mechanical designing

The preload arises in the formation of the individual cells, which try to shorten at the end of the differentiation process. Since this is not possible in the cellular network, a tensile stress is established throughout the tissue. Accordingly, the wood in the peripheral areas of the trunk is shortened in longitudinal direction when cutting a tree across the fiber.

 Mechanical Designing

This pretension status exists in every straight trunk. The principle of preload also allows trees to purposefully orient their organs or to change the direction of growth by means of a "Aufkrümmen". Interestingly, deciduous and coniferous trees follow different strategies. Deciduous trees on the top of branches and crooked trunks form the so-called pulling Wood, which, due to a higher tensile preload than in normal wood, pulls the organ upward. Coniferous trees, on the other hand, form the so-called printwood on the underside. This tissue tries not to shorten but to extend in longitudinal direction. Therefore a pressure voltage is created which pushes the organ upward.

 

The question of how the conifer can succeed depending on the need to produce both longitudinal tensile and pressure tensions is of great interest and has been employed by scientists for quite some time.

The starting point of the considerations for the clarification of this phenomenon are differences in the nano-and micro-structure between the respective cell types [12]. While the normal wood cells of the straight stem have an approximately rectangular cross-section and a small Zellulosemikrofibrillenwinkel in the dominant cell wall layer (Μ ~ 10 °), typical wooden cells have a circular cross-section and a large Mikrofibrillenwinkel (Μ ~ 45 °). In addition, in some tree species Lumenseitig also spiral-shaped columns can be observed (Fig. 4a).

 

The following simplified assumptions were made for modelling the stress structure in both cell types: a) The mechanical behaviour of the cells is mainly determined by the secondary walls; b) The Cellulose Fibrils act as long, practically unexpandable fibres; c) The matrix shows an elastic isotropic behavior; and d) A source of the verbund leads to a isotropic volume increase.

 

The calculations indicate interesting geometric boundary conditions, if the cells are differentiated into a source of the cell wall. Above all, it is crucial whether the cell and tissue structure allows a small torsion of the individual cell or completely excludes it. This connection is shown in Figure 4b. The processing of the cell wall results in a rectangle which remains rectangular when the surface is enlarged, unless torsion is possible. Since the length of the diagonal must remain constant (unexpandable cellulose fibril), a maximization of the surface leads to a square. This means a reduction in longitudinal direction for a Mikrofibrillenwinkel below 45 °. However, if a small torsion is possible, the rectangle can move into a parallelogram, which leads to a length increase in longitudinal direction in the case of a surface magnification.

 

The "normal" wood cells have an approximately rectangular cross-section, which suppresses a torsional movement of the single cell in the cellular compound. Thus, the small Mikrofibrillenwinkel leads to a longitudinal shortening of the cell when the matrix is swollen. In contrast, the round cross-section and the spiral-shaped columns could be conducive to deformation of the pressure wood cells at torsion.

 

Summary and Outlook

Trees are hierarchical in structure and have exceptional mechanical properties. One reason for this is the ability to adapt, which includes not only the external form of the organism, but also the adaptation of the molecular structure to the natural requirements. The wooden cell wall can be characterized as a composite material consisting of rigid cellulose fibrils, which are only a few nanometers thick, as well as a soft matrix of Hemizellulosen and Lignin. Investigations into the deformation behaviour of the cell wall and the generation of voltage in the cells have shown the importance of the mechanical interactions of the cellular polymers. The aim is therefore to carry out further investigations on the Nanoverbund of the cell wall in order to fathom the optimization strategies of the living plant and on the other hand the material design as such. The previously extracted principles of cell wall design are to be used to better understand the construction of tensions in cell walls and the Aktuation of organ movements in plants. The fiber-matrix concept in cell walls can serve as an inspiration for a transfer into technical applications: for the development of Anisotropic hydrogels and for novel technical fiber composites