Chameleon skin-inspired material changes color, can detect seafood freshness

Scientists have created a new type of synthetic skin that uses luminogens and a bilayer structure to artfully alter its color in response to the environment.

The new chameleon-like skin, described Thursday in the journal Cell Reports Physical Science, is designed to detect seafood freshness, but the technology could be used for a range of applications.

Most color-changing synthetic skins are formed by a single layer matrix of different light-sensitive molecules.

For the new material, researchers outfitted two hydrogel layers -- an inner core and an outer shell -- with luminogens, molecules that cause crystals to fluoresce.

"This novel core-shell layout does not require a careful choice of luminogen pairs, nor does it require an elaborate design and regulation of the complex photophysical interactions between different luminogens," study co-author Tao Chen said in a press release.

"These advantages are important to the future construction of robust multicolor material systems with as-yet-unachieved performance," said Chen, a professor at the Ningbo Institute of Materials Technology and Engineering at the Chinese Academy of Science.

By organizing luminogens into two different layers, researchers were able to achieve a more complex array of color shifts and patterns.

The scientists suggest their material's core-shell structure more closely resembles to design of actual chameleon skin.

Material scientists began by building a core hydrogel layer that fluoresced a bright red.

After soaking the core hydrogel in a series of aqueous Europium solutions, researchers exposed the initial layer to a grow solution composed of sodium alginate and blue-green fluorescent polymers.

The Europium ions from the core layer diffused into the surrounding solution, forming blue and green hydrogel layers.

Due to the organic fusion of the hydrogel layers, researchers were able to trigger color shifts from red to blue or green by exposing the new synthetic skin to different temperatures or acidity levels.

They said they estimate the design process could be tweaked to create materials capable of producing different types of color shifts and patterns.

"The proposed diffusion-induced interfacial polymerization to prepare core-shell materials proves to be general," Chen said.

"It is thus highly expected that the proposed synthetic strategy could be expanded to produce other soft color-changing materials, such as smart hydrogels or elastomers with stimuli-responsive structural color or pigment color change," Chen said.

To test their technology, researchers fashioned a chemosensor out the bilayer material. The chemosensor was designed to react to amine vapors released by microbes as seafood loses its freshness.

To test them, scientists stored strips of the novel hydrogel material in bags of frozen shrimp.

When stored at a temperature of negative 10 Celsius, the strips stayed the same red color, indicating the seafood was still fresh. When stored at 30 degrees Celsius, the strips slowly turned green, suggesting the shrimp had spoiled.

In addition to sensing seafood spoilage, the new bilayer hydrogel material could be used to build stretchable electronics, soft robots or even counterfeiting technologies.

"In the near future, we plan to utilize the developed chameleon skin-like core-shell hydrogels to prepare biomimetic soft camouflaging skins, which can be used to mimic the diverse color-changing functions of living organisms' skins and to help achieve desirable active camouflage, display and alarm functions in robots," said Chen.

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