Chemists in Japan have created light-driven polymer films that walk like inchworms and move like robotic arms.

The films, made by Tomiki Ikeda at the Tokyo Institute of Technology in Yokahama and collaborators, contain a polymer which contracts when visible light shines on it and expands again under UV light. 

The polymers respond to light because they have azobenzene groups - which contain N=N double bonds - incorporated into them. Under visible light the N=N bonds have a cis conformation which means the polymer is bent. But when the light source is changed to UV the bonds become trans and the polymer flattens.

To make the polymer walk, the group incorporated it in a laminated film with one pointed end (at the back of the 'worm') and one flat end (at the front of the 'worm'). As the polymer bends the pointed back end is dragged forward then, when the light source is changed to UV, the polymer flattens, pushing the front flat end forward. This continuous flattening-bending motion allows the film to move forward like an inchworm. 

The robotic arm also requires clever lamination, but this time the polymer layer and laminated sections are alternated which allows the film to act as a hinge joint and move flexibly. By controlling the intensity of the light and the position on the film where the light is concentrated, the researchers can make the film move as they chose.

'The polymers function with a minimum of moving parts which minimises friction and surface contact problems,' says Ikeda. 'One can envisage applications such as direct light-to-mechanical energy conversion, storage systems and in microfluidic devices.'

Graeme George, an expert in polymer science at Queensland University of Technology, Brisbane, Australia, commends 'the efficiency of the reversible photo-processes.' He adds that the time is ripe for further detailed studies of such systems to see if these photo driven polymers offer any challenge to their electroactive counterparts.

Ruth Doherty