Hygromorphic actuator from a metal oxide film driven by a nano-capillary forest structure

We have developed a hygromorphic metal oxide actuator using an electrochemical method to produce a superhydrophilic freestanding nano-capillary forest of titanium oxide with a high aspect ratio (~80). This metal oxide film has an inhomogeneous initial gap at the top and bottom surfaces between the tubes due to flexure during fabrication. The actuation mechanism is as follows. First, when a drop of water is applied on the surface of a titanium oxide nano-capillary forest (TNF), the water penetrates through the film instantaneously, and the titanium oxide nano-capillaries are pulled together by interplay of the capillary force and van der Waals force. When water has fully filled in the gaps between the capillaries, the free-standing TNF film remains unbent for ~ 2 min. Then, as the water evaporates, the film bends further in the forward direction. When the water has completely evaporated, the van der Waals force alone acts on the capillaries, and the TNF film returns to its initial state. This TNF possesses great stability and repeatability for long-term usage having a high bending energy density of ~ 1250 kJ m–3 and unique capabilities. It may lead to novel stimuli-responsive systems, including energy collection and storage, as well as robotics applications. NPG Asia Materials (2017) 9, e417; doi:10.1038/am.2017.139; published online 4 August 2017