Polymers that mimic chameleon skin

Implants are used whenever lost body parts or functions need to be replaced. Attempts to do this are generally less than perfect, because research scientists have been unable to copy Mother Nature with complete success in this context so far. On regular occasions, however, scientists also come up with inventions that are admired by both the man on the street and the experts. The invention of a synthetic polymer, for example, the property profile of which resembles biological tissue very closely and which changes colour like a chameleon when moved.

To produce a medical implant, we need to select materials with similar mechanical properties to those in biological tissues, so as to mitigate inflammation or necrosis. A number of tissues including the skin, the intestinal wall, and the heart muscle, have the particularity of being soft yet stiffening when they are stretched. Until now, it has been impossible to reproduce this behavior with synthetic materials.

The researchers have attempted to achieve this with a unique triblock copolymer. They have synthesized a physically cross-linked elastomer composed of a central block onto which side chains are grafted (like a bottle brush) and with linear terminal blocks at each end (See figure). The researchers have found that by carefully selecting the polymer's structural parameters, the material followed the same strain curve as a biological tissue, in this case pig skin. It is also biocompatible, since it does not require additives, e.g. solvent, and remains stable in the presence of biological fluids.

Another property of the material appeared during the experiments: its color change upon deformation. As the scientists have shown, this is a purely physical phenomenon, which is caused by light scattering from the polymer structure. Atomic force microscopy and X-ray diffraction experiments have shown that the terminal blocks of these polymers assemble in nanometer spheres, distributed in a brush-polymer matrix. Light interferes with this microphase-separated structure to produce  color according to the distance between the spheres; so when the material is stretched it changes color. It is the same mechanism that explains – in large part – how chameleons change color.