#auxetic

By using new geometric shapes in the shock-absorbing material, researchers at the Universities of Gothenburg and Isfahan have developed a bicycle helmet that provides better protection against head injuries. The material absorbs shock by contracting bilaterally. Bicycle helmets are important for protecting cyclists from head injuries, but traditional designs have limitations in terms of impact absorption and fit. Researchers at the University of Gothenburg and the University of Isfahan in Iran designed a bicycle helmet whose shock-absorbing material utilises what is called auxetic metastructures. The material is designed in special geometric patterns that behave differently under impact conditions compared to conventional foam liners.

Vídeo Intercanvi

A new way of designing auxetic materials

Imagine pulling on the long ends of a rectangular piece of rubber. It should become narrower and thinner. But what if it instead became wider and fatter? Now, push in on those same ends. What if the rubber became narrower and thinner? Such common-sense-defying materials do exist. They're called auxetics, and they have a raft of unique properties that make them well-suited for sneaker insoles, bomb-resilient buildings, car bumpers and clothing. Despite this great potential, auxetic products have been slow to market. Researchers at the National Institute of Standards and Technology (NIST) and the University of Chicago hope to change this. In a new study published in npj Computational Materials, they announced they've developed a new tool that makes designing materials with auxetic properties easier and faster. As an algorithm, the tool enables precise three-dimensional design of auxetics.

Versatile building blocks make structures with surprising mechanical properties

Researchers at MIT's Center for Bits and Atoms have created tiny building blocks that exhibit a variety of unique mechanical properties, such as the ability to produce a twisting motion when squeezed. These subunits could potentially be assembled by tiny robots into a nearly limitless variety of objects with built-in functionality, including vehicles, large industrial parts, or specialized robots that can be repeatedly reassembled in different forms.

The researchers created four different types of these subunits, called voxels (a 3-D variation on the pixels of a 2-D image). Each voxel type exhibits special properties not found in typical natural materials, and in combination they can be used to make devices that respond to environmental stimuli in predictable ways. Examples might include airplane wings or turbine blades that respond to changes in air pressure or wind speed by changing their overall shape.

The findings, which detail the creation of a family of discrete "mechanical metamaterials," are described in a paper published today in the journal Science Advances, authored by recent MIT doctoral graduate Benjamin Jenett Ph.D. '20, Professor Neil Gershenfeld, and four others.

Findings open up possibilities for mechanical metamaterials to be used in soft robotics and medical devices

The simplicity and elegance of origami, an ancient Japanese art form, has motivated researchers to explore its application in the world of materials.

New research from an interdisciplinary team, including Northwestern Engineering's Horacio Espinosa and Sridhar Krishnaswamy and the Georgia Institute of Technology's Glaucio Paulino, aims to advance the creation and understanding of such folded structures for applications ranging from soft robotics to medical devices to energy harvesters.

Inspired by origami, mechanical metamaterials -- artificial structures with mechanical properties defined by their structure rather than their composition -- have gained considerable attention because of their potential to yield deployable and highly tunable structures and materials.

What wasn't known was which structures integrate shape recoverability, pronounced directional mechanical properties, and reversible auxeticity -- meaning their lateral dimensions can increase and then decrease when progressively squeezed. Though some 3D origami structures have been produced through additive manufacturing, achieving the folding properties displayed in ideal paper origami remained a challenge.

Using nanoscale effects for an origami design, the team of researchers from the McCormick School of Engineering and Georgia Tech sought to answer that question. They produced small, 3D, origami-built metamaterials, successfully retaining the best properties without resorting to artifacts to enable folding.

Researchers create a new class of rate-sensitive mechanical metamaterials

Researchers at the Department of Biomechanical Engineering of Delft University of Technology have created a new class of metamaterials that can dynamically switch their mechanical behavior. It may form the basis for practical applications such as fall-protective clothing for the elderly. The results are to appear in the journal Science Advances on 17 June.

Metamaterials are artificially crafted material structures that derive their properties from their internal microstructural design, rather than the chemical composition of the material they are built up from. Metamaterials can be designed to show exceptional properties not found in simple natural materials. For example, while structures that are compressed in one direction are intuitively expected to expand in the opposite direction, a class of metamaterials called auxetic materials are purposefully designed to do the opposite.

Using a material's 'memory' to encode unique physical properties

A new study published in Science Advances found that certain types of materials have a "memory" of how they were processed, stored, and manipulated. Researchers were then able to use this memory to control how a material ages and to encode specific properties that allow it to perform new functions. This creative approach for designing materials was the result of a collaboration between Penn's Andrea Liu and Sidney R. Nagel, Nidhi Pashine, and Daniel Hexner from the University of Chicago.

Liu and Nagel have worked together for many years on the physics of disordered systems. In contrast to ordered systems, which have systematic and repeating patterns, disordered systems are arranged randomly. An illustrative example is a natural wall made of tightly packed dirt, where individual grains aren't neatly stacked but instead clump together to form a rigid structure. Researchers are interested in these systems because their randomness allows them to be easily transformed into new mechanical metamaterials with unique mechanical properties.

One important property that materials scientists would like to control is how a material responds when an external force is applied. When most materials are stretched in one direction, they shrink perpendicularly, and when compressed they expand perpendicularly, like a rubber band—when it is stretched it becomes thin, and when compressed becomes thicker.