January 1, 2007 — 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.
TORONTO -- 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.
Note: This story and accompanying video were originally produced for the American Institute of Physics series Discoveries and Breakthroughs in Science by Ivanhoe Broadcast News and are protected by copyright law. All rights reserved.
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