A packaging material inspired by squid skin that changes the way it absorbs heat when stretched could be used to keep takeaway meals at the right temperature.
Squids camouflage themselves using hundreds of cells called chromatophores, which contain sacs of pigment. The size of these sacs fluctuates according to the contraction and relaxation of their surrounding muscles, enabling squids to change colour according to the reflection of light off their surface.
Inspired by this contraction and relaxation, at the University of California, Irvine, and his colleagues have created a material made of highly reflective nanoscale copper plates embedded in an elastic polymer. When stretched, both the material and squid skin change their reflective and absorption properties. This enables squids to camouflage and changes how heat travels through the material.
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鈥淲e applied basic principles that squid have evolved over hundreds of millions of years to change their colour, to design materials that have similar tunability for transmitting and reflecting heat,鈥� says Gorodetsky.
High-resolution images of the team鈥檚 material showed that the copper nanostructures interlocked while in an unstretched state, forming a continuous surface. When they exposed the material to infrared radiation, about 99 per cent of the waves were reflected off the surface, while around 2 per cent of the radiation was transmitted through it.
Stretching the material pulled the copper nanostructures further apart, enabling more of the infrared radiation heat to be transmitted through the polymer. When stretched to be 50 per cent longer, the amount of heat reflecting off the surface decreased by about 30 per cent, while the proportion of heat being transmitted through the material increased by around 18 per cent.
Changing how heat is reflected off or transmitted through the material could regulate the temperature of the food or drink it contains.
鈥淎n application of the material could be having a temperature ruler marked on the outside of a cup and based on the temperature you want to drink your coffee at in a certain time, say聽66 degrees聽Celsius in 20 minutes time, you could stretch and clip the material to a mark that lets it cool at the right rate,鈥澛爏ays Gorodetsky.
The team hopes that the material, which is easy to manufacture and recycle, will be used in food and drink packaging within a couple of years.
鈥淭he recyclability is a huge advantage of our material compared to other metallised polymers鈥ou can use聽vinegar聽to dissolve away the copper and what you鈥檙e left with is a very common polymer [which is easy to recycle],鈥� says Gorodetsky.
While the material could be usable for about a year, existing aluminium-lined polymers may be more resistant to corrosion and therefore favourable for longer term storage, he says.
Nature Sustainability
