Protein design yields material with unprecedented two-way stretchiness

Materials such as rubber get thinner when stretched and thicker when compressed. Basically, when they expand in one direction, they contract in another, and vice versa.

But some oddballs, called auxetic materials, thicken when stretched and thin when compressed. Now, researchers have used protein design to develop a synthetic auxetic material that expands or contracts equally in two directions when pulled or compressed, respectively. That unusual property could be useful in sports equipment and body armor that resist forces from impacts.

F. Akif Tezcan of the University of California, San Diego, and coworkers created the material by combining tetramer units of a modified version of the enzyme l-rhamnulose-1-phosphate aldolase (Nature 2016, DOI: 10.1038/nature17633). The researchers modified the enzyme so the four corners of the squarelike tetramers carried cysteines. When the team oxidized the proteins, the tetramers joined up through disulfide bonds between the cysteines to form a sheetlike crystal.

Forces that stretch or compress the material in one direction cause the tetramers to rotate. The rotations open or close pores between the tetramers, making the crystal expand or contract to an equivalent extent in the other direction. Tezcan and coworkers estimate that the crystal expands or shrinks at least 24% linearly between its two extremes.

Engineers measure a material’s two-dimensional response to strain via its Poisson’s ratio. Most familiar materials, such as rubber, have positive Poisson’s ratios, while auxetic materials have negative ones. The ratios for most auxetic materials fall between 0 and –0.4, and the lowest previously reported values, –0.7 to –0.8, have been observed in specialized foams.

The new crystal has a Poisson’s ratio of –1, meaning that it expands or contracts equally in two directions simultaneously. This is the thermodynamic limit for any isotropic material, a substance that exhibits identical properties in all directions.

“This is an exciting result in the control and design of materials,” says computational protein designer Jeffery G. Saven of the University of Pennsylvania. Potential applications include single membranes that can change their porosity via applied forces, he says.

“The findings are exciting and the results beautiful and novel,” comments bio­nanotechnologist Martin Noble of Newcastle University. The material “is likely to have rare properties of deformation in response to stress and so provides an experimental system to characterize molecular behavior that has until now been inaccessible. Work is needed to find scientific or technological applications of this interesting new class of materials. But the same could once have been said of graphene.”