Analysis of the hierarchical structure of biological tissues by scanning X-ray scattering using a micro-beam

The outstanding mechanical properties of biological tissues such as wood or bone are mainly due to their hierarchical structure and to their optimization at all levels of hierarchy. It is therefore essential to characterize the structure at all levels to understand the complex behavior of such tissues. Structures down to the micrometer level are accessible to light or scanning electron microscopic observation. In the case of bone this includes, for example, morphometry of the trabecular architecture or the bone mineral density distribution in cortical and trabecular bone. To characterize the sub-micrometer structure of, e.g., the collagen-mineral composite in the case of bone or the cellulose microfibrils in the case of wood, other methods, such as transmission electron microscopy or X-ray scattering are necessary. The recent availability of extremely brilliant synchrotron X-ray sources has led to the development of the new techniques of scanning small-angle X-ray scattering and scanning X-ray microdiffraction, which are capable of providing structural information on the micrometer and the nanometer level, simultaneously. As a basic principle of the method the specimen is scanned across an X-ray beam which has a diameter of few micrometers. Measuring the X-ray absorption at each position provides an image of the specimen (microradiography) with resolution similar to light microscopy, in the micrometer range. Moreover, the X-ray scattering pattern is analyzed at each specimen position to provide parameters characterizing the structure in the nanometer range. The present paper reviews the principles of the techniques and demonstrates their application to biological materials, such as wood or bone.