Artificial "skin" changes color in response to minute force

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Changing the color of the skin-like membrane is as simple as stretching it a tiny amount

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A thin and flexible chameleon-like material developed by engineers at the University of California, Berkeley changes color when stretched or bent even tiny amounts. With potential applications in camouflage, structural fatigue sensors, display technologies, and more, the material's color changes reliably as it gets flexed thanks to rows of ridges that are precisely etched onto a silicon film one thousand times thinner than a human hair.

These ridges reflect a specific a wavelength of light, with that wavelength determined by the spaces between the ridges. When the material is flexed or bent, this spacing changes and so too does the color reflected.

Previous attempts to create flexible color-shifting surfaces had issues with reliability or control – metallic surfaces reflect only a portion of light received and non-metal surfaces have previously been too thick or rigid. The UC Berkeley team behind this new material overcame these issues by embedding its 120 nanometer-thick silicon bars into a flexible layer of silicone.

The result was a skin-like membrane that is easy to manufacture and that reflects precise and pure colors. The membrane changed color from green to yellow to orange in response to tiny amounts of stretching – just a 25 nm change in the period, or interval, of the ridges corresponded to a 39 nm change in display color (from 541 to 580 nm wavelength). Further stretching produced additional colors, but beyond this point the efficiency of the material – how much light it reflected – dropped substantially.

The next step is to scale up the design from the 1 cm2 size used in the researchers' demonstration and to see if it's possible to increase the material's efficiency and range of colors.

"At that point we hope to be able to find applications in entertainment, security, and monitoring," said co-author of the study, Connie J. Chang-Hasnain.

Specific applications might be active camouflage on the outside of vehicles, large displays for outdoor entertainment venues, and sensors that change color to indicate structural fatigue in critical components on bridges, buildings, or the wings of airplanes.

A paper describing the research was published in the journal Optica.

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