Mechanics of a Biomimetic Moisture Sensitive Actuator Based on Compression Wood

Various mechanisms of plant organ movements have been reported, including the close association of two layers with expressed differences in hygroscopic properties. Following this principle, actuator beams composed of thin veneers out of normal and compression wood cut from Scots pine (Pinus sylvestris L.) were prepared by using two types of adhesives. The mismatch of the swelling properties of the two layers in tight combination resulted in an expressed bending deflection in response to set humidity changes. The resulting curvatures were measured and analyzed by the Timoshenko bi-metal-model, as well as with an enhanced three-layer model, with the latter also considering the mechanical influence of the glueline on the actuator bending. The thermally induced strain in the original model was replaced by another strain due to moisture changes. The strain was modelled as a function of wood density, along with changes in wood moisture. Experiments with free movement of the bilayer to measure curvature, and with constraints to determine forces, were performed as well. Deformation and magnitude of actuators movements were in close agreement with the enhanced bilayer-model for the phenol-resorcinol-formaldehyde adhesive, which deviated substantially from the casein adhesive glued actuators. The obtained results are seen as critical for wood-based actuator systems that are potentially used in buildings or other applications.