Artificial muscles

Making light work of artificial muscles
The light-responsive film is made up of polymer brushes (right) that have self-assembled into a two-layer, three-dimensional array (left). Credit: Reproduced,with permission, from Ref. 1 2011 American Association for the Advancement of Science

A new form of self-assembling polymer film that bends and stretches when hit by light is pointing the way to anew family of functional materials. This flexing film is the first material to have been made by coaxing complex molecules to form large-scale, highly ordered three dimensional arrays -- a discoverythat could change the way that many active material are made, from artificial muscles to solar cells. Nobuhiko Hosono, Takuzo Aida and colleagues at RIKEN Advanced Science Institute in Wako and TheUniversity of Tokyo developed the self-assembly protocol. The researchers found that brush-shaped polymers would form an orderly film when hot-pressed between two sheets of Teflon. They made theirdiscovery while studying a polymer in which each side chain, or bristle, of the brush structure incorporates light-responsive azobenzenes-two benzene rings separated by a pair of nitrogen atoms. When hitby UV light, the bond between the nitrogens rearranges, contracting the side chain. The researchers used this photoisomerization behavior to confirm the remarkable long-range order of the polymerstructure. Because the side chains were all aligned, when those at the surface were hit by light they curled up in concert, bending the film. A second beam of light at a different wavelength reversed theisomerization process, and the film relaxed back to its original shape. The trick to making the material is to heat it between two sheets of Teflon that have been drawn tight in one direction, saysHosono. This tension orients the Teflon sheets' internal structure along a single axis, which acts as a template for the molten polymer brushes sandwiched in between. The side chains of the polymer...