Molding Fingerprints - Materials Chemists Apply Photonic Crystals to Forensics

Photonic crystals -- materials with precise patterns of gaps that make them reflect only selected wavelengths of light -- could soon replace the traditional ink-based fingerprinting. In a new silica-based, photonic-crystal material, the spacing of the gaps changes in response to pressure applied. Corresponding changes in its color reveal fingerprints with high precision -- not only the ridges in the skin, but also the depth of the ridges, the shape of the finger, and the mechanical properties of the skin.

Increased airport security ... Better police forensics work ... Even improved bridge and building safety. These are all the tremendous possibilities stemming from a new material that's 20 times thinner than a strand of human hair.

Imagine a security system that relied on something unique to every single person -- his fingerprint. Now, scientists have developed a material that makes those prints nearly impossible to forge.

At the University of Toronto inside a science lab, Materials Chemist Andre Arsenault starts from scratch making new crystals. The raw materials are a lot like opal gemstones, which reflect light.

"Opal gemstone is very nice because you get all these multi-faceted color effects," Arsenault tells DBIS.

But these crystals are microscopic. Inside a flask, they form millions of tiny silica circles. Chemists fill the spaces with a synthetic rubber and then dissolve the silica, leaving behind a thin, honeycomb-like structure called a photonic crystal. When you press on it, the holes get closer together, changing the wavelength of light that's reflected.

"As you start pressing, you're gonna gradually go through the rainbow toward the blue. So you're gonna go red, orange, yellow, green, blue, purple," Arsenault says.

With a traditional ink fingerprint, the only thing that can be seen is the ridges on the finger. Full-color prints provide so much more. "You can get information about the depth of the ridges on the people's fingers," Arsenault says. "You can get information about the shape of people's fingers, as well as even the mechanical properties of the skin."

He even made a rubber replica of his fingerprint, which might fool a traditional fingerprint scan. The new material picked up the fake.

Researchers say the photonic crystal material is inexpensive to make and could be used to improve sensors in a number of consumer products.

BACKGROUND: Materials chemists at the University of Toronto have developed a new elastic light-sensitive material that changes color based on pressure and could be used to capture data-rich fingerprints in multiple colors. The material could also be used in pressure sensors in consumer products, such as consumer electronics, airbag deployment, strain and torque sensors in high-rise buildings, or even in children's toys, where kids would press or squeeze the item to see it change color in front of their eyes.

HOW IT WORKS: Traditional fingerprinting methods involve treating samples with powders, liquids, or vapors to add color to the print, so it can easily be photographed. This process is known as contrast enhancement. The Toronto scientists engineered their new material into a thin, elastic foam that can be transferred onto any surface, such as glass, metal or plastic. If the foam is compressed, the internal structure changes, altering the wavelength (color) of light it produces and further enhancing contrast. The resulting images capture detailed information about pressure patterns and surface ridges that may not be visible to the naked eye.

WHERE THE COLOR COMES FROM: A peacock's brightly colored feathers don't get their color from pigments. Pigment molecules create colors by absorbing or reflecting certain wavelengths of light, depending on the chemical composition. Peacock feathers only have brown pigment (melanin). The bright colors we see arise from the inherent structure of the feathers, which have arrays of tiny holes neatly arranged into a hexagonal (lattice) pattern. This causes the light to refract off the surface in such a way as to produce the perception of color in the human eye; which colors one sees depends upon the angle of reflection. Physicists call these structures photonic crystals.

ABOUT PHOTONIC CRYSTALS: Photonic crystals are materials with an arrangement of atoms in a precise lattice pattern that repeats itself identically and at regular intervals. But Nature doesn't produce crystalline structures with the level of precision we need, so scientists learned to make their own version of these materials, atom by atom, to control and manipulate light. Light generally travels in a straight line, but if the atoms are organized precisely enough, certain wavelengths of light will be blocked and reflected in new directions, even turning corners. The spacing of the atoms in the lattice structure determines which wavelengths will be blocked.


