Harnessing Fluctuations in Ambient Humidity Enables Semipermanent Actuator Operation

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A research group, made up of researchers from RIKEN (Japan's comprehensive science research institution) and other institutes, has developed a film actuator that operates semipermanently by harnessing fluctuations in ambient humidity. This article is based on a report by Smart Japan, a media services provider specializing in energy conservation, storage, and generation, regarding an initiative to develop energy harvesting technologies using unutilized energy collected from the environment around us. We summarize it here with their permission.

The development of renewable energy technologies that convert green energy sources -- such as solar power, wind power, and geothermal heat -- into accessible energy is gaining momentum, in an attempt to create a sustainable society. Meanwhile, the popularization of wearable terminals, mobile computing devices, and Internet of Things (IoT) devices has raised expectations for the development of energy harvesting technologies, which use unutilized energy collected from the environment around us, instead of supplying electric power to each of them.

Against this backdrop, a research group, directed by Takuzo Aida (Professor of the Graduate School of Engineering, the University of Tokyo), who belongs to the Emergent Soft Matter Function Research Group at RIKEN Center for Emergent Matter Science, has developed a film actuator that operates semipermanently in response to subtle changes in ambient humidity.

As this film curls up and straightens out according to the amount of absorbed liquid, it shows a bending and stretching motion in response to fluctuations in ambient humidity. The newly developed film features a powerful and fast motion with a smaller amount of liquid than conventional films.

In addition, as local humidity changes can be efficiently converted into kinetic energy with high efficiency, the film responds to humidity changes too small to be detected by general-purpose hydrometers.

By depositing gold in one part of the thin film, the research team succeeded in developing an actuator that keeps moving in one direction by utilizing the ambient humidity that occurs around the drop of water as a driving force. Absorption and desorption of water is stopped at a particular site by depositing gold onto one part of the thin polymer film. When this happens, as a result of ambient humidity, the film repeatedly performs the same curling and straightening movements, autonomously moving in one direction.

As the absorption of moisture on the thin film affects heat and light, various fluctuations in the environment can be converted to kinetic energy. Furthermore, as the film quickly responds to environmental changes, when exposed to intense light, it can curl and straighten at a high speed, and even jump.

The thin film is made from a two dimensional polymer called Graphitic Carbon Nitride. Using a method developed by the research group, the film can be created by simply applying heat, using low-cost guanidine carbonate as a raw material. From this achievement, the research group demonstrated that it is possible to utilize ambient humidity, which to date has been difficult to use as an energy source, as kinetic energy. In order to use it with an actual device, it will be necessary to develop a technology that can efficiently convert the kinetic energy into a suitable form of power. Furthermore, the thin film is expected to have applications in the development of artificial muscles.

Source: Smart Japan (in Japanese)