Biological tissues show complex mechanical and optical responses that are difficult to replicate in synthetic materials. For example, skin protects the body by, for one, rapidly stiffening to prevent injury. Some organisms, such as chameleons, cephalopods and amphibians, can even change the colour of their skin in certain situations. Chameleons rapidly go from a “cryptic” (or camouflage) state, for instance, and an excited state (mainly exhibited during courtship or combat).

Researchers from the US, France and Russia led by Sergei Sheiko of the University of North Carolina at Chapel Hill, and Andrey Dobrynin of the University of Akron, Ohio, have fabricated the first chameleon-inspired elastomers that boast both strain adaptive stiffening and colour-change behaviour. They made the materials by synthesizing triblock copolymers comprising linear ends and a central block resembling a bottlebrush in which the side chains extend like bristles from a linear chain backbone. “The central bottlebrush block forms a soft matrix and the linear end chains self-assemble into rigid domains playing the role of multifunctional physical cross-links,” explains Dobrynin.

This special combination produces a rigid-while-flexible and soft-while-stiff material that has a low Young’s modulus. The fact that the two chemically dissimilar blocks are separated by microphases also improves its strain-stiffening characteristics so that it is on a par with that of skin tissue.

Synthetic pig skin

By carefully selecting the length or density of the bottlebrush’s side chains, the researchers found that they were able to control how the material is deformed. They even succeeded in “coding” in all the parameters required for it to exhibit a mechanical response that was identical to that of living tissue. For example, they produced mouldable elastomers with strain-stiffening characteristics close to those of lung, brain, skin and blood vessel tissue. “These materials are solvent free, could have mechanical properties similar to pig skin, do not dry out in air or swell in bodily fluids,” says Dobrynin.

Being able to tune the mechanical properties of a material in this way could be useful for making biological devices, such as intervertebral discs, for example, that have mechanical properties similar to those of surrounding tissue to minimize the inflammatory response, he adds.

Microphase-separated structure

Importantly, the material also changes colour when it is deformed. Thanks to atomic force microscopy and X-ray diffraction measurements, the researchers discovered that the terminal blocks of the polymers self-assemble in nanometre-sized spheres. Light interferes with the microphase-separated structure and the colour observed depends on the distance (or gap) between the spheres. This is very much like what occurs in photonic crystals, nanostructured materials in which the periodic variation of the refractive index on the length scale of visible light produces a “photonic bandgap". When the material is stretched - that is, the distance between the spheres is increased - the colour reflected goes from turquoise to dark blue.

The team, which also includes researchers from the ESRF in Grenoble, the University of Haute Alsace, the Institute of Materials Sciences in Mulhouse and Moscow State University, detail their work in Science DOI: 10.1126/science.aar5308.