Octopus Skin Inspires New Color-Changing Paint

A naturally occurring dye compound present in the skin of cephalopods – like octopuses and squid – can be used to create novel paints that change their color in response to light, according to a new study.

Published in Advanced Science, the new study demonstrates the use of a synthetic version of this cephalopod dye, which can be mixed with titanium dioxide to form a paint that reversibly shifts between being yellow or red with strong light exposure. Additional colorants can be mixed in with the paint to expand its shifting color palette, the researchers say.

Reinventing natural materials

The natural world has many examples of color-changing materials, from the chameleons of the rainforest to the cephalopods of the deep blue sea.

The key to a cephalopod’s color-changing power is xanthommatin, a natural pigment that exists in the animals’ chromatophores. By manipulating these chromatophores – tiny sacs filled with nanoparticles of xanthommatin – the animals can rapidly and reversibly change their skin’s appearance.

“Their color change is so rapid and it’s so vibrant and it’s so intense,” said Cassandra Martin, a research scientist at Northeastern University’s Kostas Research Institute (KRI). “There’s not a lot of natural systems out there that change that fast and there’s not a lot of color-changing materials that are that fast without requiring a lot of external [changes].”

Building a similar color-changing material for human use would be a challenge. But it is one that Martin and her colleagues are willing to take on.

Researchers at the KRI have previously conducted other research projects looking at color-changing materials; last year senior research scientist Dan Wilson published a paper looking at the development of wearable patches that change color when the wearer gets too much sun. These patches also used xanthommatin.

Spurred on by their success in this project, the staff at the KRI wanted to develop a material based on this naturally occurring pigment where its color change is just as rapid and reversible as that done by the squid themselves.

Mixing a color-changing paint

Titanium dioxide is a molecule that is already well-known to anybody who works with color – it is one of the most common white pigments used in producing white paints and plastics. But titanium dioxide is also a semiconductor and a photocatalyst, meaning that it can catalyze reactions when exposed to light.

Kaitlyn Flynn, a PhD student at Northeastern who was an intern/visiting student at the time of the research, was working on a related xanthommatin project when she had the idea of investigating how titanium dioxide might affect the chemistry of xanthommatin.

The researchers found that incorporating xanthommatin into a water-based polyurethane paint with different particle sizes of titanium dioxide resulted in a red paint that would quickly shift to a yellow hue when exposed to sunlight. When the light source was removed, the paint would readily change back to its red color.

Upon closer study, the researchers found that titanium dioxide served as a conductor for the color change, with the particle size and density of the titanium oxide particles controlling the speed and intensity of the color shift.

The color change in this new paint can happen in as little as five minutes. Depending on how long the paint is exposed to light, the color change can remain for as long as 24 hours.

By intelligently masking certain regions of a material coated with the new paint, the researchers found that it was also possible to create complex images in the paint that would fully fade over time. Painting a sample card and then masking certain areas of it during sunlight exposure left a visible negative image in the shape of the mask design. Even very complex mask images – including an image of Northeastern University’s wolf mascot and a university logo including small Latin text – were clearly visible and readable following this masking procedure.

“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,” Wilson said. “We can create temporary artwork or art or paint that could potentially track the weather or track the environment that it’s in.”

The paint itself is also easy to produce, taking as little as two hours, with the colorant recipe being compatible with both water- and oil-based paints.

Beyond temporary art

Color-changing paint is more than just a fun novelty, or a potential tool for creating temporary artworks. They could also be a more environmentally friendly alternative to traditional colored paints, the researchers say.

“Paints that are commercially used nowadays can have harmful chemicals in them, so they can have things that can be harmful to the people that are painting them,” said Flynn. “The fumes can be super harmful. They can be harmful long-term if you’re exposed to them for a long time. They can also leach out into the environment. Searching for a more natural way to make these paints creates a safer environment for the people using it and for the people that are going to be exposed to it.”

Following on from this new research, Flynn and Martin are working to create similar paints with an expanded color palette and different color change properties, such as a faster or slower color change rate.

Reference: Martin CL, Flynn KR, Kim T, Nikolic SK, Deravi LF, Wilson DJ. Color‐changing paints enabled by photoresponsive combinations of bio‐inspired colorants and semiconductors. Adv Sci. doi: 10.1002/advs.202302652

This article is a rework of a press release issued by Northeastern University. Material has been edited for length and content.