MIT's engineers developed a stretchy, color-changing films

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

Imagine stretching a piece of film to reveal a hidden message. Or checking an arm band’s color to gauge muscle mass. Or sporting a swimsuit that changes hue as you do laps. Such chameleon-like, color-shifting materials could be on the horizon, thanks to a photographic technique that’s been resurrected and repurposed by MIT engineers.

By applying a 19th-century color photography technique to modern holographic materials, an MIT team has printed large-scale images onto elastic materials that when stretched can transform their color, reflecting different wavelengths as the material is strained.

The researchers produced stretchy films printed with detailed flower bouquets that morph from warm to cooler shades when the films are stretched. They also printed films that reveal the imprint of objects such as a strawberry, a coin, and a fingerprint.

The team’s results provide the first scalable manufacturing technique for producing detailed, large-scale materials with “structural color” — color that arises as a consequence of a material’s microscopic structure, rather than from chemical additives or dyes.

The team found the film’s color is highly sensitive to strain. After producing an entirely red film, they adhered it to a silicone backing that varied in thickness. Where the backing was thinnest, the film remained red, whereas thicker sections strained the film, causing it to turn blue.

Similarly, they found that pressing various objects into samples of red film left detailed green imprints, caused by, say, the seeds of a strawberry and the wrinkles of a fingerprint.

Interestingly, they could also project hidden images, by tilting the film at an angle with respect to the incoming light when creating the colored mirrors. This tilt essentially caused the material’s nanostructures to reflect a red-shifted spectrum of light. For instance, green light used during material exposure and development would lead to red light being reflected, and red light exposure would give structures that reflect infrared — a wavelength that is not visible to humans. When the material is stretched, this otherwise invisible image changes color to reveal itself in red.