# Exploring the Mechanisms of Humidity Responsiveness in Plants and Their Potential Applications
## 3 Results and Discussion
### Biomimetic Application  
A biomimetic approach could also be used to create designs that are connected to natural models by mimicking the integrated aspects of biological forms, functions, structures, and materials [28,29]. Wood materials were modeled for analyzing deformation tendencies in different tangents and radial directions [30,31]. The hygroscopic swelling rate, a key parameter of the material, was measured after determining how the shape of the sample glued with dissimilar materials was bent upon moisture absorption. The dissimilar materials were glued such that the glued parts would be flexible upward and downward without any longitudinal deformation. The hygroscopic swelling of wood cut in the radial and tangent directions [32] was analyzed (Figure 5A). The hygroscopic swelling analysis of the wood with thicknesses of 3 mm, 6 mm, and 15 mm revealed that the degree of deformation increased with decreasing wood thickness, resulting in a bow-like shape upon deformation. The wood thicknesses were selected based on the visually observable degree of deformation. The hygroscopic swelling rate of 3 mm thick wood was five times higher than that of 15 mm thick wood. Bending changes occur more quickly for thin layers, which leads to an additional increase in curvature amplitude [33]. Similar to radial-direction wood, a thinner tangent-direction wood was also found to undergo a larger degree of deformation with bow-shaped bending. In general, compression is required for proper bending of wood; in particular, pressure at the end of the wood is essential. This could explain why thin wood has a greater bending angle than that of thick wood [34]. The degree of bending was determined after the wood sample had absorbed sufficient moisture (Figure 5B). The tables and graphs showed that the hygroscopic swelling  
Figure 5: Hygroscopic deformation by wood thickness (**A**) and changes in expansion upon moisture absorption by wood (**B**). rates of the samples were the largest when aluminum was glued. However, because these swelling rates were not small in other samples, strength and durability were considered when choosing the desirable samples rather than the degree of deformation. When only the epoxy-superglue was applied to wood, the deformed shape caused by hygroscopic swelling was restored to its original shape even after being wet for a long time, possibly because the absorption of water into the epoxy-superglue causes expansion. ABS superglue tends to permeate deeply into wood and harden. As the hygroscopic swelling rate inside wood changes upon superglue permeation into it, the ABS superglue was considered an appropriate material. In contrast, UV superglue lost its adhesiveness upon repeated hygroscopic swelling and contraction (restoration). We found that upon gluing two kinds of wood with their annual rings crossed with each other and only applying the wooden material to a humidifier, bending like a bow was observed as if dissimilar materials were glued to wood. However, controlling the changeability of shape caused by hygroscopic swelling was found to be difficult due to complex folding in different homogeneous [35] materials. As dissimilar materials glued to the wood have some uniformity, they have the advantage of allowing controlled hygroscopic swelling. Finally, the wood glued with dissimilar materials was examined. Although an aluminum sheet could be glued to cypress wood, metal was considered undesirable as it showed extreme swelling and contraction upon temperature changes. Considering the uniformity of the wood sample deformation, deformation controllability, and commercial value, ABS plastic was considered the most appropriate material for gluing on wood. This environment-friendly material is also used in children's toys, is very durable, and is easy to process. Polyoxymethylene (POM) can also be considered even though it was not tested, as it is stiff, has less flexibility than ABS, and can result in less bending of the sample at the time of hygroscopic swelling than that with ABS.