Squid skin inspires new “liquid windows” for greater energy savings

Artistic representation of the prototype
Zoom in / Artist’s impression of a “liquid window” prototype inspired by the texture of squid skin.

Raphael Kay, Adrian So

Squid and many other cephalopods can change the colors of their skin rapidly, thanks to the unique structure of that skin. University of Toronto engineers were inspired by the squid to create a prototype of “liquid windows” that can change the wavelength, intensity and distribution of light transmitted through those windows, thus saving substantially on energy costs . They described their work in a new paper published in the Proceedings of the National Academy of Sciences.

“Buildings use a ton of energy to heat, cool and light the spaces within them,” said co-author Raphael Kay. “If we can strategically control the amount, type and direction of solar energy entering our buildings, we can greatly reduce the amount of work we ask of heaters, air conditioners and lights.” Kay likes to think of buildings as living organisms that also have a “skin,” that is, an outer layer of exterior facades and windows. But these characteristics are largely static, limiting how much the building ‘system’ can be optimized as environmental conditions change.

Installing blinds that can open and close is a rudimentary means of lightening the load on lighting and heating/cooling systems. Electrochromic windows that change their opacity when a voltage is applied are a more sophisticated option. But, according to Kay, these systems are expensive and have complicated manufacturing processes and a limited range of opacities. Nor is it possible to shade one part of a glass but not another.

So they looked to nature for inspiration. Last year, Toronto engineers built a system with arrays of optofluidic cells inspired by marine arthropods, such as krill, crabs and fish such as tilapia, which can disperse and collect pigment granules in their skin to change color and hue. Those prototype cells consisted of a thin layer of mineral oil between two sheets of clear plastic. Injecting a little water containing a pigment or dye through a tube connected to the center of the cell creates a bloom of color. The shape of the bloom is related to the flow rate, which can be controlled by a digital pump. A low flow rate produces circular blooms; higher flow rates create intricate branching patterns:

In these optofluidic cell prototypes inspired by tilapia, krill and crab skins, dye injection at different flow rates leads to different branching patterns. Credits: Raphaël Kay, Charlie Katrycz.

Squid skin is translucent and has an outer layer of pigmented cells called chromatophores that control the absorption of light. Each chromatophore is attached to muscle fibers that line the skin’s surface, and those fibers, in turn, are connected to a nerve fiber. It’s simple to stimulate those nerves with electrical impulses, causing the muscles to contract. And as the muscles pull in different directions, the cell expands, along with the pigmented areas, changing color. As the cell shrinks, the pigmented areas also shrink.

Below the chromatophores, there is a separate layer of iridophores. Unlike chromatophores, iridophores are not based on pigments but are an example of structural color, similar to the crystals in a butterfly’s wings, except that a squid’s iridophores are dynamic rather than static. They can be tuned to reflect different wavelengths of light. A 2012 paper suggested that this dynamically tunable structural color of the iridophores is linked to a neurotransmitter called acetylcholine. The two layers work together to generate the unique optical properties of squid skin.

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