Credit: ©2008 Wiley

Finding ways to use light as an energy source is becoming increasingly important because of the current energy issues faced by society. The use of photovoltaics, which convert light into electrical energy, is well known, but another route — the conversion of light directly into mechanical energy — could also be useful. Now a team of scientists from China, Canada and Japan have shown that photoresponsive liquid crystal elastomers can be used to drive a motor1.

Liquid crystal elastomers (LCEs) are rubbery polymers that can show the orientational ordering of regular liquid crystals. When changing from a disordered to an ordered phase, the LCE molecules orient and align in a specific direction, leading to the bulk material expanding along the direction of orientation. The transition is reversible — so the material can contract on changing from the ordered to disordered phase — and is usually induced by heating or cooling, but it can also be caused by light. Azobenzene can photoisomerize between its cis and trans forms on exposure to different wavelengths of light, and when incorporated into an LCE, the isomerization can also induce a phase transition and thus an expansion or contraction.

The team, led by Tomiki Ikeda from the Tokyo Institute of Technology, created an azobenzene-LCE belt that was laminated with polyethylene and fitted it around two wheels of different sizes supported on axles. By shining ultraviolet and visible light in different places on the belt — to induce both cis–trans and transcis isomerizations — the expansions and contractions worked in concert to cause rotation of the wheels.