Scanning small-angle X-ray scattering (scanning SAXS) was applied for the first time on bone to compare results from SAXS directly with those from other position-sensitive methods, such as light and polarized light microscopy, back-scattered electron imaging, and radiographic imaging. Since scanning SAXS is a nondestructive method of investigation, images from all these techniques could be obtained from the same bone sections. Thus, it could be shown that both the collagen and the mineral crystals were predominantly aligned parallel to the trabeculae and, therefore, to principle stress directions. Moreover, the mean crystal thickness as determined by scanning SAXS was found to be different at various positions inside the trabecular and cortical structure. Finally, it could be shown that scanning SAXS is suitable for detecting local changes in bone material, e.g., due to fluoride treatment.

The wood cell wall is built with elementary cellulose fibrils (ECF) having a uniform thickness of 25 ± 2 A. This was shown by investigating the same samples independently with three different experimental techniques, transmission electron microscopy (TEM), wide-angle X-ray scattering (WAXS), and small-angle X-ray scattering (SAXS). Discrepancies between results from these techniques discussed in many earlier studies did not appear in the present work. In particular, it was shown that the size distribution measured on TEM pictures is exactly the same as the one estimated from SAXS, if the statistical error introduced by the grain size of the contrasting medium is taken into account for the evaluation of the pictures. The fact that native cellulose fibrils have a uniform thickness in the wood cell wall-which is not the case in many other cellulose preparations-could indicate a biological regulation of the thickness, possibly to achieve better mechanical stability of the cell wall.

Many biological materials, like bone or wood, are hierarchically organized and optimized at all levels for their specific mechanical function. At the lowest level, these materials are fiber composites, where the fiber direction as well as the size of the individual components varies considerably with position inside a given specimen. For bone or wood, some of these parameters can be readily obtained by small-angle X-ray scattering (SAXS) in a position-resolved way. A scanning-SAXS system based on a pinhole camera with rotating anode and area detector is presented, and first applications to the study of bone and wood are reported.

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Hydration dependent structural changes at the nanometer level were investigated in the natural wood cell of Picea abies. Using the technique of small-angle X-ray scattering (SAXS), it was possible to investigate the same specimens at different degrees of hydration, x, in a nondestructive way, which allowed us to separate the scattering from pores from the scattering of the cell wall itself. For specimens dried below the fiber saturation point, x F, the scattering from pores and other cavities dominated and considerable changes of the cell-wall structure occurred. In the native state (for x > x F), however, the structure of the cell wall was independent of the hydration. The structure function describing the relative arrangement of the cellulose fibrils was obtained for the native cell wall. It was in quantitative agreement with the prediction from a hard-disk model with packing density 0.3, corresponding to a typical spacing between fibril centers of about 40 Å.

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