When you think of a turkey, you might visualize it roasted and crispy on a table at a large family dinner. But for a group of scientists at the University of California, Berkeley, a turkey served as a different sort of inspiration. The team has created a color-changing biosensor that can detect the explosive agent TNT using a design inspired by a turkey’s wattle.
A wattle, the fleshy fold of skin that surrounds a turkey’s head and neck, is more than just a floppy chin. It can change from red to white to blue as the bird becomes excited. This color change is so striking that in Japan and South Korea, the word for turkey translates to “seven-faced bird.”
UC Berkeley materials scientist Seug-Wuk Lee, his former postdoc (now a scientist in South Korea) Jin-Woo Oh and their colleagues wanted to understand how the turkey’s wattle was able to change color so rapidly. For this, they needed to examine some turkey skin at the microscopic level. So Lee went to a local turkey farm to purchase a turkey head. Surprisingly, the farmer was willing to give the heads away for free. “So we got overnight shipping,” Lee says, “and soon our lab had too many turkey heads!”
The scientists found that the turkey skin was made of bundles of collagen enriched with lots of blood vessels. Collagen, a structural protein found in various connective tissues, is a fantastically flexible molecule. The long, thin proteins can form into bundles, cross-hatching or arranging themselves at random. In one pattern, collagen makes up the lens of our eyes. Arranged another way, it forms tendons and scars. In a third formation, it helps our kidneys filter waste.
In a relaxed turkey, the collagen bundles are small and transparent, and the blood vessels show through the skin, creating a bright red wattle, the team found. But when a turkey gets excited, the collagen bundles expand, become opaque and scatter light. With the blood vessels obscured, the turkey’s wattle goes from red to white, or even blue.
Because the collagen bundles in turkeys change size and scatter differing amounts of light, a similar bundle could be used as a sensor, the scientists reasoned: When exposed to a given chemical, the sensor could change size, and thus change color.
So Lee’s group set out to mimic the collagen bundles in turkey skin. To do that, the team used a bacteriophage, a virus that infects bacteria. The scientists developed a way to make the many copies of a long, thin bacteriophage arrange themselves in collagen-like bundles.
Bacteriophage can also carry receptors, proteins that are activated by specific chemicals. For example, the scientists created bundles of bacteriophage with receptors that are activated by methanol. If there is methanol present, it binds to the receptors on the bacteriophages, causing the collagen-like bundle to swell. Then, just like a turkey’s wattle, the swollen collagen-like bundles scatter light, and the sensor changes color. Lee and colleagues report their new sensor January 21 in Nature Communications. The scientists demonstrated that they can use the sensor to detect water, methanol and trinitrotoluene, the explosive agent known as TNT.
For very small concentrations of chemicals, say, just 300 parts per billion of TNT, the color change will be very slight. So to detect it, Lee and his colleagues built a smartphone app that analyzes pictures of the biosensor before and after exposure to see how much the color has changed. The smartphone app, coupled with some tweaks to the biosensor design, could yield an inexpensive, portable explosive detector.
The detection method could be an improvement over other current color-driven sensors. Many sensors used now involved a light-excited chemical, or fluorophore, that gives off light in response to a stimulus. These fluorophores become damaged and less effective over time. In contrast, collagen-like bundles don’t become damaged with repeated use. “The color change from collagen is based in structure,” says Lee, “so unless you destroy the structure, it will continue to work.”
For many current color-changing sensors, your viewing angle changes what color you perceive. But the bacteriophage in the new sensor, like the collagen in turkeys, are in bundles arranged at random with many different angles. “The color change depends on the bundle size, not the bundle angle,” Lee says, “so no matter what angle you view the bundles, everyone sees the same color.” This could help eliminate confusing readings.
This new biosensor is also versatile. Scientists are able to insert DNA into the bacteriophage that makes it selective for different chemicals. By inserting random repeats of DNA base pairs and seeing which will bind the tightest, the authors can pick the best possible peptides to detect their chemical of interest.
Lee is particularly interested in medical uses for the biosensor. For example, the strips could be used to monitor blood glucose in people with diabetes. Blood glucose levels can be measured in your breath, and Lee hopes that his biosensor could be adapted for this purpose, allowing diabetics to just breathe on a strip and plug the results into the smartphone app to find out how much insulin they need, rather than enduring constant needle sticks.
All this potential thanks to a little bio-inspiration from a turkey’s wattle. “Bio-inspired designs have been big in academia for several decades,” Lee explains, “but we needed the biotechnology to enable us to construct what we see in nature. Now we can use techniques to construct any type of protein structure. We can fine tune the approach to mimic how it’s done in nature.”
Lee finds a lot of inspiration from the natural world. “All the materials you look at in nature, anything we are looking at now is a champion of its environment,” he says. With recent advances in biotechnology, scientists can now learn from and mimic the first-class features of living things. The future of bio-inspiration is a bright one, Lee says. “There are so many things waiting for discovery.” After all, if we can develop a sensitive sensor from a turkey’s wattle, who knows what could be next.