The structured lattice-type material protects against longitudinal and sheer energy waves that can travel through the ground (Credit: University of Missouri)
A new metamaterial inspired by military radar ‘cloaks’ could protect buildings and vehicles from dangerous vibrations.

Energy waves travelling through solid material pose a range of serious problems to engineers. Buildings that are miles from the epicentre of an earthquake can collapse due to the ground shaking and rattling, for example.
To tackle the risks – and potentially help save lives – a team of engineers led by Guoliang Huang from the mechanical and aerospace engineering department at the University of Missouri created the new flexible material.
Known as a ‘polar material’ it is composed of a functionally graded lattice, embedded in an isotropic continuum background. The layers were 3D-printed and manually assembled.
“Our elastic material can stretch and form to a particular surface, similarly to a wrap on a vehicle,” said Huang. “It can be applied to the surface of an existing building to allow it to flex in an earthquake. What is unique about the structured lattice-type material is that it protects against both types of energy waves – longitudinal and shear – that can travel through the ground.”
The work was inspired by military cloaking material. Aircraft, submarines and satellites are often covered with radar-absorbent material, such as paint, to hide them from radar and other detection methods. The coating materials makes objects indistinguishable from their surroundings or undetectable by external field measurements.
These types of materials are technically mature because the properties of acoustic and optical waves are well-understood, said Huang, but he added that little to no research has succeeded in solving the problem of cloaking for elastic waves in solid material.
“We are the first to propose the principles of this material and also the first to design and fabricate this material,” he said. “If you have an object you want to make invisible, you design some kind of coating material around the object so that when a wave hits the object, if passes around the material with no refraction.
“If you want to hide something in solid media, this is different. In solid media, the wave is more complicated than the radar wave because… we not only have a compression wave but we also have a shear wave. In civil engineering, we deal with earthquakes – seismic waves, which have longitudinal and shear waves, and most of the damage is cause by the shear wave.”
The material could also be used by the defence industry to protect against vibrations in mechanical parts, such as aircraft or submarine engines.
“For over 20 years, no-one had a natural solution for this issue in a solid material,” said Huang said. “Now, we've designed, modelled and fabricated a new material with properties that do not exist naturally, for what we believe is a nearly perfect protective device.”
The research was funded by the US Army Research Office. “This research could lead to new strategies for steering mechanical waves away from critical regions in solid objects, which could enable novel capabilities in soldier protection and manoeuvring,” said programme manager Dan Cole.
Two studies on the polar material were published in Physical Review Letters, a journal of the American Physical Society.
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