How colour-changing camouflage tech work?

This technique opens a door to manufacturing of pressure monitoring bandages, shade-shifting fabrics, or touch sensing robots.
Interesting Engineering

Imagine decoding hidden messages by stretching color-changing films or having shade-shifting clothes in the market, or developing touch-sensing robots, wouldn't that be interesting? The good news is such color-shifting materials will soon be available and manufactured at a large scale using a unique photography technique, all thanks to MIT researchers. This video describes how they developed this technique, how they got this idea, and the challenges they faced while developing it. 

The researchers repurposed a 19th-century color photography technique called Lippmann Photography, developed by Gabriel Lippmann, a Franco-Luxembourgish physicist, and applied it to modern holographic materials to produce color-changing films. The team printed large-scale images onto elastic materials that can transform their color and reflect different wavelengths when stretched. They printed detailed flower bouquets on films that changed warm colors to cool shades when stretched. They also printed films that reveal the imprint of objects such as a strawberry, a coin, and a fingerprint. 

According to the researchers, this is a unique technique for manufacturing detailed large-scale materials with structural colors. These colors appear due to the material's microscopic structure instead of dyes or chemical additives. 

While looking for solutions to scale these structures' production, Benjamin Miller, an MIT researcher, came to know more about holography or Lippmann color holography. This technique makes 3-D images on physical materials by superimposing two light beams. Lippmann produced images through light wave interference. He placed a mirror behind a thin transparent emulsion (a substance he prepared using light-sensitive materials like silver halide). He exposed the setup to light, and the mirror reflected beams to the emulsion. The interference of incoming and outgoing light beams stimulated the emulsion to show colored patterns. This technique is laborious and time-consuming, so Miller and his team got a new idea by combining this technique with modern hologram technology. 

They adhered transparent, elastic film on a reflective mirror-like surface. Then, they placed the projector at some distance from the film and projected images on each sample, including Lippmann-Esque bouquets. As a result, the film produced large, detailed images within a few minutes, reproducing colors in the original images. They were then removed from the mirror and placed on a black elastic silicone sheet for support. When stretched, the film showed color shifting due to structural changes at molecular levels. For instance, it reflected the red color when exposed to green light and infrared wavelengths (non-visible) when exposed to red light. 

The team is now exploring various applications of this technique, such as making color-changing bandages to monitor bandage pressure when treating conditions like lymphatic disorders and venous ulcers. 

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