldstein says. Such a sensor could react to skin pH or the greater content of CO2 in breath than in air. Sensors could be made from pH-sensitive dyes, liquid crystals, and other materials.

The NRC report suggests that tiny optical-fiber segments made of glass or acrylic could be added during the production of currency paper. The optical fibers would end up embedded in a random pattern in the substrate. When illuminated with laser light, they would produce speckles of light on the bill's surface. The speckle pattern would create a unique optical signature that could be converted into a bar code and printed on the note. A machine reader could compare the bar code and the actual speckle pattern to authenticate the note.

Alternatively, the cotton used to make currency paper could be genetically engineered to alter the substrate's properties. Researchers have already grown cotton in which the fiber's normally hollow core is filled with a natural thermoplastic polyester. The core could also be filled with an iron-containing protein such as hemoglobin, which would give a banknote a measurable electromagnetic signature, the committee notes.

Trying to duplicate or even simulate such security features would stump almost all counterfeiters. "Only very high-level corporations or governments would have the resources to do that," Goldstein says. "And you're really trying to knock out the 95% of the counterfeiters who fall below that category of sophistication."

It doesn't pay to count counterfeiters out, however. "These criminals are pretty creative," Crimp says. "I learned more about the criminal mind during this than I ever cared to know."

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