Crystallization of biomimetic calcite aggregates in hydrogel systems

Abstract

The formation of biological hard tissues takes place in hydrous gelatinous environments rich in polysaccharides, proteins and glycoproteins. These organic components are arranged in compliant three-dimensional matrices that direct the crystallization of the mineral component, within which they become occluded as porous membranes and networks of fibrils. The composite nature of biological hard tissues together to their distinct hierarchical architecture provide them with enhanced mechanical properties that are highly desirable in man-made materials. Designing successful routes to produce high-performance bio-inspired organic-inorganic composites relies on our understanding of the parameters that control the occlusion of organic matrices within growing crystals and the effect that this occlusion has on the micro-structuring of the mineral. Artificial hydrogels that have traditionally been used as crystallization platforms to produce large crystals of sparingly soluble phases share numerous features with the organic matrices found in hard tissues. Interestingly, these hydrogels also become occluded within crystals during growth. The way of gel occlusion is distinct and relates to characteristics that are specific of the type of hydrogel, like the nature of the interactions that hold its matrix together and its porosity.1 Gel occlusion is further modulated by physicochemical conditions in the system, which determine supersaturation and growth rate, and their evolution.2 Here an overview of the morphological and microstructural characteristics, as uncovered by Field Emission Gun Scanning Electron Microscopy (FEGSEM) and Electron Backscattered Diffraction (EBSD), of gel-calcite composites grown in a variety of gels (silica, gelatin, agar, agarose, and gelatin-agarose mixtures) is presented. Differences in amount and organization of occluded gel polymeric matrices within the composites are discussed taking into consideration the gel mechanical response to crystallization pressure. Finally, a correlation between the characteristics of occluded gel distribution and microstructural features of the composites is stablished.