  This work reports an innovative humidity driven actuation concept based on
Bangham effect using simple nanoporous sol-gel silica thin films as humidity
responsive materials. Bilayer shaped actuators, consisting on a
humidity-sensitive active nanostructured silica film deposited on a polymeric
substrate (Kapton) were demonstrated as an original mean to convert water
molecule adsorption and capillary condensation in useable mechanical work.
Reversible silica surface energy modifications by water adsorption and the
energy produced by the rigid silica film contraction, induced by water
capillary condensation in mesopores, were finely controlled and used as the
energy sources. The influence of the film nanostructure (microporosity,
mesoporosity) and thickness, and of the polymeric support thickness, on the
actuation force, on the movement speed, and on the amplitude of displacement
are clearly evidenced and discussed. We show that the global mechanical
response of such silica-based actuators can be easily adjusted to fabricate a
humidity variation triggered tailor-made actuation systems. This first insight
in hard ceramic stimulus responsive materials may open the door toward new
generation of surface chemistry driven actuation systems.
