The strong, flexible bonds in the protein tiles allow them to rotate to open spaces. Image: Yuta Suzuki and Timothy Baker, UC San Diego.
The strong, flexible bonds in the protein tiles allow them to rotate to open spaces. Image: Yuta Suzuki and Timothy Baker, UC San Diego.

Chemists at the University of California, San Diego (UC San Diego) have created an ‘adaptive protein crystal’ with a counterintuitive and potentially useful property: when stretched in one direction, the material thickens in the perpendicular direction, rather than thinning as most other materials do. And when squeezed in one dimension, it shrinks in the other rather than expanding, and gets denser in the process.

This strange behavior could find use in a whole range of applications, including soles for running shoes that thicken for greater shock absorption as the heel collides with the pavement or body armor that strengthens when a bullet strikes.

"It's a property, called 'auxetic', that has been not been previously demonstrated at the molecular level through design," said Akif Tezcan, a professor of chemistry and biochemistry at UC San Diego. Tezcan headed a team of researchers that describe this work in a paper in Nature.

Tezcan's group created a sheet-like crystal made of proteins connected in a regular, repeating pattern. As their building block, they chose a protein called RhuA for its square shape and used it like a tile to make their material.

"We found a way to create strong, flexible, reversible bonds to connect the protein tiles at their corners," Tezcan said. This flexibility allows the tiles to rotate to open spaces, creating pores, or to close up, producing a kind of adaptable sieve.

Stretching or compressing the material in one direction causes the connected protein tiles to rotate in unison, leading to a corresponding expansion or contraction in the opposite directions. The relationship between strain in different directions is captured by the Poisson ratio, which takes a positive value for normal materials that stretch and shrink in opposition. In contrast, Tezcan's group measured a Poisson ratio of -1 for their protein crystal, a value at the thermodynamic limit of what is possible.

The crystals form perfectly with almost no tiles missing or misaligned, and the material is self-healing. Protein tiles easily pop into place, given the right chemical conditions.

"This is protein design using a highly chemistry-based approach," Tezcan said, noting that the materials are made via a streamlined, minimalistic design strategy that requires few alterations to the protein building blocks. "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."

This story is adapted from material from UC San Diego, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.