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April 7, 2013

Color-Changing Gel

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What would you do with a gel that dramatically changes color in response to heat, water and salt? As this ScienCentral News video explains, the uses could range from color-changing clothes to monitoring food safety.

Hot Fashions

MIT engineer Ned Thomas has some fashionable ideas for his new high-tech gel.

"Around the pool side you would be wearing one kind of bathing suit, when you got in the water it would be a different color," says Thomas.

That's because the fabric could change color in response to water or to the changes in the pool water's temperature or pH.

Thomas and his colleagues created a gel that responds to various stimuli by dramatically changing color. It's not a gel like the gooey hair products in salons. It's more like the thin plastic of a sandwich bag.

Other scientists have made gels that change color in response to changing conditions. But two things make Thomas' gel stand out: the color change is reversible and it can display a wide and controllable range of colors.

"I think it's fair to claim that our gels at the moment are the most responsive gels on the planet," says Thomas. "We've done something for the first time to namely structure a gel so that it interacts and reflects specific colors of light."

The Recipe

Thomas and his colleagues explain how they create the gel in the journal Nature Materials.

The gel is made by putting together alternating layers of gel and non-gel materials. The gel layer swells when exposed to stimuli such as water or acid, while the non-gel layer does not respond. This change in the gel's thickness determines whether certain wavelengths of light pass through or bounce off it — and what colors our eyes see.

"It's a gel that can shrink or expand so you have this layered sort of system and the fact that it has layers which have different optical properties causes light to be reflected from these layers," says Thomas.

Thomas says that the gel can be fine-tuned for many different uses. He says it could be tailored to respond to moisture and could be used in the food processing and distribution industries to monitor food safety and quality.

"They don't require any power. You just look at them, you see a color, and you then translate the color into whether or not you've got a lot or a little [of the stimuli being measured.]," explains Thomas.

With funding from Defense Advanced Research Projects Agency (DARPA), he has more high-tech ideas in mind, like aircraft stability.

"An alternative would be to coat this wing with a layer a thin layer of the polymer which would have a particular color when it's relaxed with the air--no airflow. When you turn on the airflow the pressure in the different parts of the wing would cause lesser or greater extensive compression. This would change the color say from red in some areas to green in others to blue in the areas where it's most compressed. And you could optically read out continuously over the surface what the local pressures were," says Thomas.

But Thomas also explains that the advantage of his gel is that it is an inexpensive, tunable visual indicator. If high accuracy is needed, other existing products would be more appropriate.

"So I think a lot of the applications for our gels are going to be those in which you're interested in whether things are in the safe range or they've gone beyond what your tolerance is," says Thomas.

Thomas says novelty items such as color changing clothes or umbrellas will probably be developed before the more large-scale applications like food safety.

The study was published online in Nature Materials, October 21, 2007 (Study Authors: Youngjong Kang, Joseph J. Walish, Taras Gorishnyy and EdwinL. Thomas); it was funded by the National Science Foundation and Defense Advanced Research Projects Agency (DARPA).

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