From Biomineralization And Self-Assembly To Lightweight And Tough Biomimetic Structural Materials

Structural materials are the most widely used materials by human being. While biomimetic structural materials have extraordinary mechanical properties because of their special micro/nano structures, the fabrication of these materials is by no means a low hanging fruit. In order to solve these problems, this dissertation reviews the research of structural biomaterials and the state-of-the-art methods for the fabrication of biomimetic structural materials. Based on the above review, the strengthening and toughening mechanisms of natural structural biomaterials such as the nutshells and spines in plants are analyzed. Then a self-confined amorphous template mechanism is proposed through the observation of in vitro crystallization process of calcium carbonate. Finally, based on the study of assembly and mineralization, a novel strategy is proposed for the fabrication of biomimetic structural materials. Through this strategy, a kind of nacre-mimic material is fabricated successfully. The detailed results are listed below.1. The strategies the plants adopt for designing structural material are analyzed by the study of the structures and the mechanical performances of nutshells and spines. The micro/nano structures of the shells of korean pine nut, pecan nut, pistachio nut and macadamia nut are comparatively studied and the mechanisms are concluded as follows:the elongated anisotropic units, the enhanced interface between the units, the interlocked structure, the interior wall-enhanced pits and the shared layers between the units. The micro/nano structures of the spines of the locust tree, bristle tooth oak and ball cactus are comparatively studied and the mechanisms are concluded as follows:the ultralong building blocks with enhanced structure, the pull-out of 2D subunits, the incorporation of layers with different structures and the folding structure. These mechanisms could provide inspirations for the design of biomimetic structural materials.2. Based on the investigations of a model reaction, a self-confined amorphous template mechanism is proposed for the anisotropic growth of crystals in the presence of amorphous phase. In the model reaction, the growth of calcite nanowires is initialized by the oriented assembly of nanoparticles. Then partly crystallized amorphous calcium carbonate (ACC) nanodroplets precipitate onto the tip of the nanowire, thus the surface of the tip area includes many crystalline surfaces. The crystallized areas in the tip area are assimilated by the single-crystalline core via axis rotation and boundary match, and the amorphous areas are left outside to form an amorphous layer coating on the side surface of the nanowire. Because ACC is more thermal dynamically unstable than crystalline phase, and both of the shell and the CaCO3 nanoparticles in the solution have net negative charge, the tip surface is more favorable for the precipitation of the nanodroplets. In other words, this ACC layer plays a role as a protective template to stabilize the morphology of the nanowire and facilitate the anisotropic growth of the nanowire along its Z axis. This is a new role of ACC in the mineralization other than the precursor and the storage. The result can be used to explain the formation of anisotropic biominerals and provide a new strategy for the fabrication of anisotropic crystals via amorphous precursors.3. In consideration of the formation of biominerals, the fabrication of biomimetic structural materials is divided into two steps, i. e. the preparation of ordered matrix and the mineralization of the matrix. First, a laminated chitosan matrix is prepared via the freeze-casting method and then acetylated to obtain an insoluble chitin matrix. Then the matrix is mineralized in a circulating system. The mineralized matrix is infiltrated with silk fibroin and then hot-pressed. The resulting material bears a striking resemblance to natural nacre:aragonite nanoparticles precipitate onto the chitin layer and gradually assimilate the chitin layer to form microscale single-crystal aragonite platelets; these platelets form a Vironoi pattern on each layer in the matrix; the layers are separated by the silk fibroin layer to form a brick-mortar structure; the bulk material is formed via the stacking of these alternating layers. The microscopic mechanical property of the resulting material is quite distinct from pure aragonite or calcite mineral, and the macroscopic mechanical performance is significantly improved in contrast to the control groups and comparable to natural nacre. This new strategy provides a reasonable method for the fabrication of biomimetic bulk materials.