Auxetic Materials

Stretch an ordinary rubber band and it grows thinner as it grows longer -- pull on almost any everyday material and the parts perpendicular to the pull tend to narrow. Auxetic materials do the opposite: stretch them lengthwise, and they actually grow thicker in the other direction instead of thinning out. This unusual behavior comes down to what happens inside the material's internal structure. Most auxetic materials are built from a repeating pattern of bowtie- or hourglass-shaped cells that sit folded inward at rest. Under tension, these folded cells unfold and rotate outward like a hinge opening, and that unfolding motion pushes the material's width outward even as its length increases. The effect isn't limited to engineered products -- researchers have identified naturally auxetic behavior in certain rock formations and even in the spongy, trabecular bone found inside human joints. Engineers have taken deliberate advantage of the property, designing auxetic foams for helmet padding and body armor, since a material that thickens under impact absorbs force more effectively than one that thins and deforms away from it. Medical researchers are also exploring auxetic bandages that expand to hug the shape of an irregular wound, and self-expanding stents that widen naturally once placed inside a blood vessel, without needing an external mechanism to force them open.