Calcium carbonate biomaterials

Hörl, Sebastian (2025): Calcium carbonate biomaterials: architecture, design and nanomechanical properties of selected mollusc, brachiopod and echinoderm skeletal elements. Dissertation, LMU München: Fakultät für Geowissenschaften

Abstract

Calcium carbonate is a chemical substance that is not only considered the most important compound for biomineralisation in marine organisms and invertebrates, but also has significant implications for industry, the carbon cycle, and biomedicine. Through evolving and adapting to ecological challenges, organisms have developed the ability to biomineralise complex hierarchical structures comprising biopolymers and inorganic minerals. Due to their outstanding material properties, such as high strength and toughness, while retaining a low density compared to the pure inorganic mineral, biomineralised materials have long been a source of inspiration for fabricating and optimising man-made materials. Accordingly, it is of great importance to understand the hierarchical design and biomineralisation principles of calcium carbonate hard tissues. This dissertation presents and discusses in great detail findings related to the microstructure, crystallographic texture, and nanomechanical properties of various calcium carbonate biomaterials. This work focuses mainly on analysing marine invertebrate hard tissues, such as bivalve molluscs shells, rhynchonellate brachiopod shells and sea urchin skeletal elements. Molluscs, particularly bivalves, are globally widespread and present in highly diverse marine environments. Bivalves are renowned for their high diversity in lifestyles and for generating hard tissues with hierarchical structures. About 15 main calcium carbonate microstructures are recognised today in modern bivalve shells. An important part of the bivalve composite hard tissue is the organic matrix, which regulates and organises the crystallographic orientation and morphology of the inorganic minerals. Bivalve molluscs can secrete four main carbonate polymorphs, namely calcite, aragonite, amorphous calcium carbonate or (rarely) vaterite and use the biomineralised hard tissues mainly for protection of the soft body, but also to pursue diverse lifestyles. To gain insight into the hierarchy, microstructure, and nanomechanical properties of bivalves, I characterised differently cut shells of 30 different species from 11 orders by EBSD analysis, thermogravimetric analysis, electron and confocal laser microscopy imaging, and nanomechanical testing. The structural results provide a detailed illustration of the microstructure and changeovers between the different layers. If the microstructures feature the same calcium carbonate phase, the changeover is generally smooth, with the initial crystallographic texture being transmitted from the shell portion of the adjacent layer. This biomineralisation mechanism is particularly fascinating for the aragonitic myostraca, the bivalve adductor muscle attachment sites, as their microstructure generation is largely influenced by competitive growth determinants rather than biological control. Bivalve myostraca are not only interesting because they enable the organism to form a strong connection to the adductor muscle fibres, but also because of their outstanding nanomechanical properties, such as a significantly enhanced hardness, compared to other shell layers or geological aragonite. The microstructure and nanomechanical properties of myostraca from different bivalve orders and bivalves following different lifestyles (such as burrowing, swimming, or attaching to substrates) are characterised. Previous studies have indicated that bivalve myostraca are strictly conservative in microstructure and texture, consistently forming large, prismatic units. However, the measurements presented in this work reveal a broad diversity in myostracal microstructures, demonstrating how the different crystal arrangements, textures, and twinning modes of the myostraca and other shell layers can influence material properties. Like bivalve molluscs, rhynchonellate brachiopods also use specialised epithelial cells to mediate crystal growth at muscle attachment sites of their calcium carbonate shells. Although rhynchonellates and bivalves share similar living environments, lifestyles and modes of biomineralisation, they are biologically distant organisms. Thus, comparing the microstructural similarities and differences between rhynchonellates and bivalves provides insight into the biological convergence of muscle attachment sites and the different approaches for forming these important organic-inorganic interface structures. High-resolution EBSD measurements for the shells of modern two- and three-layered rhynchonellates show that the general microstructure of brachiopod muscle attachment sites differs strongly from bivalve shells in both calcium carbonate phase and crystal morphology. However, the muscle attachment sites of both invertebrate classes share some characteristics that may be important for a strong muscle-shell attachment. This includes the adoption of crystallographic texture from adjacent shell layers with the same calcium carbonate phase, as well as the co-orientation of crystallographic c-axes parallel to the muscle bundles at muscle attachment sites. In contrast to the biomineralised hard tissues of bivalved invertebrates, nucleation of calcitic skeletal elements in echinoids involves intracellular processes. The latter usually comprises an intricate arrangement of highly co-oriented trabeculae into various porous, yet robust stereom architectures, historically described as single-crystalline. Applying the unprecedented EBSD data analysis method of pattern matching to different stereom architectures in the tests and spines of Cidaris cidaris and Paracentrotus lividus, this work reveals the presence and distribution of internal small-angle misorientations. As these misorientations are mostly located at the trabecular junctions, the quality of single-crystallinity appears to be influenced by the respective stereom architecture; however, sea urchin calcite cannot be considered single-crystalline invariably. Furthermore, I demonstrate the presence of poorly co-oriented, polycrystalline areas at both muscle attachment sites of echinoids (at test tubercles and the base of the spines) and the cortex, which encases the primary and secondary spines of C. cidaris. The microstructure generation of the cortex is determined by two factors: (i) The cortex crystals nucleate with c-axes oriented perpendicular to the stereom, which functions as a nucleation template. Thus, the smoothness of the outer stereom surface determines the texture of the initial cortex portion. (ii) A competitive growth mechanism determines the further microstructure and texture of the cortex. Furthermore, this study demonstrates how the EBSD pattern matching method can improve the angular precision and reveal small-angle misorientations within highly co-oriented structures, such as the sea urchin stereom. By employing this method to other calcium carbonate biomaterials, such as eggshell grains, myostracal prisms or large columns in bivalve shells, I highlight in great detail their crystallographic texture and microstructure, demonstrating the large potential of this data evaluation technique for future studies.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Themengebiete: 500 Naturwissenschaften und Mathematik ; 500 Naturwissenschaften und Mathematik > 550 Geowissenschaften
• Fakultäten: Fakultät für Geowissenschaften
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 22. Oktober 2025
• 1. Bericht­erstatter:in: Schmahl, Wolfgang