NPG Asia Materials research highlight | doi:

Light-powered motors

Materials that are able to change their shape upon light irradiation could be used on the micro and nanoscale. The first demonstration by researchers from Tokyo Institute of Technology of a polymer material that undergoes complex three-dimensional movements delivers on this promise and may lead to novel motoric functionalities¹.

Polymer materials that are capable of changing their shape upon exposure to external stimulus have been fabricated before. However, for the most part light-induced motion of larger objects is generally limited to contraction, expansion and bending motions, and a more complex functionality has been missing.

In their demonstration of a macroscopic photomobile material, the researchers used the expansion capabilities of liquid crystal elastomers that change their structure when cycled through a transition temperature. However, when azobenzene compounds are incorporated into the liquid crystals, irradiation with ultraviolet light can also induce this phase transition, while irradiation with visible light reverses this process.

In their study, the researchers used this functionality and deposited azobenzene-containing liquid crystal films onto a flexible polyethylene sheet. As an example of the capability of these films, a continuous film was placed on a rotor and illuminated with UV as well as visible light to complete a full transition cycle. The sections that were exposed to light expand while those regions away from the light  shrink, causing an overall rotating moment of the plastic film.

The rotation of the film was clearly observed (Fig. 1), representing the first demonstration of a light-driven plastic motor. Commenting on the advantage of this approach, Tomiki Ikeda from the research team emphasizes that “as light can be handled remotely, instantly and precisely, these plastic materials can work as main driving parts of light-driven actuators without the aid of batteries, electric wires and gears.”

Obviously, this demonstration on the millimetre scale only serves to illustrate the principle of this method. Indeed, Ikeda suggests that “numerous applications on the nanoscale are possible, wherever an efficient power supply without batteries or mechanical contact to the device is needed”. For example, similar motors might be used for a direct light-to-energy conversion, or to drive sophisticated MEMS systems.

This research highlight has been approved by the author of the original article and all empirical data contained within has been provided by said author.

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