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Exposing Prints

​LATENT FINGERPRINTS, or fingerprints that are not visible to the human eye, are an integral tool in crime fighting and identification. But researchers at the University of Leicester in the United Kingdom estimate that only about 10 percent of latent prints are detected through traditional methods being used by law enforcement. The researchers would like to help find the other 90 percent of prints that remain hidden.

Most fingerprint techniques focus on illuminating what researcher Robert Barker, instrument scientist at Institut Laue-Langevin in Grenoble, France, calls the “fatty residue” that fingers leave behind as prints. These existing methods often involve adding powders and chemicals, or “reagents,” to the residue that will interact with the prints so that they can be detected, sometimes through the use of UV lights. But the Leicester researchers, led by Rob Hillman, professor of physical chemistry at the University of Leicester, have developed what Hillman calls a “complementary” method, which will instead highlight the ridges between the fingerprint residue.

The researchers have added polymer to the ridges, which would essentially mean adding polymer to the surface that the prints may have been left on. The fingerprint residue acts like an insulator while the areas between it are covered in a polymer that reacts with an electronic charge, providing basically a “negative” image of the fingerprint. “But digitally, that’s perfectly straightforward to convert to a positive image,” says Hillman.

Barker explains the process: “Whereas before, we were targeting the peaks on the finger where they made contact with the surface, now we’re targeting the troughs where it hasn’t made contact and, therefore, getting that negative image.” This could be particularly helpful because, while there does need to be a certain amount of residue for the print to be salvageable through traditional methods, it takes much less latent print for the print to act as an insulator in this method, says Barker.

Additionally, if a more traditional illumination method has already been used on the prints, such as adding a chemical reagent, investigators should still be able to use this newer method because it focuses on the valleys that have not been treated with any chemicals yet, says Hillman. Likewise, someone could use a traditional illumination method after this method is applied.

The researchers have made further breakthroughs in this method by partnering with Barker to use neutrons to attach what are known as fluorophore molecules to the polymers, which will make them easier to pick up by certain lights that are already used in many police laboratories. It also makes the barrier to entry for using this new technique much lower.

One limitation, however, is that the method only works on metals for now. “It has to be a conducting surface in order for us to put the polymer down and, simplistically, that means metal. So you couldn’t do it on a painted metal surface, because you’ve colored the whole of the surface with an insulator,” says Hillman. But it would work on stainless steel knives, brass bullet casings, and items made of copper and lead.

The researchers’ method also works better in situations where the prints are older or have been exposed to ambient elements, such as heat and water, says Hillman. The “polymer methods work well for some of the scenarios that some of the existing techniques don’t work well for. And our belief at this time is that it works well for some of the cases where the…aging process has removed a significant amount of the material.” Hillman says the researchers have been in touch with the U.K. government and various police forces about testing out the method in lab settings.