## 3 Fibrous Structures
### Collagen
The diffraction rings in these figures arise from the axial periodicity (D-period) The sixth-order diffraction peak in the integrated intensity profile was fitted with Gaussian curves to determine the D-period. The fibrillar structure's lateral intermolecular spacing, the spacing between collagen molecules, was determined using Equation (2). The diffuse areas of the pattern transform into well-defined arcs in the stretched-tanned leather where collagen molecules are aligned, indicating fibrils have a smaller diameter and are more densely packed. Figures 3c and 3d show the collagen alignment in normal and stretched-tanned leather, respectively. Figure 3c shows the collagen's random alignment, which reflects the isotropy shown in Figure 3a. In Figure 3d, the collagen fibrils are aligned, reflecting the anisotropy in Figure 3b. The water molecules added to the leather act as a lubricant and decrease the stiffness. However, the stretched-tanned leather is more densely packed, inhibiting the ability of water to decrease the stiffness. Stretched-tanned leather has a larger D-spacing because of the residual strain from tanning under tension. Stretch-tanned leather has a smaller lateral spacing, indicating denser collagen packing results in higher stiffness. This orientation increased the stiffness of the leather. The fibrils were found to swell when exposed to moisture, resulting in larger D-spacing. L, the lateral interplanar spacing, increased with moisture content [63]. Meridional periodicity in collagen is an extremely useful proxy to monitor the structural change in collagen. An example is the study in which the reason collagen materials are resistant to gross structural change in the early stages of dehydration was explored. In one experiment, the effect of dehydration was simulated by exposure to various concentrations of isopropanol [65]. Dehydration was found to be a two-stage process. During initial dehydration (<90% isopropanol), a decrease in the overlap length and an increase in the gap length keep the D-period unchanged. At the second stage of dehydration, the D-period decreases and the overlap length decreases, leading to a significant decrease in the collagen fibril length. The changes in the D-period were also used to compare _in situ_ the strength of stabilization against the heat of the intermolecular structure of collagen in skins due to natural and synthetic crosslinks [66]. The D-period changes when collagen denatures. It was found that although natural crosslinks occur during self-assembly, the number of natural crosslinks decreases, and their contribution to heat stability is overwhelmed by a synthetic crosslink, chromium sulfate. Another example of the use of meridional periodicity can be found in the work of [PERSON] et al., who carried out in situ SAXS measurements using synchrotron radiation to analyze changes in the collagen structure during leather processing with different tanning agents [67]. Unlike electron microscopy, SAXS can reliably examine large areas. SAXS was used to analyze collagen in a diseased state [62]. Health professionals can better understand and remedy diseases by comparing SAXS-derived structural differences between collagen in healthy tissues and those in the diseased state [68]. For example, [PERSON] et al. used SAXS to examine variations in collagen orientations within articular cartilage and subchondral bone in horse joints to understand osteoporosis better. The collagen orientation could be determined by using a color map of the SAXS pattern [69]. The last example for the use of SAXS data from collagen to understand the structural alteration due to disease and surgical intervention and hence the biomechanical properties can be illustrated by using results from collagen in the corneal stroma, the thickest (\(-90\%\)) layer of the cornea. Collagen in this layer is arranged to make the layer transparent and tough [70]. Unlike in ligaments and bone, collagen in stroma has a layered structure, such that each lamella runs parallel to the surface.