Called Isomax, the beauty of this solid foam—in this case loosely defined as a combination of a stiff substance and air pockets—lay in the geometry within. Instead of the typical assemblage of bubbles or a honeycomb arrangement, the ordered cells were set apart by walls forming the shapes of pyramids with three sides and a base, and octahedra, reinforced inside with a “cross” of intersecting diagonal walls.
In a letter published in the journal Nature, Berger, with UCSB materials and mechanical engineering professor Robert McMeeking and materials scientist Haydn N. G. Wadley from the University of Virginia, prove that the three-dimensional pyramid-and-cross cell geometry Berger conceived is the first of its kind to achieve the performance predicted by theoretical bounds. Its lightness, strength and versatility, according to Berger, lends itself well to a variety of applications, from buildings to vehicles to packaging and transport.
The combination of the pyramid and cross-shaped cells, resulted in a structure that had low density—mostly air, in fact—yet was uncommonly strong for its mass.
The development could not have better timing. As resources become more limited and concern for energy efficiency grows, a material with this mass relative to its strength would require fewer resources to produce and less fuel to transport. The simple geometry makes it versatile enough to fabricate for a variety of situations, and, functionally graded, it can be used to create objects with varying levels of stiffness from one end to another, such as prosthetics and replacement joints, and the design is compatible with manufacturing methods from origami-like folding to bonding and 3-D printing.
The study is one of a series of steps investigating the potential of this metamaterial. Berger and team are currently following up with experimental analysis and are looking into manufacturing methods that may allow for efficient fabrication.