The building blocks of the future defy logic

A new logic-defying mathematical model could lead to materials for better skin grafts and new smart materials

Wake up in the morning and stretch; your midsection narrows. Pull on a piece of plastic at separate ends; it becomes thinner. So does a rubber band. One might assume that when a force is applied along an axis, materials will always stretch and become thinner. Wrong. Thanks to their peculiar internal geometry, auxetic materials grow wider when stretched. After confounding scientists for decades, University of Malta researchers are now developing mathematical models to explain the unusual behaviour of these logic-defying materials, unlocking a plethora of applications that could change the way we envision the future forever.

Auxetic materials have the amazing property of a negative Poisson’s ratio, becoming fatter when stretched. This comes from its structure which in the Malta-developed model is represented by a series of connected squares, technically called rigid, rotating subunits. When the subunits turn relative to one another, the material’s density lowers but its thickness increases.

Auxetics caused such colossal confusion among researchers that it went ignored for years. It was only in the 1980s that auxetics were picked up again as practical application resurfaced. Recent advances are unlocking this material’s potential.

In a paper published last week in Nature Publishing Group’s journal Scientific Reports, mechanical metamaterials, chemistry professor Joseph N Grima, together with his team at the University of Malta, have presented a mathematical model of auxetic behaviour using hierarchical rotating unit systems.

These systems take advantage of the enhanced properties provided by a negative Poisson’s ratio but also use a hierarchical system whereby the complex structures are created from the simpler units, thus creating a hierarchical auxetic metamaterials that are more versatile in terms of their mechanical properties, with experts being able to control and alter them.

Emeritus Professor Anselm C Griffin (Georgia Institute of Technology, US), notes how Grima’s work “represents a huge step forward in the conceptualization and design of a new class of metamaterials.” “With the realistic prospect of tailorable auxetic mechanical properties as described in this paper, the potential for applications of these new metamaterial structures particularly in biomedicine and catalysis is quite exciting,” he added.

Materials Chemistry Professor at Oxford University in the United Kingdom, Andrew Goodwin, notes its “exciting [applications], how fractal-like assemblies of simple [shapes] might find application in smart medical stents and ultralight responsive supports.” In principle, he said, the ideas Grima and his team are working on could also be applied to atomic-scale assemblies as they do in life-size structures. Chemists and engineers will be working closer together to develop the smart materials of the future, he added.

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