[Image above] Researchers at the Kostas Research Institute at Northeastern University combined an organic pigment called xanthommatin with titanium dioxide to create a paint that can rapidly and reversibly change color in response to light. Credit: Alyssa Stone, Northeastern University
If you are on the beauty side of TikTok, chances are you’ve seen your fill of #makeuptransformation. While these transformations often demonstrate how makeup can be used to achieve traditional beauty standards, you will also find examples of extreme makeovers that allow a person to obtain fantastical, nonhuman colorations.
Though we cannot naturally turn into a sprinkled cupcake, as in the video above, other animals are innately capable of a much broader color pallet. Imagine the vivid, color-changing skin of a chameleon or octopus, for example. How do they do it?
Compared to mammals and birds, which only have one type of pigment cell called a melanocyte, many other animals—such as reptiles, amphibians, and fish—possess a variety of pigment cells collectively termed chromatophores. Through contraction and expansion, these cells produce a temporary color that can be used for camouflage or signaling, as demonstrated in the video below.
Xanthommatin is a main pigment found in the chromatophores of fish, crustaceans, and cephalopods. Recently, this pigment has become of interest to researchers because of its sensitivity to light.
When exposed to ultraviolet or visible light, xanthommatin will absorb the light and experience an intrinsic photochemical reduction. This reaction causes the observed color to shift from yellow to red.
In February 2022, researchers led by Northeastern University in Massachusetts used this innate color shift to develop a wearable light sensor that lets people know when they have been exposed to a damaging amount of ultraviolet radiation. Now, the group has taken the research a step farther by combining xanthommatin with titanium dioxide to create a paint that can reversibly switch between yellow and red.
In the new open-access paper, published in October 2023, the researchers explain that they chose titanium dioxide due to its electronic properties and ubiquitous use as a whitening agent in commercial coatings.
Regarding its electronic properties, “the structure and size features of the TiO2 particles enhance light harvesting by the sensitizing agent, facilitating injection of electrons into the solid layer,” they write.
The fact titanium dioxide is already widely used in the coating industry means transitioning the new xanthommatin-based paint into commercial application would be easier.
The researchers found they could control how quickly the color change occurred and its intensity based on the amount of titanium dioxide mixed in. Additionally, by incorporating additional non-light-responsive colorants, they could alter both the initial and irradiated color of the paint.
In a Northeastern press release, Dan Wilson, senior research scientist at the Kostas Research Institute at Northeastern University, says the paint could be used to create temporary artwork in public places.
“We’ve imagined a scenario where if you want to have art that changes from day to day on an interior wall, like maybe in a coffee shop or something, you could use a regular projector to project a pattern onto the wall, temporarily paint in this color and this pattern or this art, and then over time that fades away and you can redo it again, ideally as many times as you want,” he says.
Additionally, because xanthommatin is a natural organic pigment, using this paint “creates a safer environment for the people using it and for the people that are going to be exposed to it,” Kaitlyn Flynn, Ph.D. student in chemistry at Northeastern, says in the press release.
The open-access paper, published in Advanced Science, is “Color-changing paints enabled by photoresponsive combinations of bio-inspired colorants and semiconductors” (DOI: 10.1002/advs.202302652).