The protein crystal with a counterintuitive property

Imagine squeezing a cube of plasticine between your fingers. In the direction you are compressing it, it would contract but then it bulges outwards in the other directions. This is the typical response of most materials. However chemists at UC San Diego University have invented what they call “an adaptive protein crystal” that when compressed in one direction shrinks in the other directions too. This property of a material makes it “auxetic”.

The material is comprised of square proteins that when put together like tiles make regular 2D sheets of material. The bonds between the corners of these tiles allow them to rotate away from each other or to fold back to make the sheet flat. The bonding between tiles has been engineered to be flexible enough to allow for these rotations. When stretched or squeezed all the bonds rotate in unison so that all the material either expands or contracts.

This is not the first “auxetic” material researched, however according to researcher Afif Tezcan this property has not “been previously demonstrated at the molecular level through design”. The material also shows other useful properties such as the fact that the material synthesised is almost perfect- there are few “missing tiles” in the structure. Also it is easy to produce due to its simple building blocks and it is self-healing.

Materials based on this technology could be used in shock absorbing technologies for use in military and sports. Also the way the material becomes more or less porous depending on the force applied on it means it could be useful in making sieves at the molecular level for which the size of the holes can be changed.

Credit: Images by Yuta Suzuki and Timothy Baker, UC San Diego