Geometric Shapes Inspire New, Stretchy Materials

Intricate designs drawn from Islamic art could help make materials that stretch in new ways

Geometric designs dating back 1,000 years could help create futuristic materials that might one day be used to build new medical technologies and advanced satellites alike. In a presentation earlier this week at the American Physical Society’s March meeting, a group of scientists from McGill University unveiled a new kind of material that draws on Islamic art to stretch in new ways.

For most materials, from cotton to rubber, pulling on them in one direction makes them contract in another, losing their original shape. That’s because stretching a material can alter its geometric substructure. When something like a rubber band is pulled, that tension stretches the rubber out, making it thinner in the process. But McGill University researchers Ahmad Rafsanjani and Damiano Pasini have discovered that by integrating certain patterns into a material it can actually get wider instead of thinner when it is stretched in one direction, Jonathan Webb reports for the BBC.

"In conventional materials, when you pull in one direction it will contract in other directions," Rafsanjani said in a presentation. "But with 'auxetic' materials, due to their internal architecture, when you pull in one direction they expand in the lateral direction."

Auxetic materials are one example of something called a “metamaterial”—materials that are engineered in ways to give them properties that don’t naturally exist in nature. In recent years, engineers have explored the possibility of creating metamaterials with all sorts of strange properties, like “meta-skin” made of silicone layers that can make objects invisible to radar or "invisibility cloaks" that can actually bend electromagnetic radiation like light. Auxetic metamaterials have the ability to expand in every direction when stretched and snap right back to its smaller shape, Shannon Hall reports for the New Scientist.

pattern that inspired metamaterial
Two patterns found on Iran's 1,000-year-old Karraqan tomb towers. Ahmad Rafsanjani and Damiano Pasini/McGill University

Rafsanjani and Pasini were searching for geometric designs that might help create stretchy metamaterials when they came across a set of designs found on a pair of 1,000 year-old towers built by ancient Iranian tombs.

"When you look at Islamic motifs, there is a huge library of geometries," Rafsanjani said during a presentation. "On the walls of these two towers, you can find about 70 different architectures: tessellations and curlicue patterns."

The two researchers found that when they cut two of these geometric designs into rubber sheets using a laser cutter, the sheets could expand when they were pulled and shrink when pushed, similar to a Hoberman Sphere.

While the designs are beautiful in and of themselves, they are unique in that they can hold both forms without help. Other “bistabile” metamaterials have been described in the past, but as Webb reports, they mostly require complex, origami-like folds and many have a hard time maintaining their expanded state. Rafsanjani says that these designs could be incorporated into everything from designing new stents to hold veins or arteries open to new kinds of satellites or unfolding solar panels.

"These designs are easier to fabricate; all you need is a laser cutter,” Rafsanjani said.

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