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Advanced materials

Octopus skin inspires new infrared reflector

07 Apr 2018
Greater blue-ringed octopus
Warning signs: the greater blue-ringed octopus changes its appearance when threatened using techniques that have inspired an adaptive infrared reflector. (CC BY-SA 2.5/Jens Petersen)

A simple device with tuneable infrared reflectivity has been made by mimicking the adaptive properties of the skin of octopuses and related animals. Chengyi Xuat, Alon Gorodetsky and George Stiubianu of the University of California, Irvine created the device using a dielectric elastomer and say that it overcomes many of the limitations of previous adaptive infrared-reflecting systems.

Reflecting infrared radiation is important for many technologies, ranging from building insulation to spacecraft components. But most of the materials used to reflect radiation in the infrared region are static: they are unable to respond and adapt to changes in the environment. Some adaptable infrared-reflecting systems have been developed, but they tend to be complex and difficult to control, while also lacking spectral tunability and requiring high operating temperatures.

Inspired by the skin of cephalopods – squid, octopuses, and cuttlefish – Gorodetsky and colleagues have now developed an adaptable infrared-reflecting system that they say is easy to control, can respond rapidly and be used repeatedly. The system also has a tuneable spectral range and works at low temperatures.

Camouflage and signalling

Many cephalopods can rapidly change the colour and patterning of their skin. This is done for both camouflage and signalling, and is enabled by pigment cells with adjustable spectral properties that can response within hundreds of milliseconds. These yellow, red, and brown cells, known as adaptive chromatophores, are packed with pigment granules and can be expanded and contracted by radial muscles. As their size and shape changes so do the wavelengths of light that they absorb and reflect.

Some cephalopods also have reflective skin cells called adaptive iridocytes. These cells contain alternating arrangements of nanostructured proteins and extracellular space. The geometries and refractive index differences of these structures can be altered by biochemical signalling, allowing the animal to adjust the wavelengths of light the cells reflect.

To mimic the functionality of these cells, Gorodetsky’s team turned to dielectric elastomers. The device they developed consists of a dielectric elastomer membrane sandwiched between two electrodes and is topped with an infrared-reflecting coating. When an electrical current is applied the membrane, it decreases in thickness and increases in area. This causes the reflective coating to expand – like an adaptive chromatophore – and alters its microstructure – like an adaptive iridocyte. These changes alter the spectral properties of the device, increasing the amount of infrared radiation reflected. When the electrical current is removed the membrane returns to its original size and shape.

Tests showed that in its relaxed state the device reflects around 71% of light in the infrared range, while absorbing roughly 28%. When fully activated reflectance increases to approximately 96%, and absorptance drops to around 3%. In both cases less than 1% of infrared light is transmitted through the device.

Autonomous operation

The team also showed that the system can operate autonomously by attaching it to a heat sensor that controlled the electrical input. As the temperature around the heat sensor increased from 26° C to 48° C so did the size of the device and the amount of infrared light it reflected.

Finally, to test the system’s ability to hide from an infrared camera the scientists created a squid shaped version of the device. They found that changing the apparent temperate of the device by just 2° C was enough to camouflage or reveal it.

According to the researchers both the changes in size and microstructure of the active area, and the change in thickness of the device are responsible for the changes in reflectance and absorptance. They found that infrared-reflecting properties were consistent and fully reversible upon repeated actuation, with only minor changes in performance after 750 cycles.

Gorodetsky told Physics World that camouflage is one application for such adaptive infrared-reflective systems, but added that they could be used in “any application where modulating the reflection of infrared radiation is important”. He adds: “Examples applications would be spacecraft, storage containers, emergency shelters, clinical care, and building heating and cooling.”

The team is now “working on analogous systems that function in the visible, as well as wearable thermoregulatory technologies”. The research is described in Science.

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