In Korea and Japan they call turkeys "seven-faced birds," because male gobblers can alter the color of their head and neck when they're seeking a mate or trying to intimidate a rival.
Now, researchers at UC Berkeley say the same principle that allows turkeys to cycle through shades of red, white and blue can also be used to ferret out explosives.
In a paper published Tuesday in Nature Communications, researchers said that after examining fresh turkey heads from a local farm, they determined that the microscopic arrangement of collagen fibers, as well as blood vessels, was responsible for the color show.
According to senior study author Seung-Wuk Lee, an associate professor of bioengineering, spacing between the collagen fibers changes when the turkey's blood vessels swell and contract. This change in collagen fiber spacing alters the way light waves are scattered, changing the color of the animal's flesh.
The researchers hypothesized that they could create a sensing material with similar bundling patterns that would change color when exposed to specific chemicals, such as the explosive TNT.
Lee and his colleagues turned to a particular bacteria-attacking virus: bacteriophage M13. The fast-replicating virus does not affect humans, but its long, narrow shape is similar to that of a collagen fiber.
When the researchers coated silicon wafers with the virus and exposed them to volatile organic compounds (VOCs) like isopropyl alcohol, methanol or TNT, the viruses swelled rapidly, resulting in a visible color change.
"Arrays of differently colored phage matrices, termed phage litmus, rapidly swell or shrink on exposure to external chemicals, resulting in color changes similar to those seen on turkeys when they get flustered," the authors wrote.
The phage litmus also reacted to humidity, becoming redder with moist air and bluer with drier air.
While it remains unclear exactly why the viruses swell, researchers fine-tuned the effect by genetically altering the M13 bacteriophage so they responded to one specific chemical, the high-explosive trinitrotoluene.
Then, to enhance their ability to detect subtle color changes, the researchers used an iPhone app to analyze the litmus color.
Researchers wrote that they were able to detect TNT in a gaseous phase down to 300 parts per billion. (One part per billion is roughly analogous to a single kernel of corn in a cylindrical silo 45 feet tall and 16 feet in diameter.)
Researchers said they were able to isolate the effect as well so that other VOCs and humidity did not mask the TNT gas.
Although the TNT-sensing system was used to demonstrate proof of concept, the study authors wrote that the sensor could be modified to detect "a variety of harmful toxicants and pathogens to protect human health and national security."