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Invisible ink has the right chemistry

05 Nov 2002

An invisible coating allows the surface of pills or sweets to be marked with a low-power laser.

A small UK company has developed a range of food additives that allow a low-power laser to mark the surface of pills and sweets. The patent-pending coating chemistry is the brainchild of Sherwood Technology in Nottingham, UK.

The additives, which come in the form of a dry powder or water-based solution, are applied in a coating process prior to laser marking. On exposure to a low-power 10.6 µm beam from a CO2 laser the additive changes colour to give a permanent visible image on the surface of the product. According to Sherwood, the process offers "huge cost savings compared to alternative high-maintenance technologies such as ink-jet printing and embossing."

"In the future, dosage levels and product names are likely to have to be printed on the surface of drugs," explained Sherwood commercial director Trevor Wilson. "Our technology will allow information to be applied quickly and cheaply." Potentially, the technique could also allow confectionery manufacturers to apply logos and simple graphics onto the surface of sweets.

Sherwood has also come up with invisible inks that allow a wide range of flexible food packaging such as crisp packets and chocolate wrappers to be laser-marked without the need for black ablation patches. The inks, just like the food additives, change colour when exposed to infrared light. The company says that the resulting marks are more attractive and much faster to write than those produced by traditional ablation-based laser marking.

According to Wilson, just 2-3 W from a CO2 laser should now be sufficient to mark food wrappers, compared to the current required level of several tens of watts. "Because you're using a third or less laser power, you get a far faster writing speed," he said. The ink is sandwiched between the laminated materials used in food packaging prior to marking, and according to Wilson the technique's total cost is comparable with that of current coating methods such as ink-jet and thermal transfer.

Although Sherwood was founded in 1988, it has only recently brought products to market following a round of venture capital founding two years ago. The company currently has a staff of 12 and manufactures the inks in-house.

Oliver Graydon is editor of Optics.org and Opto & Laser Europe magazine.

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