Learning from Nature: From a Master of Disguise to an Intelligent Color-Changing Material
Squid-inspired artificial skin withstands extreme cold and kills microbes
The squid's ability to change its skin color based on complex environmental information has fascinated scientists for decades. Now, Chinese researchers have taken inspiration from squid to create a new type of artificial skin that not only protects against extreme cold but also kills bacteria and fungi, opening the door to a range of potential uses.
Like other cephalopods, tropical fish and chameleons, squid possess periodic photonic nanostructures that allow them to change skin color in response to external environmental stimuli as well as for camouflage, communication and courtship.
Squid have thousands of cells called chromatophores just below the surface of the skin, which are connected to the nervous system. At the center of the pigment cell is an elastic sac filled with pigment. Muscle contractions control the size of the pigment cells, changing the squid's skin color and enabling them to change patterns to match nearby rocks or corals. Reflector proteins found in some squid species refract light and cause the animals' dynamic pigmentation and iridescence.
In the past, researchers have used the process to create insulating skin , develop devices that monitor sun exposure , and make human cells transparent . Now, researchers at Dalian University of Technology in China have been inspired by the innate ability of squid skin to create a new, flexible artificial skin that can withstand extreme temperatures and bacteria.
"The biological skin converts environmental information into bioelectrical signals and transmits them to the nervous system to sense external pressure, touch, vibration, temperature, etc.," Niu Wenbin, corresponding author of the study, told Tech Xplore."In addition to bioelectric signals, especially the skin of cephalopods can further actively perceive complex environments through color changes."
The researchers mimicked the arrangement of reflexins in squid skin to create a new type of photonic ion skin they call PIskin. When PIskin is exposed to external stimuli such as surfaces, its photonic (light-manipulating) nanostructures cause rapid color changes. At the same time, ion transport in the skin changes, allowing mechanical and thermal stimuli to be converted into electrical signals.
"Inspired by the color-changing squid skin, we introduced photonic nanostructures into the electronic skin, greatly enriching its sensory capabilities,"Niu said."In addition to providing quantitative feedback, recording and analyzing stimulus changes through electrical signals, more complex information such as the location, shape and distribution of stimuli can be intuitively identified through color."
To further enhance the skin's properties, the researchers added glyceryl monolaurate (GML), a compound with powerful antimicrobial properties, and polyethylene glycol 200 (PEG-200), an industrial-grade surface Active agents, emulsifiers and detergents. GML enables PIskin to kill virtually all bacteria and fungi, while PEG-200's low freezing point means skin can withstand cold temperatures without freezing and is less likely to dry out.
The researchers found that the skin performed well under harsh conditions and accurately measured strain, pressure and temperature.
The creation of PIskin opens the door for future applications in wearable medical devices, soft robotics, prosthetics, and human-machine interfaces. It also encourages researchers to observe other color-changing animals.
"There are many interesting animal species that have this color-changing ability,"Niu said. "In our next work, we will further explore the biological structures of species other than squid and develop corresponding biomimetic skins. Ultimately, these skins could be used in wearable devices, interactive sensing, and other real-world applications."
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"The biological skin converts environmental information into bioelectrical signals and transmits them to the nervous system to sense external pressure, touch, vibration, temperature, etc.," 
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