Laboratory hard X-ray microscopy studies on pollen and diatoms

This PhD thesis was focused on the application of the nano-XCT in study of biological materials and the development of the methodologies on study the structure and mechanical properties of biological materials. The whole unstained pollen was chosen as an example of carbon-based biological objects and imaged non-destructively. Specifically, both surface and inner structure of the unstained Pinus pollen grain and dandelion pollen grain were clearly visualized by nano-XCT. With the segmentation and rendering of the reconstructed 3D data from Pinus pollen grain according to the clear boundaries among the pollen wall and the inner structures, the detailed structures and the volumes of these different parts (exine, intine and cellular structure) were analyzed and calculated. Furthermore, Zernike phase contrast mode provides the necessary contrast for imaging of unstained carbon-based biological objects such as whole Pinus pollen. As one of the noncarbon-based biological objects, the morphology and mechanical behavior of diatom frustules and diatoms were studied by nano-XCT. A detailed comparison study of the 3D morphology of the frustules from D. geminata using nano-XCT and SEM/FIB showed that the tomographic data from nano-XCT allows arbitrary cross-sections through diatom frustules to access detailed morphology information. By using the 3D data of diatom frustule, it is able to reveal the information of the pore structure and the pore size. Nano-XCT combined with in-situ mechanical compression tests studied the 3D morphology of the frustules and, at the same time to correlate the structure with the mechanical properties of the whole individual frustules. The results show that the maximum loading force is related to the dimension of the frustule and the valve face, as well as the linking region of the epivalve and hypovalve are the weak spots. Besides, in order to study the biological objects at real or near-real environmental conditions, a house designed sample holder was integrated into an X-ray microscope and used to study the wet biological sample, i.e. to study the morphology, the volume changes and the mechanical properties of the whole diatom cells while changing from wet state to dry state. The results show that about 16% volume shrinkage of the diatoms after drying and the maximum loading force is in the range of several hundred μN for the diatoms in the wet state and single-digit mN in the dry state. Furthermore, the normalized stiffness of diatoms in the dry state is much higher in wet state. Compared with the diatom cells in dry state, the area to connect the valve face and the girdle band is the weak spots under the compression in the wet state, while the valve face and the girdle band itself could crack under the compression in the dry state.