Artificial skin changes colour to order

The artificial skin changes colour when force is applied.
Engineers at the University of California at Berkeley have created a thin material that can be made to change colour when a small amount of pressure is applied.

The multi-coloured material has potential for display technologies, colour-shifting camouflage, and sensors that can detect defects in buildings, bridges, and aircraft.

Structural colour is used in nature to create an iridescent display of colour in butterflies and beetles. Controlling the surface features on a very small scale allows them to interact and reflect particular wavelengths of light.

Connie J. Chang-Hasnain, a member of the Berkeley team and co-author on a paper published today in Optica, The Optical Society's (OSA) journal, told the magazine:"This is the first time anybody has made a flexible chameleon-like skin that can change colour simply by flexing it".

By etching features that are smaller than a wavelength of light onto a silicon film one thousand times thinner than a human hair, the researchers were able to select the range of colours the material would reflect, depending on how it was flexed and bent.

In place of slits cut into a film, the engineers etched rows of ridges onto a single, thin layer of silicon. Rather than spreading the light into a complete rainbow, however, these ridges, or bars, reflect a very specific wavelength of light. By tuning the spaces between the bars, it is possible to select the specific colour to be reflected. Unlike the slits in a diffraction grating, the silicon bars were extremely efficient and readily reflected the frequency of light they were tuned to.

Spacing of the bars is the key to controlling the colour they reflect. The researchers realised it would be possible to subtly shift the period, and therefore the colour, by flexing or bending the material.

Metallic surfaces are easy to etch, but reflect only a portion of the light received. Other surfaces were too thick, limiting their applications, or too rigid, preventing them from being flexed with sufficient control.

By forming grating bars using a semiconductor layer of silicon approximately 120nm thick, the material's flexibility was imparted by embedding the silicon bars into a flexible layer of silicone. As the silicone was bent or flexed, the period of the grating spacings responded in kind.

The semiconductor material also allowed the team to create a skin that was incredibly thin, perfectly flat, and easy to manufacture with the desired surface properties. This produces materials that reflect precise and very pure colors and that are highly efficient, reflecting up to 83% of the incoming light.

Their initial design, subjected to a change in period of a mere 25nm, created brilliant colours that could be shifted from green to yellow, orange, and red, across a 39nm range of wavelengths.

"The next step is to make this larger-scale and there are facilities already that could do so," said Chang-Hasnain. "At that point, we hope to be able to find applications in entertainment, security, and monitoring."

Caroline Hayes

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