Normally when a material is stretched it thins out, and when it's compressed it thickens up. Chemists at the University of California at San Diego (UCSD) have just engineered a substance that does exactly the opposite, and it could lead to better bullet-proof materials – or improved running shoes.

The researchers created their material by working with a protein called RhuA, which has a square molecular shape. By laying out the proteins and connecting them at their corners, they made a grid pattern that created a sheet-like crystal.

When that crystal is stretched or compacted the protein tiles rotate in the opposite direction of the force, which gives it its unique property. It's a bit like a checkerboard where all the squares can flip up. If the checkerboard was stretched, the tiles would move to a vertical position, thus thickening the playing field. When compacted, the tiles flip to a horizontal position, making the material thinner when viewed from the side. This property is known as "auxetic."

"The crystals form perfectly with almost no tiles missing or ajar, and the material is self-healing," says a UCSD report about the research. "Protein tiles easily pop into place, given the right chemical conditions."

As examples of how the material could one day be used, the report says it could be built into the soles of running shoes that would thicken instead of spreading out when striking the pavement, or it could be used to create body armor that would thicken when stretched from the force of a bullet.

"These materials are very easy to make, yet provide many new research directions both in terms of materials applications and understanding the fundamental principles of nanoscale self-assembly," says Akif Tezcan, a UCSD professor of chemistry and biochemistry who led the research.

A paper describing the strange material was published online in the journal Nature this week.

Source: UCSD