Secrets of the deep – how fish scales could confound counterfeiters

Scientists have devised a method of using fish scales to convey encrypted messages. Not only would the technology divert seafood industry waste from landfills, it should also be less costly than existing options such as special inks.

The technique was developed by researchers at the National University of Singapore, led by Prof. Sow Chorng Haur. Potential applications include the production of authenticity-verifying tags on product packaging, which would be hard for counterfeiters to read and even more difficult for them to copy.

The process involves precisely heating discarded fish scales, which are composed of a mineral known as hydroxyapatite along with interlaced chains of collagen. High temperatures permanently alter the molecular structure of both substances, causing them to fluoresce in a bright cyan color when subsequently exposed to ultraviolet light.

If not heat-treated, the scales will only fluoresce weakly in a dull blue. And importantly, even when the scales have been heat-treated, they look just like a surrounding field of untreated scales when not illuminated by UV light.

The scientists stuck mainly to the scales of the widely farmed red tilapia for their study, although experiments with the scales of other fish species indicated that they would work just as well. After some trial and error, it was found that heating the scales to 270 ºC (518 ºF) for three minutes delivered the best results. Higher temperatures produced an even brighter fluorescing effect, but caused the scales to char and thus become unusably brittle.

The technology can be used to display characters (such as letters, numerals, symbols, etc) in either of two ways.

First of all, multiple heat-treated scales can simply be arranged so they combine to form the shape of a given character. Again, in this scenario, they would be camouflaged against a background of untreated scales.

Alternatively, an ultra-fine-beam laser can be used to heat a pattern into just one part of a single scale, creating a microscopic character that can only be imaged utilizing both a UV light and a microscope. It's sort of like giving the scale a tiny tattoo. And yes, it does bring the serial-number-stamped python scale from the movie Blade Runner to mind.

Extending the scales' possible uses, heat-treating them also makes their surface more porous, allowing them to selectively adsorb a toxic industrial dye known as Rhodamine B from polluted water. In lab tests, treated scales removed 91% of the chemical from tainted water samples over a contact period of just 10 minutes. Those scales could be ultrasonically "emptied" and then reused in water purification systems, or in test kits which would indicate the presence of the dye in water samples.

What's more, when the scales do contain adsorbed Rhodamine B, they fluoresce orange when exposed to green light. This functionality could be combined with the scales' ability to fluoresce cyan under UV light, adding an extra layer of security to their use in packaging or similar applications.

"Globally, an estimated 7.2 to 12 million tons of fish waste is projected to be discarded yearly. This makes fish scale waste an abundant resource for upcycling," said Prof. Sow. "By reevaluating waste streams, fascinating properties and multifunctionalities can be discovered in materials that may have been overlooked previously."

A paper on the research has been published in the journal Nature Communications.

Additional potential uses for otherwise-unwanted fish scales include flexible "electronic skin," wound dressings and a broken-bone-healing material.

Source: National University of Singapore