CSI: X-Ray Fingerprints

Micro-X-Ray Fluorescence Also Provides Spectroscopic Information

Ordinary invasive fingerprinting techniques, such as dusting, are prone to damaging evidence. Micro-X-ray fluorescence images fingerprints without touching them. By stimulating atoms to emit signature wavelengths of light, MXRF also provides chemical information -- such as traces of soil or saliva left in the fingerprints -- in addition to the print pattern itself.

Popular television crime shows solve cases in an hour. But in real life, cracking a case isn't a quick, easy game -- especially when it comes to finding fingerprints.

...And it was no game when thieves robbed Tatiana Bonilla's home, stealing pricey jewelry. "The police didn't find anything ... It was never solved, and it's been a year," she says.

Police dusted for fingerprints in Bonilla's home, but some fingerprinting techniques can alter a print, erasing valuable clues. Now, chemists have a new, non-invasive way to detect prints -- using X-rays to find chemicals within print patterns.

"You can also get chemical information in addition to the print pattern itself, so you can tell, for instance, that there's some unusual element that's located in that fingerprint," Chris Worley, an analytical chemist at Los Alamos National Laboratory, tells DBIS.

The process, called micro X-ray fluorescence (MXRF), zaps a print with a tiny X-ray beam that mixes with atoms left behind from sweat or evidence. Next, the atoms give off information, revealing what chemicals are present. Chemicals, like potassium, then form an image of a fingerprint.

"This is a new way of visualizing fingerprints in cases where perhaps we couldn't detect a fingerprint with the traditional methods," Worley says.

Scientists say the MXRF technique could be used to better track down missing children. Children's fingerprints are more difficult to detect -- the new method could better detect prints based on chemicals left behind in a child's fingerprints due to food, soil or saliva.

BACKGROUND: Scientists at Los Alamos National Laboratory have developed a new fingerprint visualization technique using X-rays that leaves prints intact and reveals chemical markers that could give investigators new clues for tracking criminals and missing persons. Traditional fingerprinting methods involve treating samples with powders, liquids, or vapors to add color to the print, so it can easily be photographed. This process is known as contrast enhancement. However, dusting for fingerprints can sometimes alter the prints, erasing valuable forensic clues. Children’s fingerprints are especially difficult to detect.

HOW MXRF WORKS: The new technique uses a process called micro-X-ray fluorescence (MXRF), which rapidly reveals the elemental composition of a sample by shining a thin beam of X-rays onto it without disturbing the sample. All chemical elements emit and absorb radiation at a "signature" frequency of light. For instance, sodium emits primarily orange light, while oxygen (used in neon lights) emits green light. Scientists can pass collected light through an instrument called a spectrograph to spread it into a spectrum, much like visible light spreads into a rainbow of colors by a prism. By carefully studying how the spectrum becomes brighter or darker at each wavelength, scientists can tell what chemical elements are present in a given sample.

WHAT THEY FOUND: The researchers used MXRF to detect the sodium, potassium and chlorine from salts excreted in human sweat – which is sometimes present in detectable quantities in fingerprints. Since those salts are deposited along the ridge patterns in a fingerprint, it is possible to use the elemental analysis to produce a visual image of that fingerprint for analysis. It is especially useful for tracking down lost or missing children: the new method can detect prints based on chemical markers left behind in the child’s fingerprints due to the presence of food, soil or saliva, and this information can be used to track down evidence of the child’s movements.

ABOUT X-RAYS: Like visible light, X-rays are wavelike forms of electromagnetic energy (light) carried by tiny particles called photons. The only difference is the higher energy level of the individual photons, and the corresponding shorter wavelength of the rays, which make them undetectable by the human eye. X-ray photons have energies that range from hundreds to thousands of times higher than those of visible photons. X-ray machines image the outline of bones and organs, while a CT scan machine forms a full three-dimensional computer model of the inside of a patient's body. Doctors can even examine the body one narrow slice at a time. The X-ray beam moves all around the patient, scanning from hundreds of different angles, and the computer takes all that information to compile a 3D image of the body.